Thermostat or zone valve issue?

[Georgej10]

Hi,

Last week my central heating developed an issue and I’m struggling to figure out what has caused it. I have a combi boiler with 2 digital thermostats, one in the lounge, the other upstairs main bedroom.

I have a fairly decent knowledge of plumbing and heating but From Last week the when calling for heat, the downstairs stat wouldn’t get the boiler to fire up. The only way I can get the boiler to fire is by having the upstairs stat calling for heat. Now this is where it gets confusing for me. Both stats seem to be controlling there zone valves and they appear to open & close as you’d expect when calling for heat or not. As long as the upstairs stat is calling for heat the downstairs heating works fine and as you’d expect. To the extent that once downstairs has reached the required temperature on the stat it will then turn downstairs heating off.

From the above I’d come to the conclusion that I had a dodgy stat downstairs. I thought before buying a new one, as I can remove my stats from there back plate I would plug the upstairs one in downstairs and expected it to work but it didn’t! This has now left me at a dead end as to what to look at next. Is it possible that the Downstairs stat works to an extent that it can control the valve as it should but for some other reason it can’t ask the boiler to fire up? I don’t know enough about stats and the differences between opening zone valves and asking the boiler to fire. Where does the demand come from to the boiler to ask it to fire, is it the stat or the valve?

Any help would be appreciated.

Thanks

 

[John.g]

The stat(s) open/shut the valves, the valve actuators control the boiler ie there is a "end switch" in the actuator one side of which is permanently live so when the actuator/valve opens then this end switch closes and fires up the boiler. From what you are saying it looks like that the permanent supply to the D/stairs switch might be "broken" even though the valve is opening/closing. I think the grey wire is permanently live to the switch and the orange wire completes the circuit to the boiler, when closed.

 

[ShaunCorbs]

Zone valve head / unit is faulty

 

[John.g]

Looks that way alright, I misread the post and thought the actuators were changed round.

 

[SJB060685]

As above the downstairs zone valve head is most likely at fault. Either a permanent live is missing like john said or the end switch is faulty like Shaun said.

 

[Georgej10]

Thanks for you input. With it sounding like it’s the valve head I can replace without the need to drain down can’t I? Just a case of removing the broken one and install the new one while leaving the existing valve in place?

 

[SimonG]

Testers out and follow the power. As said though faulty head for me.