How Does a Boiler Cycle?

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Just pondering ...

Conventional combi boiler, not a condensing one.

Heating kicks in, house comes up to temperature, TRVs restrict flow until they close off. Assume the room stat has been set way too high so never does it's thing.

Somehow the boiler knows there isn't a demand for heat and stops. A bit later, the house has cooled, and by magic the boiler kicks back in to bring things back to temperature.

Can anyone point me to a source that can explain how the boiler detects the need to stop and restart please?
 
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What is the room stat set at ?
Does lowering it cause boiler to stop ?
 
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If the roomstat is calling for heat, the boiler will never stop but the burner will cut in /out if the heat demand is lower than the minimum output of the boiler, the burner will cut out when the boiler water (flow) temperature exceeds its SP (setpoint) by 5C and cut back in when the boiler flow temperature falls to its SP-5C and the anti cycle time (which may be 3 to 5 mins) has elapsed, the circulation pump stays running so the house shouldn't cool down. Oil fired boilers can't modulate so spend their entire time cycling.
 
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A modern boiler will use modulation to avoid short-cycling.
It will but all boilers can only modulate down to a minmum output which may be 5 to 8kw, a very few can go as low as 2kw, so if a boiler has a minimum output of say 5kw and the heat demand is 4kw, the boiler temperature will continue to rise. and,
the burner will cut out when the boiler water (flow) temperature exceeds its SP (setpoint) by 5C and cut back in when the boiler flow temperature falls to its SP-5C and the anti cycle time (which may be 3 to 5 mins) has elapsed, the circulation pump stays running so the house shouldn't cool down. Oil fired boilers can't modulate so spend their entire time cycling.
 
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If the roomstat is calling for heat, the boiler will never stop but the burner will cut in /out if the heat demand is lower than the minimum output of the boiler, the burner will cut out when the boiler water (flow) temperature exceeds its SP (setpoint) by 5C and cut back in when the boiler flow temperature falls to its SP-5C and the anti cycle time (which may be 3 to 5 mins) has elapsed, the circulation pump stays running so the house shouldn't cool down. Oil fired boilers can't modulate so spend their entire time cycling.

Interesting. I'm guessing there's a thermistor somewhere that provides water temperature for the boiler to work with. Also with the pump running continuously, the boiler is closer to measuring true water system temperature than a localised stagnant water temperature for a boiler sitting in a garage say.
 
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When the boiler switches off due to water being too hot the pump continues to pump water around the system, so once water cools the boiler will kick in again, so water alternates between the two temperatures.

With more complex systems other factors can come in, pump speed, and timers, and anti-cycle software, exactly how depends on the boiler.

As @Johntheo5 says oil is slightly different, the latest ones may modulate as they say condensing, but most are simple on/off, so the combi boiler just has a small reservoir inside, it means the circulating water always floats between two limits.

With a gas modulating boiler setting the lock shield is easy as you have time to take two readings input and output, but with oil since the water is not at a constant temperature one really needs to measure in and out temperatures together.
 
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Interesting. I'm guessing there's a thermistor somewhere that provides water temperature for the boiler to work with. .
There is a thermostat on the flow pipe, and I think it's usually soon after the flow comes out of the heat exchanger.
 
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The original question has just made me think of another I've never fully understood. Why is short cycling actually inefficient in the first place? I've seen the figures based on cycle time, and I know cycling increases wear and tear, but I can't find a good explanation of why it is inefficient. Some reasons I have thought of. Does the burner burn less efficiently in short cycles, like a car engine getting up to temperature? Does more of the heat produced get lost through the flue, or get left in the heat exchanger?
 
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Interesting. I'm guessing there's a thermistor somewhere that provides water temperature for the boiler to work with. Also with the pump running continuously, the boiler is closer to measuring true water system temperature than a localised stagnant water temperature for a boiler sitting in a garage say.
Yes, it's called the control-stat, and it's part of the boiler
 
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The original question has just made me think of another I've never fully understood. Why is short cycling actually inefficient in the first place? I've seen the figures based on cycle time, and I know cycling increases wear and tear, but I can't find a good explanation of why it is inefficient. Some reasons I have thought of. Does the burner burn less efficiently in short cycles, like a car engine getting up to temperature? Does more of the heat produced get lost through the flue, or get left in the heat exchanger?
I believe the efficiency loss is < 1.5% if the cycle ON time is 3 minutes or more, I would think most of the loss is caused in the post purge and pre purge where the fan will cool down the Hx, some boilers actually used this as a method with pump overrun to cool down the Hx after roomstat or cylinder stat shuts the boiler down. The main problem IMO with gas boiler cycling is because they fire up at 60/70% of max output and to avoid burner trip at SP+5C the anti cycle time has to be fairly long to get the flow temp low enough to avoid this on re firing, this isn't actually wasteful but can lead to poor heating control. Oil fired boilers cycle all the time with burner cut out at flow temp SP and will refire at ~ SP-8C, no anticycle time, normally, these fire up at 100% output but because the main Hx is a metal "box" containing 25/30L of water which acts as a buffer then will not reach SP and cut out on refiring.

Wear & tear, don't know, I have a 16/17 year old oil fired Firebird that has never required a single item replacing, daughter has a 21 year old one, ditto replacements. all these oil fired boilerd must cycle thousands of times during their lifetime so why should a gas boiler be different with minimal cycling?
Gas boiler manufacturers don't seem to have any concerns when used with Evohome which continually cycle the boiler if all the room temps are within < 1C?? of their SP even when the boiler is perfectly capable of firing continuously once the heating demand is > the min boiler output.
 
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I think purging sounds like the most likely reason for the inefficiency.

Your second point is something else I've been wondering about. On the one hand, we're advised to minimise cycling for efficiency, but then a basic TPI algorithm room thermostat might run a boiler for 1 minute, six times an hour, to keep the temperature bang on the room stat SP, rather than one burst of 6 minutes. At this time of year, anyway.
 
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>a basic TPI algorithm room thermostat<

Yes, which I suppose why Opentherm/Weather Comp is now the preferred means to avoid overshooting target temp as the boiler modulates fan speed. Plus devices like Nest are setup to to know if the heating is using rads or under-floor for further optimisation.
 
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Going back to my query, if the roomstat was set higher than the TRVs cutting demand, then the cycling would still continue as the system (pipe) cools below SP-5(say). The only way to totally stop my (simplistic) boiler cycling would be when the roomstat cuts out, and being a switch in series with the timer, cuts the power.

And if TRVs and cycling could run the heating demand, it doesn't feel like the roomstat makes much of a fuel saving.
 

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