Vaillant boiler always exceeds set temperature

[kilodelta]

Hello. I have a Vaillant EcoFit boiler that is a few years old. We had our annual service last month and ever since then I’ve noticed something unusual.

The radiator mode temperature is set to 65C but it always exceeds that, hits 70C, and then stops and the the egg timer comes on. If I check the boiler status, I see either S.07 (pump overrun) or S.08 (anti cycling mode).

Is this normal? I called the engineer and he said it was. I’ve noticed the egg timer would sometimes come on if the Nest turned the heating on and off in quick succession, but not every time it fires up.

Thanks.

 

[kilodelta]

The HW (unvented cylinder ) temperature setpoint temperature should always be set lower than the boiler water temperature, ideally by 10C lower but certainly at least 5C lower.

The attacher is on the cylinder… it’s set to 65C (I think it always has) and this is higher than the boiler flow temperature. This should be dialed down 5-10C?

 

[ShaunCorbs]

The attacher is on the cylinder… it’s set to 65C (I think it always has) and this is higher than the boiler flow temperature. This should be dialed down 5-10C?

Boiler should be 65 cylinder stat wants reducing to 60

 

[kilodelta]

Still happening with the boiler set to 64 and the cylinder to 50.

What is the expected behaviour? Is the boiler supposed to turn off when the cylinder signals it’s at temperature (but the Nest indicates it’s still on, waiting to signal
To the boiler that it needs to heat its water?)

Should I call an unvented cylinder engineer or should I try reducing the boiler to 8KW?

 

[ShaunCorbs]

Nest will still signal it needs heat as they don’t talk to each other the stat controls the temp

 

[kilodelta]

So as long as the Nest has the hot water on, the boiler be on to heat the water in the cylinder? What happens when the cylinder’s water reaches the cylinder’s set temperature?

 

[ShaunCorbs]

Once the nest is set for water on eg calling the cylinder stat takes over and controls the temp

 

[John.g]

Suggest running off say 10 to 15 litres of HW with tap opened for 5 minutes or so, then programme the CH + the HW to come on together then switch off the CH only after 3 or 4 minutes and see if the boiler continues to fire until the cylinder is fully heated.

 

[kilodelta]

Thanks for all your help everyone. The house is thankfully warm and we still have hot water so I can’t complain at the moment, even if it doesn’t seem to be operating as it should.

I dropped the power to 8kW to see how things runs.

I assume it’s safe to play with the power output?

 

[ShaunCorbs]

Yes as it’s just settings will soon tell you no if it doesn’t like one

 

[V1ks44]

Do you have the spec for the ufh / what flow rates do the ufh flow meters show with everything on / calling ufh wise ?

Attaching photo of the UFH located under my stairs, i cant rem for 100% the loop lengths of the pipe run. Kitchen is split in into 2 circuits i know one was about 70m and the other was about 30-40m. Same with the living room they were about 75m each.

I have been playing around with the Flow rates recently but at present they are set to around 3l and pump speed to 3. I have recently tried to have the rates at around 2l with the pump speed on 2 but the kitchen seem to take a very long time to warm up due to higher heat loss because i have very large 3 panel bifold doors.

Let me know if u need any other info...ill try my best to get this..

 

[V1ks44]

Suggest changing the Kw from Auto to 6kw, also the anticycle time d.002 might have to be increased from its present settiung of 20 mins, you should see a table somewhere that gives the actual anticycle time as it looks at the target flow temp and the set anticycle time to give the actual time.

Would doing this have any negative impact on the boiler?

Attaching the photo of the table for anticycle times, so at 60c as set currently would it be?

 

[ShaunCorbs]

Works out to around 6 kw on the ufh maybe

You want a difference of 7 degrees between flow and return on each circuit when warm so alter the flow meters to achieve this if you can

 

[John.g]

No, it shouldn't, 60C+20C(time) = anticycle time of 6 minutes, 60C+25C=7.5minutes & 60C+30C=9minutes.

I would increase that (d.002) initially to 25, a 7.5 min anticycle time, as it might reduce the starting temperature low enough to get the boiler away without exceeding the target temperature.

 

[V1ks44]

Works out to around 6 kw on the ufh maybe

You want a difference of 7 degrees between flow and return on each circuit when warm so alter the flow meters to achieve this if you can

yes thats what i have been trying recently to get, i dont know what my return temp are to be honest is there a way to check this?

when i checked the diagnostic menu on the boiler d.41 (i think it was) it was showing me like 55c, im not entirely sure if this is the return from the manifold or the whole system? as it seemed very high because when i touch the return copper pipe (one on the left in the pic i uploaded) its not very warm even when the UFH has been on for few hours. it could be due to the way the pipes are laid (single serpentine).

I dont have a temp gauge on the return part of the manifold, im certain the difference is higher than 7c because the UFH pipe layout is Single serpentine (this was a total mess by the plumber who did it and i didnt know anything about ufh at the time to correct anything). however i cant change this now so im just trying to find a way to get this best as i can. I will increase the flow rates and see if this changes.

What are your thoughts on the suggestion by John.g to change the boiler to 6kw and increase the Anticycle time. Im thinking to give this a try tomorrow morning and see how the system behaves...

 

[V1ks44]

No, it shouldn't, 60C+20C(time) = anticycle time of 6 minutes, 60C+25C=7.5minutes & 60C+30C=9minutes.

I would increase that (d.002) initially to 25, a 7.5 min anticycle time, as it might reduce the starting temperature low enough to get the boiler away without exceeding the target temperature.

Thanks John.g - i will give this a try tomorrow morning and report back.

I know a buffer tank is an option but i just dont have space near the boiler to fit this as this would solve the anticycle issue and allow the boiler to run in fully condensing mode.

