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Hi all, I have an Ideal Logic max 35 and have just connected my Nest thermostat via opentherm. The boiler modulates the water temp fine but as the heat reaches the the target temp on the thermostat, the boiler turns the flame off, keeps the pump running and then every 90-120seconds refires the flame for about 20 seconds before repeating this process for sometimes a few hours. During this time, the water temp is around 45 deg and drops down to 24/25 deg before relighting. Of note, it never did this when it was connected via on/off. Wondered if this is normal or if it’s actually bad for the boiler to constantly cycle so often. Many thanks.
 
Do you have thermostatic rad valves throughout?
I guess either flow throught boiler is restricted by trvs or your heat demand is much lower than the boiler can modulate to so it cycles.
Shouldn't be a problem for the boiler tho.
 
Hi Knappers, thanks for the reply! All rads have TRVs and are fully open. Is the boiler essentially ‘short cycling’ by doing this? I just don’t want it to reduce the lifespan of the boiler by constantly switching the burner on and off?
 
It's not going to break it but will add a bit of wear so not ideal. I'm not sure on the settings for the nest, if there's any advanced settings to change heat curve or anything but If there is then maybe have a look at that?
 
I have the exact same thing with the exact same setup - though some of my TRV's were shut some of the time which may have been my issue - testing now.
 
I have the exact same thing with the exact same setup - though some of my TRV's were shut some of the time which may have been my issue - testing now.
Be interested to see how you get on mate! Mines still doing it so I wonder if the boiler is indeed trying to modulate at too low a temp for my system or whether the nest is doing it to just keep a bit of residual heat pumping through to maintain the requested room temp.
 
It's not going to break it but will add a bit of wear so not ideal. I'm not sure on the settings for the nest, if there's any advanced settings to change heat curve or anything but If there is then maybe have a look at that?
Unfortunately I don’t think you can change the heat settings through nest which is frustrating
 
Be interested to see how you get on mate! Mines still doing it so I wonder if the boiler is indeed trying to modulate at too low a temp for my system or whether the nest is doing it to just keep a bit of residual heat pumping through to maintain the requested room temp.
Still doing the same with all TRV's open...no idea whether this is working 'as designed' or an issue now!
 
Still doing the same with all TRV's open...no idea whether this is working 'as designed' or an issue now!
How long have you had it running with this setup mate? I’ve had mine setup for about a week and a half and the last day or two, it’s done it a bit but not as much. It seems nest maybe ‘learning’ what water temp is required in order to maintain the temp of the thermostat. Rather than cycling as often it’s now holding around 48 deg flow temp as it approaches the target and then shuts off. I’ll keep an eye on it and let you know if it continues.
 
Firstly how many rads do you have ? The Ideal logic 35 kw max has a modulation rate from 8 - 24 kw in its heating mode , there must be a heat load loss through the boiler at all times we dont know how it's installed so let's say you need at least one decent size radiator with no trv fitted is this the case ?
 

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How long have you had it running with this setup mate? I’ve had mine setup for about a week and a half and the last day or two, it’s done it a bit but not as much. It seems nest maybe ‘learning’ what water temp is required in order to maintain the temp of the thermostat. Rather than cycling as often it’s now holding around 48 deg flow temp as it approaches the target and then shuts off. I’ll keep an eye on it and let you know if it continues.
I've had this setup for a week. No idea if the boiler behaved like this before as I was not really looking! I've messed around with the boilers 'Max flow temp' but that doesn't seem to make a difference.
 
Firstly how many rads do you have ? The Ideal logic 35 kw max has a modulation rate from 8 - 24 kw in its heating mode , there must be a heat load loss through the boiler at all times we dont know how it's installed so let's say you need at least one decent size radiator with no trv fitted is this the case ?
I've got 10 rads & a couple heated towel rails. TRV's on all the rads but are fully open downstairs where the thermostat is located. The majority of the radiators are pretty old so would not surprise me if they not kicking out a huge amount of heat.
 
Opentherm should help stop the boiler cycling so much I am using it on my own system via a Nest thermostat , heating the system to a maximum of 60 °c over a longer period the boiler modulates to a very low output and condenses all the time , are you 100% sure you stat is operating in opentherm? it needs to be configured in the settings on the stat , you can then test it in the equipment setting. Kop
 

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Opentherm should help stop the boiler cycling so much I am using it on my own system via a Nest thermostat , heating the system to a maximum of 60 °c over a longer period the boiler modulates to a very low output and condenses all the time , are you 100% sure you stat is operating in opentherm? it needs to be configured in the settings on the stat , you can then test it in the equipment setting. Kop
I was 100%, but your post prompted me to double check...I hadn't realised that the stat settings needed to be changed & was set up in on/off mode. Will see what happens now, but wanted to thank you Kop - I'd totally missed the stat set up part.
 
