Explanation of thermostat vs control temp

[karlbbb]

You have, I think, a 35kw combi, with 26kw to CH, when it fires up, it probably, like most gas boilers, fires at, say, 65% of max output, in your case, 35X65%, 23kw, even if it starts to modulate down within seconds of firing then there is no way IMO that it can reduce in time to avoid hitting SP+5C from firing up at SP-5C, the anticycling time must be increased to allow the temp to fall much further before refiring, if the flowrate is 25LPM then the dT is 13C (at 23kw) so the temperature must fall to ~ 42C to avoid the flow temp reaching 55C (SP+5) and burner cut out, this minimum temperature becomes 38.5C at 20LPM, 33C at 15LPM and a almost impossible 22C at 10LPM.
Have a look at the anticycle time and I would suggest a minimum of 3 to 5 minutes and see how you get on, I have watched a Vokera vision 20S cycling on a 2.5/3.0kw demand (boiler min output ~ 5.5kw) with a 3 minute anticycle time refiring with no problems with flow temp only exceeding the SP by 1 or 2C, it will then run until eventually the flow temperature rises to SP+5C with burner cut out and repeat.

Thanks for those workings, as they definitely highlight the issue with oversized combi boilers.

As for the anti-cycle time, it appears this is not changeable on the Ideal boilers so I'm not able to go down that route shy of getting a brand-new boiler.

 

[JonathanM]

You have, I think, a 35kw combi, with 26kw to CH, when it fires up, it probably, like most gas boilers, fires at, say, 65% of max output, in your case, 35X65%, 23kw, even if it starts to modulate down within seconds of firing then there is no way IMO that it can reduce in time to avoid hitting SP+5C from firing up at SP-5C, the anticycling time must be increased to allow the temp to fall much further before refiring, if the flowrate is 25LPM then the dT is 13C (at 23kw) so the temperature must fall to ~ 42C to avoid the flow temp reaching 55C (SP+5) and burner cut out, this minimum temperature becomes 38.5C at 20LPM, 33C at 15LPM and a almost impossible 22C at 10LPM.

Using your calculation, I think with a flow rate of 15 LPM, the dT would be 22C. If the boiler modulated down as quickly as 10 seconds after firing, does that mean it would have raised the temperature by 10/60ths of 22 degrees, so about 3.5C. If so, that would give a flow temperature of 48.5C, even if flow was still at 45C when firing started? I have no idea whether 10 seconds is feasible for a boiler to modulate down! Just trying to see how the figures work.

 

[Harry Bloomfield]

Unfortunately, there doesn't seem to be a cheap fix. How much for a smaller combi that would be closer to suitable but still likely not to modulate down enough? How much hassle and thousands of pounds to re-plumb the house and install a hot water tank and switch to a system boiler which could definitely be more suitably sized, as long as there was some kind of DHW priority available? And even with those changes, what's the payback period? There's a slightly bigger argument in favour of the re-plumbing of the house with a tank as it would set the house up ready for PV including a diverter to heat the water on sunnier days, but it's still a lot of outlay in preperation for further outlay for PV (+ battery maybe).

I wonder what a heatstore would cost? Boiler heats the heatstore, then the heat is circulated as needed to your radiators, with not need for the boiler to run.

 

[Johntheo5]

What is a Ideal Logic anti cycle time fixed at?.

 

[vulcancontinental]

You have, I think, a 35kw combi, with 26kw to CH, when it fires up, it probably, like most gas boilers, fires at, say, 65% of max output, in your case, 35X65%, 23kw, even if it starts to modulate down within seconds of firing then there is no way IMO that it can reduce in time to avoid hitting SP+5C from firing up at SP-5C, the anticycling time must be increased to allow the temp to fall much further before refiring, if the flowrate is 25LPM then the dT is 13C (at 23kw) so the temperature must fall to ~ 42C to avoid the flow temp reaching 55C (SP+5) and burner cut out, this minimum temperature becomes 38.5C at 20LPM, 33C at 15LPM and a almost impossible 22C at 10LPM.
Have a look at the anticycle time and I would suggest a minimum of 3 to 5 minutes and see how you get on, I have watched a Vokera vision 20S cycling on a 2.5/3.0kw demand (boiler min output ~ 5.5kw) with a 3 minute anticycle time refiring with no problems with flow temp only exceeding the SP by 1 or 2C, it will then run until eventually the flow temperature rises to SP+5C with burner cut out and repeat.

I think it's perfectly feasible in a well insulated house to stretch that 5 minutes further, Vokera's go up to 20 minutes. Dependant on model it's also possible to reduce the ignition fan speed (which is lower than 60% of the un-range-rated fan speed) but I think that's best left to a Gas Safe engineer.

The Ideal Logic is probably the biggest selling boiler in the UK, dumbed down with minimal adjustment to be put in and left for an installer and manufacturer who know their market and who do not have to live with any inconvenience due to inflexibility or the running cost.

