Central Heating stat not firing boiler

14 Jan 2013
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United Kingdom
Hi, first time here so please be patient..

The central heating's not working. Stelrad gas boiler fires up when hot water turned on, but does not fire up when CH thermostat calls for it. Sounded like the thermostat to me so replaced with a new one. Still same problem. Current goes to the stat and the stat clicks when turned and current switches to switch load terminal but boiler does not fire.

Only way to get heating working is to turn on hot water to fire boiler then turn on CH to get radiators warm (and seems to much heat in all rads for it to be simple circulation - but could be wrong of course). Boiler then shuts down when water gets hot enough.

So all seems to be working (programme unit, room stat, hot water, boiler...)except the boiler don't seem to be listening to the room stat.

Any suggestion? What can I check or test - either to repair myself or have some idea of how big a job it would be if I called somebody out.

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I assume that you have a motorised valve in the pipe supplying the central heating. Inside the motorised valve is a microswitch that is operated by the valve when it opens to turn the boiler on. My guess is this switch has failed. A new motorised valve or valve head should fix it.

The orange wire from the motorised valve is the one that sends a live supply to the boiler signalling it to run. If you are capable of testing it safely using a multimeter, remove the orange wire from its connection to the wiring centre and isolate it safely. Then set the heating set to 'on' and the orange wire should be live, if it's not the switch is faulty.

The valve is obviously opening because it does allow the radiators to heat up when the hot water is on and operating the boiler, so the room stat will be opening it OK, its just that the switch inside the valve is not starting the boiler up.

No need to ask us to be patient, that's a good post with all of the necessary information :D
Thanks, that's really helpful.

I looked at other posts on this forum and was starting to think that it could be the motorised valve so I took the box ("head"?) off the valve. I can turn the spindle on the valve a little bit by hand (read somewhere that it's only supposed to turn about 20deg) so I guess the valve bit is OK which would be a great relief (seriously don't fancy draining system, cutting pipes etc)

I haven't checked the power to the actuator bit yet but it looks seized because I can't turn it or change the auto/manual setting.

Now expecting that I shall have to get a new box (actuator? head? - not sure of proper terms). Late in the day so only BandQ open but they only sell the whole thing with valve. Planning to have a cold night then check out proper plumbing places tomorrow to see if I can get the part I want and make sure it is compatible with the valve (it's a Honeywell 3 port system).

Thanks for the help. I'll let you know how I get on.
Take the head off the valve, provided HW is calling for heat you may be able to get some flow through to the rads by gently manipulating the spindle so valve is open to both ports. You'll have no thermostatic control over CH or HW with valve in that position, but it should get you some CH until valve can be fixed, providing HW is calling and firing boiler/pump.

Good news is Honeywell parts should be easy to obtain. :) If you do need to change entire valve, then new valve will fit in place of existing. Drain system, undo 3 nuts on the connections, slip old valve out, new valve in and retighten nuts. Wiring must be like for like in junction box or it wont work!
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I had similar problem - when I traced what was going on I found the the controller on my motorised valve had broken. The thermostat is wired up to the valve - and only if the valve opens will the boiler get the demand signal - I guess to avoid the boiler pumping against a closed valve. If you get someone to click the stat to demand whilst standing next to the CH valve you should hear it whir into action - you can also remove the plastic cover to see if the valve opens. It sounds like this needs replacing - not too bad in the grand scheme of CH repairs.
Only way to get heating working is to turn on hot water to fire boiler then turn on CH to get radiators warm (and seems to much heat in all rads for it to be simple circulation - but could be wrong of course).
Unless the valve is jammed partially open, that suggests to me that the CH valve motor is winding it open, but its just not operating the switch to keep the boiler running when the hot water is satisfied. Either way, motor or switch a new head will sort it.
YEESS! Bought new Honeywell head off the shelf from Plumbcenter. Easy to fix new one in, system checks fine.

Only one small cheat with rewiring. I used a junction block to make sure I had like for like wires and because the main terminal box is virtually inaccessible (hard enough to remove cover to check for current - didn't fancy having to work back to front with mirror)

Many thanks to you all stem, Hugh Jaleak, 45yearsagasman and Captain-pugwash.

What a great forum

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