Viessmann Vitodens 100-w trouble maintaining central heating temp

[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.