Boiler activating too often

[Hicks]

We're about to sell our house, and sod's law dictates that the central heating system picks this exact time to start playing up. Problem is, the boiler is activating too often. So much that the radiators are melting off the wall. I'm guessing this is some kind of thermostat problem, although I don't fully understand how it works. The room thermostat seems to have no effect on the boiler i.e. you can turn it down low and the boiler still comes on. There is also a dial on the boiler itself, which we've set to very low, and this has prevented the boiler from firing up. Normally this dial is set to a mid setting.

Question is, how do these two thermostats interact, and what exactly are they meant to be controlling? Do you think the wall thermostat is faulty? Worst of all, will we have to replace the boiler?

Thanks. Andrew.

[Andy Hall]

it does sound like the room thermostat.

If the boiler continues to run when it's turned right down, then almost certainly so.

The effect of turning the boiler thermostat down is to reduce the temperature of the water going to the radiators and thus the heating output. This is not good for the boiler on a long term basis.

As a first step, try replacing the room thermostat. Check the wiring carefully, because some have two wires (possibly plus earth) and are a simple switch; others have a third wire which is used for a tiny heater in the thermostat used to improve its response. In a replacement, you don't need to buy a three wire one necessarily, but just note which wire goes where.

From your description, it sounds like this is the likely culprit. If the room thermostat were doing something but the boiler was getting too hot then the boiler thermostat would be the problem, most likely.

[Andrew Gabriel]

It sounds like a faulty wall thermostat, which is fortunately about the simplest and cheapest bit to replace. A really cheap one, as you might fit to a house you are just about to sell, would cost under a tenner ;-)

[Michael Chare]

Or just disconnect it. If the boiler stops it is likely that you have found your culprit.

Usually you can hear the thermostat clicking as you turn it up and down.

Once disconnected you should be able to see the contacts opening and closing as you adjust the dial

If you were not selling the house I would suggest replacing with a programmable thermostat.

Michael Chare

[The Natural Philosopher]

The room stat is supposed to control the overall room temp, the boiler stat controls the outflow temp of the water used to do it.

and what exactly

Possibly.

Also you may have a stuck motorised valve. These open the water flow and actually switch on the boiler, on demand from teh timer/thermostat, which simply switches THEM on.

YMMV.

No.

[John Stumbles]

Seconded: I had that problem with my 3-port motorised valve. What actually seemed to be happening was that it wasn't diverting water to the hot water cylinder, so the HW cyl stat was constantly in demand but the water was actually going round the CH side.

OP: try turning down the thermostat on the hot water cylinder, and/or setting the programmer to Hot Water: Off and see if the radiators still get hot.

(This is where he comes back to say it's a combi :-)

[Hicks]

No it's not a combi. That much I *do* know :) Thanks for the advice everyone. Much appreciated.