I just moved into a new house and wanted to get a WiFi thermostat installed so that I could control the a/c from anywhere I had internet. The house was built in the late 1980s and appeared to still have original Honeywell thermostats with mercury inside. I purchased a Honeywell WiFi thermostat on Amazon and followed the instructions as well as I could. Uninstalling the existing thermostat seemed like no problem, but I was a little over eager and broke it as I was taking it off the wall. So I had to get the new one working, or we were without heat.
The instructions were very clear that I needed a "C" common wire to provide power to the new thermostat. And of course, I didn't have one. After spending a few hours searching the web and tracing wires out at the heat pump, none of the colors made sense and I even moved some around, which was a mistake I regretted later. I could see a terminal labeled "C" on the circuit board, but I had no idea how I was going to get a wire to it and it appeared to already have a wire on it that was being used, so I gave up. I called my handyman who came by the next morning, but he was not sure how to solve it and suggested it would be easier just to buy a thermostat that didn't require a C wire (all the WiFi thermostats I could find do require one).
By this point, it was Monday and I did not have time to work on it until the next weekend. Unfortunately, we were entering the coldest week of the year, including freezing temperatures that are somewhat rare in Gilbert, Arizona. I still had my basement heater working, so I cranked that one up, but even at 70 degrees downstairs the main floor was a cool 61 most of the week. Needless to say, we were ready to get it fixed.
I spoke to an electrical engineer friend at work who explained that I needed to connect a wire to a negative terminal somewhere so that the 24 volts of electricity could flow through the thermostat at all times. Then it was just a matter of finding that terminal. I learned that my unit is actually split between a heat pump outside and an air handler in the attic. So Saturday morning, I climbed into the attic, took off the cover of the air handler, and stared at a bunch more switches and wires withe no idea what to do. After a bit of Googling, I found a website that offered to connect me with an A/C expert by chat for a small fee, and I gave it a try. It turned out pretty well - he was able to understand my problem and had me take pictures of the inside of the air handler to help find the right spot. He pointed me to the 24 volt transformer and specifically the negative terminal on the 24 volt side. I purchased a connector and an extension thermostat wire and connected my extra black wire to that spot. The hardest part ended up being getting my extension wire out of the air handler box - the holes for wires were pretty tight and not easily loosened. Once I had that in place, I used the black wire as my C wire and everything worked. Well, almost everything. Because I had moved some wires out at the heat pump, my heater was still not kicking on. But since I had documented the entire line of wires and colors, I was able to deduce which ones I had switched and as soon as I switched them back, all was well...and warm.
Thank you Norm (Wvfan79) for your help.