Boiler in constant anti cycle ?

[Graham J W]

Hi, I have a Vaillant ecoTEC plus 424 ( 24KW ) boiler feeding three circuits. Down stairs heating, upstairs heating and hot water via thermostats and motorized valves. There are twelve rads and two towel rails. All rads have TRVs except the one in the entrance hall. Both towel rails on lock-shields.
Target heating flow temperature 62
Partial load set at 18KW
Anti cycle time set at 25 minutes.
Room stats set at 21.
TVRs set between 3 and 4 ( Drayton TRV4s ).

Boiler fires up ok and gradually ramps up to target flow temperature and modulates for a while until the room TRVs start closing. Stats still calling for heat so motorized valves open. Boiler quickly goes into overheat and into anti cycle. Anti cycle times out at around 6 minutes, boiler fires up and back into overheat and the cycle begins again.

Is it just a Vaillant thing ?. what do other boilers do when most or all TRVs close ?. It seems to be in a continuous circle of "over" heating just the pipework ?.

 

[Notch7]

I though Vaillant could modulate down quite a lot.

I wonder if the boiler is a bit big.

Hopefully a pro will be along to go through the relationship between flow temp, TRV settings and room stat settings …..as it seems you have the relationship between them wrong

 

[Graham J W]

I though Vaillant could modulate down quite a lot.

I wonder if the boiler is a bit big.

Hopefully a pro will be along to go through the relationship between flow temp, TRV settings and room stat settings …..as it seems you have the relationship between them wrong

Thanks Notch 7. Something is clearly wrong and yes hopefully a pro can help.

 

[Madrab]

what do other boilers do when most or all TRVs close

If a stat is still calling for heat but there is no flow then the the boiler temp will rise and the boiler stat will shut it down, pump etc will still run on and once the flow cools down enough it will fire back up again and it will continue to cycle like that. When you say overheat, is the flow reaching 80+ deg or just reaching it's set temp?

That's one of the reasons why there should always be no TRV on the rad where the stat lives, so they don't fight one another and allows a min of flow through the boiler and the pump isn't running against a closed head. Only issue is the boiler will continue to cycle.

If the boiler anti cycle is set to 25mins then it shouldn't really be timing out @ 6mins.

 

[Graham J W]

If a stat is still calling for heat but there is no flow then the the boiler temp will rise and the boiler stat will shut it down, pump etc will still run on and once the flow cools down enough it will fire back up again and it will continue to cycle like that. When you say overheat, is the flow reaching 80+ deg or just reaching it's set temp?

That's one of the reasons why there should always be no TRV on the rad where the stat lives, so they don't fight one another and allows a min of flow through the boiler and the pump isn't running against a closed head. Only issue is the boiler will continue to cycle.

If the boiler anti cycle is set to 25mins then it shouldn't really be timing out @ 6mins.

Thanks Madrab, Boiler goes to set temp of 62 and then pass to 67 and into anti cycle. Room stat has rads with lockshields, no TRVs. Anti cycle set at 25 mins but Vaillant graph shows in real time this equates to 6 mins ??

 

[Madrab]

Anti cycle set at 25 mins but Vaillant graph shows in real time this equates to 6 min

That sounds more like the boiler is cooling itself back down to the point where, as there is still demand, it fires back up, like any other boiler without an 'anti cycle' setting. The boiler isn't really overheating where the HL stat's kicking in, rather the boiler's just reaching it set temp and shutting down

That sounds like there isn't enough capacity still open on the system. If you say there are 3 rads (2xtowel rads) that are open all the time then there may not be enough demand through the rads and as the boiler can't modulate down low enough to maintain the set temp, it reaches that and shuts down.

 

[Notch7]

Thanks Madrab, Boiler goes to set temp of 62 and then pass to 67 and into anti cycle. Room stat has rads with lockshields, no TRVs. Anti cycle set at 25 mins but Vaillant graph shows in real time this equates to 6 mins ??

What Madrab explains very clearly is exactly what happens to my boiler.

That sounds more like the boiler is cooling itself back down to the point where, as there is still demand, it fires back up, like any other boiler without an 'anti cycle' setting. The boiler isn't really overheating where the HL stat's kicking in, rather the boiler's just reaching it set temp and shutting down

That sounds like there isn't enough capacity still open on the system. If you say there are 3 rads (2xtowel rads) that are open all the time then there may not be enough demand through the rads and as the boiler can't modulate down low enough to maintain the set temp, it reaches that and shuts down

In basic terms the system isn’t taking much heat out of the water so the return temp is not much different to the flow temp……what is your return temp when the boiler gets to its set point of 67.

So the boiler stops firing but the pump continues as the room stat is still calling for heat, the water continues to pump around and when it calls down enough the boiler fires up again.







It’s a shame your system is not configured to hot water priority, which would allow you to lower the flow temp below 62 deg (if your boiler can set a different temp for hot water and central heating).


The only thing you can do with your current setup is to open the TRVs fully so that more heat goes into the house and that lowers the return temp.


TRVs are actually the wrong way to control a system, the most efficient system is where maximum heat is transferred from the central heating into the fabric of the house, TRVs throttle the ability of a heating system to do its job. The control of the system should be by flow temperature and correctly sized radiators - if you think about it, if radiators were sized to warm up each room to its needs then TRVs would be hardly necessary.


Heat Geeks explains the concept well

 

[Johntheo5]

Thanks Madrab, Boiler goes to set temp of 62 and then pass to 67 and into anti cycle. Room stat has rads with lockshields, no TRVs. Anti cycle set at 25 mins but Vaillant graph shows in real time this equates to 6 mins ??

