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Adding Transformer for common wire.

We take possession of a vacation property next week and I would like to add a couple Wi-Fi thermostats so I can monitor the temps in the winter, and warm the place up when we are heading there. It’s a bungalow with attached shop on a slab with hydronic heat. The shop Is on one thermostat and the house has a tstat for each zone, 6 in total. They are all 2 wire connections. I want to add a new tstat in the shop and one in the house. Pulling new wire will be a bit challenging and I’m wondering if there are any issues with adding a 24v transformer in a junction box near the thermostat locations.

Thanks, and as always, I appreciate any and all comments.


    HVACNUT Member Posts: 5,886
    Just wiring a transformer into the circuit won't do much if you need 24v and switching in the thermostat, which of course you do. I've heard Nest has a transformer you plug into a wall outlet near the thermostat and run exposed wiring from the transformer to the Nest. Another option for a power robber thermostat is installing a resistor. The thermostat manufacturer should be able to help with that. And for reliability, security, and damn pretty if I do say so my damn self, the Honeywell Prestige with EIM. You only need 2 wires (24v powered) at the thermostat(s).
    IMO the Presige is the only option. Check it out.
  • HomerJSmith
    HomerJSmith Member Posts: 2,509
    On a Nest thermostat the charging circuit is between the R and C connections. A stand a lone transformer is your only option if you have a two wire supply line to the thermo. To use the relay transformer you need a three wire supply line to the thermo.

    Some time installers will cut the unused conductors close to sheath.
  • Tim Potter
    Tim Potter Member Posts: 273
    edited November 2019
    I use a Sensi Smart Thermostat wifi with an external transformer on a 2 wire system at my shop, it works like a charm

    c & rc for 24volt
    w & rh for 2 wire control

    (edit: remove rc to rh jumper)

    Winter Park, CO & Arvada, CO
    HVACNUT Member Posts: 5,886
    > @BroEliz16 said:
    > hello,
    > Does common wire work better with transformer?

    You'll have to explain the question. A transformer with a 24v A/C secondary needs 24v power, and a Common (ground).
    Why do you ask?