low voltage boiler with drayton za5 spst switching how do I connect to nest 3rd generation

[MelbaDT]

Hey neville, 

Sorry for the delayed response but thanks for reaching out. 

Could you provide the exact make and model of your boiler/furnace as well as your old thermostat? Include also a photo of your current thermostat's wires. You can also check your system’s compatibility before purchasing a Nest thermostat with our online Compatibility Checker.

Best, 

Melba

[dhx227]

Not sure about what parts of your system you are referring to by "low voltage" nor the specific details on the drayton za5.

From what I can find with a quick search, the Drayton za5 is a type of motorized valve and relay.   It appears to require 230vac to drive the motor, but I can't find any specs on what's it needs to operate the relay (eg, is it 24vac?).  If it's relay is in fact operated by 24vac (and that's what you mean by "low voltage"), then chances are very good that you can use the Nest to operate it.

If it needs some other level of voltage (like is often the case on millivolt-based gas furnaces), then you can always use an external relay (like a Function Devices RIB-U1C) coupled with an external 24vac transformer.  You would then use the 24vac transformer to power the Nest and operate the relay.

[dhx227]

Actually, I found another article that better describes the drayton za5.   It takes 230vac as it's input to operate the motor.  When the valve is then fully opened, it engages a switch that signals the boiler to come on.

So...  IF you are a DYI-kind of person and know at least a little bit about electrical stuff and have a multimeter, then read-on.  Otherwise, do NOT buy a Nest and/or call a technician.

You would at least need an external relay to control the 230vac input to the Drayton za5.   A relay like the Function Devices RIB-U1C would be a good choice for that type of operation and can handle that voltage/amperage across it's contacts.   The spec sheet is here if you are interested: https://www.functionaldevices.com/downloads/datasheets/RIBU1C.pdf

Then you would need to determine if the power the boiler is using for it's control system is 24vac or some other voltage.   If the boiler is using 24vac, then you can merely tap into that to power the Nest and drive the RIBU1C.   However, if it's another voltage, then you will need to add an external transformer to power the Nest and operate the RIBU1C relay.    This post refers to doing a similar thing, but for controlling a millivolt gas furnace:

https://www.googlenestcommunity.com/t5/Nest-Thermostats/Google-nest-hookup-for-a-millivolt-gas-heate...

Note: I also am doing the same thing to operate 230vac in-wall electric heaters.