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?

 

There should be an R and/or an RC terminal for 24V power.

 

There should be an R and/or an RC terminal for 24V power.

I have RH and RC on the thermostat, and they are jumpered together. And those jumpered terminals are connected to a RED wire coming in from the HVAC unit. I think that's "hot" AC from the HVAC unit?

Should that red wire not be connected to the thermostat?

 

I think that's "hot" AC from the HVAC unit?

Yes, that is likely the AC power.
So why do you think you need to add an external power source?

 

So why do you think you need to add an external power source?

It works as a thermostat only with just batteries. But it needs 12-24 AC/DC for the WiFi to work.

I used to have a 15V AC brick connected to it between the RC+RH & C terminals, but now want to connect a more efficient switching DC power supply to it.

 

But it needs 12-24 AC/DC for the WiFi to work.

That should be what the red wire from the HVAC provides.
You haven't explained why you think that won't work.

 

The battery may be a backup so that in case of a power failure, the thermostat settings and WiFi parameters aren’t lost. In that case, you don’t want to eliminate the battery. Replacing it with a power source that depends on building power defeats the purpose of the battery.

 

That should be what the red wire from the HVAC provides.
You haven't explained why you think that won't work.

HVAC provides >30V AC on the red wire. I haven't connected anything to the C terminal on the thermostat as that would complete the circuit and provide power way out of spec (24v max).

Do I not need the red wire? I thought that the relays inside the thermostat complete the circuit between cool/heat and the red wire. So if they are operating at >24v not a big deal as they are isolated from the "brains" of the thermostat.

 

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?

The "C" terminal is COMMON,not ground. And usually the supply is AC, not DC.
If the voltage is too high, how high is it? Some systems have a thermostat switching mains voltage That is a different world with different rules entirely. Other than that, all of the normal systems use nominally 24 volts, or sometimes 28 volts.

 

Do I not need the red wire? I thought that the relays inside the thermostat complete the circuit between cool/heat and the red wire.

Your supply would replace the red wire to the thermostat.
Leave the red wire unconnected.

 

The "C" terminal is COMMON,not ground. And usually the supply is AC, not DC.
If the voltage is too high, how high is it? Some systems have a thermostat switching mains voltage That is a different world with different rules entirely. Other than that, all of the normal systems use nominally 24 volts, or sometimes 28 volts.

Voltage is around 31V unloaded and around 30V loaded with a resistor to about 3w. I've read online about someone else with the same thermostat trying to use it with a similarly "high" voltage, and they said it would not work, beep and alert about an overvoltage. I don't want to simulate this condition myself and risk blowing anything.

 

Your supply would replace the red wire to the thermostat.
Leave the red wire unconnected.

So If I were to use a DC power supply, (+) would go to the jumpered RC/RH terminal on the thermostat, and (-) to C terminal on the thermostat?

If heat or cool is called, how would would the relays inside the thermostat complete the circuit? Or do I need to connect the C terminal from the HVAC system to the C terminal on the thermostat?

 

What is puzzling me about how to power the thermostat is that the only difference of a wifi thermostat is the wireless access to program it. The system control connections would all be the same., except that it probably would need the white power common wire .
I suggest that if the system power voltage is 31 volts that probably that transformer should be investigated, because that voltage is excessive. It may be that the primary has a tap for 100 volt mains that was accidentally connected to the 120 volt mains supply.

Yes, the scheme described in post #12 is how you would do it. But if you are powering the rest of the system then you will need to be sure that you are using a suitable voltage DC to control any solenoid gas valves. I did that once for a relative several years back. A long term power outage stopped his steam heat boiler from operating. So I borrowed the 12 volt battery from his travel trailer to power the 24 volt AC gas valve. It worked very well for the week before his power could be restored.

 

If heat or cool is called, how would would the relays inside the thermostat complete the circuit?

The relay coils controlled by the thermostat would get power from your external supply through the C and R terminal(s).
The relay contacts are isolated, and close the circuit to the furnace, which is still powered by the 28Vac.
The power and common connections are just to power the thermostat, which you are powering externally.

But MB2 brings up a good point.
The furnace may have a faulty or incorrectly connected 28V transformer.
You should not be getting that high a voltage.
How are you measuring it?

 

I'm measuring it with a good Fluke RMS meter. It's not my HVAC unit, and I can't even look inside at what the transformer says, since the maintenance door is sealed up with aluminum tape. Pure genius. It's not old, maybe 4-5 years old at most. It's not a fancy unit, so the board is probably all analog and works just fine with a little overvoltage.

Here is what the installation manual for the thermostat says:

externally powered with a power source rated from 12V to 24V, AC or DC, at 300ma or greater. If used, connect to the C and RH terminals (no polarity)

So if I keep the >30v RED wire coming in from the HVAC system connected to the jumpered RH/RC terminals on the thermostat, and connect (+) or (-) from my external DC power supply to the same RH/RC terminals, and connect the other wire from the external DC power supply to C terminal on the thermostat, that should do it?

Trying not feed DC into the HVAC control board by accident.

 

So if I keep the >30v RED wire coming in from the HVAC system connected to the jumpered RH/RC terminals on the thermostat

Absolutely not.
Didn't you read what I said in post #10?

If you connect your power supply to the RH/RC terminals along with the red wire you will likely short one or both supplies.
Only one can be connected at a time.
In this case you remove the red wire coming from the HVAC (leave it floating) and connect your supply to the RH/RC terminals.

 

I read your #10 post but did not understand it.

If the HVAC board is operating on whatever the transformer is providing, say 30V AC, would leaving the RED wire coming from the HVAC board unconnected effectively lead to the thermostat not being able to complete the circuit using internal relays?

If there is only HEAT + COOL + FAN wires, and no "hot", which is the RED wire, how it does it work then?

 

would leaving the RED wire coming from the HVAC board unconnected effectively lead to the thermostat not being able to complete the circuit using internal relays?

No

If there is only HEAT + COOL + FAN wires, and no "hot", which is the RED wire, how it does it work then?

Your supply will provide the "hot" power to the HVAC circuits from the R terminals, as long as C (common) is connected to your supply and the HVAC.

The diagram below shows how the connections are made through the thermostat relay contacts (it doesn't show the C terminal on the thermostat which goes to the yellow connection (blue wire in post #2) on the transformer output to complete the circuit with your external power).

Make sense?

 

Is this how it should look then? (if I were to use a 12v DC power supply)

 

Is this how it should look then? (if I were to use a 12v DC power supply)

Yes, as long as the HVAC 28Vac relays will operate from 12Vdc (they likely will).