My central heating has just stopped, don't know why but I have a suspiscion that it's the room thermostat although I bought a new one last year. I've checked everything else out but the pump isn't being turned on. It's coming on for the water and I've also checked the isolator valve which manually turns the pump on, but it doesn't stay on. When I look at the wiring the power isn't getting to the pump which means that something isn't turning it on and I can only assume that it's the thermostat. Before I go and buy another one, does anyone else have any suggestions?
Unless you have a gravity HW system, with the pump *only* being used for the CH, the room stat doesn't *directly* control the pump - but controls a motorised valve which, in turn, controls the boiler and pump.
Tell us a bit more about your system. How many motorised valves does it have, and of what type? There will likely be one 3-port valve (Y-Plan) or two two-port valves (S-Plan). Tell us what you've got, and we'll tell you what to check.
OK, so for some reason, the valve is not moving either to the mid position or CH position when you would expect it to. When you move it with the lever, the microswitches in its actuator operate and switch on the boiler and pump. But when you let go of the lever, the return spring moves it back to the HW position - in which the boiler and pump don't operate unless there is a HW demand.
You may be able to understand better how it operates by studying the Y-Plan circuit diagram in
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you can move the valve with the lever and, more importantly, if the return spring can move it back, we can discount the seizure of the 'wet' part of the valve, and the problem lies elsewhere. The most likely answer is that the motor in the actuator has failed. The other possibility is that the room stat has failed. The way to determine which is to check the voltage at terminal 5 of the wiring centre which is connected to the white wire in the valve's acuator. If this is live when the heating should be on, the stat is ok and the problem is in the valve. If it isn't live, there's a problem with the room stat. In order to determine whether it's live, you need to measure the voltage between pins 5 and 2 (neutral) with a voltmeter suitable for measuring at least 250v AC. Forget about screwdrivers with neon indicators in the handle - they are hopelessly unreliable.
If the actuator *is* at fault, it's usually quite easy to replace. With most valves, the actuator can be removed without any loss of water simply by undoing a couple of screws. Make sure that you connnect the 5 wires of the new actuator in exactly the same way as the old one.
It's quite likely that the motor in the 3-port valve has gone down. These are fairly easy to replace. The motors are readily obtainable, online or in the average plumbers' merchant. Probably not avilable in the smaller sheds.
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