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Matthew
Matthew, HVAC Technician
Category: HVAC
Satisfied Customers: 1994
Experience:  14 years experience in the HVAC field of service, installation, Manual J & D calculations
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my 5 amp fuse keeps blowing. the low voltage wires are clear

Customer Question

my 5 amp fuse keeps blowing. the low voltage wires are clear of shorts, so I don't know what to do next and it's 15 degrees
Submitted: 3 years ago.
Category: HVAC
Expert:  Matthew replied 3 years ago.

Matthew :

Hello, My name is XXXXX XXXXX X will do my best to help you today.

Matthew :

Check at the outdoor unit. You have to eliminate one part of the equation at a time. At the furnace take the wires off that come from the outdoor unit see it blows. If it doesn't then the problem is a wire to the outdoor unit or a part in the outdoor unit. Put the wires back and then go to the outdoor unit and unhook the wires there. If it doesn't blow then the problem is a 24v part in the outdoor unit. It it blows then the problem is in the wires to the outdoor unit. Same thing with the thermostat.

Matthew :

If you unhook all wires from the furnace and put a jumper from R to W and the fuse blows then the problem is internal to the furnace control board or transformer.

Customer :

i skipped to the jumper part to test the board first and it has been running for 10 minutes without tripping. Do you think it is determinable to eliminate this portion and move on to the next test?

Matthew :

you took the wires off from the thermostat and the outdoor unit and just put a jumper from R to W on the control board?

Matthew :

meaning you took the wires off at the furnace

Customer :

yes

Customer :

i went ahead and replaced the thermostat wires now and i'm testing the termostat with the furnace now. When I turn the "heat" mode on the thermostat, the furnace won't run. The furnace will turn on when I put the thermostat on the "fan" mode though.

Matthew :

Is this a gas furnace?

Customer :

no

Matthew :

Heat pump then with an electric back up furnace?

Matthew :

jumping R and W at the control board of an electric furnace is only going to energize the electric strips and the furnace not the outdoor unit. So it could still be that the outdoor unit is causing the direct short. It is a bit more complicated to track down with a heat pump and electric furnace then it is with a gas furnace and conventional outdoor unit.

Matthew :

Another thing with an electric furnace is that when you jump R and W you are running the blower call through the heat sequencer. When the thermostat calls for regular heat it energizes the outdoor heat pump and the furnace blower but the blower is energized through the blower relay in this case. So you see it could be this blower relay, it could be the outdoor unit, it could be the thermostat, it could be the wires. You have to unhook and hook wires one at a time at the furnace until you see which causes the fuse to blow.

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