Vaillant boiler always exceeds set temperature

[John.g]

Stopping for a brief moment or two is probably normal behaviour, wouldnt be worried as long as the pump runs continuously for the anticycle period.
All boilers will cycle if the system demand is lower than the minimum boiler output, in your case ~ 12kw and because the rads+T.rad demand is probably a little less than the boiler min output then not nearly as much as the UFH demand where its output is probably around 6kw.
The biggest challenge is to get the boiler to modulate down to its minimum output before the flow temperature exceeds its setpoint by 5C. The boiler will fire up at ~ 60% of its maximum output and Vaillants hold this firing level for up to 60 secs, a 37kw boiler will fire at a output of ~ 22kw so its best to get the system temperature as low as possible before refiring by a suitably long anticycle time, if the system temp can be got down to say 35C, then the boiler flowrate must be 10.5LPM to limit the boiler to a almost instant flowtemperature of 65C and keep it from rising to 70C during this 60 sec period.
Even though the boiler flow rate should be the same as the UFH (cold) flowrate, it may still be < 10.5LPM and may prove very difficult to achieve with the UFH only, easier if each flowmeter can be set to 2.8 to 3LPM.
Initially would suggest testing the system with both rads+T.rails &UFH on, there should be no problems in achieving this and getting the boiler to modulate down to its minimum output.
Then try it on rads+T.rails only and again would expect no problems, finally try it on UFH only.

 

[V1ks44]

Stopping for a brief moment or two is probably normal behaviour, wouldnt be worried as long as the pump runs continuously for the anticycle period.
All boilers will cycle if the system demand is lower than the minimum boiler output, in your case ~ 12kw and because the rads+T.rad demand is probably a little less than the boiler min output then not nearly as much as the UFH demand where its output is probably around 6kw.
The biggest challenge is to get the boiler to modulate down to its minimum output before the flow temperature exceeds its setpoint by 5C. The boiler will fire up at ~ 60% of its maximum output and Vaillants hold this firing level for up to 60 secs, a 37kw boiler will fire at a output of ~ 22kw so its best to get the system temperature as low as possible before refiring by a suitably long anticycle time, if the system temp can be got down to say 35C, then the boiler flowrate must be 10.5LPM to limit the boiler to a almost instant flowtemperature of 65C and keep it from rising to 70C during this 60 sec period.
Even though the boiler flow rate should be the same as the UFH (cold) flowrate, it may still be < 10.5LPM and may prove very difficult to achieve with the UFH only, easier if each flowmeter can be set to 2.8 to 3LPM.
Initially would suggest testing the system with both rads+T.rails &UFH on, there should be no problems in achieving this and getting the boiler to modulate down to its minimum output.
Then try it on rads+T.rails only and again would expect no problems, finally try it on UFH only.

hi John,

Yes the pump does run during the anticycle period, it only stops as i said earlier for that brief moment (about 1min or so). Thanks for the details info, i feel a little better now knowing that this anti cycle process is a normal thing and what you explained above clears my head in many ways. This is all a great learning for me via this forums.

Shall i change the D.00 to 12kw? This morning i had changed it to 6k for testing. or should i test with both 6kw and then 12kw respectively with the flow rate being 65C (for both 6kw and 12kw) as you said in your previous post and keeping the anti cycle time to 30min.

the flow rates on my UFH has been set to 4LPM for the 2 kitchen loops, due to the higher heat loos in the kitchen anything below 3LPM it struggles to maintain the 18c on the thermostat. For the living room its around 3LPM for the two loops because i have very low heat loss. >> happy to reduce the kitchen ones back down to around 2/3 for the above test?


Cheers

 

[John.g]

I would leave d.00 at 6kw, the boiler will only go to whatever its set minimum output is anyhow, it might be a little lower than 12kw so let it at 6kw.
Do everything at 65C + 30 min anticycle time.
UFH flow at, 2X4 + 2X3, 14LPM is fine, leave as is.

Note d.40&d.41 while in anticycle and especially at the end of the anticycle time, note them again after refiring until (if you have the patience) recycle again and time taken.

