Central Heating issues - Thermostat or 2 way valve?

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

chiny

Demand (for heat) comes from a heat measuring device (thermostat) that has dropped below the set point. The signal for heat is then used to switch on the boiler (which has its own set of thermostats to control itself) and operate zone valves. All this is subject to the programmer, which decides whether anything should happen at all (itself subject to an over-riding frost stat).

I've always found zone valves (in my case a single 3-way valve) the most unreliable component, probably as the electrical actuator (motor) is located on a hot copper pipe. The valve itself is reasonably reliable.

nofoollikeold

1. It isn't the thermostat which calls for the boiler to fire, its the motorised valve.
2. If the motorised valve actuator doesn't travel far enough, it won't close the micro-switch which in turn send power to the boiler.
3. If the micro-switch itself is faulty, same problem.
4. If the valves are close to each other, are of the same type and have removal actuator heads, try swapping them and see if the problem shifts to the other zone.
5. If you can remove the actuator head of the valve which controls the downstairs heating, see if you can move the spindle by hand. It is likely to turn not much more than 20 to 30 degrees from open to closed. If it is very stiff, it may be preventing the motor from turning the actuator mechanism far enough. In this case, try turning it backwards and forward with pliers a few times. Might loosen it up enough for a temporary fix.
6. Any further troubleshooting requires a multi-meter, the experience to use it, and a healthy respect for mains level (lethal) voltages.

stator

Are the thermostats wireless or wired?


Do you also have a controller, or is the controller integrated with the thermostats?

Georgej10

nofoollikeold wrote: »

1. It isn't the thermostat which calls for the boiler to fire, its the motorised valve.
2. If the motorised valve actuator doesn't travel far enough, it won't close the micro-switch which in turn send power to the boiler.
3. If the micro-switch itself is faulty, same problem.
4. If the valves are close to each other, are of the same type and have removal actuator heads, try swapping them and see if the problem shifts to the other zone.
5. If you can remove the actuator head of the valve which controls the downstairs heating, see if you can move the spindle by hand. It is likely to turn not much more than 20 to 30 degrees from open to closed. If it is very stiff, it may be preventing the motor from turning the actuator mechanism far enough. In this case, try turning it backwards and forward with pliers a few times. Might loosen it up enough for a temporary fix.
6. Any further troubleshooting requires a multi-meter, the experience to use it, and a healthy respect for mains level (lethal) voltages.

Thanks for your input. The valves are quite close to each other so I could swap the heads over. By this do you mean unscrew them and simply swap them over so it would mean using the upstairs stat to control the downstairs demand?

I have already removed the actuator head from the spindle and managed to move this by hand. It did turn 90 degrees from open to closed, is this what you’d expect to happen? It wasn’t stiff at all? The valves are just over 5 years old.

The stats are Danfoss tp5000si. I believe these are wired with batteries?

Thanks

Myser

Yes, swap the actuator heads over rather than the valves themselves. If the problem moves to the other zone then your actuator head is faulty. As you've correctly already checked that the valve spindle rotates freely.