Viessmann Vitodens 100-w trouble maintaining central heating temp

[dr911t]

I have a 2017 Viessmann Vitodens 100-W condensing gas boiler connected to a radiant floor heating manifold. No external pump. 120m2 floor area. Recently had the annual service. Boiler has been on for about a week now heating the underfloor cement slab.

It is my impression that the boiler seems to have trouble maintaining the central heating temperature (at any rotary dial setting) on the digital display.
Burner starts and heats up water (central heating side), only 1 notch on the modulation indicator, heats to ~40-50C for 30 seconds then stops and water temp goes down always to 24-27C for some minutes then cycle starts again ... Same on all dial settings 1-2-3-4-5.
Outgoing water pipe is hot. The return of the central floor heating water is colder to the touch. Return pipes temp ~25-26C. Floor temp is around 22-24C. Temperature in the rooms 20.5-22C.
Room thermostat on MAX in order to keep the flow open (could this be an issue? - I wanted to see how hot the rooms would get). Rooms further away from the boiler seem colder by ~2C. Didn't notice this last year.
Outdoor temp here now is around +14C day +2C night.
No outdoor sensor installed. No error codes.
Previous years have had it set between 3-4 and that gave a constant flow temp of around 35-45C as I remember. Do not recall this odd start/stop pattern. Previous heating seasons rooms at 23-24C and floor ~25-27C.
Domestic hot water is functioning fine. Set between 4-5. Slightly slow to heat up but gets to 60-70C and holds there temp for showers.

Any thoughts or am I just being worried for nothing?

 

[Gasmk1]

It’s not enough flow through boiler the difference between flow and return temp is not enough

 

[dr911t]

It’s not enough flow through boiler the difference between flow and return temp is not enough

Boiler is only connected to domestic hot water and radiant heating system, no radiators. No external pump..
Any thoughts what might be causing the flow issue... Circulation pump or some dirt blocking something in the boiler?

 

[king of pipes]

Has the boiler been down rated on the heating side it could possibly be trying to add to much heat to quickly you probably need around 12- 14 kw so you can loose the heat through the slab and keep the return temperature low i would set the boiler heating at 60°c , another thing to check is the heating manifold are the actuators opening up allowing heated water through the heating loops these can become stuck? You could try removing a couple of acuator heads to see if the boiler behaviour changes are the room thermostats operating, is manifold pump. Kop operating

 

[John.g]

You should be able to see the flow target temperature in the menu somewhere, when the boiler fires up, watch the flow temperature and see if it exceeds the target by +5C or more before modulation, if so, then the boiler will cycle on/off..

 

[Ric2013]

Flow is 45ish and return is 25ish?

How is this 'not enough' temperature difference between flow and return?

I'm wondering whether all UFH heating zones are receiving sufficient flow or whther some have airlocked and so this is restricting flow though the boiler. Or is the UFH pump setting has been altered, assuming you aren't relying on the boiler pump to power the whole system?

It is possible the boiler is receiving insufficient flow and going into an anti-cycle mode in which it switches off for a bit to cool down, but with the very good modulation ability of a Veissmann, the flow would have to be very low indeed to cause this.

 

[John.g]

Looks like cycling mode alright, burner may be tripping because of what I said above or the deltaT is > 30C which will also probably cause cycling, I'd say the former is more likely.
I don't think there is a UFH manifold pump "No external pump", probably because of no rads.

 

[Gasmk1]

It is the flow not being enough I had this on one I fitted years ago the temperature rise is too fast for the flow so shuts off and drops back to 40 or 50 then comrs back on just a constant cycle.

 

[Deleted member 123817]

It should be easy enough to get the flow rate from the individual flow tubes on the loops.

 

[dr911t]

Thanks for all the answers and help troubleshooting.

Increased the central heating flow temp dial to middle of 3-4. Burner fires 1 bar, flow seems to go up to 50-60C then stabilize down to ~27.5C. This takes about 1 minute. Then it waits around 4-5 minutes at this temp with the burner off and then the whole loop starts again.

Flow temp out of the boiler goes to around 45C then slowly settles down toward 27.5
When flow temp settles it’s flow 27.5 - return 25.5
When flow temp settles start loop 26.5 - end loop 24.4
All thermostats open. Actuators function well. Flow dials show 1L/min in most. 0L/min if i turn off one of the actuators.

The Viessmann certified guys did a cleaning on it a few weeks ago but I don't recall them cleaning the heat exchanger plate. Could a blocking there or some incomplete cleaning cause the flow issues inside the boiler? They did however really quickly replaced the UFH logic/thermostat control board outside the boiler as central heating setting wasn't starting at all. Was a bit more cost than I was expecting for the service.

Has the boiler been down rated on the heating side it could possibly be trying to add to much heat to quickly you probably need around 12- 14 kw so you can loose the heat through the slab and keep the return temperature low i would set the boiler heating at 60°c , another thing to check is the heating manifold are the actuators opening up allowing heated water through the heating loops these can become stuck? You could try removing a couple of acuator heads to see if the boiler behaviour changes are the room thermostats operating, is manifold pump. Kop operating

I'm on the middle floor and the place downstairs is under renovation, no heating working there but it's been like that for years ever since we've installed our underfloor heating and Viessmann. Probably solid loss of heating from the slab but don't recall this boiler behavior in previous heating seasons. 60c would really be pushing it as I was advised to not run the UHF more than 45c even.

You should be able to see the flow target temperature in the menu somewhere, when the boiler fires up, watch the flow temperature and see if it exceeds the target by +5C or more before modulation, if so, then the boiler will cycle on/off..

It's a Viessmann Vitodens 100-W B1KA-26, so no menu for heat setting just the turn dials with 1-2-3-4-5-6 setting. No outside sensor installed so as I understand, no heating curve. There is no fixed temp in the documentation for each of those settings so I'm not sure which is the target temp for each setting.

Flow is 45ish and return is 25ish?

How is this 'not enough' temperature difference between flow and return?

I'm wondering whether all UFH heating zones are receiving sufficient flow or whther some have airlocked and so this is restricting flow though the boiler. Or is the UFH pump setting has been altered, assuming you aren't relying on the boiler pump to power the whole system?

It is possible the boiler is receiving insufficient flow and going into an anti-cycle mode in which it switches off for a bit to cool down, but with the very good modulation ability of a Veissmann, the flow would have to be very low indeed to cause this.

Correct on the temps but the flow stays at 45 for very little time. Maybe half a minute if that.
It's the boiler pump only, no external pump. 120m2 coverage across 6 loops. Wasn't a problem in previous heating seasons.

Looks like cycling mode alright, burner may be tripping because of what I said above or the deltaT is > 30C which will also probably cause cycling, I'd say the former is more likely.
I don't think there is a UFH manifold pump "No external pump", probably because of no rads.

Maybe pump is failing?

It is the flow not being enough I had this on one I fitted years ago the temperature rise is too fast for the flow so shuts off and drops back to 40 or 50 then comrs back on just a constant cycle.

Exactly. What was the solution?

It should be easy enough to get the flow rate from the individual flow tubes on the loops.

~1.0L/min

 

[dr911t]

Here are two videos of one of the cycles and the strange noise.

 

[ShaunCorbs]

How many zones ?

 

[dr911t]

How many zones ?

