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Viessmann Vitodens 100-w trouble maintaining central heating temp

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dr911t

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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 said:
It’s not enough flow through boiler the difference between flow and return temp is not enough
Click to expand...

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?
 
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
 
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..
 
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.
 
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.
 
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.

king of pipes said:
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
Click to expand...
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.

John.g said:
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..
Click to expand...

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.

Ric2013 said:
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.
Click to expand...

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.

John.g said:
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.
Click to expand...

Maybe pump is failing?

Gasmk1 said:
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.
Click to expand...

Exactly. What was the solution?

johntheo said:
It should be easy enough to get the flow rate from the individual flow tubes on the loops.
Click to expand...

~1.0L/min
 
How many zones ?
 
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
 
ShaunCorbs said:
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
Click to expand...
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).
 
Last edited:
Could be your main heat exchanger is dirty / slightly blocked, does it have the same issue on hot water ?
 
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.
 
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
 

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John.g said:
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.
Click to expand...

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 )

king of pipes said:
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
Click to expand...

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?
 
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.
 
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.
www.plumbersforums.net

Viessmann boiler short cycling

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...
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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
 

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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?
 
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
 
ShaunCorbs said:
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
Click to expand...
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.
 
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
 
king of pipes said:
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
Click to expand...
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.
 
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?
 
johntheo said:
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?
Click to expand...

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.
 
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.
 
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