Unclear on how to power a WiFi thermostat

Thread Starter

MikeA

Joined Jan 20, 2013
362
Yes, as long as the HVAC 28Vac relays will operate from 12Vdc (they likely will).
But doesn't this configuration then feed 12V DC into the HVAC control board that was supposed to run on 24/28V AC? How do I know they are going straight to the relays and nothing else?
 

Reloadron

Joined Jan 15, 2015
7,501
Well alrighty then.
I have a thermostat that I have used in the past with an external power supply on a system that did not have a C wire at all. But now I'm trying to connect it to a system that does have a C wire, but the voltage is too high. The thermostat is designed to work on 12-24 AC or DC.

If I understand it right, the C terminal on the thermostat is the "ground". So if I were to use a DC external power supply, would (-) go to C and (+) to ..... where?
Nice unit and they seem to cover all the bases. While I am not sure why you are seeing 30 volts the manufacturer is pretty helpful where they cover ...
"How to use an external 24VAC power supply instead of a C-Wire.
An external 24VAC transformer that is rated for 0.5A to 1A can power the Wi-Fi thermostat.
First, unplug the 24VAC transformer from the mains power supply such as a wall outlet. Remove the AA batteries from a CT50 thermostat.
Then, connect the power adapter wires to the "Rh" terminal and the"C" terminal on the thermostat. Either wire can connect to either terminal.
Finally, plug in the external 24VAC transformer to the mains power supply such as a wall outlet. Verify the thermostat has successfully turned on, and then replace the AA batteries in a CT50 thermostat".

While you do now have a C wire you also have a higher than suggested voltage. I would just go with an external supply. Really a 24 VAC 500 mA transformer is inexpensive and you really do not want any surprises.

Ron
 

Thread Starter

MikeA

Joined Jan 20, 2013
362
It is a very nice unit indeed. It has an API exposed to the local network that can be integrated into open source smart home software.

This is the connection layout that first came to my mind. The C wire from the HVAC control board and the C terminal on the thermostat are not connected. The relays on the HVAC control board get their 30V AC, and the 12V DC from the power supply doesn't go anywhere except the thermostat itself since terminal C on the thermostat is isolated. Am I missing something?

hvac.png
 

crutschow

Joined Mar 14, 2008
34,285
the power supply doesn't go anywhere except the thermostat itself since terminal C on the thermostat is isolated.
Okay, that may work if the C wire to the HVAC is not connected..
The 28Vac thus should be isolated from the thermostat electronics.
 

Thread Starter

MikeA

Joined Jan 20, 2013
362
I have the thermostat connected up and running like in my last picture, but the C wire from the HVAC board is flapping in the wind, and nothing is connected to the C terminal on the thermostat.

I just tried measuring voltage between the C terminal and the FAN + COOL + HEAT terminals, and each combo has 31V AC on it. Maybe it's not as isolated as one would think?
 

crutschow

Joined Mar 14, 2008
34,285
I just tried measuring voltage between the C terminal and the FAN + COOL + HEAT terminals, and each combo has 31V AC on it. Maybe it's not as isolated as one would think?
That's normal because you are measuring the voltage between the 28Vac connected to the R terminals which goes through the thermostat circuit to C, and to the HVAC relay coils, which are connected to the HVAC common, so that measured voltage is expected.
Basically you are seeing the C terminal floating on the R AC voltage.

If you measure across the R and C terminals you should see no significant AC voltage.

What you want isolated is the C thermostat common from the HVAC common, (which you have) to keep the 28Vac from appearing across the R and C terminals.
 

Reloadron

Joined Jan 15, 2015
7,501
Crutschow's thinking parallels my own. I have been looking at several WiFi thermostats and something I noticed at a glance is some do and some do not require a C terminal use. I have yet to get down and open up our system in the basement. I also have not looked closely at our existing thermostat.

Ron
 

crutschow

Joined Mar 14, 2008
34,285
I noticed at a glance is some do and some do not require a C terminal use.
Since the operating current is typically quite low, the ones without a C terminal could conceivable use diodes to provide a common connection through any unenergized relay coil in the HVAC unit.
If all the relays are turned on during an operation cycle, a small rechargeable battery could provide power for the time it is on, and then recharge when at least one of the relays is off.
 

Thread Starter

MikeA

Joined Jan 20, 2013
362
Thanks folks! Got it up and running. Everything is working just fine now. Got it logging and calibrated against a 1wire temp sensor.

hvac_7682_radio_thermostat_f.png
 
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