Can be a pita to configure sometimes you need to uninstall the app and start a fresh you should see something like this once your set up . Regards Kop
 

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Can be a pain to configure sometimes often it's better to uninstall the app and download it again , once your connected you should be able to see these
 

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Firstly how many rads do you have ? The Ideal logic 35 kw max has a modulation rate from 8 - 24 kw in its heating mode , there must be a heat load loss through the boiler at all times we dont know how it's installed so let's say you need at least one decent size radiator with no trv fitted is this the case ?
Thanks King, I have 12 rads in the house, a couple are fairly large column ones and all TRVs are in the fully open position. I’ve balanced the system as best I can with an average differential between flow and return to the boiler between 13-15deg.
 
Opentherm should help stop the boiler cycling so much I am using it on my own system via a Nest thermostat , heating the system to a maximum of 60 °c over a longer period the boiler modulates to a very low output and condenses all the time , are you 100% sure you stat is operating in opentherm? it needs to be configured in the settings on the stat , you can then test it in the equipment setting. Kop
Hi kop, mine is setup exactly like this and does run for longer periods at lower temps but as the room temp reaches the target, the boiler doesn’t shut off, instead the pump runs and the flame ignites for a bit then goes out and repeats this continually. This maintains the target temp but wondered if it was normal or whether it should shut the boiler off and then restart it as the target temp drops a bit?
 
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
 
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.
 
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)
 
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
 
Trying running on speed 2 eg II
 
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 :(.
 
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.
 
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.
 
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.
 
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?.
 
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?.
The range of fan speeds based on the min and max fan menu options (used for commissioning) are 1639 and 3960 rpm. There are no settings available in the menu for adjusting recycle time. I wonder if there is an 'expert' menu where this is available? There are basically no adjustments there at all. Yes there is an ABV a basic looking Myson, it is screwed fully in.
 
You can access the installer mode here, and you can, at least run the burner at minimum rate, you could then take two gas meter (photo) readings say exactly 3 minutes apart, subtract one from the other then X 20 to give the gas consumption in m3/hr, X this by 11.0 to give the gas consumption in kwh, X this by say 0.88 (boiler gross efficiency) to give the boiler minimum output in kw.
Also, for interest, if possible,note the fan speed at this minimum output.
a
How many rads have got and ~ total output (T50, rated output) and are TRVs installed? have you zoning?


1703854865902.png
 
Thanks again. I did the measurements / calculations correctly (recorded a video for measurement) and the minimum output at the 1639 fan speed equals 5kW. I've 13 smallish mainly single convector radiators. I used an online tool to input all the details and gives a figure of 5.5kW for T50. I observed the boiler some more. The fan does modulate down to minimum speed. It took 90s for the fan to drop from Max to minimum when doing the output test! The NEST will ask for a target of around 38c when at or near set temperature and this is where the issues start. Min is a later boiler the controls are different.
20231229_142434 Medium.jpeg
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It’s dt27 on the info you’ve given that your rads at outputting at would look at upgrading a few / all of them to either k2 or p+ etc
 
With flow/return temps of 53C/43C, dT 10C, a T50 rad will emit 47.1% of its rated output, a flowrate of 8.0LPM gives a total output of 8*60*10/860, 5.58kw but this then means that the T50 rad(s) output is, 5.58/0.471, 11.85kw, average of 912watts/rad, you calculated a total T50 output of 5.5kw, 423watts/rad, this gives a total output of 2.88kw at 53C/48C/dT5.2C/8LPM.
If you try to run at 38C then the output is 1.33kw, 38C/35.6C/dT2.4C/8LPM, if the T50 rad(s) output is 11.85kw then, at 38C the output is 2.62kw, 38C/35.6C/dT2.4C/8LPM.

To put it another way. if the T50 rad(s) output is 5.5kw then a flowtemp of 71C would be required to keep the boiler just above its minimum output (4.9kw) and if the T50 output is 11.85kw then a flowtemp of 50.3C will keep the boiler running just above its minimum output (4.9kw).

IF, and only IF the data below is accurate then the T50 output of those rads has to be 11.85kw.

20231229_142407-medium-jpeg.86331
 
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