Heating & Air Conditioning Electrical Wiring

My Fixer-Upper & Two Thermostats...

5 years ago

So, we bought my house about five years ago. It was built in 1978. When I bought it, it had one programmable thermostat and then about 15 feet over in the next room was an old mercury Honeywell thermostat. The mercury thermostat seemed to do nothing.

About two years ago, I installed a Nest thermostat, replacing the programmable one. Now this week when we were painting, my wife decided the other thermostat had to go. Now I don't have heat or AC. My fan still goes on based off the Nest thermostat.

Does that mean the old mercury thermostat was an in-line thermostat with the new one and by removing it the circuit was broken, hence no heat / ac? How would I go about re-wiring it, or is it better to just reconnect a thermostat and call it a day? Why would the wiring matter if the actual thermostat doesn't matter?

Any thoughts are appreciated. Also, for context, every single project that was done by the two previous owners was done wrong in both just small cutting a corner to major totally wrong work being done.

Thanks

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Comments (5)

mike_home
5 years ago
It is hard to tell have the how the mercury thermostat was connected to the Nest. The fact that the fan turns on means you are getting 24 V (R terminal) and the fan connection (G terminal). D

Does the Nest thermostat have a C wire connection? If it does not then that could be the source of the problem. Nest thermostats don't always work properly with a C connection. The old thermostat may have been connected in parallel and may have compensated for the lack of the C connection.

Are you able to run new wiring from the Nest to the furnace?
HU-681240837
Original Author
5 years ago
First picture is the nest wiring / troubleshooting guide, second is the thermostat that was replaced by the nest.
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HU-681240837
Original Author
5 years ago
Definitely able to run wiring if needed.
mike_home
5 years ago
The trouble shooter shows a C wire connection. I am not sure how accurate this troubleshooter is for electrical connections. It may detect a wire, but does it actually connect back to the furnace?

Is it possible you accidently disconnected a wire that could be running over to the Nest? Were you able to control heating and cooling from the mercury thermostat prior to its removal?
wdccruise
5 years ago
last modified: 5 years ago
Does the area that contained the Honeywell thermostat have two cables, one that runs to the furnace and a second that runs to the Nest? If so, then I'd think that wires from the furnace were "spliced" to wires to the Nest by connecting them both to the Honeywell. When you removed the Honeywell you broke the "splice". So if you could figure out which wires from the furnace were "spliced" to those from the Nest, you could connect those wires together using wire nuts, discard the Honeywell, and make the furnace work again.