Boiler not firing for heating but heats the water

[Big Norm]

Hi,
My boiler has stopped firing when the temperature has dropped below the point when the thermostat should kick in. The boiler does however fire when the water cylinder drops below a certain temperature and the thermostat on the tank kicks in. When this happens the radiators heat up during the period that the boiler is on heating the water, but not to the temperature you would expect.
I have a Danfoss thermostat ret230p. When I adjust the dial it clicks but the boiler does not fire. I am unsure whether this is a failure of the thermostat, a motorised valve, the boiler itself or something else. My thoughts were that because the boiler doesn't even attempt to fire, that it is not recognising the signal from the heating thermostat, but then again, why do the radiators go warm when the boiler heats the water cylinder. Any help would be appreciated. Thanks.

 

[terryplumb]

Motorised valve fault most likely. Is your system Y plan or S plan ?

 

[Big Norm]

Thanks for the quick reply Terry, I think you are right.
This is my elderly mother's house and the hot water cylinder is located downstairs in the same room as the boiler and I am not that familiar with it. I have had a look and there is a mass of pipework with what appears to be two motorised valves on different sections.
Last year at my house I had a problem with one radiator getting hot in the summer when the hot water cylinder was heating. The spring on the lever on the valve head (3 port) did not appear to be working and was not returning so I replaced the valve head, but it made no difference so I drained down the system and replaced the whole valve which resolved the problem.
Having checked the two valves on my mother's system, although I cannot tell from the setup which serves the hot water cylinder and which the heating, I can tell that the spring on one valve is not returning the lever. It doesn't look easy to isolate the valve and replace it but I will try and buy the same valve and see if I can just get away with replacing the head. If not I will drain down and replace the whole valve. My assumption that it has the two valves that it is S plan.
Many thanks.

 

[terryplumb]

Follow the pipework from the valves ,one of them will connect into the hot water cylinder. Obviously the other one would go to central heating and is the one that needs attention.

 

[Mottie]

Or see which one lets hot water through when the hot water is on (you may have to temporarily turn the hot water up if it’s already up to temperature) and that will identify which valve is which.

 

[oldbutnotdead]

It's possible that it's only the microswitch on the motorised valve that is failing- proof would be fire up the hot water with the room stat set very low, give it 5 mins (rads should not get warm) then turn room stat up to very high and see if rads get warm (with hw still calling for heat)- if all that happens then replacing the head is much easier than replacing the whole valve

 

[Big Norm]

Yes thanks for that. I actually did it the other way around several hours apart. With the stat turned up high the radiators came on quite quickly once the boiler started heating the water. When I turned the stat right down later in the day the valve did not appear to open at all and there was no hear whatsoever going to the radiators. I have ordered a new head.

 

[gas112]

Yes thanks for that. I actually did it the other way around several hours apart. With the stat turned up high the radiators came on quite quickly once the boiler started heating the water. When I turned the stat right down later in the day the valve did not appear to open at all and there was no hear whatsoever going to the radiators. I have ordered a new head.

sometimes cheaper to buy whole valve depending on the make and just use the head