Heating stuck on

[Roger Mills]

OK, so you can forget what I said about S-Plan - 'cos you've got a Y-Plan system.

Remind me again. Was it all working ok - and suddenly started doing this without anything having been touched/changed?

The only other possible explanation I can offer is this:

Assuming that the boiler controls the pump, in order to provide pump over-run, the boiler will have a permanent live and a switched live wire connected to it. The permanent live is - like it says - live all the time regardless of what the programmer, thermostat and valve are doing. The switched live is controlled by the programmer and/or valve (depending on mode) and should only be live when there's a demand for heat.

It seems to me that the switched live has somehow got itself connected to the permanent live - maybe a short somewhere or a component failure. This could be in the external wiring or within the boiler itself.

So you need to identify the switched live wire, and disconnect it from the boiler. If the boiler and pump *still* run, the fault has to be inside the boiler. If it doesn't, it's an external problem - and you need to find out why this wire is live when it shouldn't be.

[marble]

Try disconecting the "demand wire " from the boiler. If the boiler still runs there is a fault (short) in the boiler. Otherwise replace demand and disconect demand from the valve at the junction box if boiler is still running then short is in the cable so replace that. Otherwise from what you said ie "removed all stats and timers and still runs" then its the valve head.

[The Natural Philosopher]

I am not familiiar with 3-way valves, but shouldn't it return to 'all off' when no demand is called for?

[Roger Mills]

I nearly posted along similar lines - and then I remembered that Y-Plan systems have to have a HW-off signal - so there'll always be one live wire going to the valve even when the programmer and stats are off. Whether there is a mode of failure (like a shorted out micro-switch) which could bring the boiler and pump on under these circumstances, I'm not sure.

[Alex DV]

Update: Fixed!

I finally had the boiler fixed. In the end I had to concede, it got the better of me, so I got a local plumber to look at it. It turned out to be a faulty "pump overrun stat" within the boiler itself. A replacement stat has fixed the problem. Many thanks to all posters.

Alex.