120m2 floor area
6 thermostats
9 actuators on a 9 port Fornara Manifold

 

[ShaunCorbs]

So the heat load / capacity of the floor approximately 5kw at 40dc if all zones are 1lpm eg 9lpm total

So yep flow issue eg boiler too big / load isn’t enough for min output

 

[dr911t]

So the heat load / capacity of the floor approximately 5kw at 40dc if all zones are 1lpm eg 9lpm total

So yep flow issue eg boiler too big / load isn’t enough for min output

I see. The thing is this system has been working 'normal' for last 4 heating seasons. This problem appeared at the start of this year's heating season and after a "cleaning/service". Could there be a pump issue that's slowing down the flow rate? I don't recall what the rate was before but maybe closer to 2L/min. Not sure exactly. I posted a couple videos above of the cycle the boiler goes through as well as the noise (second video in conversion queue).

 

[ShaunCorbs]

Could be your main heat exchanger is dirty / slightly blocked, does it have the same issue on hot water ?

 

[dr911t]

Could be your main heat exchanger is dirty / slightly blocked, does it have the same issue on hot water ?

Domestic hot water is perfect. Runs smooth to ~80C and holds temp. Boiler fires on high for hot water.

 

[John.g]

1LPM seems very low alright (9 LPM total) but as it was ~ 2LPM last year then except some new restriction in the system its possible that some setting or other was changed in the recent service the most obvious is the pump speed changed from its default speed3 to speed1, while this still shouldn't half the flowrate it would certainly knock it back a lot, this mightn't have a huge effect on DHW output especially if the boiler temp is 80C. It looks as if the boiler isn't able to modulate down fast enough after ignition to its SP (+5C). It may also be worth looking to see if its range rated and then reduce it down initially to say 5kw or so which should help to get that boiler away.
It doesn't mention the minimum flowrate, only the max flowrates for the different models and I don't see the minimum boiler outputs either. A 30kw boiler might fire at 70% output for 30/60 secs before modulating so a flowrate of 9LPM will mean a initial deltaT of 33C so theoretically at any rate the boiler SP would need to be ~ 55/60C assuming return temp at 25C, on fire up. If the UFH flowrate was 18 LPM then the deltaT might only be 17C allowing a SP of 35/40C.
You could try setting the SP at say 65C but only for a few minutes (as its UFH) to prove that it will fire continuously. You could also increase the UFH flowrates but I would exhaust the other possibilitias first.

 

[king of pipes]

You simply don't have enough flow through the system especially if the heating is off on a lower floor ? post some pictures of the heating manifold is there a pump and blending valve fitted to the to push the heating through the loops as below ? or are you relying on the boiler pump and running the boiler at a low temperature

 

[dr911t]

It looks as if the boiler isn't able to modulate down fast enough after ignition to its SP (+5C). It may also be worth looking to see if its range rated and then reduce it down initially to say 5kw or so which should help to get that boiler away.

I just checked the Vitodens 100-W B1KA-26 has a maximum heat output settings and it's on 5 bars (high fire). However, as showed in the video, when the burner fires in the central heating, it only fires on 1 bar during the short cycles. Would changing the max heating output to 3 bars ( page 35/36 https://viessmanndirect.co.uk/files//461be7f2-8610-4b7f-a942-a9d1010d8602/Vitodens 100-W B1HA B1KA Installation and Service Manual.pdf )

You simply don't have enough flow through the system especially if the heating is off on a lower floor ? post some pictures of the heating manifold is there a pump and blending valve fitted to the to push the heating through the loops as below ? or are you relying on the boiler pump and running the boiler at a low temperature

The 100-W B1KA-26 combi-condensating has a built in circulation pump for the central heating side. I believe it is the Grundfos UMP3 15-75. There is no external pump on the manifold. In regards to the speed, it seems it cannot be changed if not using the outside temperature sensor.
For heating mode without outside temperature sensor: Rated heating output in kW 19 26 30 35 Speed in % 72 80 86 100 The speed cannot be changed.

Lower floor is neighbor. The boiler and UHF is all on a 120m2 one floor apartment. Worked very well for last 4 heating seasons without any noise, short cycles but maybe since there was no external pump, this had put too much strain on the internal pump and it's not failing/clogged?

 

[John.g]

If you take the low fire literally then the boiler ignites at ~ 6.5kw which @ 9 LPM gives a deltaT of 10.4C which should allow a SP of 35C (25C return) without problems, Viessmann can advise as to whether it actually does ignite at these low settings or not, if it doesn't then IMO that is the likely culprit for this very rapid cycling.
Also as was suggested above if the UFH demand is < 6.5kw then the boiler will also cycle but should certainly fire for number of minutes except in the very unlikely event that no heat is being extracted by the UFH.

You can reduce the max output as shown in pages 35/36, the instructions are not quite clear to me as it shows a tap symbol with 60C, if they are clear to you then I would reduce the output to minimum as well and see does it help to get the boiler away.

Not having a dedicated manifold pump shouldn,t affect the life of the boiler circ pump in any way.

 

[John.g]

Long thread here re re short cycling, the poster states that this boiler does fire up at 70% output and will cutout at SP+7C.

Viessmann boiler short cycling - UK Plumbers Forums: Plumbing Advice | Plumbing and Gas Engineer Forums

Hi, I have noticed my boiler Viessmann Vitodens 100-W(B1HC-26) makes a series of short cycling before starting to heat. Boiler is controlled by Baxi uSense room thermostat connected via Opentherm. I installed some monitoring equipment to get the better understanding of the problem. Bellow is...

www.plumbersforums.net

 

[king of pipes]

I've never had much success with ufh without its own pump if I'm honest there is so much resistance on the ufh loops for the internal pump to circulate it doesn't take much to stop the flow there's probably nothing wrong with the boiler it's the ufh the waters not flowing through the loops fast enough the one below worked just fine on a 37 kw system boiler , I'm fitting a Viessmann 100 w 35 kw combi on Monday I'll study the manual and see if there's a solution to your problem . Regards kop

 

[dr911t]

Update on the problem - here is what the Viessmann tech did so far:

• Replaced plate heat exchanger
• Flushed floor heating & added ADEY MC3+ to the floor heating system
• Replaced boiler internal circulation Grundfos pump
• Power flushed with chemicals/acid the main heat exchanger

Total spent: €475 parts + €325 labor.

Next step diagnosis from Viessmann tech: clogged heat exchanger - replace it.
Estimation: €650 parts + labor.

Is this standard way troubleshooting is done at Viessmann and on gas boilers, part by part replacement at the customer's expense whether or not that particular part is the problem? I'm very shocked that the heat exchanger would 'clog' after 5 years especially after the 2-3h of chemical power flushing did zero to improve the symptoms.

I asked the tech if we order the new heat exchanger and it turns out to not solve the issue if they would take it back so that's left to be seen what they answer. What do you guys suggest we do? Update the Christmas gift to myself to a new boiler&install and throw it in the garbage or take the chances continuing the whack-a-mole with the parts?

 

[ShaunCorbs]

You could of bought a new boiler by now at them costs you would still have to carry out a flush but no that’s not the right way to fault find just throw parts at it

 

[Deleted member 123817]

Inchrease each loop flowrate to 2 LPM and see how it goes,

 

[dr911t]

You could of bought a new boiler by now at them costs you would still have to carry out a flush but no that’s not the right way to fault find just throw parts at it

That's the stage I'm getting at bit by bit. I previously lived in warmer climates where electric or HVAC was enough. Gas boilers are new to me.
First the tech came and said it's the plate heat exchanger, replaced that.. didn't help. Let's add some ADEY chemical in the whole system to clear it out.
Then they came back and said well it's probably the pump since there's still no flow. Replaced that... boiler operation was the same.
Then they said it's scale/debris in the heat exchanger so let's do an acid pressure clean, so we did that and boiler operation was the same.
You can see how we go to this but at which point should I have put my foot down and said enough part replacement, new boiler time. A new boiler plus install is not €1000 btw.

