Explanation of thermostat vs control temp

[vulcancontinental]

If your boiler fires for more than three minutes you can live with it. As for oil boilers if a feature or consequence is unavoidable then you build your machine to withstand it but the flexibility of gas boilers and the variations in application is huge in comparison.

 

[karlbbb]

Ideal boilers seem to have very few options.

Very short cycles reduce efficiency, but I don't know in practice how much of a problem short cycling is, as regards wear and tear. I know that most modern thermostats use a TPI algorithm to control the temperature, and they fire the boiler in short cycles on purpose. Also, a poster pointed out to me that oil boilers cycle a lot more without apparently causing a problem.

My thermostats can do TPI but it's off by default from the factory. I've not enabled it, and I'm not sure if I will, though I suppose it gets me a "fake" OpenTherm at the expense of actual heating on/off cycling, with an option for 3 or 6 cycles per hour.

 

[karlbbb]

If your boiler fires for more than three minutes you can live with it. As for oil boilers if a feature or consequence is unavoidable then you build your machine to withstand it but the flexibility of gas boilers and the variations in application is huge in comparison.

I sat and measured recently, and the firing time varied from 20-110 seconds with a flow temperature of 50C set. This may change as the winter months roll in as it's still very mild and house temperatures haven't dropped much below 18C.

 

[JonathanM]

If your boiler fires for more than three minutes you can live with it. As for oil boilers if a feature or consequence is unavoidable then you build your machine to withstand it but the flexibility of gas boilers and the variations in application is huge in comparison.

That makes sense now.

 

[JonathanM]

I sat and measured recently, and the firing time varied from 20-110 seconds with a flow temperature of 50C set. This may change as the winter months roll in as it's still very mild and house temperatures haven't dropped much below 18C.

That seems really short. And I think your situation will be extremely common. Many popular combis don't seem to go below 7KW, and with people being advised to run their boilers much cooler, there could be an epidemic of short cycling.

Not to labour the point, but is that short cycling definitely happening because of the boiler hitting its flow temperature? There's no thermostat or other external controller anywhere that could be causing it?

 

[Harry Bloomfield]

Ideal boilers seem to have very few options.

Settable options was a major part of my decision of which replacement boiler to specify and a boiler with an LCD display. A Worcester installer thought it amusing that I was insisting on the LCD display, which could provide lots of information about what the boiler was actually doing and access to lots of the settings.

 

[karlbbb]

That seems really short. And I think your situation will be extremely common. Many popular combis don't seem to go below 7KW, and with people being advised to run their boilers much cooler, there could be an epidemic of short cycling.

Not to labour the point, but is that short cycling definitely happening because of the boiler hitting its flow temperature? There's no thermostat or other external controller anywhere that could be causing it?

100% because the boiler hits the flow temperature. It seems to vary +/- 5C either side of the set point. For whatever reason, it occasionally fired up, and the flow temperature started rising, but I could hear the boiler modulate back, and the flow temperature then only crept up very slowly, until it hit SP+5, at which point the flame extinguished until SP-5, then fired again. It just seems on some fires, the boiler couldn't modulate down quick enough and hit SP+5 very quickly causing the cycle.

And just to be clear, it's definitely a flame/ignition cycle, as the boiler showed that the heating demand was "on" the whole time, indicated by the radiator icon on the front display of the boiler (which also shows flow temperature in use).

 

[Harry Bloomfield]

That seems really short. And I think your situation will be extremely common. Many popular combis don't seem to go below 7KW, and with people being advised to run their boilers much cooler, there could be an epidemic of short cycling.

Ability to modulate down to a reasonable level, was also a part of my specification for my last boiler. It's an 18Kw, open vented, with a 1:6 modulation ability, so can modulate down to 3Kw. Others can modulate even more than that.

 

[JonathanM]

100% because the boiler hits the flow temperature. It seems to vary +/- 5C either side of the set point. For whatever reason, it occasionally fired up, and the flow temperature started rising, but I could hear the boiler modulate back, and the flow temperature then only crept up very slowly, until it hit SP+5, at which point the flame extinguished until SP-5, then fired again. It just seems on some fires, the boiler couldn't modulate down quick enough and hit SP+5 very quickly causing the cycle.

