Ideal boiler cycling with Nest & Opentherm

[flyingsailor]

I don't think it should cycle so frequently even when it reaches its temperature set point mine doesn't Opentherm should modulate your boiler down to its lowest setting possible , review your settings on your Nest as my previous post , what boiler do you have ? it maybe possible to set a period of time your boiler heat between switchings ? Regards Kop

All settings are the same as yours - opentherm, combi, 60 deg etc. I have an Ideal Logic Max 35. It just seems odd to me that it won’t hold just hold mid 40 deg flow temp (it can do this as I’ve checked the sensors on the boiler) until it reaches target then shut off. It stops the flame as it’s very near to target temp and keeps the pump on with the occasional kick of flame to increase the flow temp for about 20 secs.

 

[flyingsailor]

I don't think it should cycle so frequently even when it reaches its temperature set point mine doesn't Opentherm should modulate your boiler down to its lowest setting possible , review your settings on your Nest as my previous post , what boiler do you have ? it maybe possible to set a period of time your boiler heat between switchings ? Regards Kop

Hi kop, update - when the heating kicks in it will modulate low (around 37-40 deg flow) but if there is a demand for DHW when it’s running that low, it never returns to that temp and that starts the cycling. It’s as if the flame for CH starts up and gets too hot too quick (hits around 41 ish and then cuts out)

 

[speedyuk]

Hi, I'm joining in on this thread as I've just had an Ideal Logic Max Heat 2 15 boiler fitted. On day one it was commissioned in S Plan using the NEST Gen 3 that had been fitted to the previous boiler. Day 2 it was switched over to PDHW / Opentherm. It operates correctly in the two different call for heat modes but I'm getting the dreaded short cycling when the room stat approaches target temperature - grr. I went down this route after having a perfect working BAXI 600 Combi and Tado setup in my previous home. That setup was fantastic, didn't miss a beat and modulated the boiler perfectly. The 11 year old 4 bed detached property I'm in now already had unvented cylinder and 10mm microbore. It has a Grundfos UPS 15/60 pump. Pump was on III with previous boiler and left like that with new. The boiler reports a flow rate of 11l/m on III and drops to 9l/m on I. If anyone has any suggestions / new info on a cure I'd love to hear it. Thanks

 

[ShaunCorbs]

Trying running on speed 2 eg II

 

[speedyuk]

Trying running on speed 2 eg II

Hi, I've run it on 2 and now I'm down to 1 as the boiler actually reports a better flow / return differential on 1. The boiler is still short cycling. I suspect a) it can't modulate low enough for the demand and b) even if it can in certain cases it still fires too high and doesn't modulate down quick enough to catch the required output .

 

[John.g]

If its running at minimum output, 4.9kw?, then the dT at 11LPM should be 6.4C and at 9LPM, 7.8C,
If the burner is tripping before target temp then you may have TPI stat control, if the heat demand is less than the boiler minimum output then it shouldn't trip until it reaches Targettemp+5C.
Some boilers fire up at ~ 65% of max output, if yours fires up at 9.75kw, then a flowrate of 11LPM gives a dT of 12.7C so the flow temp on refiring has to be at least 12.7C below the target temp or if the lowest practical flow temp after anticycling is say 30C then the minimum target temp cannot be less than 43C, this still leaves +5C to allow the boiler to modulate down before the burner trips which is pushing it a bit but might be OK.

 

[speedyuk]

If its running at minimum output, 4.9kw?, then the dT at 11LPM should be 6.4C and at 9LPM, 7.8C,
If the burner is tripping before target temp then you may have TPI stat control, if the heat demand is less than the boiler minimum output then it shouldn't trip until it reaches Targettemp+5C.
Some boilers fire up at ~ 65% of max output, if yours fires up at 9.75kw, then a flowrate of 11LPM gives a dT of 12.7C so the flow temp on refiring has to be at least 12.7C below the target temp or if the lowest practical flow temp after anticycling is say 30C then the minimum target temp cannot be less than 43C, this still leaves +5C to allow the boiler to modulate down before the burner trips which is pushing it a bit but might be OK.

Hi John, thanks for the detailed reply. Yes the minimum output is specified at 4.9kw. The Nest is operating in Opentherm mode with True radiant enabled. It should and is call for lower flow temperature rather than just on / off the boiler in TPI mode (which I had to google). I think the issue is like you say, the start up fan speed (which seems to be how the rate is adjusted) is too high and so the returning flow temp is already above the set temperature and so it just cuts out . I’d like to be able to set a higher minimum set temperature on the Nest so at least the boiler carried on firing until the room set temperature was reached. Shame there aren’t more overrides on Nest.

 

[John.g]

Can you see any typical low(est) flowtemp on the boiler display/menu?.

 

[speedyuk]

Can you see any typical low(est) flowtemp on the boiler display/menu?.

With a room temperature set to 20.5 and once up to or approaching that Nest I’ve seen a requested / set flow temperature of 38. I’ve just done a few experiment, as I say the burn rate is controlled by the fan speed. Max fan is 3900 rpm. It seems it will fire at 3600 rpm. I’ve seen it drop to around 2400. The software is obviously not specific to this heat only boiler as the display shows a variable pump speed even though it doesn’t have one! The Nest does keep the pump running all the time it calls for heat, so there is circulation there while the boiler is doing its cycling. Thanks for you help.

 

[John.g]

Boiler output is fairly proportional to fan speed so if 3900rpm = 15.1kw then it fires at ~ 13.9kw, on the other hand 2400rpm = 9.3kw which is hardly its minimum output?.
Can you see a settable anticycle time (and (pump) overrun time)?, the circ pump should stay running on any boiler during anticycle, if the recycle time is long enough, say 10 minutes or so then it might be possible to get the flow/return temp down to 25/28C and then just maybe the boiler can get away.
Is there a manual or automatic (ABV) bypass installed?.