 

[dr911t]

Inchrease each loop flowrate to 2 LPM and see how it goes,

all loops are fully open and only getting 1L/min not matter how many thermostats are open or closed.

 

[Deleted member 123817]

Is there a automatic bypass valve (ABV) installed. Some boilers also have a adjustable internal bypass,

 

[dr911t]

Is there a automatic bypass valve (ABV) installed. Some boilers also have a adjustable internal bypass,

No ABV, and I'm not aware of the Viessmann 100-W having an adjustable internal bypass either.

 

[king of pipes]

I'd be very surprised if the boilers main heat exchanger is blocked it's water ways are huge, as said before it's a lack of circulation the boilers pump isn't man enough you need a system pump in my view . Kop

 

[dr911t]

I'd be very surprised if the boilers main heat exchanger is blocked it's water ways are huge, as said before it's a lack of circulation the boilers pump isn't man enough you need a system pump in my view . Kop

The tech kinda blew air through the flow and said it's not getting through to the return easily hence the diagnosis of a blocked heat exchanger causing lack of circulation.

@king of pipes - me too. the tech ran a high pressure pump with and acid solution through the flow and return of the boiler for 2 hours. Quite a bit of yellowish lime scale came out but I'm just surprised the boiler behaves the same (not better not worse) right after this procedure.We are stumped at what is causing lack of circulation, the system was running fine for 4 heating seasons. The original internal Grundfos UPM3 15-75 was just replaced a few days ago before the chemical flush.

 

[Deleted member 123817]

Is there a HX pressure loss chart available for this boiler, that pump should give a 6m head for almost double that flow rate. Also if another pump is installed then might need to be done with TMV/circulating pump and increase the boiler flow temp to say 65C, the boiler circ pump will then only need to supply 30 to 50% of the total circulating flow rate required?

 

[dr911t]

Is there a HX pressure loss chart available for this boiler, that pump should give a 6m head for almost double that flow rate. Also if another pump is installed then might need to be done with TMV/circulating pump and increase the boiler flow temp to say 65C, the boiler circ pump will then only need to supply 30 to 50% of the total circulating flow rate required?

The heating side works continuously without short cycling at boiler flow temp of 50+C return is about 30C.. flow-rate 1L/min.
Anything under that short cycles as boiler temp overshoots.

Maybe here on page 9? https://viessmanndirect.co.uk/files...a6dd00a6c20c/B1HA_B1KA_111-WTechGuide (1).pdf
Internal pump was suspected faulty and replaced with new original Viessmann Grundfos UPM3 15-75 so the same pump specs.

 

[Deleted member 123817]

The boiler should be delivering 12.5Kw at a dT of 20C & 1 LPM.
The plump at 100% speed is a 6.8M.
Assuming 100M of 12mm piping/loop then the residual pump head available is 5.3M and the pipe line losses - 1,5M allowng 20% extra. You should have no problem in increasing the flow rate to at least 1.5LP. It does state "Upper operational limit" and shows a max of 2M head so maybe enquire re this.

 

[Deleted member 123817]

1 LPM or 9LPM for 9 loops

 

[ShaunCorbs]

1 LPM or 9LPM for 9 loops

Jon please use your original account

 

[Deleted member 123817]

Can someone delete the "johntheo" account ?

 

[John.g]

That "upper operational limit is only the pump end of curve conditions so as the pump is new it does point to a restriction in the Hx or else the loops are full of sludge. You may have to prove both clear .

 

[dr911t]

That "upper operational limit is only the pump end of curve conditions so as the pump is new it does point to a restriction in the Hx or else the loops are full of sludge. You may have to prove both clear .

No way to increase flow really as pump speed is controlled by boiler and all loops are fully open. Viessmann tech suspects calcified main heat exchanger due to limescale buildup but theory doesn't explain boiler noise nor still fluctuating tap temp with brand new plate heat exchanger.

I'm leaning toward these pieces of magnetite floating around the circuits causing blockages.
There was black oxide in the old plate heat exchanger so that was cleaned out once but possibly coming from boiler parts corrosion as we have no rads? Thinking to install a Magnaclean or Fernox TF1 or equivalent on the return hopefully it picks up whatever is lying around.

 

[John.g]

If you are handy yourself, maybe connect mains hose to UFH manifold with return disconnected, you will then get a feel for head required to give a flow rate of 1LPM/Loop by cracking open the mains.
You said originally no problems with DHW on old Hx but now some problems with new HX?

 

[dr911t]

If you are handy yourself, maybe connect mains hose to UFH manifold with return disconnected, you will then get a feel for head required to give a flow rate of 1LPM/Loop by cracking open the mains.
You said originally no problems with DHW on old Hx but now some problems with new HX?

Will try.
New circulation pump inside the Vitodens had zero improvement or aggravation of the symptoms. Same low flow rate, same short cycling overshooting of heating temp on lower settings and sometimes luke warm/hot at the DHW taps/shower. Funny metallic gurgling noise from back of pump (both old and now) when boiler is heating but when also idle with pump operational.

Thinking hoses are flaking and constantly obstructing the plate HE, main HE and pump. Possibly in the UFH loops as well.

 

[dr911t]

@John.g - I aired every UFH loop via tap mains pressure and it was ridiculous how low of a head pressure was required for 1L/m! At full blast the gauges were flat down at 4L/min. So it's definitely a circulation problem. Although some of the loops had quite a bit of air in them, even after the slighty tedious work to refill them all and air them one by one, the problem is the same and the boiler behaves identical. Maybe the floor heating has better coverage and more even heating. Definitely a circulation issue, likely boiler side as no blockages in the UFH.

Any ideas what I should do at this point?

• find yet another Viessmann service partner to come and look and pay the call out fee to get an additional opinion
• agree to replace the heat exchanger and hoses €1200+... and pray it's the actual problem as "no return policy on the HX" and if that doesn't work...
• buy a new boiler + install @ at least double the cost of the HX replacement, try to sell the old one for parts to make up some of the $ spent on the part replacement.

What worries me about the heat exchanger suggestion is the strange bubbling noise behind the pump, maybe due to the alleged blocked heat exchanger some water is flowing backward or bubbles somehow making this noise? I'm worried it could also be something with the AquaBloc which is all one big (expensive as usual) piece on the Viessmann which causes some obstruction or problem with circulation.

 

[John.g]

Maybe try and figure something out from the DHW (combi?) side, set the boiler HW setting to 3 or4? then open a hot tap fully or even two, measure these flow rate(s) and measure the DHW temp, see if you can can read off the boiler flow&return temps or just the flow temp if only one available, may be able to figure/confirm something from these readings.

 

[dr911t]

Maybe try and figure something out from the DHW (combi?) side, set the boiler HW setting to 3 or4? then open a hot tap fully or even two, measure these flow rate(s) and measure the DHW temp, see if you can can read off the boiler flow&return temps or just the flow temp if only one available, may be able to figure/confirm something from these readings.

DHW Rotary Knob Setting: 4
Hot water tap on full: 10.25L/min
Hot water temp: 40C
Boiler temp: 65-80C

Heating Rotary Knob Setting: Between 4 & 5
Heating flow: 47C
Heating return: 27.5C
Boiler temp: 49-50C
Manifold flow meters: ~1L/min flow meters open fully - all thermostats on, 9 loops.

