any one know the answer to an electrical issue = wall thermostat (homes, floor)

[sausages]

just took off old thermostat Edison, it has 3 wires the new one I have has four any thoughts please. Also replacing old floor wall heaters with two new hydronic heaters, having difficulty get both to work... wiring is a little complicated. I think I messed up the thermostat in the process.

Thanks

[frank754]

Your new one may have a pair of contacts for both heat and air conditioning.
If you just use it for heat, you will probably only need 2.
Try touching two of the wires that come out of the wall together, and see which two turn the heat on (don't worry, it's low voltage).
Then get a multitester, VOM, continuity tester, etc., and find the two wires on the thermostat that show continuity (closed circuit) when you start at the highest temperature setting (it should then be "off" and slowly turn the dial down (at some point it should come on).
Those two would be the ones you connect to the two "hot leads" of your furnace circuit.
You should also be able to see what it does by looking at the inside, follow the wires to the contacts that close at each position. I don't know if they use mercury anymore, but that's the same concept.
Good luck.

BTW - This is for normal furnaces, they use low voltage relays. If you are using 240v baseboard heaters be real careful and you must be sure the new thermostat is rated for those voltages as well.

[sausages]

the thermostat that we bought has 4 wires and the one on the wall has 3. any thoughts ? tks

[psu_mri]

Here's how I deal with t'stat replacement. Shut off the associated breaker before you do anything. Do you have electric base board heat or a furnace? With BB heat, they're most likely 220, and in most cases, the wires coming to the stat are #12, in romex or BX cable, and the t'stat would be mounted to a conventional electrical box.. Could be as few as three, black, white and a bare ground. That would use a line voltage t'stat. Some homes use relays to control the 220. They would send a lighter gauge wire, #18 or so, to a t'stat that may be just screwed to the wall surface, via a base plate. These I consider Low Voltage units. All the furnaces/boilers I've had use LV t'stats. The lighter gauge wire and mounting scheme is OK because the relays switch with low voltage. I've seen LV t'stat installs with as many as 5 wires in a single jacket, the extras being available if one fails or you add A/C.
Your current t'stat may have it's ratings stamped on it. If you see anything like 20A, 25A, or 30A, I'd say you're dealing with a line voltage t'stat.
If you're still not sure, bring the old with you and match your replacement that way.

[mathjak107]

Originally Posted by sausages

the thermostat that we bought has 4 wires and the one on the wall has 3. any thoughts ? tks

What are the letters on the terminals marked? y,r,w,c?