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Heating & Air Conditioning Electrical Wiring

Re-wire furnace only thermostat from 2-wire to 5-wire

2 years ago

last modified: 2 years ago

Is it possible to re-wire a heat only thermostat from a two wire to a five wire setup? With the yellow (AC) wire just taped up since I don't have an air conditioner? My current thermostat has only two wires: red and yellow. I bought a new roll of wire with 5 wires to be used with an EcobeeLite thermostat. I do not have an air conditioner at this point in time. So the thermostat would just be running my furnace. We will be adding an air conditioner in a few months, so I wanted to upgrade from a 2-wire to 5-wire setup. I planned on replacing the 2 wire with a 5 wire, and taping up and not connecting the yellow wire for now since we will not be using an air conditioner.

New wire that I want to wire to the furnace and EcobeeLite.

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Comments (4)

Elmer J Fudd
2 years ago
If pulling a new wire is difficult or a problem, you might want to consider the Honeywell/Resideo RedLink product line. The thermostat has a wireless connection to the furnace - not Wifi, but a proprietary RF capability that connects wirelessly to a receiver unit near the furnace. The control/receiver unit is mounted near and wired to the furnace as if it were the thermostat, then it handles the communications to/from the thermostat via its own wireless send/receive capability.

I have two Redlink stats for two different systems in my house, they work great. An optional internet interface allows control from a smartphone, tablet, or PC.
mike_home
2 years ago
Your description is a little confusing. Do you want to replace a thermostat which currently is connected with five wires with a heat only thermostat which only uses two wires? That may be possible.

Post the model numbers of the thermostat you currently have and its used connections (R, W, etc.), and the thermostat you would like to use.
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is this a trailer or modular home so you have to go to the furnace to switch it to cooling and fan? If so your programable stat will not work on that system. You would have to run new 4 wire and do a lot of rewiring at the furnace. Later Paulbm
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Integrating unmatched systems with different circuit boards and especially two transformer systems is never easy. Sorry to say, not knowing more about the whole system including how the heat pump changeover and controls operate, I doubt you will get an answer on this. Based on my past experience on instances like this, it would take all the wiring diagrams of all the components, a lot of scratch paper and an hour to figure it out to assure everything integrates and nothing burns out before making the connections. In addition, sometimes without fossil fuel kits, additional sensors are involved. "C" is a common, many times for the stat to power lights, indicators, screens, power unit so that the batteries are back-up only and on occasion operate additional accessories, including the remote if you have that option.
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I disagree with Petey (and with Bus). There is nothing wrong with the thermostat in this application. 410A would have been a better fit but all you're doing is leaving out the second pole on the 410B. You've wired it wrong, though for the life of me I can't think of how you did it. If you'd hooked up the wrong terminals it would be OFF all the time. I suspect you've somehow shorted the wires together separate from the thermostat itself. The color of the wiring isn't particularly obvious. I got it wrong myself (my dumb fault for not reading the instructions). There's not much to this device, the operation is purely mechanical which closes the two switches.
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The back of the thermostat usually has a diagram of how the thermostat should be connected. Can you take a picture of the back? A picture of the front and the brand and model number will also be helpful. I think this thermostat is wired to a 220 V circuit, whereas your other baseboard heaters were wired to 120 V. If it is 220 V then connecting two line wires would cause a dead short and probably fry the wiring. Don't wire anything together until you determine exactly what you have. Is the office heater bigger than the others? If it is then that would explain the different wiring.
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HU-571307274
Original Author
2 years ago
THank you both for responding, sorry I was not clear. I want to replace my thermostat and the thermostat wire. I currently have a 2-wire (yellow and red) going to my old Honeywell thermostat. I want to change the wire to a 5 wire to an EcobeeLite thermostat. I only have a furnace right now, so I want to know if I can tape up the yellow wire and just not connect it to anything for right now. That way we have the 5-wire in place for when we do get an air conditioner. I updated my post with pictures too.
mike_home
2 years ago
Having extra used wires is a common practice and is not a problem. You can leave them unconnected for feature expansion.

In addition to the R and W connections, the EcobeeLite will require a third wire for the C terminal. You should be able to find the C terminal on the White Rodgers 50A50-473 module. Normally W is the white wire, R is red, Y is yellow, G is green. You can make the C connection blue. The colors don't matter as long as the connection is correct at both ends.

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