After turning off the hot water, if I put one hand on the top of the heating flow pipe coming from the boiler and the second hand 1 meter lower, I can feel that the heated water comes through very slowly.

Attached is a recording of the strange noise behind the circulation pump inside the boiler coming somewhere from the Aquablock, behind the pump.
-no water leak or any air from airvent nor the pressure release valve
-system and boiler were recently vented.
-pump is a Grundfos UPM3 15-75. Replaced recently as was the suspect problem, however boiler behavior and noise same as with original 4 year old pump.
-noise reduces in intensity but does not go away after a few moments when pump circulating at lower power in central heating mode.

 

[king of pipes]

It does sound dodgy the only way to find out is strip it out which will need a engineer with a spare pump , as I've said before in my opinion you need a external pump on the heating manifold to push the heating water through the heating loops and back to the boiler , you haven't posted any pictures of your system as requested so we're all stabbing in the dark really.

 

[John.g]

Certainly sounds like a bag of nails and metallic like noise.

From DHW tests the boiler output was 21.5kw (assuming 10c mains) difficult to say what the primary flowrate was but would estimate ~ 10 LPM, what is a bit strange is the variation in boiler temperature, 65C/80C, I would have thought that the boiler would just modulate down to maintain that 21.5kw, did you notice if the boiler ran continuously during this test or was it cycling?, if so then one would expect DHW temp fluctuations as well.
Maybe just repeat this test sometime with DHW setting to max, I would expect the boiler to run continuously as the DHW heating demand is then ~ 36kw, don't know what your boiler output is.

From heating tests the boiler output was 13.8kw at a flowrate of 9LPM.

 

[dr911t]

@king of pipes Here are the photos of the installation. I am aware it is quite a hack job - not done clean and well. Looking back I should have done more research on the installer and asked why no mixer valve, external circulation pump, pressure and temp gauges were installed, which inhibitor or water treatment they used for the fill and asked to do a neat organization of cables and floor. At least I know more now about it for the next project... Live and learn 🙂

RE: external pump... what i'm wondering is why it worked 'well' for 4-5 years with decent flow rates, no short cycling, ok steady temperature control

 

[king of pipes]

I can't answer that but what I can say is if you look at my first ever post answering you there a picture of how it should be done ? , a small low loss header with a separate pump fitted would also do the job on your existing set up look at the Viessmann installation guide it may give you a better understanding . Kop

 

[John.g]

Is that the system with the manifold circ pump and where you run the boiler at say 60/65C and UFH at 35/35C which reduces the primary flow rate substantially?. Your post #23.

 

[Gasmk1]

I can't answer that but what I can say is if you look at my first ever post answering you there a picture of how it should be done ? , a small low loss header with a separate pump fitted would also do the job on your existing set up look at the Viessmann installation guide it may give you a better understanding . Kop

Totally agree

 

[John.g]

If the pump speed is limited to 72% then the 7.5M pump becomes < 4M which probably isn't enough for 12mm piping but strange that the system worked OK apparently for years.

 

[dr911t]

I can't answer that but what I can say is if you look at my first ever post answering you there a picture of how it should be done ? , a small low loss header with a separate pump fitted would also do the job on your existing set up look at the Viessmann installation guide it may give you a better understanding . Kop

@king of pipes - wholeheartedly agree. After all the research and help from this forum I know much better of what a proper UFH should look like.
The OEM circulation pump inside the boiler is the Grundfos UPM3 15-75 which also comes as a UFH pump. If I were to add another pump to the system, wouldn't it basically be running two pumps? Is it because of the thickness of pipes that so much pumping pressure would be needed? If we had say 10 rads in the flat instead, would an external pump not be necessary then because of larger pipes? We are on a one level 120m2 with 16mm PERT-AL pipes.

Is that the system with the manifold circ pump and where you run the boiler at say 60/65C and UFH at 35/35C which reduces the primary flow rate substantially?. Your post #23.

@John.g - are you referring to @king of pipes ' setup?

If the pump speed is limited to 72% then the 7.5M pump becomes < 4M which probably isn't enough for 12mm piping but strange that the system worked OK apparently for years.

Piping is 16mm. 120m2.
Agree it's strange that it worked but I'm not so sure the manifold area is the problem since the noise behind the pump and possible circulation seem to be an issue also when just DHW is being used (if I turn off central heating).

 

[John.g]

Well, I would be quite surprised if one pump couldn't give adequate flow through 16mm piping, my daughter has a mixture of rads/UFH serviced with a 16 year old Grundfos selectric 6M (one) pump, all heating working perfectly.

Don't know what length the loops are but assuming 100M then the pressure drop through each loop is 0.152M @ 1LPM, 0.54M @ 2LPM and 0.82M at 2.5LPM, these numbers become 0.18M, 0.66M & 1.0M for 120M loops.

I know you are in a dilemma after spending all that money but if I were you (and should have suggested this previously) I would go away and buy a 0 to 10M pressure gauge, install it in the UFH flow side of the manifold and run your system, that will tell a lot and may save you a pile of money at the end of the day. Also find out if that circ pump is restricted to 72% speed.

Yes, I was referring to KOPs post/photo.

 

[dr911t]

Well, I would be quite surprised if one pump couldn't give adequate flow through 16mm piping, my daughter has a mixture of rads/UFH serviced with a 16 year old Grundfos selectric 6M (one) pump, all heating working perfectly.

Don't know what length the loops are but assuming 100M then the pressure drop through each loop is 0.152M @ 1LPM, 0.54M @ 2LPM and 0.82M at 2.5LPM, these numbers become 0.18M, 0.66M & 1.0M for 120M loops.

I know you are in a dilemma after spending all that money but if I were you (and should have suggested this previously) I would go away and buy a 0 to 10M pressure gauge, install it in the UFH flow side of the manifold and run your system, that will tell a lot and may save you a pile of money at the end of the day. Also find out if that circ pump is restricted to 72% speed.

Yes, I was referring to KOPs post/photo.

Thanks.. I would assume also loops are less than 100M since some of the larger 20-40m2 rooms were split into 2 or 3 loops but I don't have the schematics used during construction. Circulation pump is at 80% speed.

 

[John.g]

Didn't use proper wall thickness for that 16mm piping so I think a ID of 12mm so the corrected numbers for 100M loops are 0.32M @ 1LPM, 1.16M @ 2LPM & 1.75M @ 2.5LPM so the boiler circ pump running at 80% should still give a flow rate > 2LPM.

 

[Ric2013]

I've seen far worse installations. In this case no mixing valve is required since the boiler is itself supplying the water at the required low temperature. Short pipe runs to the manifold mean not too much loss of head. If it used to work, then the basic design may not be at fault.

If the OP isn't getting more than 1 lpm even with only one circuit open, there is very obviously something wrong. The fact that water is flowing at all would seem to rule out airlocks in the UFH pipework, but there may be some kind of algal blockage that has developed over the summer season. Nothing to stop the OP isolating the UFH manifold from the boiler and mains flushing a circuit using a hose to see what the water is like and if there is any blockage.

EDIT If number of circuits open does not appear to affect the flow through the circuits that are open then pump would appear to be on a constant pressure setting.

 

[John.g]

Have you ever noticed/noted the manifold pressure (if PG installed) on a UFH system without a manifold pump.

 

[Ric2013]

Have you ever noticed/noted the manifold pressure (if PG installed) on a UFH system without a manifold pump.

If that's a question for me, then afraid the answer is no, but it would be very useful information if someone here has it.