Quite normal but the Vaillant's can be a right pain to get them to refire successfully after cycling because they hold their ignition settings for 60 secs after refiring before releasing the controls to modulation, by which time the boiler temp has exceeded its targettem+5C so repeat recycle,
All boilers fire up at ~ 60% of their maximum output but then atart to modulate down once the targettemp is reached, within around 10 secs or so, from observing my daughter's Vokera Vision 20S.

Your anticycle time of 6 minutes may be too short, you need to get the flow (&return) temps as low as possible before refiring, the next time it goes into recycle, just watch the flowtemp and see what it falls to just before the anticycle time has elapsed, you can then maybe increase the anticycle time to say 40 minutes which gives a actual anticycle time of ~ 10 minutes @62C and then observe the flow&return temps again before the anticycle time has elapsed, they should equalize and then fall, theoretically to room temperature, if the anticycle time is long enough.

Have you got a automatic by pass valve (ABV) installed, the further away from the boiler the better.

 

[dilalio]

What temp are the rooms maintaining?
Does this under and overshoot (seesaw?).
Try all TRVs fully open as they really are not that good at accurate control of room temp.

 

[Graham J W]

That sounds more like the boiler is cooling itself back down to the point where, as there is still demand, it fires back up, like any other boiler without an 'anti cycle' setting. The boiler isn't really overheating where the HL stat's kicking in, rather the boiler's just reaching it set temp and shutting down

That sounds like there isn't enough capacity still open on the system. If you say there are 3 rads (2xtowel rads) that are open all the time then there may not be enough demand through the rads and as the boiler can't modulate down low enough to maintain the set temp, it reaches that and shuts down.

Morning Madrab, thats my thinking as well. I have tried taking a couple of TRVs off and using the Drayton " manual" cap so effectively two more rads stay open with a trickle demand. Made some difference but not enough to stop the boiler from shutting down / anti cycle ?.

 

[dilalio]

using the Drayton " manual" cap

Do you mean "decorator cap"?
These shut the rad Off!

 

[Graham J W]

Do you mean "decorator cap"?
These shut the rad Off!

Polished chrome valve caps for manual opening and closing of Drayton TRVs. Suitable for chrome towel rails and radiators. ( Screwfix )

 

[Madrab]

Polished chrome valve caps for manual opening and closing of Drayton TRVs

TBH - These are just a chrome version of the decorators cap - TRV's would be really difficult to get them to 'trickle', the pin only moves a couple of mm's from open to closed, so it would be extremely difficult to be able to set them manually to do that.

To stop the boiler from cycling like this you would need to ensure there is still enough demand from the rads (Kw's) when everything is up to temp, to ensure that the boiler will keep running at its lowest modulation point, would need to check how low the 424 can modulate but I think it's around 3.5Kw.
This all comes down to setting the system up properly, ultimately the system should run until the stat is satisfied, that then shuts the boiler down, that should be the controlling factor, not the TRV's. If the rooms are up to temp before the stat stops calling for heat then it needs to be setup differently or the stat needs to be moved to allow the stat to shut down when there is no more need for heat.
Alternatively, as before, the system needs to be setup in such a way where there is always a load that corresponds to the boilers lowest modulation setting, if that makes sense.

 

[Johntheo5]

I would agree that its best to get the demand higher than the minimum output, the boiler makers themselves were well behind the curve in getting their minimum outputs lower while weather compensation, zoning, house insulation, years ago, were leading to demands as low as 2.5 to 3kw.
However most boilers can deal with this cycling, bearing in mind that evohome and others awitc the boiler off and on up to 6 times/minute to give pinpoint control of room temperatures but Vaillant for the reason I gave above have the greatest difficulties, they may have changed this 60 sec hold on combustion conditions now though?.
You are better off by having a high flowrate through the rads/boiler and reducing the flowtemps to give you the same low rad output as with TRV'v but this means a far better chance of getting the boiler away after a burner reset.

 

[Graham J W]

TBH - These are just a chrome version of the decorators cap - TRV's would be really difficult to get them to 'trickle', the pin only moves a couple of mm's from open to closed, so it would be extremely difficult to be able to set them manually to do that.

To stop the boiler from cycling like this you would need to ensure there is still enough demand from the rads (Kw's) when everything is up to temp, to ensure that the boiler will keep running at its lowest modulation point, would need to check how low the 424 can modulate but I think it's around 3.5Kw.
This all comes down to setting the system up properly, ultimately the system should run until the stat is satisfied, that then shuts the boiler down, that should be the controlling factor, not the TRV's. If the rooms are up to temp before the stat stops calling for heat then it needs to be setup differently or the stat needs to be moved to allow the stat to shut down when there is no more need for heat.
Alternatively, as before, the system needs to be setup in such a way where there is always a load that corresponds to the boilers lowest modulation setting, if that makes sense.

Point taken Madrab. Thank you for the ongoing advice which is appreciated. I most likely need a "pro" to come and check / set the system up as there looks to be an imbalance somewhere. I am doing some load calculations presently and that the downstairs rads equate to 10.18 Kw. The TRVs are closing down before the ground-floor thermostat.
The upstairs thermostat closes before the TRVs mainly because its well insulated up there. So I think the boiler is mainly dealing with the down stairs circuit and the hot water. Incidentally, when the hot water kicks in ( timed ) the boiler is happy for that period which also points to not enough demand or poor setup.