Viessmann Vitodens 100-w trouble maintaining central heating temp

[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??.