Ideal boiler cycling with Nest & Opentherm

[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?.

 

[speedyuk]

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.

 

[John.g]

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.
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How many rads have got and ~ total output (T50, rated output) and are TRVs installed? have you zoning?

 

[speedyuk]

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.

 

[ShaunCorbs]

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

 

[John.g]

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.

 

[speedyuk]

I'm really grateful of the support. Here is the full details of the current rads. The lounge as you can probably guess is where the two Double Plus (or K2 I believe) are. I presume you would have to upgrade all the radiators to keep the system design balanced? So this would mean larger physical radiators in the lounge? I guess I should have done my research better regarding the minimum output rate of the boiler, I went with the Ideal ones as they were a heat only boiler that did support DHWP with Opentherm. Not sure other / better choice there was given the system as is? I guess I should just be greatful that I'm in a well insulated house that doesn't need much energy to keep warm.

 

[John.g]

There must be someway of limiting the minimum flow temperature surely to say 48/50ish to get the boiler away?, also maybe ask Ideal if there is some method of extending the anticycle time, if required.
What is the flow temperature on DHWP and does the boiler cycle when the cylinder temperature nears its SP of 60C?.

 

[speedyuk]

Hi John, yes I think you are spot on, if the minimum set flow temperature was around 50 degrees the boiler wouldn't overshoot. The room temperature would just be reached a bit sooner and the boiler would just be told to turn off in my case for probably around 30mins. This would be far preferable to the short cycling and longer time to get to room temperature. I've pinged an email off to Rishi Chandra to request the option. Will be interesting to see if I get a reply. The NEST Gen 3 is an old product now and I suspect the dev team have long since moved on to something else. I have been researching Opentherm gateways to act an intelligent middleman but I'd really rather not have some Heath Robinson contraption in the system.