Knowledgebase

Gas Furnace - Blown Low Voltage Fuse

Article ID: 280
Last updated: 19 Apr, 2018

Gas Furnace

Re: Blown Low Voltage Fuse

The low voltage fuse provides protection for the control board and other low voltage components in the case of high amperage caused by a low voltage short or excess component load. 

► The fuse must be sized per the "VA" of the transformer.

  • Amps = VA / Secondary Voltage
    • 75VA Transformer: 75(VA) / 24(Volts) = 3.2 Amps

Too large of a fuse will cause damage to the transformer or control board in the case of a low voltage short.


Qty Possible Causes (From Most to Least Likely):
Mis-Wired Low Voltage
Thermostat Wires Touching Metal
Shorted Thermostat Wire
Shorted Pressure Switch Wires (A/C & Heat Pump)
Shorted Contactor (A/C & Heat Pump)
Shorted Reversing Valve Coil (Heat Pump)
Shorted Control Board
Bad Thermostat

1. Install Mini-Breaker

  • Turn off power to the furnace.
  • Replace the fuse with a resettable mini-breaker.
    • Requires two short pieces of wire, (2) female and (2) male spade terminals.
    • Remove the blown fuse from the control board.
    • Insert male spades into the fuse holder.

2. Check Wiring

  • Furnace
  • Inspect the low voltage wiring at the furnace.
    • Confirm that the wires are connected to the correct terminals.
    • Note the colors used for each terminal.
    • Make sure no wires have come loose.
    • Look for any spots where the control wiring could rub against metal (especially penetrating the cabinet).
  • Thermostat
  • Inspect the low voltage wiring at the thermostat.
    • Confirm that the wires are connected to the correct terminals.
    • Confirm that the colors correspond with the furnace wiring.
    • Look for any bare spots on the wiring touching metal or other wires.
  • Air Conditioner
  • Inspect the control wiring at the air conditioner.
    • Confirm that the wires are connected correctly.
    • Look for any bare spots that could touch metal (especially at cabinet penetrations).

3. Test for a Short

  • Turn the thermostat system switch to "Off" and the fan switch to "Auto".
  • Re-apply power to the furnace (it should be necessary to push the blower door switch in to get power to the transformer).

4. Test Thermostat Wire

  • Turn off power to the furnace.
  • Identify the wiring cable connecting the furnace to the thermostat.
  • Remove the Red wire from the furnace control board "R" terminal.
  • Reset the mini-breaker if tripped.
  • Re-apply power to the furnace.

5. Test Thermostat

  • Turn off power to the furnace.
  • Re-attach the Red wire to the "R" terminal of the furnace control board.
  • Remove the thermostat from wiring sub-base.
  • Reset the mini-breaker if tripped.
  • Re-apply power to the furnace.
    • Mini-Breaker Trips = Shorted Thermostat Wire
    • Mini-Breaker Ok = Bad Thermostat

6. Test Control Circuits

  • Test each furnace input (one at a time) by placing a jumper wire from "R" to the Control Input.
  • DO NOT JUMP "R" to "C"!
Wires Action Likely Short Reason
R to G Fan
  • Thermostat Wire
R to W Heat
  • Thermostat Wire
  • Bad Gas Valve
R to O (Heat Pump) Rev Valve
  • Thermostat Wire
  • Reversing Valve Solenoid
R to Y Compressor
  • Thermostat Wire
  • Contactor
  • Pressure Switch Wires

This article was:   Helpful | Not helpful
Report an issue
Article ID: 280
Last updated: 19 Apr, 2018
Revision: 9
Access: Public
Views: 126
Comments: 0
Also listed in