Thinking about it, it's not uncommon to see a 2kW radiator run on a 10m or more run of 15mm copper tube which has similar internal bore to 16mm MLCP UFH pipe and this could be achieved at a fairly low head. A UFH circuit may be much longer, but won't be 2kW. So there oughtn't be a lack of pressure to drive at least one circuit (...unless the pump ports/boiler H Ex etc. are obstructed) or the UFH circuits are themselves obstructed.

If an inline flow gauge is an option, what if the OP were to create a short circuit by attaching a braided hose across the two hose connectors at the end of manifold? If a decent flow were then obtained, that would point to the issue being in the UFH circuits themselves rather than in the boiler itself?

 

[John.g]

His low flow rate may be partly be because the "Maximum speed setting in the delivered condition" for the boiler is 80% so this makes it a 4.5M pump, don't know if this setting can just be changed to the 100% rating of 6.8M which the previous pump may have been set to?. It also mentions residual head, not sure if this means the head available after allowing for the Hx. Setting may be changeable from the boiler menu??.

 

[Ric2013]

Hi John,

To probably slightly mis-quote "The Lion the Witch and the Wardrobe", the OP states the system worked correctly for 4 years and so either he is lying, or he is telling the truth, or he is mad (or mistaken).

I think we need to work on the assumption that the OP's statement that the system has worked for 4 years is correct as otherwise this is a waste of everyone's time. I don't see why we are trying to prove the technical feasibility of a no external pump setup when 4 years of working order has already done this. My own UFH manifold at my own home tees off the primary flow in 15mm which is wrong by every rule of thumb and I had to defend the decision on technical grounds when discussing with colleagues until after I'd installed it, after which, the fact that it functioned perfectly made such defence of my 'undersized' pipework unnecessary.

While it may be interesting to explain why the OP's system should work, if it cannot be made to work, then some of the others may be right to say such a single-pump system can't work, but that implies that the OP is not correct to say it formerly did work. We cannot all be correct.

Looking at your chart, we can have up to 5.5lpm total before we fall below a 4 metre head, and if the circuits will have enough pressure on a 3m head (minus resistance of primaries and boiler itself) to run properly, then we have 13lpm available flow which should give each circuit about 1.5lpm which ought to be enough really. The problem is we don't know if a 3m head is available at the circuits, but an undersized pump would be indicated by dropping off of flow to each individual circuit as more and more circuits are opened. Looking at the data for the UPM3(K) AUTO XX-50 130/180 (N) which is, I think, the kind of pump that might be fitted to a UFH system, its highest constant pressure setting is only 3m anyway, so unless the boiler/primaries in the OP's case are robbing more than 1.5m head, he should have at least enough oomph to run 2 circuits at 1.5lpm with the pump you have shown the curves for.

At this point, we need to wait to hear back from the OP. From that funny noise in the boiler, I'm thinking the UFH circuits are not at fault and something is up with the boiler (which was the original assumption) but it would be nice to rule out the UFH, and it might be something really silly like someone had fiddled with the flow adjusters on the manifold, or an isolator valve is remaining partly shut although it looks open.

What about this kind of approach?:

How might the OP prove the manifold is at fault? I think your idea of testing the flow at a specific working head is good, but may be a job for a plumber rather than a DIY one, though I suppose we could guide the OP through the process.

How might the OP prove the boiler is at fault? It would be very easy to fit a low loss header and then find, if the boiler is at fault, this has 'cured' the flow issue through the manifold, but things still wouldn't work properly and it would probably reduce the boiler efficiency as well as be an expensive experiment.

 

[John.g]

The cheapest way to see what the actual head is, is to install a PG which the OP I would think is capable of doing since he flushed out the manifold, the other approach is to ask Viessmann if the circ pump can be set to 100% because I don't think anyone would be happy to run a UFH with a single 4.5M pump which this pump effectively is at 80% speed. The original pump may well have been set to 100% which would go a long way to explaining normal performance for a few years.

 

[Ric2013]

He could quite easily install a pressure gauge on the 3/4" BSP hose connector isolators. It would presumably show differential pressure relative to atmospheric so he'd need to subtract the system charge pressure from the pressure created by the pump in action, but it would give an idea.

Does the following sound right? If the assumed blockage is between the return rail and the pump inlet on the return side, you'd see a good pressure measured at the flow rail and at the return rail. If the blockage is the UFH circuits themselves (which might just be acting normally due to their inherent resistance), you'd see a lower pressure on the return rail than on the flow rail, and if there is not a decent pressure on either rail then the blockage is between the pump outlet and the flow rail, or the pump is not running properly for some reason.

If that's your plan, I think it's an excellent way of locating the problem and will help even if the problem proves to be internal to the boiler.

 

[John.g]

Yes, you are correct, he would really need 2 PGs, one on the flow, the other on the return, for some reason or other I thought it was a OV system so the PGs would need to be 0 to 3bar which makes it difficult to get a accurate differential reading of say 0.2bar between the two but worth trying at any rate?.

 

[dr911t]

OP here. Thanks for all the idea brainstorm.

I've isolated the manifold and flushed every loop for 5' with tap mains. Water is clear. No blockage. Bit of air in some loops but not much. Flow rates way over 4LPM (max of the gauges). I don't think there is a blockage in the manifold itself because as previously mentioned, even with one circuit open, flow rate is max 1.5LPM. If the boiler was fine but the manifold or UFH was the issue then hot water would be running perfect when CH is off however the boiler runs hot... 75-85C at times as it modulates up and down.

Viessmann & the tech's current theory is blocked main HX (calcified). Pump cannot get enough pressure through the main heat exchanger to move the water around. The sound possibly has something to do with it. Pump is brand new (few weeks old). This theory might be plausible although not sure if it explains the sound from the hydroblock. However Viessmann tech has done a 2h power wash with acid (don't ask me which) to remove limescale in the HX, didn't change anything in terms of boiler behavior or noise. There's nothing else behind there besides the PRV and automatic air vent. Could it be some back-flow into the pump due to the alleged 'blockage' in the HX?

Another strange thing, we do not have an outside temperature sensor and weather dependent mode on (installer couldn't get access to drill on a proper outdoor place or some reason like that) and the manual says without wether dependent mode, pump speed can't be changed, however I seem to be able to still do it. I can change the speed up to 100%. At 100% it just makes more noise like a car engine on high revs, but LPM flow doesn't change!

I'm going to look into finding some pressure gauges and fit them at the fill/drain connections of the manifold.

@Ric2013 I might qualify for the mad/mistaken part but if my memory serves me right, this noise never existed in previous years, boiler was running at low temps in CH without issues at 35-45C flow and flow meters were showing more than 2LPM so something went bonkers this year. All flow meters are fully open during this troubleshooting.

 

[John.g]

Why should the main Hx be calcified, its a closed system and except there is a constant leak somewhere and the system is being topped up with a auto fill PRV. If the system is manually topped once/twice/annum and the filling valve(s) are then shut then calcified Hx should not be a problem. Was this Hx ever changed?. Can understand if it was the DHW Hx was getting calcified due to constant mains water through one side of it.

One simple test you can do is to carefully note the boiler pressure before fire up and circ pump start up, as soon as the pump starts up, again carefully note the boiler pressure, if the E.vessel is teed in before the pump suction then the pressure should rise by the pump head, if teed in after the pump then the pressure may fall by the pump head.

Another manifold test which may be worth doing is to disconnect the flow and return, get one 0 to 1bar PG and install it in the flow side of the manifold or install it in the mains filling hose, then open the mains filling hose isolating valve slowly and note the pressure with flow through all the loops on together at say 1LPM/loop, 1.5LPM/loop, 2LPM/loop & 2.5LPM/loop. This should give a indication of the pump (differential) head required at these flow rates.

