AC thermostat malfunction

[MiamiCuse]

I have the old style thermostat on my AC that is giving me some trouble. I have played with it a bit and have isolated the problem to the point I think may be someone can tell me if it can be fixed or have to get a new one.

It is the old style with a mercury bulb inside, the manually adjust a dial to the desired temperature. There are two switches:

- fan ON or AUTO swicth - I leave it on AUTO

- HEAT OFF COLD switch

Down here in Miami, I pretty much leave the HEAT OFF COLD on COLD. I set it to 72 degrees and that works fine. If it dips below 72 the AC kicks on.

Last two weeks we had a cold front so I turned on the heat, I might use the heat once every two year or so, last may be a week here, a week there.

If I set it on HEAT the heat does not kick on no matter how I adjust the temperature settings.

I then removed the cover, and adjust the settings left and right and I can see the mercury drop rock left and right.

However, if I set the switch from HEAT to OFF, wait 2 seconds, then set it back to HEAT, if I then adjust the temperature setting I can see the mercury drop moved to one side and when it hits the wire filaments inside a very faint spark, and the heat comes on. It stays on until the temperature is reached then it goes off. It will stay off from now on unless I turn the switch to OFF and back to HEAT, and adjust the temperature then it will come on again.

Any ideas what is wrong?

Thanks for any comments.

MC

[pinkogal]

i had the same trouble with my old mercury thermostat when the seasons changed. We are in north Georgai, and have a heat pump, when we went from air cond. to heat, it was malfunctioning. I called my heat and air guy and he said the old thermostat was not sending the right signal to the unit, we simply replaced the old thermostat with a new one, you can get them at HOme Depot.

[RBM]

I'm not sure that it has anything to do with the thermostat. It may be an issue with whatever the thermostat is controlling. It sounds like you are describing a Honeywell T87 with Heat-Cool subbase, but there are several varieties of them. You need to pop the plastic ring off, loosten the 3 screws, remove the stat from the subbase, then describe all the terminals that have wires connected to them

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[Tony Hwang]

Hi, If it is really 'stat problem(probably tarnished contacts acting intermittent), easiest is forget about the oldie, get a programmable digital(I like Honeywell Vision Pro series and replace the existing old one. More reliable and precise and if you program it you may save some $$ on your power bill.

[Oren]

Replace the T-stat with digital. Save the mercury and poor into drilled out bullets.

[RBM]

Is that what it takes to kill a vampire... or an environmentalist?

[MiamiCuse]

Is it easy to wire in a new thermostat? Do I just pull it off and the new one will match up to the existing wiring?

[Oren]

Yes, if you don't short someting out. See directions in the new box.

[Oren]

Depends! How busy are you?

[Tony Hwang]

Fist turn the power off to the system. Then mark the wires as it comes off the old 'stat(not by the color, by the letter on the terminal such as G, Y, W, R, etc.) and mount new one as you reconnect wires. Also there will be installation guide witrh new one. If get into trouble, folks here can help you. Good luck.

[Stormin Mormon]

For vampires, it's silver. Mercury? I guess that's environmentalists or dentists.

[Stormin Mormon]

Should just match up the wire colors. Look where they go on the old stat, and wire the same on the new. Be sure to ask for guidance on the usenet group alt.hvac where they regularly field questions about thermostats.

[mm]

If it goes above *72* you turn the AC on. Why not open the window, have fresh air, and not waste electricity.

[RBM]

You first have to identify what type of thermostat you have, which you can do pretty much by identifying the letter terminals that are on its base. Once you know what you have, you get a new stat with the same or more lettered terminals that it has. When I replace thermostats, I leave the existing unit attached to the cable(s), and with power to the heating/cooling systems off, I cut the cable(s) close to the stat, then mount the new unit and wire it terminal for terminal by looking at the old unit

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[trader4]

It's hard to imagine a simple old mercury thermostat behaving the way that is described. If it turns on and correctly works each time it's moved from off to heat, but then doesn't work a second time, I can't conceive of a problem with the thermostat that would make it behave that way.

[Mark Lloyd]

[snip]

Digital is good, but you may want to consider these can be MUCH harder to set.

[Mark Lloyd]

[snip]

Be sure to determine how it s connected BEFORE removing the old one. You probably have 4 wires (24VAC, heat, cool, fan) but you need to know which is which, color coding is unreliable.

[RBM]

It's hard to imagine a simple old mercury thermostat behaving the way that is described. If it turns on and correctly works each time it's moved from off to heat, but then doesn't work a second time, I can't conceive of a problem with the thermostat that would make it behave that way.

I can't either. That's why I suspect whatever heating system its attached to may be locking out

[MiamiCuse]

I will try to match up the old and new.

I wouldn't dare post to alt.hvac. I did once I almost got my head twisted off I think that's a trade only group LOL.

MC

[MiamiCuse]

Thank you!