Boiler in constant anti cycle ?

[Graham J W]

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.

Thank you also Johntheo5, I am glad to hear my interpretation of a overheat / anti cycle concern is more like the boiler sensor saying no heat required and going into anti cycle. I must admit that it has probably been doing this for some time. It looks to be a demand to minimum output setup as you say.

 

[Graham J W]

Thank you also Johntheo5, I am glad to hear my interpretation of a overheat / anti cycle concern is more like the boiler sensor saying no heat required and going into anti cycle. I must admit that it has probably been doing this for some time. It looks to be a demand to minimum output setup as you say.

Hi Johntheo5,

Just called Valliant technical support and they tell me my boiler a 424 ecoTEC plus, modulates between 24Kw and 12Kw. Ie 12Kw is the lowest. That cant be correct can it ?????

 

[Johntheo5]

Can't find any numbers in the 424 MIs but the 624 shows a modulatrion of 9.0kw to 24kw, so its probably certainly not lower than this, shocking really, a 2.67 to 1 turndown.
What might give you a idea is to go to d.00 and see what is the lowest setting you can enter there, change from auto.

You might find this useful in searching for data.

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[Graham J W]

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.

After conversation with Vaillant technical who said my 424 ecoTEC plus has an output range of 12kW to 24kW the manual in fact says

VU 246/6-5 OVZ (H-GB) 24kW

Effective output range (P) at 50/30 °C 6.3 to 25.7 kW

Effective output range (P) at 80/60 °C 6.1 to 24.6 kW

6.1kW still seems high. I would have thought that most heating systems would be below 6.1kW when all or most TRVs have shut off ? My whole downstairs circuit is 10kW when fully loaded.

 

[Johntheo5]

After conversation with Vaillant technical who said my 424 ecoTEC plus has an output range of 12kW to 24kW the manual in fact says

VU 246/6-5 OVZ (H-GB) 24kW

Effective output range (P) at 50/30 °C 6.3 to 25.7 kW

Effective output range (P) at 80/60 °C 6.1 to 24.6 kW

6.1kW still seems high. I would have thought that most heating systems would be below 6.1kW when all or most TRVs have shut off ? My whole downstairs circuit is 10kW when fully loaded.

Yes, but as I said all the Boiler makers are way behind the curve but cycling in itself is no big deal, oil boilers spend their whole life doing it, it isn't a big deal either for most gas boilers except Vaillant who have(or at least had) this "daft" routine of maintaining ~ 60/65% of full output for 60 secs on firing.

 

[Madrab]

As suggested - cycling of the boiler isn't the end of the world. Most boilers will cycle once everything is up to temp, the trick is to maintain the load to minimise that cycling and maintain the boiler at the lowest modulation and then once everything reaches set temp then the boiler shut down and only fires back up once the stats call for it.

I am also a little surprised by their info re the lowest modulation, given the specs on their page state up to a 1:6 (6:1?) modulation rate? Which should be ~17% of max (approx 4kw), or am getting it totally wrong? Being Vaillant it wouldn't surprise me.

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[vulcancontinental]

To be absolutely clear, when you say anti-cycle OP that would mean no firing for 25 minutes, the system circulator will be turning the whole time. The Vaillants are quiet, you not going by sound are you but by watching the flame icon?

I'm not sure how one can maintain the load however, it is what it is between the comfort temperature and external temperature at any one time. I know what you intend Madrab but if the boiler is firing up at 60% of 24kW for 1 minute after 6 minutes where is the heat going to go, straight back to the boiler via the return.

If it is firing after 6 minutes and the OP is sure the setting is set for 25 the first problem is the boiler. This is dusting off memory cells and posts from Alec Morrow regarding micro-firing. Maybe a search for old posts might help.

 

[Johntheo5]

The Vaillant anticycle times depend on the set anticycle time and the boiler target flowtemp, a target flowtemp of 60C and a set anticycle time (default setting) of 20c gives a actual anticycle time of 6 minutes but if the target flowtemp is increased to 70C then the anticycle time is only 2.5 minutes, if a anticycle time of 20, actual 6 minutes, with a target temp of 60C then if increased to 35 minutes will give a actual anticycle time of 10.5 minutes and gets the flow&return temps down lower which gives the boiler a better chance of getting away after one cycle, with Vaillants one needs a high flowrate and a relatively long anticycle time.

The pump over run time is fixed to where ever you set it, between 1 and 60 minutes, default time 5 minutes.

 

[Graham J W]

The Vaillant anticycle times depend on the set anticycle time and the boiler target flowtemp, a target flowtemp of 60C and a set anticycle time (default setting) of 20c gives a actual anticycle time of 6 minutes but if the target flowtemp is increased to 70C then the anticycle time is only 2.5 minutes, if a anticycle time of 20, actual 6 minutes, with a target temp of 60C then if increased to 35 minutes will give a actual anticycle time of 10.5 minutes and gets the flow&return temps down lower which gives the boiler a better chance of getting away after one cycle, with Vaillants one needs a high flowrate and a relatively long anticycle time.

The pump over run time is fixed to where ever you set it, between 1 and 60 minutes, default time 5 minutes.

Thanks Johntheo5,

Flow temperature set at 62C. anti cycle time 20C, actual 6 mins. Boiler flow temperature at end of anti cycle 38C. Boiler then fires up and within seconds hits 67C and back into anti cycle. I have already increased pump over run time to keep circulating whilst in anti cycle. Possibly still looking like a confliction between zone thermostats and TRVs restricting demand and effectively strangling the boiler. ABV opens and return temperature rises, boiler over temperature sensor comes in and anti cycle loop starts ?. PS I am learning.

 

[Johntheo5]

Presume its the anticycle time you increased and not the pump over run time?

 

[Graham J W]

Presume its the anticycle time you increased and not the pump over run time?

Well, the pump was stopping before the anti cycle time elapsed. So my thought was that the heating water still needed to circulate rather than the pump keep stop starting as well ?.

 

[Johntheo5]

I think the pump might stop briefly when the anticycle time has elapsed and then restart with the burner refiring as well, Im not 100% sure but suggest you increase the anticycke time to say 35 minutes which will give a actual anticycle time of 10.5 minutes and a lower flow temp before refiring.

 

[Graham J W]

I think the pump might stop briefly when the anticycle time has elapsed and then restart with the burner refiring as well, Im not 100% sure but suggest you increase the anticycke time to say 35 minutes which will give a actual anticycle time of 10.5 minutes and a lower flow temp before refiring.

Thank you. I will give this a try tomorrow and let you know.

 

[Johnytuono]

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.

John, on the Valiant anti cycle chart, what do the top numbers actually refer to (5,10,15,20 etc), also, when you say "fall, theoretically to room temperature", do you mean the boilers internal temp, or the ACTUAL room temperature (say 20c) ?
Sorry to hijack btw but just curious

 

[Johntheo5]

Its a look up table that Vaillant devised based on a 20 minute (default) anti cycle time (5,10,15 etc) along the top will give a actual recycle time of 20 minutes if the target flowtemp is set to a ( unlikely) 20C, if the target flowtemp is set a more likely 60C then the set 20 minute anticycle time will result in actual anticycle time of 6 minutes, but if you think this time of 6 minutes is too short you just change the set anticycle time to a higher number, setting it to 35 minutes will give a actual anticycle time of 10.5minutes, where the vertical 35C and the horizontal 60C lines, intersect, and so on.

The boiler flow&return (internal) temperatures will (have to) eventually equalize to the room temperature if the anticycle time is long enough, (a very very long time or say, if you switch off the boiler at night, all the temperatures will eventually be the same.