 

[king of pipes]

To many if buts and maybes going on here all the info the op needs is in the manufacturers instructions it's just a matter of studying them closely and putting it into practice, the boiler needs to be installed and configured to supply the correct amount of flow through the heat exchanger in my opinion it isn't .

 

[John.g]

Can you send on a link please KOP to that manual or just the pressure loss through the HX.

The OP states that he has seen flow rates of 2LPM= 18LPM/9loops or a boiler flow rate of 1080 LPH. My calculations show a required head of 1.16M (@2LPM) assuming 100M/loop, allow another 25% for elbows etc then the required head is ~ 1.5M or allowing another 72% still results in a required head of "only" 2M.
You can tell from your manual, the pressure loss through the (clean) Hx. I don't know what this is but assuming that the total pump head required is 4.5M then in your opinion should the boiler supply this without any problem or not?, ie a flowrate of 1080LPH @ 4.5M or so?.

dr might inform of the output of his boiler, if not already done so.

 

[king of pipes]

Here you go John busy on breakdowns I don't think it will I could be wrong thinking it needs a low loss header ? Kop

 

[dr911t]

Why should the main Hx be calcified, its a closed system and except there is a constant leak somewhere and the system is being topped up with a auto fill PRV. If the system is manually topped once/twice/annum and the filling valve(s) are then shut then calcified Hx should not be a problem. Was this Hx ever changed?. Can understand if it was the DHW Hx was getting calcified due to constant mains water through one side of it.

Closed system. Annual top-ups. No previous flush of heating water. Unknown what original installer put in the water if anything. The UFH is 120m2 so with an average of 5m of pipe per m2 and 0.15-0.17L per m of piping I'm estimating around 90-100L of water in the UFH. Is that enough to calcify a main HX? It's the original one. Boiler was new and installed 5 years ago.
The plate heat exchanger was recently replaced. Looking back I don't think it needed to, a flush and some chemicals would have been enough. It didn't look too calcified.

dr might inform of the output of his boiler, if not already done so.

It's the 2017 100-W B1KA 24kw combi.

it needs a low loss header

I was under the impression these are more for commercial setups with multiple boilers providing separation from multiple circulating pumps. I think if they would have done the job properly or had I had more knowledge at the time, then an external circulating pump, mixing thermostatic valve, low loss header, pressure/temp gauges, a filter, all should have been fitted.

 

[John.g]

Lack of inhibitor may cause Hx blockage due to corrosion.
Still can't find the dP across the main (clean) Hx.


Did you note the change in boiler pressure on circ pump start up.?

 

[hometech]

You have a return temperature back to the boiler that is not a true reflection of the system temp.

Hot water flow is returning back to the boiler too early without dissipating heat which means the flow is mixing with the return and raising the temperature of the return higher than it would otherwise be.

This will always be a function of the pipework route. If the route has not been changed then a valve is causing the issue.

Buy a digital thermometer and measure the temperatures along the pipe, at valves, joints etc. Where the temp rises or drops within a few centimeters is your problem point. it only takes a very small rise in return to create this issue.

As you are using low temperature water for circulation, your problem will be unimaginable to 99% of plumbers who only know about basic combi boilers and room thermostats set at 22 degrees.

If you want a more scientific approach rather than the random guesses, have a look at heat geek's site. Go back to basics, its so refreshing

Renew the heating industry - HeatGeek

Heat Geek offers expert advice on all aspects of the heating industry. Browse our knowledge hub online now.

www.heatgeek.com

 

[dr911t]

Lack of inhibitor may cause Hx blockage due to corrosion.
Still can't find the dP across the main (clean) Hx.


Did you note the change in boiler pressure on circ pump start up.?

CH On - Pump 70% - 1.5Bar - Flow ~1L/min
CH On - Pump 100% - 1.6Bar - Flow ~1L/min - pump noise increases. - boiler temp drops by about 4-5C after a few minutes.
Boiler Off - 1.4Bar
DHW On - CH off - 1.5Bar after pump start

The pressure meter does seem to wiggle a bit between 1.5-1.6 Bar during CH operation... bit odd.

In regards to adding a pressure gauge to the manifold: found one that has a 1/8 male connection, problem is the manifold intakes are 1/2 male connection too so having some trouble finding an adapter. I'd rather not attempt to remove the auto air vents. I

 

[dr911t]

You have a return temperature back to the boiler that is not a true reflection of the system temp.

Hot water flow is returning back to the boiler too early without dissipating heat which means the flow is mixing with the return and raising the temperature of the return higher than it would otherwise be.

This will always be a function of the pipework route. If the route has not been changed then a valve is causing the issue.

Buy a digital thermometer and measure the temperatures along the pipe, at valves, joints etc. Where the temp rises or drops within a few centimeters is your problem point. it only takes a very small rise in return to create this issue.

As you are using low temperature water for circulation, your problem will be unimaginable to 99% of plumbers who only know about basic combi boilers and room thermostats set at 22 degrees.

If you want a more scientific approach rather than the random guesses, have a look at heat geek's site. Go back to basics, its so refreshing

Renew the heating industry - HeatGeek

Heat Geek offers expert advice on all aspects of the heating industry. Browse our knowledge hub online now.

www.heatgeek.com

I think the problem is the opposite. Too much temperature drop due to not enough flow.

Heating Rotary Knob Setting: Between 4 & 5
Boiler temp: 49-50C
Heating flow: 47C
Heating return: 27.5C
Manifold flow meters: ~1L/min flow meters open fully - all thermostats on, 9 loops.

I have it set like this (for now to have heating until the issue is fixed) because it only operates on these temps and higher, any setting below just causes it to short cycle.

 

[IND_Nick]

As already discussed it definitely appears to be a circulation issues, I don't deal with Viessmann but are there any gauze filters on the return side of the boiler. Many manufacturers have them in the return valve or close by. If missed during a flush it can stay blocked for a while and force flow through the plate to plate rather than the desired heating circuit. Just a thought, may not be relevant on the Viessann though.

 

[dr911t]

As already discussed it definitely appears to be a circulation issues, I don't deal with Viessmann but are there any gauze filters on the return side of the boiler. Many manufacturers have them in the return valve or close by. If missed during a flush it can stay blocked for a while and force flow through the plate to plate rather than the desired heating circuit. Just a thought, may not be relevant on the Viessann though.

Don't think there is a gauze filter, at least it's not visible and nothing in the manuals.

I could close all loops on the UFH, drain the boiler from the return side via the return manifold to and see if a lot of junk comes up against the usual direction of flow. Maybe some flakes or something are stuck in the hydroblock. Then again, the Viessmann tech did a power wash from the flow to the return for a few hours in both directions and everything was the same after.... Running out of ideas.

 

[John.g]

That boiler gauge test only indicates that the pump head is at least 2M but of course is not conclusive.
There are really only a few constructive tests left to do IMO.

First, I don't think using two pressure gauges will tell you anything as I stated previously because of the need to install 0 to 2bar gauges and very difficult to see a accurate differential between the two.

I would attach a 0 to 1bar PG on a T piece on the mains hose and do the tests as suggested on the manifold.
I don't know if you can back flush the boiler Hx from the connections to the manifold as the boiler diverter valve I think defaults to the DHW side, you can try it but don't exceed 1 bar, if you do establish a flow then set the mains pressure at exactly 4M and measure the flow rate into a bucket for exactly 1 minute, this may give some indication of the state of fouling (if any) of the main Hx, it may of course depending on the diverter valve position mean that you are measuring the pressure loss through both the main Hx and the DHW PHEW but worth a test anyhow.