 

[V1ks44]

I would leave d.00 at 6kw, the boiler will only go to whatever its set minimum output is anyhow, it might be a little lower than 12kw so let it at 6kw.
Do everything at 65C + 30 min anticycle time.
UFH flow at, 2X4 + 2X3, 14LPM is fine, leave as is.

Note d.40&d.41 while in anticycle and especially at the end of the anticycle time, note them again after refiring until (if you have the patience) recycle again and time taken.

So yesterday evening i tested the system at 6kw with the 4 x UFH circuits + 4 rads + Towel rails >> the boiler got to about 45c and then was going up and down so to me this suggest that the d.00 needs to be higher for all 4 rads and ufh ? I waited about 45minbefore just stopping the test.

Then i tested the above with 12kw and the boiler never hit the 65c flow but it kinda stayed around 50-55c constant flow and it never went into the anticycle mode, however it did drop to around 50c and just stop for like 30 seconds before firing again and it did this twice, during this time the Rads were warm i would say little less then the day before but rooms did get to their desired temp. in terms of time the boiler was running for about 1 hour 10min before i stopped the test.

With the above test i saw the flow and return temps were the same.

This morning i tested again with 12kw - this time it was 3 x UFH circuits and 2 Rads, with 65C flow and 30min anticycle timer >> I found that the boiler would get to about 68c and go into anticycle (pump is running) and then we it got to about 43C it stopped completely (pump is not running) with the timer sign flashing away and it would sit on this screen for about 4min before firing again.

With the above test i saw that when the system stopped complete for that 4min the flow was obv 43c and the return was 41c. Generally during the anticycle i noticed the return temp was always about 2-3c less the flow.

Generally i never had all my 4 Rads and UFH on together as usually only 2 rooms are used mainly.

UFH normally kicks in around 4am > mainly the kitchen circuits and 1 in living room and then the CH will come on around 6am to 7am for one bed room and then 6.30am to 7.30am for the second bedroom so between 6 and 7.30 is when the the output would be at its max

I am just happy to keep the boiler at 12kw and just see how it goes, i guess lowering may put strain on the boiler if and when the UFH and all 4 rads run together.

When someone says boiler should be running in condensing mode, when does this actually happen in a normal operation of a boiler> im still trying to get my head around this..

 

[John.g]

So yesterday evening i tested the system at 6kw with the 4 x UFH circuits + 4 rads + Towel rails >> the boiler got to about 45c and then was going up and down so to me this suggest that the d.00 needs to be higher for all 4 rads and ufh ? I waited about 45minbefore just stopping the test.

What ~ were the flow/return temps for the 45minutes? above.

Then i tested the above with 12kw and the boiler never hit the 65c flow but it kinda stayed around 50-55c constant flow and it never went into the anticycle mode, however it did drop to around 50c and just stop for like 30 seconds before firing again and it did this twice, during this time the Rads were warm i would say little less then the day before but rooms did get to their desired temp. in terms of time the boiler was running for about 1 hour 10min before i stopped the test.

Don't understand why the boiler should stop for 30 secs.

With the above test i saw the flow and return temps were the same.

Are you saying the flow/return temps were the same for over 1 hour, above.?

This morning i tested again with 12kw - this time it was 3 x UFH circuits and 2 Rads, with 65C flow and 30min anticycle timer >> I found that the boiler would get to about 68c and go into anticycle (pump is running) and then we it got to about 43C it stopped completely (pump is not running) with the timer sign flashing away and it would sit on this screen for about 4min before firing again.

How long, roughly, did it take for the boiler to reach 68C?

With the above test i saw that when the system stopped complete for that 4min the flow was obv 43c and the return was 41c. Generally during the anticycle i noticed the return temp was always about 2-3c less the flow.

Above, normal IMO.

Generally i never had all my 4 Rads and UFH on together as usually only 2 rooms are used mainly.

UFH normally kicks in around 4am > mainly the kitchen circuits and 1 in living room and then the CH will come on around 6am to 7am for one bed room and then 6.30am to 7.30am for the second bedroom so between 6 and 7.30 is when the the output would be at its max

I am just happy to keep the boiler at 12kw and just see how it goes, i guess lowering may put strain on the boiler if and when the UFH and all 4 rads run together.

