Hysteresis setting on thermostat

skycatcher

What is the recommended setting for this? Mine is defaulted 0.4c ie the boiler fires when the temp drops this much below the target.... but it does feel a noticeable change in the room comfort. Is there a problem changing this to a smaller margin?

FreeBear

My thermostat is set to fire the boiler up when the temperature drops 0.3°C below the set point and shuts it down at 0.2°C above - This gives me a hysteresis of 0.5°C. Reducing this would mean that the boiler fires up more often, but for shorter periods.

With a modern condensing boiler, it would make more sense to modulate the output down, but I don't have that luxury... Cycling the boiler on/off more frequently would put extra stress on some of the components inside which could increase the chance of a failure (ignition components & fan spring to mind).

lohr500

I am no expert but I think if you set it too low, the boiler will short cycle. ie keep firing up and switching off on a very frequent basis. I suspect that probably does nothing for efficiency as a longer burn time will allow the boiler to settle down and condense effectively (assuming you have a condensing boiler and the water return temperature is below 54C). 

matelodave

As said above, it's not a good idea to short cycle the boiler, not only does it add extra stress to components but it will also increase your fuel consumption because the boiler will run at full blast every time it relights sending most of the energy out the flue until it settles down.

Qyburn

I don't think it's just the hysteresis causing this effect, it's also the delays inherent in a system of boiler, pump and radiators.  If you just have an on/off thermostat then when the room reaches the "off" threshold the boiler and pump switch off within a few seconds. However the radiators in the room remain hot for some time, continuing to heat the room. This causes the temperature to overshoot, which means it has further to cool before it reaches the "on" threshold. Furthermore there's another delay between the thermostat switching the heating on, before the radiators are properly back up to temperature meaning that the room continues to cool even after the thermostat has switched back on.

The proper solution is a proportional control that starts to back off the heat input before the setpoint is reached.

Old school room thermostats used to contain a tiny heating element, this provided a degree of anticipation so the thermostat switched off before the room was truly up to temperature. I don't know if modern "smart" stats do anything similar, I wouldn't be surprised to find they don't.

Reed_Richards

If you have Load Compensation enabled you can set the hysteresis very low and you boiler will maintain the set temperature very accurately without short-cycling.  But for that feature you need an Opentherm controller for your (gas) boiler or the possibly the boiler's own-brand controller.

Astria

My thermostat requests heat at 0.1c below the target temperature and turns off the heat at the requested temperature.

The boiler is currently running at 50c at the moment, which means the property drops by about another 0.1c before it starts climbing again, and typically has turned off before the thermostat has stopped asking for heat, with the residual heat bringing the rooms upto temperature, as the boiler runs for about 15 minutes before shutting down then ignores requests for heat for 10 minutes.

So it doesn't overshoot by more than 0.2c and typically doesn't undershoot by more than 0.2c either.

InvertedVee

0.4C is certainly high enough to feel too cold and too hot.

My Tado thermostat maintains temperatures to within 0.1C. I have set my Worcester Bosch 30i boiler to an anti-cycling time of 10 minutes.

skycatcher

InvertedVee said:

0.4C is certainly high enough to feel too cold and too hot.

My Tado thermostat maintains temperatures to within 0.1C. I have set my Worcester Bosch 30i boiler to an anti-cycling time of 10 minutes.

Wasn't aware of this setting...I shall dig out my manual!

Astria

skycatcher said:

InvertedVee said:

0.4C is certainly high enough to feel too cold and too hot.

My Tado thermostat maintains temperatures to within 0.1C. I have set my Worcester Bosch 30i boiler to an anti-cycling time of 10 minutes.

Wasn't aware of this setting...I shall dig out my manual!

I also changed the maximum output of my boiler to 10kW which means that it takes longer to heat up the water in the system which means it's on longer and seems overall more efficient than blasting 20 kW, hitting the target temp and switching off in 5 minutes.

InvertedVee

Astria said:

I also changed the maximum output of my boiler to 10kW which means that it takes longer to heat up the water in the system which means it's on longer and seems overall more efficient than blasting 20 kW, hitting the target temp and switching off in 5 minutes.

Good move. And much nicer to live with if your boiler is in the kitchen like ours is (much less noisy).