 

[JonathanM]

Thanks for those workings, as they definitely highlight the issue with oversized combi boilers.

As for the anti-cycle time, it appears this is not changeable on the Ideal boilers so I'm not able to go down that route shy of getting a brand-new boiler.

The Logic Max has a ten year warranty. Do you know if that is still in operation?

The fact that sometimes it modulates down, and sometimes it doesn't, makes me wonder if there is a fault. I would expect it to do one or the other every time, as it seems logical that it should be following the same firing pattern. I think you said when it modulates you hear it happen almost straight away. You said sometimes it only fires for as little as 20 seconds, and other times up to 110 seconds. When it's the longer burns like the 110 seconds, is that when you hear it modulate down?

I don't know whether you've mentioned how long you have lived there, but have you ever monitored it through colder weather to see what happens then?

EDIT: and when it's on for just 20 seconds, does the flow rise by 10C, from SP-5 to SP+5, in those 20 seconds?

 

[JonathanM]

What is a Ideal Logic anti cycle time fixed at?.

I don't know the answer to that. But I was wondering what you think about this?

It looks like the only setting you can change at all on the Ideal Logic is the pump speed. It has a modulating pump which automatically varies its speed. You have just one option. By default, it is set to vary from a minimum of 70% up to 100%. The option you have is to set it to run continuously at 100%. Presumably, this might make the water flow faster through the heat exchanger. If so, would the water heat up more slowly and give more chance for the boiler to modulate? But, with my limited knowledge, I would worry that running the pump at that power might do some damage in a relatively small system.

I'm really proposing this only as a test to see if the boiler will modulate more.

EDIT: I know instinctively this is a really silly idea, but changing the pump speed is the only option on this boiler. I won't be offended if anyone tells me why it's stupid. It's a 7m head pump at 100% power.

 

[vulcancontinental]

You'd close the ∆T across the emitters and boiler.

 

[Johntheo5]

Your wrong instincts are right, definitely give that a go because potentially you will improve the flow rate by up to 40%

 

[Johntheo5]

You'd close the ∆T across the emitters and boiler.

But you will cool the water more on anti cycle and the rads will emit more normally, a help as well IMO.

 

[JonathanM]

You'd close the ∆T across the emitters and boiler.

It would just be about buying time when the boiler fires up for it to modulate. We might only be talking about another ten seconds. By my simplistic thinking, the hot water leaving the boiler will take longer than this to return to the boiler, and by the time it does, the boiler might be modulating. I realise it sounds a bit mad.

 

[JonathanM]

Your wrong instincts are right, definitely give that a go because potentially you will improve the flow rate by up to 40%

Thanks. Although this is a suggestion I'm making for somebody else to follow. I'm not the OP. Just wanted to see what you thought.

 

[vulcancontinental]

It would just be about buying time when the boiler fires up for it to modulate. We might only be talking about another ten seconds. By my simplistic thinking, the hot water leaving the boiler will take longer than this to return to the boiler, and by the time it does, the boiler might be modulating. I realise it sounds a bit mad.

And if there are a couple of radiators a few metres from the boiler?

 

[JonathanM]

And if there are a couple of radiators a few metres from the boiler?

I see what you mean.

 

[karlbbb]

The Logic Max has a ten year warranty. Do you know if that is still in operation?

The fact that sometimes it modulates down, and sometimes it doesn't, makes me wonder if there is a fault. I would expect it to do one or the other every time, as it seems logical that it should be following the same firing pattern. I think you said when it modulates you hear it happen almost straight away. You said sometimes it only fires for as little as 20 seconds, and other times up to 110 seconds. When it's the longer burns like the 110 seconds, is that when you hear it modulate down?

I don't know whether you've mentioned how long you have lived there, but have you ever monitored it through colder weather to see what happens then?

EDIT: and when it's on for just 20 seconds, does the flow rise by 10C, from SP-5 to SP+5, in those 20 seconds?

Given we have no evidence of services, I think it's safe to say any warranty is now gone. I know that'll draw you even closer to there being a fault with the boiler... but on the times were it'd fire for a small amount of time, I did hear the boiler start to modulate back down, so I don't believe there is a fault, I just think sometimes it's been able to "catch" itself quick enough to be able to control the burn a lot longer. Yes in the 20 seconds it'd rise up by 10C.

My guess is that on the time it fired for a short period, it raised the flow temperature enough to hit SP+5, but a lot of that heat didn't soak into the rest of the system, so when SP-5 came back around the temperature in the loop was dropping a bit quicker than last time. Then on the next fire, thanks to the loop being a bit cooler, it was able to "catch" itself properly and modulate down to the lowest flame which kept the burn up for a lot longer this time. Rinse and repeat.

We've only been here since July so absolutely no reference on it running through colder weather. It's also our first experience with a properly insulated house, after leaving a 1902 Victorian terraced