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Moved to a house which has a myson bolier (approx 15 yeras old). We have had thermostatic valves fitted upstairs (not bathroom). Since then the bolier heats up fine but after reaching the temp set by the central theromostat (in hall) and turning off it then turns on again too quickly (say within 5 to 10 minutes) and then will go off for 5 minutes before coming on again. This can go on for most of the day. I had thought that the temp had to drop by at least 1 or 2 degrees before it kicked in again! Not sure if this is related but I am having to bleed the bathrrom rad daily due to excess air (it makes a clunky noise most of the time). Any help appreciated as I'm concerned the boiler is overworking and therefore will be reducing its life. Many Thanks.

If your boiler is piped to give heating and hot water the continuous cycling may be a combination of heating and hot water demands. It is unusual for the room thermostat to control the boiler directly. On CH mode the room stat would normally control the pump or a motorised divert valve.

Depending on how the room stat is wired, and its functionality, the on/off span could vary between 2 and 4 degrees. The location of the room stat is crittical in respect of what temperature variations are influencing it.

The problem with the constant air getting into the system is usually caused by the cold water feed and the system expansion pipework wrongly connected at point of entry to the system.

May be an idea to have a heating engineer have a look over the system.

Many thanks for your reply.

The boiler does control both CH and hot water - we have been leaving it on constant (both CH and hot water) for the last couple of weeks turning the heating down from approx 19C to 15C at night to see if it settles down with little success.

Any other ideas to try before I resort to calling out an engineer?

Me again...been doing a little more experimenting and it does seem that the room thermo does control the bolier (when the room thermo makes a rather loud clunk the bolier then kicks in a couple of seconds later).

Also, I'm not sure if the linkage to hot water is now true as after the bolier turns on (shortly after turning off!) it will turn off again if I move the room thermo down just one degree!

Any help much appreciated as it is starting to drive me mad!!!

Anyone advise how often a boiler should cycle each hour on a cold day (say max 5 degrees outside)? I'm starting to think it may be due to the conditions rather than the boiler itself.

It will be the frost stat making the boiler ignite

The frost stat will signal the boiler to fire as Steve the gas states but I cann't see this cycling the boiler on/off as quickly as you say. The fact the room stat is still controlling is the problem.
Think you would be advised to have an engineer have look to prove the system wiring and component function.

What about the boiler thermostat setting?

Set it on high so that the sytem is controlled by the room stat and cylinder stat (if you have one).