24V Thermostat Troubleshooting Quiz

Ten questions on tracing a call through the 24-volt control circuit. Pick an answer to see whether you are right and why. Related reading: The Code: controls.

1. During a cooling call you measure 24 volts directly across the contactor coil terminals, but the contactor does not pull in. The most likely fault is:

   1. The thermostat
   2. A failed contactor coil or a mechanically stuck contactor
   3. A blown fuse on the furnace board
   4. No power at the transformer

   With 24 volts across the coil, the control circuit is doing its job. If the contactor still will not pull in, the coil is open or the contactor is stuck. Check the coil's resistance with the power off.

2. A blown 3- or 5-amp fuse on a furnace control board usually indicates:

   1. Normal end of life
   2. A short in the low-voltage wiring or a shorted component
   3. Low line voltage
   4. A dirty filter

   The fuse protects the transformer. A nicked thermostat wire touching sheet metal or a shorted contactor coil will blow it. Find the short before you replace the fuse.

3. With the system calling, you read 24 volts across the two terminals of a condensate float switch. The switch is:

   1. Closed
   2. Open
   3. Shorted
   4. Disconnected from the circuit

   Across an open switch you read the full circuit voltage. A closed switch has almost no voltage across it.

4. The thermostat is blank and nothing runs. The first measurement should be:

   1. Compressor amps
   2. 24 volts between R and C at the control board
   3. Refrigerant pressures
   4. Static pressure

   No 24 volts means no control power. Check the transformer, fuse, door switch and the line voltage feeding the equipment before suspecting the thermostat.

5. With the low-voltage wiring disconnected, the transformer secondary reads 0 volts while the primary has correct line voltage. The transformer has most likely:

   1. Nothing wrong with it
   2. Failed
   3. Been overloaded by the blower
   4. Been wired backward

   Correct primary voltage with no secondary voltage, and nothing connected, means the transformer has failed, often because of a short in the control wiring. Find that cause before installing a new one.

6. Jumping R to G at the furnace board turns on the blower. This tells you:

   1. The board and blower respond, so the thermostat, its settings or its wire are the likely problem
   2. The blower motor is bad
   3. The transformer is bad
   4. The capacitor is bad

   Jumping at the board bypasses the thermostat and its cable. If the blower runs, look upstream. Only jump terminals when you understand the circuit.

7. Why should you never jump R to C?

   1. It turns on emergency heat
   2. It shorts the transformer secondary, which can blow the fuse or burn out the transformer
   3. It resets the thermostat
   4. It reverses the heat pump

   R and C are the two sides of the transformer's 24-volt output. Connecting them directly is a dead short.

8. A system shuts off and restarts repeatedly during a cooling call. Which low-voltage device is a common cause?

   1. A safety switch that is tripping, such as a high- or low-pressure switch
   2. The color of the C wire
   3. The filter size
   4. The thermostat display brightness

   Safety switches open the control circuit when pressures go out of range. Find out why it is tripping, such as a dirty condenser or a low charge, instead of bypassing it.

9. During a cooling call you read 24 volts from Y to C at the thermostat but 0 volts from Y to C at the outdoor unit. The problem is most likely:

   1. The compressor
   2. A break in the Y or C conductor, or an open safety switch between the two points
   3. The refrigerant charge
   4. The indoor blower

   The signal leaves the thermostat but never arrives. Trace the conductors, splices and any safety switches wired in series between the two points.

10. A smart thermostat with no C wire loses power and resets during calls. A common fix is to:

    1. Install a bigger fuse
    2. Run a C wire, or use an approved common-wire adapter
    3. Jump R to C
    4. Remove the batteries

    Many smart thermostats need continuous 24-volt power from C. Stealing power through other terminals can cause resets and short cycling.