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doowright · Oct 30, 2005, 12:07 PM

I'm replacing an old honeywell thermostat with a new digital. The old one has the standard two wires - R and Wh, but then it has two C wires. I think they just provide power for the clock, and I'm wondering if I can just close off that circuit. The clock has two metal brush connections that close the circuit.

Can I just close that connetion ? Or should I wire them to two other labeled options?

Thanks.

labman · Oct 30, 2005, 12:25 PM

The red wire should connect to RC and be jumpered to RH. The white to W. That is all many programmable thermostats need to control the heat. I don't quite understand how the programmable thermostats work without a common, but many do. If yours requires a common, you will need to identify which of the 2 other wires connects to opposite terminal of the transformer from the red wire, and connect it the common on the thermostat. Otherwise, just tape up the 2 unused wires.

doowright · Oct 31, 2005, 08:27 PM

Thanks.

I got that far, taped off the C wires (which had a current for the clock) and then figured out that the new thermostats have different connection terminals than the old thermostats.

Do new thermostats have connections like slip-in telephone type wires? The one I bought is a honeywell RTH 110B - not even programmable.

I didn't have a small enough screwdriver to loosen the tiny screw of the insert point. Then I thought that perhaps this is not how the new ones work and that the wire I had was too big.

It's not a junction box in the wall, and it's a smaller wire than a standard electrical current wire;

I wouldn't be surprised, however, if this thermostat was put in 30-40 years ago. It's got the mercury tube, etc.

Still a little mystified. Old houses present many strange situations. My current thermostat is an old Honeywell Chronotherm T808A (that was the only relevant number I could find), and there's a limit of 30v Max.

Advice?

Also, does one save a lot by having a programmable thermostat?

Thank you.

labman · Oct 31, 2005, 09:13 PM

Most thermostats have transformer that reduce the voltage to 24 volts and control mostly using relays or low power coils. I thing the wire used to be 20 gage, but may now be 26 gage. The low voltage, low power circuits don't require near as much to avoid killing people or burning the house down.

Most thermostats I have seen have ever smaller screws on the back for the wires. Could be a problem if a new one has slip in connections meant for 26 gage wire and you have old 20 gage wire. Might have to ge a little 26 gage wire and splice it in.

I am not sure how much money programmable thermostats save. It certainly would be nice to let the house cool off over night while you sleep under a pile of blankets, and then have the furnace blasting warm air out while you are shaving in the morning. Then shut off and let the house cool off again while you are at work all day, and have it cozy just as you come home.

doowright · Nov 1, 2005, 08:05 PM

Thanks for your help!