I have an oil furnace that feeds hot water to baseboard in four zones in my house and also to a hot water tank for tap water.
There is a 24vac transformer with two output wires that feed power to the electric zone valves.
I want to change one thermostat in a bedroom to a Wifi unit that needs a c-wire. I have read that I can connect the c-wire to the transformer (power stealing), but that is not recommended. I have 5 wires running to the old thermostat, but only Rh and W are connected.
I think a good option would be to just replace the old transformer with a modern one. Why is it that I cannot find an HVAC transformer that has a dedicated common terminal? To me this makes more sense then adding a second transformer.
Any recommendations?
Buy a 24v transformer with a dedicated common c-wire feed
thermostatthermostat-c-wiretransformerwifi
Related Solutions
You guys down voted the only answer that is correct and up voted one that is completely wrong!!! You CANNOT use just one wire from a transformer and expect to get it to do a thing for you. The simple answer to your question is that if you add a transformer you want to tie the lines from it to Rh and C. Most modern thermostats I know of expect to pull power between Rh and C. You do NOT want to tie Rh and Rc together since your heating and cooling are on separate transformers and you don't know if they are truly floating.
The complex answer is that you should check to see if the control board of your heating system doesn't, in fact, have a common terminal and nobody bothered to run the wires for it. If it does, it might be simpler to just pull 18/3 wire from the heater to the thermostat. Depending on the run, you can use the old two conductor wire to pull the three conductor wire through.
It appears according to the manual for the Mastertrol Automatic Balancing System (MABS) EZ Zone (EZ-2 and EZ-4) Control Panels (PDF), that M1 is used for C.
Click for larger view
This does not appear to be the manual for your device, so this information may not be correct.
The manual for the MABS 2 Zone Panel (PDF), also shows M1 being used for C.
UPDATE:
After further review, you should be able to connect the C wire directly to the transformer terminals. Measure for Voltage between the T5 terminal of the zone you're working in, and the 1 and 2 terminals in the 24V 40VA Transformer section. One should measure 24 Volts, while the other measures 0 Volts. The one that gives you 24 Volts, should be able to be used as C.
Best Answer
Um, I don't know where you got the term "power stealing". What you describe as that, is exactly the normal way to do that thing, and the right way... unless you are aware of some capacity issue on the transformer (as is sometimes seen in transformer-relay combos).
As far as the R and C terminals, any HVAC transformer has two pins on the 24V side... They are R and C. The one that is not R is C.