When someone says boiler should be running in condensing mode, when does this actually happen in a normal operation of a boiler> im still trying to get my head around this..

Condensing doesn't begin until the return temperature falls to 50C, will require < 45C to get any meaningful condensing.

 

[V1ks44]

Hi John,

So at 6kw, i recall Return temp was 5/6c less the flow is what i observed, i rem the flow temp will go up and down. I am just running another test now as i type this (started at 11.40am)>>> at 6kw with everything switched on and the boiler has been on for 15min and flow temp is 37c while return is 31c. at 12:00 I turned off 2 rads (to see if this helps with flow temp increase) and at 12.02 the flow increased to 38c flow and 33c return. I will let this run for the next 30min and the may be stop as i dont think with UFH and Rads on the boiler can keep up to the demand?

Then during the 12kw test yesterday>> If i remember correctly the return temp was only 2-3c less the flow so nowhere near the difference you would expect but i can test this again once i stop the above testing which is in progress to confirm this.

I also dont know what the boiler just stopped and then fired again but it did it twice...

Then this morning to get to 68c - i didnt really measure it but happy to check this again also...

 

[V1ks44]

At 12.11 pm >> the CH zone valve (closed) i think as it made the noise and the Hive receiver CH light was flashing for about 10 seconds and then i heard the zone valve open again

At 12.41 same as above....

Flow temp is 43c at present and return is 39c.

 

[V1ks44]

John,

Ive left things back to how they were, if i keep it at 6kw with UFH and 2 rads +TowelR the boiler never got to to the target flow. I could keep it at 10kw or 12kw but not sure if i gain anything vs keeping it on Auto?

If u keep it at 12kw and say only using UFH will the boiler automatically reduce the kw which i presume is what the Auto mode would be doing?

 

[John.g]

Keeping it to ~ 12/15kw is probably best, in auto or say set to 25kw the boiler will modulate, only problem is if the actual flow temp is below the setpoint is that the boiler will then fire flat-out to whatever you have set and because it won't start to modulate until the flow temp is > the SP it will take longer to modulate down than from a restricted output. Flowtemp may then exceed the SP by 5C and cause burner trip and recycle.
Auto setting isn't just allowing unrestricted output, some say it does weird things, I think you said yourself that it was taking ages to get the boiler up to temperature?

 

[V1ks44]

Keeping it to ~ 12/15kw is probably best, in auto or say set to 25kw the boiler will modulate, only problem is if the actual flow temp is below the setpoint is that the boiler will then fire flat-out to whatever you have set and because it won't start to modulate until the flow temp is > the SP it will take longer to modulate down than from a restricted output. Flowtemp may then exceed the SP by 5C and cause burner trip and recycle.
Auto setting isn't just allowing unrestricted output, some say it does weird things, I think you said yourself that it was taking ages to get the boiler up to temperature?

Ok understood regarding keeping to a set kw rather than Auto.

My setup is that my UFH runs longest vs CH rads, it was after reading various forums and how these modern boilers should be more efficient if set correctly led me to investigating my own.

So the first thing I noticed is that on the default setting with just UFH heating on it would get to the desired flow temp (e.g 70C) then it would go into the anticycle mode for x minutes and the fire up again, this would be the cycle even when running UFH + rads. At times it was also overshooting by 5c as mentioned earlier.

This lead me to ask my self is the boiler running efficiently? Hence all my questions here and then trying the various settings you and others have suggested.

I also keep reading on various other forums/websites is that in order to run UFH with a boiler efficiently where it would go into a proper condensing mode,.ideally you need a buffer tank which will allow the return temperature to be much lower?

So to your question in Auto mode I think things were working as I didn't know any different, I didn't have issues with boiler taking ages, that only happens when dropping it to 6kw.

Im / I was - trying to find the setting where the return would be lower than 54c so it can go into that efficient zone...and I'm not sure if it does this during the Auto setting.

I came across this thread and decide to ask what I was seeing...