Yes, it seems that if there is a way to use the T3 thermostat closing to provide a 12 volt signal to a small relay

I'm not so sure from those voltage readings in post #14. The second photo down shows zero volts across the T3 connector when that zone is activated. That indicates to me the output of the thermostat is a closed relay contact. I believe those voltage readings are just residual from the optocouplers.

 

I'm not so sure from those voltage readings in post #14. The second photo down shows zero volts across the T3 connector when that zone is activated. That indicates to me the output of the thermostat is a closed relay contact. I believe those voltage readings are just residual from the optocouplers.

Using the voltage developed when the thermostat contacts are closed would be the voltage between the input and the supply common, not the voltage on the contacts

 

Same as before but contacts wired to T1 and T2.
View attachment 227609

Thank you for the interpretation. Easy job from now.

 

to use the T3 thermostat closing to provide a 12 volt signal

It can be done by only external power supply, right? If yes I would not take the risk of ruining the controller or the thermostat electronics by the external current. According to my humble opinion to go on the 240 VAC DPDT relay way as suggested before seems safer.

 

Looking at the photos of the circuit board, I see optical isolators on the thermostat inputs. So it is not clear how much current would be available to operate a 12 volt relay. And I was considering that such a relay would be powered by the internal power supply on the board, not any external supply.
The benefit of using the 220 volt relay is that the relay and connections can all be external to the zone controller, only requiring one additional par of conductors. In fact, the relay could be powered by the zone 3 valve output and not require any additional connections to the controller. That would work easily if all of the zone valves are located at one area, and not spread around the system and located in different areas. I have seen both schemes, so it might be either.

 

So it is not clear how much current would be available to operate a 12 volt relay.

Correct. That was my concern in my previous post. However from viewing the last two photos it's possible that the left terminal on T3 is a 12 volt supply line although it's reading negative with respect to ground. I see the red probe + on the ground terminal.
When Zone 3 is activated there should be a 12 volt reading on the right terminal of T3 and if that's the case a 12 volt relay can be connected there.

 

if that's the case a 12 volt relay can be connected there.

Can I check this out somehow? What should I measure?

 

Can I check this out somehow? What should I measure?

As you did before. Connect volt meter to GROUND and the right terminal on T3, then activate Zone 3 via the thermostat.

 

Again after further examination of the PC board the GROUND terminal is for the 240Vac input, may not be the same ground for the internal 12 volt supply.

 

Once again, if all of the zone valves are close to each other then adding an external relay to disable zones 1 and 2 could be done just by using the zone 3 valve controlsignal to also operate the relay, causing no changes to the controller assembly at all, thus no risk of damage to it.

 

adding an external relay to disable zones 1 and 2 could be done just by using the zone 3 valve controlsignal

You mean like this?

 

You mean like this?
View attachment 227727

YES! Instead of using the 12 volt signal, use the 220 volt output. AND, same relay contacts BUT wired in series with the LINE outputs from the Z1 and Z2 terminal sets, presuming that they are all in the same locality near the boiler. That would leave the more sensitive thermostat wiring untouched, thus eliminating connections to the more sensitive section of the system.

 

INDEED! The Z3 L and N terminals, which also power the zone 3 zone valve would then operate the relay to remove power from the zones 1 and 2 zone valves when zone 3 would be active. And that wiring can be done in the area where the zone valves are located, and thus not add any connections at the zone controller area.