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The power went out for a few hours one night. Not clear why but the entire neighborhood was out, including the nearby street lights. When the power came back online, our house air conditioner would no longer work.
It took me a while to figure out the problems and fix them. Each time I found a problem I had to wait for a replacement part to arrive so I could move on to finding the next problem.
For all the time it took to troubleshoot this, the problem was down to two failed parts.
- The contactor on the outdoor unit had failed
- That caused the 240 VAC to 24 VAC transformer on the indoor unit to fry
Replacing both parts at the same time fixed the A/C.
But it wasn't quite that simple to figure out. There was no ground conection going to the indoor thermostat so I didn't have a way to confirm the thermostat was seeing 24 volts on the supply. That has been remedied for future generations and now the termostate neutral is actually connected.
Second, the transformer on the indoor unit actually had a 3 amp in-line fuse connected. It had blown during the power failure. Proud of myself for finding it, I replaced the fuse only to see the transformer putting out nothing. I replaced the transformer, turned everything on, and smelled burning from the transformer. It was fried.
What should be 240 VAC from my power company is actually 252 VAC. That is right on the borderline of the +/- 10%. I chased that for a while but ultimately the incoming voltage was not the issue.
The fuse was still intact so I dropped it down to 2 amps, which is the right limit given the transformer provides 40 VA with 24 VAC (actually 29 VAC thanks to my 252 VAC input). Thus 2 amps (29*2=58 VA) was about as close I could get to properly protecting the transformer.
So the problem was in whatever the transformer was powering. If I just ran the house fan and not the compressor, things worked fine. As soon as I told the thermostat to engage the compressor for actual cooling, the transformer fried again. Yes, the new one and despite the 2 amp fuse.
The problem was thus at the outdoor unit. The 24 VAC line goes to the contactor. The contactor had a switch you could use to manually engage it. When pressed, it worked fine and the compressor sprung to life. I replaced the transformer (2nd new one), left the 24 VAC lines connected to the contactor but removed the 240 VAC lines, and tried again. Contactor fried.
There must have been a short in the electrial leads that flipped that activate the contactor with with 24 VAC despite the fact that it works fine when manually pressed.
I replaced the contactor and transformer (3rd new one). I fired everything up and it all worked. Thankfully the transformers are like $25 with next day prime shipping available. But that was still annoying.
One other note, there is a switch next to the indoor unit that switches off the 240 VAC going to the AC. It also has a 15 amp fuse for each 120 VAC leg. I did some research and this is common practice when the circuit breaker is rated for something higher than the device is. These inline fuses protect the device itself while the circuit breaker protects the house. No issue so far.
I actually opened up the electrical box as part of my troubleshooting. And what do I find? One of the terminals on the switch is broken off. Whoever installed the switch + fuse device, accidentally broke the switch. They proceeded to leave switch controlling one 120 VAC leg with fuse protection. The other leg they just connected directly without going through the switch or fuse. If you shut off this switch, you expect all 240 VAC to turn off. What you actually got was 120 VAC turned off, the other leg was still HOT and with no fuse protection!
I ordered a new switch+fuse box. It was extremely expensive ($250), which I presume is why the original installer didn't fix their mistake. I will install it once it gets here. How frustrating.
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