Thermostat replacement?

September 25, 2005 10:19 AM

Installing a new thermostat for electric baseboard heat - can you help?

My thermostat apparently broke because the heat comes on/goes off randomly even though the dial is at "off". So, thinking I' m all crafty, I went out and bought a new one to install myself.

I pulled the old one out and it has only two wires - red and black - coming from it and attaches to the 2 wires from the wall (also red and black).

The new thermostat has 4 wires coming from it - 2 red and 2 black, so I can't figure out if a) I am supposed to cap the 2 black to the one black coming from the wall or b) the thermostat I bought is simply incompatible with old wiring (1984).

There was no indication on any packages at the store that a certain kind was different than another - they all just said "for electric baseboard heat". Can someone help - I'm cold.

posted by tristeza to home & garden (5 comments total)

There are a couple of possibilities here, and not enough info to help you much beyond basics. But probably, the unit you bought is general purpose device for controlling HVAC systems that have both cooling and heating functions. One set of connections (one red/black pair, or red/red or black/black) is for the temperature sensing part of the thermostat, the other is for switched fan functions (fan on/off, auto).

The old unit is the minimum thermostat required for baseboard heat, which doesn't typically have a fan switch connection, since most baseboard heating is not equipped with a forced air fan. Depending on whether your baseboard heaters are pure electric resistive heat, or water circulated, there could be other parts of your system causing intermittent heat delivery.

There should be some wiring directions included with unit, may be available on the maker's Web site, if not packed in with it. It might make sense to just test the old thermostat before trying to replace it. Installing a new one, unless the new one has programmable timer functions, or some other feature you want, may not do much to solve your heating problem, if the old one is working to properly close its contacts at the set temperature.
posted by paulsc at 11:10 AM on September 25, 2005

Thanks, paulsc. The package on the new one says "for baseboard electric heat only" and doesn't have anything indicating a fan/blower, etc. And the instructions are completely Greek. -sigh- I think I'll suck it up and hire a guy. :)
posted by tristeza at 4:28 PM on September 25, 2005

tristeza this should be really easy. I can't for the life of me figure out what the extra wires would be for[1]. Actual installation should be mearly a matter of joining the wires from the wall to wires on the stat. Try getting a different model of stat or even the different one of the same model.

[1]I wonder if you don't have one with a defect or that's been wrongly packaged.
posted by Mitheral at 6:43 PM on September 25, 2005

OK, if you're sure you have electric baseboard heaters, you probably have the wrong type of thermostat. You have a "4 pole" type unit, but need a "2 pole" type. A "2 pole" type is just going to have a single pair of wires (one red, one black) and you hook it up, like your old one.

But you should be aware that thermostats can be "line voltage" or "low voltage" types. Most simple electric baseboard systems use "line voltage" types, where the thermostat directly switches 120 VAC or 240 VAC to the heaters. The wires in the wall will be fairly thick (10 or 12 guage) because the current they are conducting will be up to 15 amps - the amount of power required to run about thirty (17) 100 watt light bulbs (for a 120 VAC 1750 watt system). "Low voltage" types only send a control current to a remote relay that maybe mounted in the baseboard units, so the wires in the wall can be very light guage, say 22 guage.

If the wires in the wall are pretty thick, your thermostat is switching line power, and you need to be careful dealing with it, since if you don't kill the circuit correctly before starting your installation, there is enough power in it to be dangerous. If the wires coming out of the wall are thick and stiff, you shouldn't try this if you aren't experienced in working with electricity, and have a meter to test the circuit .

Hire a pro, and live.
posted by paulsc at 7:07 PM on September 25, 2005

Dude, I'm so getting a pro to do this - bless you.
posted by tristeza at 8:02 PM on September 25, 2005