Wifi Thermostat to Reznor Garage Heater. No “C” wire connection on terminal strip

heaterhvacthermostat-c-wire

I just had a 75000 btu Reznor UDAP garage heater installed. Can I run a "C" wire for a smart thermostat if I don't have a C connection on the terminal strip? There is a C connection on the circuit board and an unused W2 connection on the terminal strip.

There are 4 connections on the terminal strip R, G, W1, W2. The current thermostat is connected to the R and W1 terminals. The G is connected to the circuit board from the terminal strip, but looks to be unused tight now. The W2 has nothing hooked to it, i.e bare terminals.

My question is, Can I use the G terminal (since it's connected to the circuit board already) to connect my C wire up (to provide 24V power to my smart thermostat), or should I connect a wire from the unused W2 terminal and connect it to the C terminal on the circuit board, and us that to run 24V power to the smart thermostat. Or is the heater just incompatible and I'll need to buy a battery operated wifi thermostat?

Glen.

Best Answer

After more carefully reading your question:

Neither G nor W1 are suitable to use as a common (C). The C connection is there in the unit as you said, but not present on the rear terminal strip.

Based on the manufacturer schematic, it does appear that "W2" on the rear terminal strip of the single stage unit is truly not connected to anything, so you should be able to extend your "C" terminal from the interior circuit board to W2 on the rear terminal strip without any adverse consequences as you mention. It would probably be a good idea to relabel "W2" on the rear to "C" as well.

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How to connect a WiFi thermostat to the furnace’s primary control

Judging by the Honeywell R7184A Controller manual, you have one of these:

illustration of R7184

You described it as terminal 4 but the diagram just shows two terminals labelled "T", but that is fine:

I found a manual for an EnviraCom device which shows terminals 2 and 3 are 24vac power:

enter image description here

This means we have the right connections, and according to the R7184 manual:

EnviraCOM™ Current Available: 150 mA

So the liming factor here is simply the current available. I can't find any specs at all for the thermostat you posted, but so long as it needs 150 mA or less (at 24 Vac) then it should work. You would make the following connections:

Burner             Thermostat     Desc
Terminal 4 T       W              Heating call
Terminal 3 T       R or Rh        24Vac
Terminal 2         C              24Vac "Common"

Note: your current wiring may not have W and R connected correctly, because with the two-wire system it doesn't matter. Now that you need a C wire, it is important to have R connected to constant power. If wrong, your thermostat simply won't get power.

If your thermostat draws more than 150 mA, you're going to run into various strange problems that may range from occasional glitches to your burner not working at all, and I'd highly advise against doing this.

If you do need more than 150mA, normally you could upgrade the transformer -- but in this case, it's all an integrated solid-state unit. I'm actually not sure you could wire this up without damaging the burner controller. The safest thing would be to use a separate circuit with a relay, but that is far beyond the original scope so I won't post how do to that unless necessary.

HoneyWell Wifi Thermostat and C wire

I just got off the phone with Honeywell. If you only have a 4-wire older system, you'll only be able to use the fan in the "auto" position. It won't let you use the fan-only selection. You need a 5th wire for power for the wi-fi side of the thermostat. Call Honeywell and they'll walk you through it for free.

Best Answer

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