Switched thermostat from heat to cool, heat won't come back on!

Old 04-18-10, 12:18 AM
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Exclamation Switched thermostat from heat to cool, heat won't come back on!

I bought a foreclosure, and have been renovating it over the past years. I replaced the old furnace with a used Carrier 58STA/STX furnace (110K BUT). I replaced the old mercury thermostat with a new Honeywell thermostat (also replaced the wiring with new wiring). I wired the thermostat as instructed, and the heat kicked on!

Well, a few months have passed, and it is now Spring. It became warm, so I decided to kick on the FAN. So I switched it to HEAT to COOL, and turn the fan to ON. Nothing happened, and I didn't think much further.

The last few days, it started to get cold. I switched it back to HEAT and AUTO and ON, but the heat didn't come on. I turned the furnace off at the switch, then at the breaker, and nothing came on. Before, if I did that, the blower would kick on for thirty-seconds as it runs through all the tests. It didn't do that. The Carrier furnace has a solid red light. (None blinking).

Any ideas would be great. It's coooold!


Old 04-18-10, 10:13 AM
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So basically, the bottom line is it has no function whatsoever for the last few months?

If so, we have to start with the basics:

Confirm 120 volts getting to the furnace and/or simply confirm 24 volts at the secondary side of the transformer wiring.

Make sure bottom blower door is on good. You might want to remove it and resinstall to be sure.

If you have a voltmeter, locate the low voltage terminal strip in the furnace (often in blower compartment), turn up the thermostat, tape the blower door switch so that it remains pushed in, then test in ac volts between R an C. You should have 24 volts. Then test between W and C. Again you should have 24 volts, with a call for heat.

If you have an ignition control module, you can check to see if say you have at least 24 volts there with a call for heat. If you have 120 into furnace and 24 volts at transformer, and through the stat per test above - yet no power at control module, you could have a stuck relay that converts the 24 volt ac current to 120 volts, to get the inducer motor to run, which should be first in the sequence.

Or you could even have a 3-wire pressure switch that is stuck open, and will not allow the safety circuit to close, to power the inducer.

Or something else is still open in the safety circuit - like a limit, roll out, or spill switch.

We can speculate more if you post back what you know about the questions I have rasied.

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