My AC Went Out

My central air conditioning went out Sunday. Law of AC in Texas says: AC go out only on weekend. Remembering the sage words of an old AC man that "90% of AC problems are electrical in nature", I got out my trusty volt meter. I determined pretty quickly that there was no juice to the thermostat. I checked the usual suspects, the breakers, and all was OK there. I decided to dig deeper and removed the panels on the air handler to get at the control board. Sure enough, it had a blown fuse. That meant a quick trip to the hardware store for a couple of replacement fuses. I popped one in and flicked the fan-only switch and the fan fired right up. Next I called for cooling and bam, the fuse blew again. This suggested that something was shorted related to running the compressor (but not the fan).

Next I pulled the wires from the control board that go to the contactor (relay) in the compressor unit outside on the roof. The contactor is the device that switches the high voltage to the compressor on or off in response to low voltage signals from the thermostat. I called for cooling again at the thermostat and the fuse did not blow. A quick check across those pair of wires to the roof with the meter showed 1 ohm of resistance. Mr. Ohm says: E = IR so I = E\R and if E is 24 volts that means an "I" (current) of 24 amps. Since the fuse was rated at only 5 amps, that was not good.

So I went up to the roof to check on that contactor and to rule out that the wire going to the contactor was not shorted. Oh yeah, other Rule of AC in Texas: When time to work on part of AC in hottest location, sun will be very high in sky. After removing the panel to reveal the contactor and checking out both the resistance across the contactor coil and the wire back down to the control board I confirmed that it was indeed the contactor that was the problem. Being that it was Sunday I knew I could not get a replacement that day.

It was going to be a long hot afternoon and night with no AC. I solved the hot afternoon problem by spending it at the Ice House. Still hot there, but the beer was plenty cold enough. Fortunately, back in early June I had installed a small window unit in my office. I was able to blow some of that cool into my bedroom with a box fan and that combined with my new ceiling fan made for fairly comfortable sleeping.

I did call my AC repair guy on Sunday anyway, figuring since I had sussed out the problem I could get the job done pretty cheap and not have to endure any more time on that hot roof. However, by noon on Monday I had not heard back from him. The house was starting to get pretty hot and I was getting antsy. I decided to Google-up my nearest AC parts place and give them a call with the part number. They did not have the exact part but one close enough to work. Contactors are pretty simple things and as long as they have the right number of poles (connections), the right load rating and coil voltage they will work. Just to be sure I pulled the faulty part to take with me (again, here I am at high noon on that hot roof).

I got the contactor, hooked everything back up and lo and behold: cool air again! The total cost for this repair was $10 for the part and about $2.25 in fuel for the round trip to get it. I think I might have saved some money! Funny thing is that on the way back from the parts house my AC guy finally called me back. I told him what I was up to and that if things did not work out I would call him back. Thankfully, I didn't need to.