Old thermostat wiring

January 15, 2009 9:37 AM

Replacing the old mercury thermostat in our old house with a programmable one for an oil furnace. The old one is a two wire hook up, one red/white and the other black. What would be the correct configuration to the new thermostat? Black to R, Red/White to W? I've searched high and low for the answer.
posted by Heatwole to home & garden (16 comments total)

My programmable thermostat had specific instructions on how to convert the number of wires and types from old system to the new one. Was there not one included with yours? If it helps mine was a Hunter thermostat and you can find the manuals online (in the off chance this is standardized).
posted by mmascolino at 9:45 AM on January 15

Unfortunately, nothing that specifies wires with these colorings.
posted by Heatwole at 9:47 AM on January 15

seconding mmascolino's answer. The instruction manual for the thermostat that I installed had all the information to determine which wires went where. What model do you have?
posted by Brettus at 9:47 AM on January 15

Does it matter? Doesn't a mercury thermostat (i.e., a mercury switch rotated by a bimetallic strip) simply close or open the circuit at a temperature greater or lower than the setpoint? There wouldn't be any polarity issues for such a configuration.
posted by Mapes at 10:13 AM on January 15

The black is normally your ground wire. As for the other two I would look it up online just to make sure. You don't want to be hooking up the wrong wire and causing a fire!
posted by Mastercheddaar at 10:21 AM on January 15

I tried to do this, and I had to revert of a "regular," non-programmable two-pole thermostat because there was just no other way, with a two wire set-up.

I hope that you have better luck than I did. I had to return the programmable stat that I had bought and go with the simpler one.
posted by Danf at 10:27 AM on January 15

Can you post the thermostat brand and model#? It would be easier to work through this if we could find the manual online in PDF or something.

You probably just need to figure out which two terminals on the thermostat are used in 2-wire arrangements. They will probably be marked R, to which the wire that is hot all the time should technically be connected, and W (or W1 if the thermostat is meant to control a multistage furnace), to which the other wire should be connected. It probably doesn't really matter which wire you connect to which terminal, but if you want to do it right then you can either use a multitester to figure out which of the two wires is hot, or you can trace the thermostat wire back to the furnace itself and see which terminals each wire is connected to there.

In a 2-wire setup, all the thermostat can do is turn the furnace on or off. More wires lets the thermostat do fancier things, like control the air conditioning, run the fan without running the burners, maybe operate a whole-house humidifier, etc. You don't need to worry about any of those other terminals.
posted by jon1270 at 11:02 AM on January 15

Forgot to mention, if the thermostat has a C terminal (common), it may require 3 wires to work properly. You can buy 3 (or 5, or 7)-conductor wire at Lowe's or HD, and it's not difficult to change it over if you can get it pulled through the wall/floor to get it where it needs to go.
posted by jon1270 at 11:15 AM on January 15

I also installed one of these, and it came with a very thick manual explaining how to install for almost any combination of wires, including two-wire situations like yours. But notice the packaging at the hardware store (eg Home Depot, Lowes, etc) -- not all programmable thermostats will work with all kinds of furnaces. It is possible that you bought a more restrictive sort of thermostat, that won't work with your set-up.

If so, just return it and swap it for one that claims to be totally universal. It might cost $5 more, but that's a small price to pay for it actually working.
posted by Forktine at 12:43 PM on January 15

Also, it may not be as simple as just which wires to connect. With the one I installed, there was the question of wire connections, and then there were different little jumpers and switches that had to be moved around depending on what kind of furnace you had (gas vs oil, for example). So the answer you need is likely to be more complicated than just "red wire to X, white to Y."
posted by Forktine at 12:44 PM on January 15

When we first bought our house (about 3 years ago) one of the first things I did was change the thermostat to a digital one (Honeywell). I remember having some difficulty installing it and having to make a call to their tech help and they were able to sort me out. Basically I told them what I had before and where they went to before and they me the new wire hookups.

Try giving the tech help people a call for the brand of thermostat you bought.
posted by boomcha76 at 1:30 PM on January 15

I did this recently. Hooked it up as you state, got nervous and unhooked it, called an electrician, and he charged $150 to hook it up the same way as I had before. Good luck.
posted by vilthuril at 1:43 PM on January 15

Hook it up the way you have stated (Black to R, Red/White to W).

This will work.
posted by davey_darling at 2:26 PM on January 15

I've done this too. Polarity doesn't seem to matter, you just need the 2 wire set up.
posted by pointilist at 9:21 PM on January 15

And remember to safely recycle the old thermostat since it has a big blob of mercury in it.
posted by exphysicist345 at 10:02 PM on January 15

Echoing exphysicist345! Some scrap recyclers even get a payment for safely recycling mercury-containing thermostats, so shop around and find someone who wants to take care of it for you!
posted by greekphilosophy at 10:13 AM on January 16