Thermostat Wiring R&W only?

[c5_nate]

Hello everyone,

First time posting, long time browser of this forum. Some of you have amazing garages! I will post one of mine soon. Onto my question...

I have a Modine Hotdawg 80 000btu propane heater (cant link)

It runs 2 wires to the existing heat only thermostat (R and W I believe)

I have been looking into installing a wifi thermostat with a c wire adapter but cannot figure out if this will work. The thermostat I am looking at is a Honeywell 6580

with a c wire adaptor from amazon. (Cant Link)

The problem is that Honeywell's website says if you dont have a G and Y wire, that the thermostat wont work. My existing setup only has an R and W wire. Any the C wire adapter seems to have 2 wire that it provides, one being C wire and the other RC.

Quote from Honeywell's website:
"If the thermostat is controlling a two wire, heat only system, a second 24 VAC transformer can be connected to the RC and C terminals. Please Contact A Pro in this situation to help safely install and wire a transformer."

Does anyone know if how how to make this work? I want to be able to turn my heater on from inside the house.

 

[Innovate1]

The thermostat cant get power when it turns on the unit with only two wires - There is no voltage across the thermostat when it is on. Often the thermostat can use the other wires to get voltage as heat and cool will not be on at the same time. It takes a very low current and it goes through the relay coils in the HVAC unit. Not enough to trigger the relay but enough to power the thermostat. For the two wire case they are saying you need to connect 24VAC power to the other terminals. If you have access to the transformer in the unit (if it has one and it 24V) you may be able to tap off the 24VAC there. Otherwise a 24VAC transformer is pretty cheap. It can be a low current one since it is powering only the thermostat.

I just googled "smart thermostat no C wire" and came up with tons of info.

Here is one that looks like it covers things:
https://www.thermostastic.com/how-to-install-a-wifi-thermostat-without-a-c-wire/
although it doesn't include a wiring diagram for connecting the added transformer. It shouldn't be to hard to find all the info you need to do this.

 

[Innovate1]

Then there is this:
https://hvac-talk.com/vbb/showthread.php?1505131-Trying-to-connect-WIFI-Thermostat-to-Modine-heater

 

[Caddybill]

We have used these before.
https://www.fast-stat.com

 

[Innovate1]

I don't see the point of using a C wire adapter box unless the wire between unit and thermostat is hard to upgrade to 3 wires. The unit has a 24V transformer. It's just a matter of getting the connections right. The 24V connection may not be brought out to the thermostat terminals which would just be a wire to another point in the heater. But the link in post #3 seems to show that all the connections are there.

 

[c5_nate]

The thermostat cant get power when it turns on the unit with only two wires - There is no voltage across the thermostat when it is on. Often the thermostat can use the other wires to get voltage as heat and cool will not be on at the same time. It takes a very low current and it goes through the relay coils in the HVAC unit. Not enough to trigger the relay but enough to power the thermostat. For the two wire case they are saying you need to connect 24VAC power to the other terminals. If you have access to the transformer in the unit (if it has one and it 24V) you may be able to tap off the 24VAC there. Otherwise a 24VAC transformer is pretty cheap. It can be a low current one since it is powering only the thermostat.

I just googled "smart thermostat no C wire" and came up with tons of info.

Here is one that looks like it covers things:

although it doesn't include a wiring diagram for connecting the added transformer. It shouldn't be to hard to find all the info you need to do this.

So Ive been reading similar stuff as well before posting, but am still confused. The C wire adaptor I found says its a 24VAC C wire adaptor that plugs into the wall and provides C and Rc wires. The heater has R and W. So if I hook up these 4 wires to that thermostat will it work?

 

[c5_nate]

I don't see the point of using a C wire adapter box unless the wire between unit and thermostat is hard to upgrade to 3 wires. The unit has a 24V transformer. It's just a matter of getting the connections right. The 24V connection may not be brought out to the thermostat terminals which would just be a wire to another point in the heater. But the link in post #3 seems to show that all the connections are there.

Any idea how to wire this properly?

 

[cahilj]

Older HVAC systems had two separate transformers and used Rh, Rc, W, etc. The Rh is the "heating" transformer, Rc is the "cooling" transformer. The Honeywell 6580 has terminals R,C,G,W,Y,Y2,W2,Rc. You'll see a small jumper between R and Rc. That's because modern HVAC systems only (for the most part) use one transformer to power all the low voltage requirements. If you had a cooling only system with it's own transformer in your shop (in addition to the unit heater), you would use the Rc terminal. Since you are only hooking up the heat side (W), all you need is R,W,C.

The link to the HVAC-Talk thread is NOT the model heater you have, it's a much older one. The 6580 needs transformer power to work (as opposed to batteries). The unit heater has a transformer, so you can use it to power the 6580. If you're able to, I recommend replacing the thermostat wire with a 4 or 5 conductor (to allow for fan control) as opposed to using the add-a-wire kit. Connect the terminals directly as labled, leave in the R-Rc jumper. If you're not able to replace the wiring, using something similar to the Fast Stat above is the only way, since the 6580 and heater need to have a shared 24v supply to work (as opposed to a separate transformer powering the 6580).

 

[Innovate1]

Everything cahilj said seems good to me. You should be able to find the other side of the 24 VAC in the unit and connect it to common on the thermostat. It will have 24 VAC when measured to the R terminal at all times. Is there a circuit diagram inside the unit. How about some pictures of the unit control area and the circuit diagram?