Forced Air Furnace: No-Heat
Bad Thermostat Diagnostic Tree (Non-Communicating)
* These diagnostic trees are primarily for training purposes and may not cover all possible scenarios.
1) 24 V on W → C at the furnace control board?
Probe W and C at the control board. Expect ~24 VAC on a call for heat.
2) 24 V present between R → C at the furnace control board?
Probe R and C at the control board for ~24 VAC.
Go To: Verify Control Voltage Logic Tree → link
3) Is thermostat in HEAT mode with setpoint higher than room temperature?
Initiate call for heat, then click "Continue".
4) Time delay passed? Note: Some thermostats will go through a time delay of a few minutes after a power interruption. If a zoning system is installed, time delay may be as long as 5–10 minutes.
Wait for time delay and return to Step 1.
5) Check for 24 V at the thermostat R terminal (R → C). Note: If no C wire is landed on common, any other wire not being powered can be used as a common reference for your multimeter (all circuits go back to the transformer common).
6) Jumper R to W at the thermostat. Does the furnace run?
Jumper R to W at the furnace control board. Does the furnace run?
Not a thermostat problem.
Test W wire for break (OL) or resistance > 1 Ω.
7) Is this a smart thermostat or a battery-only thermostat?
Replace batteries & retry. Does the furnace run?
Problem resolved. Batteries were dead.
Is a common wire connected to C?
Verify thermostat is “power stealing” or add common wire.
If a power-stealing stat isn’t working reliably on this system, add a common wire (or approved power-extender) and retry. Does the furnace run?
Add a common wire between thermostat C and furnace C terminals, then retry. Does the furnace run?
Does the thermostat also have batteries installed?
Remove batteries and retry.
Some smart thermostats will not function properly even with a common wire if dead batteries are installed. Leave batteries out and retry. Exception: The Google Nest Thermostat (2020) budget model with the mirrored face uses two AAA batteries and may still require batteries to be installed even if a C-wire is connected.
Problem resolved.
Bad Thermostat. Replace.