Thermostat on a one pipe system

8 May 2012
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United Kingdom
Can someone help me understand how my central heating system should work? I've just moved house and am not sure if my system has a fault, been wired incorrectly, or I'm missing something.

The system has a back boiler with a single pipe system to the radiators.
It seems that before going to the radiators, the water goes through the hot water cylinder. There is a thermostat on the cylinder that links back to the mess of wiring for the control. From this there is also wiring to a pump and what I assume to be a motorised valve, (although I'm struggling to see from its positioning how this affects the movement of water within the pipes.)

The control panel has timers for hot water and central heating. Starting from cold, the central heating seems to warm the hot water and then the radiators ok.
However, when the control panel started the 'hot water' first and took it to temperature, later on the central heating control wouldn't heat the radiators.

Could this be the thermostat or something else is broken? Perhaps the thermostat is wired to the central heating control, when perhaps it should be the hot water control (or vice versa)?

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Your theory sounds feasible, it suggests that once the hot water temperature is reached it also stops the heating working.

If you have a thermostat on the hot water cylinder it may have a changeover contact (see diagram below) that is wired to keep the boiler running when heating is required and the hot water is up to temperature but it is not doing so. However this is only a possibility, without seeing the pipe layout and wiring its impossible to diagnose really.

Is this a coal fire with back boiler?

These need to be connected correctly to be safe.

I would recommend you get a professional to check it.

Also if coal ( or wood ) you should have an electronic CO alarm!


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