And just to be clear, it's definitely a flame/ignition cycle, as the boiler showed that the heating demand was "on" the whole time, indicated by the radiator icon on the front display of the boiler (which also shows flow temperature in use).

Thanks, that's really useful for me to know for when I change my boiler, as I also want to run cool.

Have you seen the chart showing theoretical efficiency loss vs cycling time?

 

[JonathanM]

Settable options was a major part of my decision of which replacement boiler to specify and a boiler with an LCD display. A Worcester installer thought it amusing that I was insisting on the LCD display, which could provide lots of information about what the boiler was actually doing and access to lots of the settings.

The Worcester installer who quoted me was the same. He said I needed a 30KW boiler, when I know an 18KW would do, and thought I was being silly when I asked about range rating and modulation. Said I needed a big boiler that would cruise along, rather than a small one that works hard.

 

[JonathanM]

100% because the boiler hits the flow temperature. It seems to vary +/- 5C either side of the set point. For whatever reason, it occasionally fired up, and the flow temperature started rising, but I could hear the boiler modulate back, and the flow temperature then only crept up very slowly, until it hit SP+5, at which point the flame extinguished until SP-5, then fired again. It just seems on some fires, the boiler couldn't modulate down quick enough and hit SP+5 very quickly causing the cycle.

Hopefully, then, when the weather is colder, it will modulate every time. I don't know how warm it is where you are. I'm in Yorkshire, but a poster down in Bedfordshire hardly had the heating on at all yet, and his house was still warmer than mine. The weather seemed to be 2-3C warmer night and day.

 

[Harry Bloomfield]

The Worcester installer who quoted me was the same. He said I needed a 30KW boiler, when I know an 18KW would do, and thought I was being silly when I asked about range rating and modulation. Said I needed a big boiler that would cruise along, rather than a small one that works hard.

He was also very insistent that I should change from open-vented with cylinder, to a combi - as we all know a combi needs to be an even greater Kw rating, to cope with instantly heating the HW.

 

[karlbbb]

Hopefully, then, when the weather is colder, it will modulate every time. I don't know how warm it is where you are. I'm in Yorkshire, but a poster down in Bedfordshire hardly had the heating on at all yet, and his house was still warmer than mine. The weather seemed to be 2-3C warmer night and day.

I'm just over in Liverpool so weather shouldn't be too far from yours. My biggest issue is it being a modern house (timber frame with lots of insulation) - I'm seeing the house lose less than a degree of heat overnight even with external temperatures below 10C. Great for the bills overall, but certainly makes the already-oversized boiler even more hilariously oversized.

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).

 

[JonathanM]

I'm just over in Liverpool so weather shouldn't be too far from yours. My biggest issue is it being a modern house (timber frame with lots of insulation) - I'm seeing the house lose less than a degree of heat overnight even with external temperatures below 10C. Great for the bills overall, but certainly makes the already-oversized boiler even more hilariously oversized.

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).

It's no comfort, of course, but your situation really highlights this dilemma, in a way I hadn't really understood to date. There must be millions of people in well insulated, newish houses, with overpowered combis going through the same thing. Most of whom will have no idea why it's hard to get the house comfortable. Maybe developers should ditch the combis, but planning rules have made new UK houses so small, they wouldn't want to waste the space.

It's strange how sometimes it decides to modulate down and others it doesn't. If only that could be fixed it would help a lot.

It sounds like you really need an extreme modulating boiler.

 

[Johntheo5]

100% because the boiler hits the flow temperature. It seems to vary +/- 5C either side of the set point. For whatever reason, it occasionally fired up, and the flow temperature started rising, but I could hear the boiler modulate back, and the flow temperature then only crept up very slowly, until it hit SP+5, at which point the flame extinguished until SP-5, then fired again. It just seems on some fires, the boiler couldn't modulate down quick enough and hit SP+5 very quickly causing the cycle.

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.