I have 6 electric 220V heaters powered by two separate 20A circuits. All heaters are wired to a single thermostat. The thermostat is labeled 24V, and only has a small red and white wire running to it (far smaller than 10# or 12# wire.) I would like to replace this thermostat with a progammable. Originally, I bought a programmable thermostat for line voltage, but that appears not to be the correct option. Then I bought a standard Honeywell thermostat (RTH6300B,) and hooked that up. Now only one half of my living room heat (1 breaker worth) works. I'm assuming that somewhere in my wall, I have a 220V/24V relay (or two,) but I have no idea where. So my question is: why is this not working? Do I have the wrong thermostat (again?) Does this little battery powered thermostat not provide enough power to flip these 24V relays (if they exist?) Also, the old thermostat had a little green light that would light up when the room was dark, so I know the little red and white wires must be carrying some power from somewhere... This appears to be a pretty good one! Thanks for any help!

There could be a single relay, but most likely there are two.

The thermostat just provides a contact closure. e.g. connecting the red and white wires just cause the heaters to come on.

It's not code to hide the relays.

With the right tools, the low voltage wires can be traced.

The high voltage wires might also be traced with an AC voltage detector.

Having the right tools will help immensely.

One thing to consider is whether your original stat was a 2-stage thermostat.

Mark:

How with only 2 wires? Inquiring minds would like to know.

Kiss is on the right track. Have you found the relay(s) yet?
When you say 2 separate 20A circuits... do you mean 2 20A dbl poles?
I suspect so. If one leg of the 2 breakers drops out, you shouldn't get any heat.
The two relay scenario fits... so far. Please provide more details.