Central Heating: Radiators only hot when hot water tank being heated

I have an old Thorn Apollo boiler, a Randall 4033 timer and a 3 wire thermostat. Until now this system has been very reliable but now the radiators now only heat up when the boiler is heating the hot water tank.
The live connection on the room thermostat is live and the switched connection becomes live when I turn the thermostat up but neither the boiler nor circulating pump come on. The boiler is in my garage and I have a frost prevention thermostat - if I turn this up the boiler and circulating pump come on and the radiators heat up but the boiler doesn't seem to respond to the main thermostat.
Any ideas what's wrong and how to fix it? I would be grateful for any suggestions.
Thanks,
Gareth.
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On 07/11/2017 23:28, Gareth wrote:

I suggest that you make sure than any zone valve that you may have is working properly and is not stuck.
--
Michael Chare

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On Tue, 7 Nov 2017 23:56:43 +0000

Agreed. it sounds like a valve not making the limit switch, either because the valve is stuck, or the switch is faulty. Remember that there is probably a zone valve controlled by the tank thermostat, depending on the arrangement.
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Davey.


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Got to agree that it is probably a faulty zone valve. If you need heat whil st waiting repair you should be able to manually open the valve by moving a lever on the motor housing as long as it not the valve itself that is stuc k. Manually opening the valve means you will not have any temperature contr ol except for those radiators fitted with TRVs but the boiler stat will con tinue to control water temp and by turning any manual valves down you shoul d be able to ensure you do not get runaway temp rises. This all assumes the switch in the zone valve which switches on the pump is not what is at faul t. A good indicator of a faulty switch is the motor turning but the pump no t switching on.
Richard
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On 08/11/2017 09:06, Tricky Dicky wrote:

I have just had two fail myself. I think usually if not always you can prise the motor from off the valve. You can then see if the motor switches according to the central heating and hot water demands. You can also check that you can twist the valve off and on quite easily with your fingers. A stiff valve can lead to the motor failing. Much easier to change the motor than the valve body! In my case it was the motors that had failed. By taking the motor apart I was able to disconnect the cable from old motor and then attach it to the new one.
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On 08/11/2017 09:06, Tricky Dicky wrote:

Thanks for all the replies.
It was the switch in the zone valve. The valve seems to move all the way across but the switch wasn't quite closing. I've bent the springy level on the switch a little bit and the boiler and pump now respond to the room thermostat and still respond to the cylinder thermostat.
Thanks to everyone who replied.
Gareth
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On Wed, 8 Nov 2017 22:55:14 +0000

Glad it worked out ok.
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Davey.

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