When I installed my new programmable thermostat I could not get it to turn on the AC. I wanted to verify that the connections to the thermostat were the same as the terminals on the furnace itself. Follow along to see how I checked my thermostat wires and for some good steps for thermostat wiring troubleshooting.
- Honeywell RTH2300B Programmable Thermostat
- Evcon Industries (Coleman) Direct Vent Forced Air Furnace Model DGAT075BDD
- Evcon Thermostat 7670-368-A 65A-753-662
Thermostat Wiring Troubleshooting
I thought this installation would be pretty straight-forward because the wiring labels on the old thermostat matched the labels on the new thermostat. When I installed it, as detailed here, I expected it to work right away, so when it didn’t I checked the wiring.
Step 1: Double-check the original wiring connections.
I always take pictures before I take something apart and this is the wiring of the original thermostat:
I had everything wired correctly so I wanted to trace the wires back to the thermostat and verify that the red wire was actually connected to the furnace’s RH connection, that the white wire was actually connected to the W connection, that the blue wire was connected to the Y connection, and the green wire is connected to the G connection.
Step 2: Locate your furnace.
This manufactured home has a mobile-home type furnace that is located off the kitchen and has a metal door:
Step 2: Shut off power to the furnace.
Caution: It is not recommended that you go any further without turning off the electricity to the furnace. Turn off power to the furnace at the circuit breaker:
Step 3: Find the location of the control board in the furnace.
When you open the door, you can see where the control board is located based on where many of the wires converge:
I wrote another article on running an extra wire for thermostats that require power (or the “C” wire), which you can read here, and it shows you another furnace configuration:
Step 4: Remove the cover from the control board.
There is a small panel with one screw on the front left and one screw on the top left:
Once you remove the two screws, the cover tucks into the furnace in two places as seen on the right of the picture above. Careful lift up on the left side and untuck the tabs and remove the cover to expose the furnace electronics:
Step 5: Locate the thermostat terminals on the lower left:
The W to white wire, G to green wire, R to red wire, and Y to blue wire matches the connections on the thermostat, so we have that correct. So, it turns out that the wiring wasn’t the issue.
Step 6: Check the thermostat wires for an open circuit.
I didn’t perform this step because my old thermostat still worked so I felt like there wasn’t anything wrong with the wiring. But if you thought you had a broken wire you could check your thermostat wires for an open circuit by first carefully noting what color wires connected to what terminals at the furnace and then disconnecting them all. Then twist two wires together and then check the resistance of that “loop” from the other end. If one of the wires is broken you will measure infinite resistance. To narrow down which of the two wires is actually the open circuit you can switch the pair of wires you are checking and then if it shows a closed circuit you know it was the other wire.
Step 7: The conclusion.
I ended up sending the thermostat back after consulting an HVAC technician and feeling like my wiring was correct and thinking that the thermostat was bad. This thermostat also had dust behind the LCD display, which made me want to send it back anyway. When the second one came it had dust behind the LCD too but when I hooked it up, I learned that you need to give it more time while “Cool” flashes before the furnace actually kicks on. Well, you learn something every day and it wasn’t all lost since I can share what I learned with all of you.
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