KOP kindly supplied a graph of the main Hx pressure loss which I only spotted after going back through the posts and it shows a pressure loss of 2M at 1080LPH (=2LPM/loop) and ~ 3.8M loss at 1350LPH (=2.5LPM/loop). my calculated manifold losses are 1.16M@2LPM and [email protected], so with a clean main Hx you theoretically require a pump head of 3.16M@2LPM and a pump head of [email protected].
A flow rate of 2.5LPM/loop is not achievable even with the pump at 100% but 2LPM is achievable at a pump speed of 86% or higher which bears out what you were getting in former times.
KOP also supplied the recommended limits for the use of a LLH which are < 400LPH or > 1400LPH so a flowrate of 2LPM(1080LPH) does not require the LLH.

 

[IND_Nick]

Don't think there is a gauze filter, at least it's not visible and nothing in the manuals.

I could close all loops on the UFH, drain the boiler from the return side via the return manifold to and see if a lot of junk comes up against the usual direction of flow. Maybe some flakes or something are stuck in the hydroblock. Then again, the Viessmann tech did a power wash from the flow to the return for a few hours in both directions and everything was the same after.... Running out of ideas.

Fair point, what about in the UFH Manifold, inlet valves, etc. If the boiler works on DHW, surely that indicates something on the heating circuit. I appreciate you have flushed each loop but may be worth dismantling flow and return to check. If it worked previously there must be something different now. Obviously easy to say, but harder to find.

 

[John.g]

He is experiencing some problems with DHW after new circ pump installed, post #42 (and others).

 

[dr911t]

I would attach a 0 to 1bar PG on a T piece on the mains hose and do the tests as suggested on the manifold.
I don't know if you can back flush the boiler Hx from the connections to the manifold as the boiler diverter valve I think defaults to the DHW side, you can try it but don't exceed 1 bar, if you do establish a flow then set the mains pressure at exactly 4M and measure the flow rate into a bucket for exactly 1 minute, this may give some indication of the state of fouling (if any) of the main Hx, it may of course depending on the diverter valve position mean that you are measuring the pressure loss through both the main Hx and the DHW PHEW but worth a test anyhow.

I believe when boiler is off the diverter valve is on the CH side but I'd have to double check this.

Just to be clear, the suggestion is to turn off the boiler, close the manifold loops ( flow meters and actuators ), drain the boiler water and run mains pressure through the manifold back into the boiler without isolating the manifold?
Which direction do you suggest the mains into the return side of the manifold so water flows into the return pipe of the boiler, up through the heat exchanger and back down through the diverter valve into the flow side of the manifold?

 

[John.g]

No.
Regarding the boiler, First isolate the manifold in whatever fashion you know best, remove the boiler flow and return where they connect to the ufh manifold then connect hose to return and back flush main Hx and let water flow to drain from the now disconnected flow pipe. When complete connect hose to the (disconnected) boiler flow, let the return go to drain, set the mains pressure to 4M and then measure the flow rate by putting the return into a bucket or whatever for a exact known time, measure this and convert to LPM. Carry out the manifold flow test as described above, both tests done completely disconnected from each other.

 

[John.g]

Got a bit mixed up above.
Re boiler flushing, to back flush (if required) hose to flow side, return to drain.
Normal flush and Flow test, hose to return side, flow side to drain.

 

[John.g]

Alternative Test?.
If you don't want to be messing about purchasing/installing PG(s) perhaps connect hose to boiler return, flush complete system then measure flow rate using the (now disconnected) manifold return at the boiler and using the boiler pressure gauge for monitoring test(s).

 

[hometech]

I think the problem is the opposite. Too much temperature drop due to not enough flow.

Heating Rotary Knob Setting: Between 4 & 5
Boiler temp: 49-50C
Heating flow: 47C
Heating return: 27.5C
Manifold flow meters: ~1L/min flow meters open fully - all thermostats on, 9 loops.

I have it set like this (for now to have heating until the issue is fixed) because it only operates on these temps and higher, any setting below just causes it to short cycle.

where are you getting this accurate data readings from? You have analogue dials yet mention very accurate readings. 20 degree drop is way too high BTW (heat geek videos worth watching). I had a similar problem with low temperature heating that was resolved only this week

 

[dr911t]

Alternative Test?.
If you don't want to be messing about purchasing/installing PG(s) perhaps connect hose to boiler return, flush complete system then measure flow rate using the (now disconnected) manifold return at the boiler and using the boiler pressure gauge for monitoring test(s).

I'll probably be able to do this test although removing the pipes isn't something I would venture into.

I would hope it would be sufficient to just close the loop's actuators and flow meters and feed water at mains pressure through the return side of the manifold - up through the boiler/HX - and back down through the flow and out the manifold flow drain...

 

[dr911t]

where are you getting this accurate data readings from? You have analogue dials yet mention very accurate readings. 20 degree drop is way too high BTW (heat geek videos worth watching). I had a similar problem with low temperature heating that was resolved only this week

the 100-W B1KA's setting dials are analog but the boiler temperature display is digital
the other temperature readings are from a digital IR thermometer
drop is way to high, agreed, likely due to low circulation, been trying point out the circulation fault without spending 1000+ on a new HX...

how was your issue solved?

 

[king of pipes]

You seem to be going round in circles and getting nowhere it's pretty obvious that this problem is a circulation issue, the boiler is getting confused and modulating down the pump speed is also decreased so you have reduced flow you don't want that on ufh, what does the boiler manufacturer say ?? It's in the manual you have no way of knowing what your flow rate requirements are ? So what do you do in this situation read the manufacturers instructions what does it say ? Fit a low loss header Eureka We're getting some where, this will need a constant fixed rate pump with time and temperature control wired to work in conjunction with the boiler, DIY is not a option here get a competent engineer to make the necessary changes needed , it may have worked previously but it's not now is it and why is that simple it's not installed correctly and to the manufacturers instructions ,I very much doubt there's s restriction your hot water works ok? And the water ways in s Viessmann are huge compared to other boiler manufacturers you can fiddle all you like unless you make changes you will not improve the situation. Sorry if this seems a bit harsh but your getting nowhere at the moment . Regards kop

 

[John.g]

I'll probably be able to do this test although removing the pipes isn't something I would venture into.

I would hope it would be sufficient to just close the loop's actuators and flow meters and feed water at mains pressure through the return side of the manifold - up through the boiler/HX - and back down through the flow and out the manifold flow drain...

Do it whatever way you think most convenient and also try and estimate/measure the flow rate at a boiler pressure of 5M (0.5bar).

 

[hometech]

the 100-W B1KA's setting dials are analog but the boiler temperature display is digital
the other temperature readings are from a digital IR thermometer
drop is way to high, agreed, likely due to low circulation, been trying point out the circulation fault without spending 1000+ on a new HX...

how was your issue solved?

so low temperature heating had worked fine on automatic save mode setting (boiler decides based on air intake temp and return temp). In the summer, I changed the pipework (went to 15mm) leaving the exact same arrangement (replaced like for like) and it caused circulation issues (radiators would have flow then they would not have flow and this would then alternate. One radiator had no flow at all when other radiators on the same manifold worked fine. I had to keep raising the circulation temperature and increased the pressure but it seemed to make things worse and still it was not sufficient to keep the property warm. I then noticed that at a setting of 65 degrees (almost normal room thermost heating temp) some of the radiators were still luke warm and the property was cold and yet the boiler was not firing (it sensed that the return temp was sufficient so switched off). Checking the boiler return, it was 2 degrees higher that the flow temp. After pressure checking and temp checking, I discovered that the pipework that had been a former heating zone was creating a short circuit back to the boiler. I capped the pipework so I broke the continuous loop (it became 2 spurs for flow and return). Opened the isolation values and instantly the entire circuit came back to life, all radiators working and back to being able to have the boiler decide what to do. Property nice and warm.

