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Reecey · Oct 10, 2005, 01:35 PM

I'm in the process of selling my house and a home inspection was just performed. Supposedly everything went okay and no problems with heating/ac were noted on the report. However, when I got back in town, the house was warm. The AC wouldn't come on when I adjusted the thermostat (on auto or on position). I checked the breaker box and the breaker was on. I'm wondering if the inspector could have tripped something or turned something off and forgot to turn it back on. I've never had problems with the system. It was serviced last year for general maintenance and found to be in good working order. Any suggestions?

labman · Oct 10, 2005, 05:41 PM

For the A/C to work, the following must be on, the breaker for the furnace, the breaker for the A/C, the outside disconnect at the compressor, and any switch at the furnace. Breakers can look all right, and still be off. Turn them both clear to off, and then back on.

The blower in the furnace and the compressor are operated by relays. If neither runs, chances are you have a problem with the low voltage, perhaps a fuse on the control board.

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

caibuadday · Oct 11, 2005, 06:26 PM

Originally Posted by labman

For the A/C to work, the following must be on, the breaker for the furnace, the breaker for the A/C, the outside disconnect at the compressor, and any switch at the furnace. Breakers can look all right, and still be off. Turn them both clear to off, and then back on.

The blower in the furnace and the compressor are operated by relays. If neither runs, chances are you have a problem with the low voltage, perhaps a fuse on the control board.

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

also check... the filter cover (door) it may not run if not properly close