 

[ShaunCorbs]

Clip on thermostat is the best tbh

Depends on the load as if it’s set right atm 3lpm you have 6kw on just the ufh without the rads

Tbh combi boiler or system eg hot water tank ?

 

[V1ks44]

Clip on thermostat is the best tbh

Depends on the load as if it’s set right atm 3lpm you have 6kw on just the ufh without the rads

Tbh combi boiler or system eg hot water tank ?

ok will try that...I have a system boiler Vaillant 637 with Horizontal 180L Unvented tank which is next to the boiler.

.someone suggested the below Actuators to me...they maintain the 7c difference (they have some clips on the which u attach to flow and return pipes on the manifold) and i can just leave all the flow meter fully open ...not sure if anyone on here has heard of them or any experience...they are around £20 each + del so not massively expensive...thought id share it as its in my list of things to try.

Salus Actuator – Auto Balance & Energy Saving​

 

[John.g]

I think the min output of that boiler is ~12kw so the boiler will probably cycle anyway on either with rads only on or UFH only on so the boiler temp must be got as low as possible before refiring after a recycle.

 

[ShaunCorbs]

ok will try that....someone suggested the below Actuators to me...they maintain the 7c difference (they have some clips on the which u attach to flow and return pipes on the manifold) and i can just leave all the flow meter fully open ...not sure if anyone on here has heard of them or any experience...they are around £20 each + del so not massively expensive...thought id share it as its in my list of things to try.

Salus Actuator – Auto Balance & Energy Saving​

Tbh should be easy to do with a clip on temp meter

 

[V1ks44]

Tbh should be easy to do with a clip on temp meter

ok cheers, ill try that.

 

[ShaunCorbs]

7-10 is ok better with as close to 7 as you can get

 

[V1ks44]

7-10 is ok better with as close to 7 as you can get

ok let me increase the flow rates i guess to the max and get this clip thermostat and then ill report back after testing over few days. I will also reduce the kw to the minimum ( i think it showed me 6kw) but i will try 12kw first and see.

Thank you both for the prompt responses this evening.

 

[V1ks44]

Just reporting back...

So i changed the D.00 to initially 12kw and then tested both UFH only and CH and it was still going into Anti cycling mode, with the UFH it did it more frequently then it did with CH.

Then i changed the setting to 8kw and same result and this morning i dropped it even further to 6kw and also dropping the Flow to 55C to see if this would have any impact. but still It went into anti cycle and the Flow would hot 65 before the change and then 60 so basically overshooting by 5c.

So today the UFH has been on longer due to kitchen being much cooler (also the -2/3 outside) than my living room, so what i noticed was at 6kw and 55C flow, the boiler would get to about 60c and then hit the anti cycle mode and then go down to about 40c, then it would stop complete when it got to 40c with the time only flashing (pump is not running at this 1-2 min period) and then will sit on this screen for about 1-2 min before firing again and going and heating to the desired flow temp.

I also noticed the return temp were above the desired 54c (needed for the boiler to be most efficient) regardless of the flow temp being at 55c, 60c or 65c.

I have come to a conclusion after reading various forums over the last 24 hours where there are may others on the same boat as me....THAT this is the best im going to get due to limitation in the setup i have, i believe the only way i can solve this is to have a buffer tank installed (as suggested by many others on various threads on the internet). But sadly i just dont have the space as i live in a terrace house with not much space to put one.

i assume with this usage the boiler wont be as efficient as one would like it to be but i dont see any other way...

@Shaun - i am still to find out the return temps on the loops, do you have suggestion on which clip thermometer i can buy? I have the attached Infra red guns but im not sure how accurate the readings are because when i did try it was giving much lower readings e.g. the incoming temp from boiler was 60C and when i pointed at the copper pipe just below the manifold as a test it would show 31c/ and then the flow pipes would on the manifold was showing 27c.

So i may get the clip ones you suggest - any recommendation?

 

[John.g]

How many rads?, have you tried it with both rads and UFH on together?, would also suggest increasing the flow temp to 65C and the anticycle time d.002 to 30mins, this gives a actual anticycle time of 6.5mins, if you do this then note that the actual time is 6.5 minutes when it goes into recycle, the pump should also continue to run whatever the anticycle time is.

 

[V1ks44]

How many rads?, have you tried it with both rads and UFH on together?, would also suggest increasing the flow temp to 65C and the anticycle time d.002 to 30mins, this gives a actual anticycle time of 6.5mins, if you do this then note that the actual time is 6.5 minutes when it goes into recycle, the pump should also continue to run whatever the anticycle time is.

I have 4 Rads and 2 Towel rails (in Bathrooms), let me try increasing the temp to 65c later on today, I have Hive TRV on all 4 rads so i will set the temp to say 20c and change the d.002 to 30min as you say and the test how this goes along with the ufh being on as well.

After the boiler has gone into anticycle mode should it stop completely for the brief moment as mentioned above? Would a video help, i can upload something and share it.

on a side note am i right in thinking that when running just the UFH say at 60c and the manifold TRV set to 45/50 - this setup is returning the water too hot back to the boiler hence it could be going into anti cycle mode or it doesn't matter what flow i set on the boiler this will always happen, or isit normal for the boiler to be in anticycle mode x many times during the running of UFH. At the moment im just confused as to what is a "normal" setup?

When i check d.41 - return temp on the boiler whilst the boiler is in the process of heating the water back to the desired flow temp (e.g. 60c) i find the return temp will start going from 54c to nearly 60c and the boiler will continue to overshoot to 65c and then when it stops at 65c (flow) then return would be at around 60c....then anti cycle kicks in and the flow starts to drop down along with the return...as you would expect >>>> is this expected behavior?

 

[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...