 

[Ric2013]

You seem to be going round in circles and getting nowhere it's pretty obvious that this problem is a circulation issue, the boiler is getting confused and modulating down the pump speed is also decreased so you have reduced flow you don't want that on ufh, what does the boiler manufacturer say ?? It's in the manual you have no way of knowing what your flow rate requirements are ? So what do you do in this situation read the manufacturers instructions what does it say ? Fit a low loss header Eureka We're getting some where, this will need a constant fixed rate pump with time and temperature control wired to work in conjunction with the boiler, DIY is not a option here get a competent engineer to make the necessary changes needed , it may have worked previously but it's not now is it and why is that simple it's not installed correctly and to the manufacturers instructions ,I very much doubt there's s restriction your hot water works ok? And the water ways in s Viessmann are huge compared to other boiler manufacturers you can fiddle all you like unless you make changes you will not improve the situation. Sorry if this seems a bit harsh but your getting nowhere at the moment . Regards kop

Inclined to agree, at this point. To some extent anyway.

While the page you have provided suggests a LLH must be fitted when the maximum flow of the system exceeds the maximum flow of the boiler (which is 23 lpm) and this is probably not going to be exceeded (hence I'm still against the idea of a LLH as a fix, for now), but we don't know what flow was anticipated as a total for the UFH as we didn't design it.

That said, maximum flow or not, the boiler should give adequate flow for at least a couple of circuits, and if I remember this long rambling thread correctly, it isn't doing that. So I don't know what to make of it.

I've suggested some basic tests, but the OP either hasn't done these, or hasn't told us he's done them, hometech is talking about his own system which seems an unlikely comparison (though idea of a short-circuit - perhaps an internal bypass - is something we'd need to be there to rule out), and JohnG and the OP are talking about flushing the system and testing the flow rates in the same paragraph and confusing the life out of me (it's taken me a week to catch up). If they understand each other, fine and well and indeed probably better than my suggestion, and if the OP wants to test the pressure/flow characteristics of the UFH at the manifold, this may be useful information. But until that is done, any further comment is a waste of time.

If the OP can come back to us with a statement that x pressure at the manifold gives 2lpm flow on each circuit (I would be surprised if any circuit required greater flow), then we might be getting somewhere. If it needs 0.5bar pressure, we need a LLH, whereas if we need a 0.1bar pressure, a LLH might not be needed as maximum flow would be 18 lpm and therefore the system pump might be sufficient. But all that would then prove is you'd need a decent heating engineer on site. But I think I've already said this and we are, therefore, indeed, going around in circles.

 

[Gasmk1]

It is just going in circles the flow is not enough the viessmann does not like a large difference in flow and return temp so will cycle the most obvious way is to follow manufacturers instructions and fit a low loss header.

 

[Ric2013]

It is just going in circles the flow is not enough the viessmann does not like a large difference in flow and return temp so will cycle the most obvious way is to follow manufacturers instructions and fit a low loss header.

The manufacturer's instructions state to fit a LLH in certain conditions. We don't know that the OP's installation meets those conditions.

 

[king of pipes]

Ric read what it says bud it states to fit one if it exceeds maximum flow or minimum flow or you don't know what you have it can't be any clearer. Merry Christmas 🎄 kop

 

[John.g]

Ric read what it says bud it states to fit one if it exceeds maximum flow or minimum flow or you don't know what you have it can't be any clearer. Merry Christmas 🎄 kop

I would say that we do know to a reasonable degree what the required flow rate is, as the OP did see all the loops flowing 2LPM, 9 loopsX2 equals 18LPM or 1080LPH and viessmann states that the use of a LLH is only required where the flowrates are < 400LPH or > 1400LPH, what could be clearer?. My simple calcs suggest that this flowrate of 1080LPH is achievable with a pump head of ~ 3/3.5M.
Again, all I would suggest is that the OP, if happy/confident to do is, is to measure this flowrate as suggested above using the boiler gauge as a aid.
Viessmann (like most gas boilers) does not like a big flow/return dT but if the flowrate is as calculated then this will not be excessive.

 

[hometech]

The important rule of thumb with water in pipes...that is that it must go somewhere...lol.

If the flow had its heat extracted to such an extent that the returning water was so cold it would damage the hot exchanger and so the boiler closes down (as is being suggested) then how is this happening so quickly (as in the video). That is not the answer.
Boiler shut down on return temp (which is what is happening) can only be
a. the water is actually at the temp for cut-off
or b. the temperature probe has failed.
If you can establish the resistance (ohm range for the sensor) from the manufacturer then that would be an easy test to do.

 

[Ric2013]

Ric read what it says bud it states to fit one if it exceeds maximum flow or minimum flow or you don't know what you have it can't be any clearer. Merry Christmas 🎄 kop

I see what you're saying, but we have inline flow gauges so we do know the flow (though you might have a point about the minimum flow). And since the system isn't working even if we reduce the number of circuits being fed, I think it's fair to say the issue at hand is more the insufficient working head than that the boiler maximum flow is being exceeded. But we can also check the required head and then we'll know if the boiler is doing its job.

This is a high efficiency boiler but it will never run in condensing mode if we couple it to a LLH. Which is why, for me, that would have to be a last resort.

I'm not sure we can take that 'if the flow is unknown' comment literally. It would apply if we thought the flow might fit within the boiler parameters most of the time but might be exceeded on occasion or we couldn't prove we weren't pushing the envelope and I think what Veissmann is saying is fit a LLH if there is doubt about the flow being excessive.

But 'doubt' surely doesn't apply to a case where we have flow metres fitted. It might apply to a boiler serving 4 heating zones or multiple TRVs where we could assume flow is unlikely to exceed x lph but where it might on occasion giving rise to excessive flow / return delta t or if we thought the UFH might require as much as 3lpm per circuit. Take the comment literally, however, and you'd fit a LLH when performing a boiler swapout on a 2 bedroom flat because you won't know the exact flow rates required on the existing radiators that could be 50 years old.

Surely if it were that clearcut, Veissmann would already have said so?

 

[hometech]

any circuit will need a differential in pressure/flow for the water to circulate. The differential pressure/flow required to create this circuit is always very small, so a minor differential can have major impact..which then impacts something else and so on. I would not have believed just how small a difference can have an impact until it happened to me.

Water takes the path of least resistance so if the path is easier back to the boiler than through a circuit, that's what will happen and that is what is happening here. (well, the boiler is saying that)
The massive drop in temp will not be possible over these short distances so rule that out as Its only the hot temperature that is telling the boiler to stop heating.

If the pressure/flow is too high, a water brake will help in the balance but the water will still have the same characteristics (easiest path).

You either have a short circuit or the thermistor is faulty because this is a return water temperature issue.
Multi meter rules out faulty thermistor, IR thermometer shows actual temp.
Prove those first.

 

[Gasmk1]

The return temperature is too big a difference because the flow is not enough.

 

[ChrisMMM]

Did this ever get resolved? I have the same issue ..

 

[Lou]

(Just bumping a few older threads, they will drop off what is new soon)

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