Dull question: CH cycling or constant?

30 Dec 2008
Reaction score
United Kingdom
Bit of a dull question:

Is it better to leave the thermostat on a constant setting, say 18, and leave the central heating to start up every 15mins (for 5 mins) and then stop again etc... and leave it cycling?


Can you get a thermostat which will enable you to increase the range of temperatures, i.e. it will turn off the CH when temperature is 20, but won't come on again until temperature falls to say, 16? This would increase the periods it is on & off, resulting in less firing. At the moment, this approach would have to be done manually.

It just seems to be a lot of firing for the boiler every 15mins to keep the house temp at 18, only to run for 5mins. Admittedly it is 2 degrees outside.

Apologies if this post makes you sigh!

Sponsored Links
It is wiser to let the room thermostat control the boiler so it does not cycle (having achieved ambient tempertaure needs). Boiler cycling is wasteful.
If the room thermostat controls it, then it will be more likely to cycle, surely? Unless there is a thermostat that can increase the ranges between turning on/off the CH.
Boiler fires from cold. Circulating water is heated in the heat exchanger. As the heated water leaves the boiler, a probe checks the temperature. If water temperature greater than as set on boiler thermostat, burner goes out.

Room thermostat waits until room aquires design temperature (hall 18, bedroom 16, lounge 20 and bathroom 25 degrees C). If room thermostat installed in hall, and boiler now running at higher temperatuer to get designed heat output from radiator (it being baltic outside conditions for which the radiator should have been sized), at 18 degrees ambient temperature the boiler will be switched off.

On the other hand, if the boiler is chugging away with boiler thermostat set on low, boiler will keep on cycling as room thermostat fails to be satisfied. If only TRV fitted, rooms get to desired temperature but boiler stays on line as nothing tells it to 'stand down'
Sponsored Links
Thanks for your reply DP.

It is not so much that the boiler is reaching temperature early and that the room temperature is not satisfied.

It will usually reach room temperature before the boiler will halt because it has reached water temperature.

It is more of a case of: the boiler will re-light every 15mins because the room thermostat has dropped off (because it is so cold). Re-lighting every 15mins to satisfy the room temperature just seems excessive. TRVs are fitted on every radiator - with variable levels as required. I've got all the cavity wall insulation, floor & loft insulation (car port beneath me).
You should not have TRV,s on every rad, there should be at least 1 without :eek:
There are programmable room thermostats such as the Heatmiser PRT which have adjustable settings for...

...the temperature differential between switching ON ond OFF

...the output time delay (up to ~30 mins), to avoid rapid cycling.

In addition the heatmiser can do tricks like calculating the time it needs to fire up the boiler in the morning to get the house to the desired temperature on time.
This doesn't surprise me. My house is 1 year old and the plumbing company who installed all of the plumbing (now out of business) really didn't do well with it.

x 2 leaks in bathroom in 10 months - x 1 new carpet. Both due to fittings not adequate/faulty.

x 1 new boiler (due to flue restrictor installed when the flue is 2m long - boiler drowned in fluid - rust holes eveywhere)

No access to stop cock

Banging CH pipes (living with this at the moment)

TRVs on every radiator - if this is an issue?

x 1 Burst pipe resulting in car port below being pulled down to locate leak - pipe joints were only just soldered together.

Thanks Jackthom for info.
The trouble with having TRV;s on every rad is that if all the rooms get up to temp the TRV is set at before the room stat reaches its set point, all the TRV;s will close and the water will not be able to circulate around the system. This will cause the boiler to cut in and out all the time as the water inside the boiler will just heat up instantly to the set temp and then cut out because there is no flow through it. When the boiler water cools, it will do the same again.

However Some systems DO have an adjustable bypass line that would allow the flow to continue with all TRV's shut :?:

Have you tried increasing the set point on a few of the TRV's to see if this helps with the rapid cycling problem?
Everytime your boiler's gas valve operates you lose about 20% of the initial volume of unburnt gas out of the flue so boiler cycling is not a good idea if you want to save fuel/money.
The room where the wall thermostat is located must not have a TRV fitted otherwise the boiler will keep on running trying to heat the room even though the TRV may have reduced the flow to the rad.

If there is a TRV on that rad have it fully open/set on highest temperature or remove the operating head from the valve. This will allow the wall thermostat to do its job and should reduce cycling.

You also need to check the wiring to ensure there is an interlock shutting down the boiler when it the thermostat has reached the desired temperature. Some older systems are not wired up like this.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links