How I Fixed My AC When it Stopped Working

My AC, heater and thermostat stopped working one day.  It wasn’t just one piece of the system, but everything and I wasn’t in the mood for a huge repair bill.  I got out my multimeter, screwdriver, voltage detector and started looking and asking questions.  This is how I solved the problem.

One of the first things I did was to check the batteries in the electronic thermostat; since I have two AC units in my home I also swapped out the thermostats since they were easily interchangeable – This did not fix the problem, the suspect thermostat worked fine and had fresh batteries now.

Next I began to look outside at the condenser.  It gets kind of scary at this point because you are no longer investigating a 24 volt system, but 110 and 220 volts – So I don’t like to touch anything.  Not knowing exactly how all of the pieces in the system are connected proved a little frustrating at first, but I did identify that 110 v was going to the unit.  Eventually I was able to confirm that the outdoor unit worked and there was evidently no problem with the contactor or any of the outdoor unit components.

I then started looking at the pieces in the attic,  I looked at the blower – it did not seem to be stuck, but it would not start – the door switch seemed to operate fine as well.

I spoke with a few others that were more knowledgeable about these systems and was able to get some good guidance, they seemed to suspect that either the problem resided in the furnace control box or the power to the system was off.   Sometimes people do inadvertently turn off the power to their AC systems when they find a light switch in their attic and are unsure what it does.  I had discovered that switch some time ago and mine was turned on.

I began to investigate the furnace control box – After turning off the power to the system and making everything was off, I opened the box.  Almost immediately I found the problem. The 110 voltage lines going into the furnace control box had burned and one was broken in half.

burned wires in a furnace control box

I can’t really determine what happened other than that the wires evidently got very hot and burned, the wire nuts were melted and one of the wires eventually burned in half which shut the whole system down.

I found out that furnace wiring is not regular wire that you would find at your local hardware store – unfortunately most of them don’t carry it.  The problem is that the wire you find in most hardware stores is rated only up to a certain temperature, 90 degrees Celsius, whereas wire that will be used in the furnace generally requires a higher temperature rating of 105 degrees Celsius.  This wire was somewhat harder to find, but I eventually found some high temperature wire on the internet and I decided to go with 12 awg wire instead of the 14 that was originally there.   I got some wire nuts at a local store, some wire strippers for a buck at a garage sale, and after making sure all the power and electricity was off to the whole system; went to work replacing the burned wires and connections.

Once these burned wires were replaced, the system worked fine again.