Smart thermostat with external transformer

Question

I have separate heat and cooling systems with wiring to Y, W, G, Rh and Rc. I don't have a C wire and therefore I will be using an external 24 V transformer. For the external transformer, where do I attach the two wires. C and ?. Pulling a C wire from the boiler/AC is not an option. Thank you

Answer 1

Try an add-a-wire

One way to attack this is to use a thermostat add-a-wire accessory. One of the common use cases for this accessory is to provision a 5-wire thermostat when only 4 conductors are available. If we reserve one conductor in your cable for C, then this is exactly the boat you're in! You might reasonably hope for success with an add-a-wire device and wiring like below. I believe this arrangement will leave you with no ability to run fan-only mode, but if you don't use fan-only anyway, then you wouldn't miss it!

• conductor 1: boiler R to thermostat Rh
• conductor 2: boiler W to thermostat W
• conductor 3: cooling R to thermostat Rc; also connect add-a-wire R to cooling R
• conductor 4: add-a-wire blue (color may vary) to diode pack at thermostat
• conductor 5: use for thermostat C. Consult thermostat manual to learn whether C should attach to the heating or the cooling equipment.

Additional wiring:

• diode pack at thermostat: one wire to Y; the other wire to G
• add-a-wire accessory at cooling equipment: yellow to Y; green to G; C to C

Adding new transformer

One could, in theory, power the thermostat from a "local" transformer (meaning, local/near to the thermostat rather than built in to the HVAC equipment). If the local transformer is phase-matched with the heating/cooling transformer, it can provide a "virtual" C wire. The 5 thermostat wires are probably already wired as below, but if not, make the necessary changes.

• conductors 1 and 2: thermostat Rh and W to boiler R and W
• conductors 3-5: thermostat Rc, Y, G to cooling R, Y, and G

As in the add-a-wire scenario, you'll have to consult the thermostat manual to learn whether its C is supposed to come from the heating or the cooling transformer.

Supposing the manual says C is supposed to match with Rh, pick one of the local transformer's wires and connect it with Rh. Then, using a volt meter set for AC volts, take a series of measurements. Because this is AC the red and the black meter leads are interchangeable.

• between the local transformer's two wires. Expected measurement is on the order of 24-30 volts. If not then the meter is not set up correctly, the local transformer is not powered, or there's some other problem you'll have to solve before proceeding.
• between the W and the local transformer's loose wire. If the reading is high, perhaps 40-60 volts, then disconnect the local transformer's first wire from Rh and connect its second wire there instead. Then repeat this measurement.
• When the reading between the W and local transformer's loose wire is low, perhaps 5 volts or less, the local transformer is phased correctly. Connect the loose wire to thermostat C.

Answer 2

Basic principle of operation is :

The boiler or/and AC have 24 volt relays.

Those relays get 24 volt from the thermostat pending the function selection and are already connected to the C.

The other C is only need by new type thermostat that have to perform other tasks like WiFi and do not run on battery.

You can not mixed match two different sources of 24 volt. R and C have to come from same transformer.