How often should boiler switch on and off

[wavemeister]

Our heating seems to be be very good at maintaining a steady temperature in the house but seems to switch on and off a lot. We have the thermostat set to about 22°C. I have digital wireless sensor in the lounge linked to a weather station so I can monitor the room temperature.

The room temperature hardly seems to drop before I hear the thermostat click and the boiler fires up. It then will run for two or three mins before I hear the the room thermostat click again. The pump continues to run for 2 mins before switching off. It will then come back on again again 2 or 3 mins later. Is this normal or is the thermostat faulty?

The boiler is a conventional gas boiler with thermostatic radiator valves fitted and the house is well insulated

 

[JohnD]

is it a modern boiler? Condensing and self-modulating?

Do you know the heat output of your boiler, and of your radiators?

An older boiler is either "on" or "off " whereas a modern one will modulate its power up and down depending on demand (mine varies from 8kW min to 24 kW max) so it doesn't go on and off much.

You may be able to turn down the boiler stat so that the output is nearer the demands of the rads, then it will tick quietly along.

A boiler that switches off and on a lot is less economical, but I am not a pro, and I don't know the figures.

You mention you have TRVs, and a wall stat as well, which is good.

Yours is certainly going on and off too much. You mighty do better with a thermostat which has wider hysteresis (sp?) so that it allows a wider gap between on and off. Is your wall stat too close to a radiator or a draughty window? Does the rad in that room have a TRV (it shouldn't)?

 

[wavemeister]

The boiler is 13 years old, not condensing it is a Potterton Suprima 50 with the updated pcb fitted, output is 11.4 to 14.7 KW. When we had it serviced in the summer the engineer said we had the boiler stat set too low and said it should be on max. before that I had never set it higher than 3/4. The room stat is in the lounge and is not in a draught. The two radiators in the lounge both have trv's but are both turned to full

 

[JohnD]

if the stat is near one of your rads, it may be worth turning the TRV down on that one. What I have in mind is that the heat from the nearest rad might be overwhelming the stat so that it gets too warm, then once the rad goes off, it cools quickly.

The other thing that occurs to me is that the thermostat might have an "accelerator" heater fitted. the idea of this is that it warms the stat up by a small electric element, before the room comes up to temp, to remove the delay caused by the rads warming up, then the heat travelling to the stat.

If you are fond of minor electrical work you could turn off the power and have a look under the stat cover, or post a photo of it and someone will know.

Experimentally, turn down the boiler stat and see what effect that has.

If the output of the boiler is greater than that demanded by the rads, it may be possible to have it adjusted to run at the bottom of its range. This will reduce cycling.

I think I have run out of ideas now