Crutschow's comments above are spot on

In that case I think the controllers could drive the coils of respective DPST relays. Pole 'A' of each relay would switch the respective pump. Poles 'B' of the relays would be commoned to switch the boiler. Does that sound right?

 

The 2 controllers are 24v (They are plugged into the mains and have their own internal 24v transformer). But essentially I am dealing with the 240v switched outputs that go to the boiler.

So control the two relays from the 24Vac control signals and run the 240V from the pump modules to the relay contacts, which then goes to the boiler (diagram below).

The 240Vac can also come directly from the mains supply, if that's easier.

 

https://heatingcontrols.honeywellho...df/Heating-Controls-Wiring-Guide-Issue-18.pdf
seems to be how they are usually wired. All the ”logic” is 230V AC using micro switches and relays, so, it’s likely that the two ”call for heat” wires could be connected together to the boiler. You would have to check the wiring diagrams to confirm that it doesn’t feed 230V back somewhere it shouldn’t!

 

Actually, I think I need to simply get rid of the supplied pump modules altogether (single relays with shared output terminals for boiler and pump). Replace them with a DPST relay for each circuit. These could then use the 24v signal from each controller to switch 240v to boiler and respective pump. Then Pole 'A' of each relay would switch the respective pump. Poles 'B' of the relays would be commoned to switch the boiler.

Each DPST relay can be housed in a plastic junction box (similar to the existing pump modules). Existing 2 core 24Vac wire simply connects in. And existing live, neutral earth wires in the pump modules are also readily available to wire into the new relays.

So in effect all that is happening is the existing single pole relays which carry out 2 tasks are being replaced by DP relays so that each task is carried out by a separate pole terminal. So one task can be common to both circuits whilst the other task can be kept discrete for each circuit.

Love it.

 

the existing single pole relays which carry out 2 tasks are being replaced by DP relays so that each task is carried out by a separate pole terminal. So one task can be common to both circuits whilst the other task can be kept discrete for each circuit.

Sounds like a working plan.

 

Well thank you, guys. I chanced upon your forum. Helpful and willing responses. I enjoyed benefiting from your knowledge and hope you enjoyed the challenge of unravelling my little puzzle. All the best
Mike

 

I set up a system to do exactly what the TS is asking for a few years back, except that there were six thermostats and six pumps, and one boiler. Each pump controller, (Honeywell brand) included a relay with heavy contacts to switch the pump power, and a second set of contacts that had not been used. So I connected all of the six sets of low-current normally open contacts in parallel, and they control the 24 volt circuit to operate the boiler gas valve. The result was an immediate reduction in gas consumption, because previously the boiler had simply used a water thermostat to hold the water temperature constant, operating from October through March here in Michigan. Changing to only heating when there was a demand for it provided a great improvement of efficiency.
So it the controls that the TS has include a second set of contacts then a similar arrangement will be easy.
This seems to be very much like what the TS describes in post #24

 

That's a good point and I had been thinking that I should just have a close look at the existing "pump module" to see if there is an additional separate output port. I'm not optimistic because the boiler feeds and pump feeds had been wired together on one terminal - but you never know until you know!
Thanks

 

I did repairs on another system that had four zones and one pump and changed it so that thepump and burner will only run when at least one zone is calling for heat. Each zone valve has a switch that comes on when the valve opens by command from a thermostat.