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.

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).
  • Mini-Breaker Trips --> Step 4. Test Thermostat Wiring
  • Mini-Breaker Ok --> Step 6. Test Control Circuits

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.
  • Mini-Breaker Trips = Shorted Control Board
  • Mini-Breaker Ok --> Step 5. Test Thermostat

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"!