Just another C wire problem

Question

Sorry for the long post but if you can make it to the end, I would greatly appreciate it. I have what I believe to be an uncommon situation that I haven't been able to solve. I have a thermostat that controls only my a/c. I wanted to upgrade the thermostat and picked up a Honeywell Lyric T5 secondhand. I then discovered that I don't have a C wire.

I currently have Rc, G, and Y. When I connect them to those spots in the thermostat, it does nothing. I went to the attic and found that the Rc wire is connected to the C terminal in the air handler. So I moved it from R to C in the thermostat. Now the thermostat will power on and acts like it's working but only the air handler turns on, not the a/c compressor.

So my question is, is there a way to add a jumper at the thermostat? If not, I can run new thermostat wire but I'm not sure how I can hook it up at the air handler since C is being used. I don't remember what other terminals were available at the handler but I can fnd out. I'm pretty sure the air handler is original to the house from 1970 so I don't know if things have changed since there. Thanks in advance for any help.

Answer 1

Run a new cable with more wires. Then at the air handler connect them as follows:

• Red to Rc
• Yellow to Y
• Green to G
• Blue to C

If there's already a wire attached to the C terminal, don't worry. Just attach the blue wire from your thermostat cable there as well. The other wire is likely from the condensing unit outside. There may also be a wire on the Y terminal, which also likely goes out to the condensing unit.

At the thermostat, connect then wires as follows:

• Red to Rc
• Yellow to Y
• Green to G
• Blue to C

You should be all set.

Answer 2

It finally cooled off enough that I was able to get up into the attic to investigate a little further. This system is a Fedders brand that I'm assuming is original to the house and the house was built in 1970. The labels at the air handler didn't match the accepted labels that are in use today. I started by pulling a new, 5 conductor cable (I needed 4 conductors but the old wire only contained 3). The labels in the attic are X, F, T, V, C, H. After studying the wiring diagram and thinking it over a little, I figured it out. F = fan, V = voltage, and C = cool. I was able to verify this by looking at where the old wires were attached to the old thermostat. I assume that H = heat but my system doesn't have a furnace, it's a/c only. The wiring diagram showed me that one side of the 24V transformer went to V, the other side went to T (I haven't figured out what T is supposed to stand for). So I hooked my common wire to T and my Honeywell Lyric thermostat is now working properly. The air handler and the a/c compressor both come on as they should. Sorry for the long post but hopefully this information will be of some help to somebody out there somewhere.