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We've got a central cooling air conditioner that was installed in 1984 with a thermostat from that same year. We are replacing the thermostat with a new programmable one (which has a 100% compatibility guarantee).

Following the instructions from Home Depot I located the circuit breakers for the unit and switched them off (the circuit breakers on the panel inside the house, not the main ones for the entire house). Nothing happened. The AC unit is still running and blowing cold air.

Am I missing something here or could the breakers be broken? Or worse, could the unit be installed improperly? Feel free to point out the obvious. :-)

More info if it matters: We have a Trane central air unit with a heater pump and an emergency electric heater. We are in Central Florida

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3 Answers

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You are missing something. Either you have not found the correct breaker yet, or the breaker is not inside the breaker panel for your house.

Walk through each breaker in the breaker planel for your house. You could have a mislabled breaker. (The breaker you are looking for is likely a two-pole 240V breaker, maybe 30 amps or better.)

Since this is a central cooling unit, it might be powered from its own breaker box. Follow the electrical supply wires (cable) back from the unit. Where does it go? You might find another breaker box dedicated to the cooling unit alone. There might also be a breaker in your meter box protecting the unit in addition to the main breaker for your house.

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After turning off all of the breakers in the breaker panel to no avail (I still wonder what the 2 breakers labeled "Air Conditioner" do). I found a black box connected to the unit with a door on it. Opening the door revealed a handle with the label "Miscellaneous Switch." I gave it some thought as to what it could be; determined that it isn't going to launch any nuclear weapons; then yanked on it. Voilà! My energy-saving programmable thermostat is installed and working great. Thanks. – David Murdoch Feb 11 '11 at 14:30
Side Note: on Trane AC units the "B" wire is actually the "C" wire and the "O" wire is the "B" wire (well, sometimes). – David Murdoch Feb 11 '11 at 14:32
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The two breakers (double pole breaker) labeled "Air Conditioner" are likely protecting some other 240 volt device in your house, such as a hot water heater, dryer, range, baseboard hearters, etc. You should probably find out what that breaker controls and label it correctly.

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Mis-labeled breakers are VERY common. Don't be shocked that you've found one. There's probably only a few 240v devices in your house, so it shouldn't take too long to work out which one the breaker you found actually goes to. At the very least, cross out where says 'air conditioner' so that you know for the future. – Michael Kohne Mar 4 '11 at 0:47
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Some AC units have an on-delay timer that prevents the system from turning on until 2 minutes after power is restored.

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He's talking about turning the system OFF by removing power. An on-delay timer won't affect this. – Michael Kohne Mar 4 '11 at 0:48

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