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Matthew
Matthew, HVAC Technician
Category: HVAC
Satisfied Customers: 2014
Experience:  14 years experience in the HVAC field of service, installation, Manual J & D calculations
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3 amp fuse blown on furnace control board. What can be done?

This answer was rated:

Furnace blows 3 amp fuse on control board after several years of operating just fine. Can you help? Thank You.

Matthew :

Hello, My name is ***** ***** I will do my best to help you today.

Matthew :

A low voltage short is a lot like finding a needle in a hay stack. First off, does it blow the fuse every time immediately on start up or is it random?

Customer :

Hi Mathew, it blows the fuse every time at start up!

Matthew :

good that will make it easier

Matthew :

You have to eliminate one possible source at a time until you find the short. The first thing to do is take all the low voltage thermostat wires loose from the furnace; both the wire from the thermostat and the wire from the outdoor unit.

Matthew :

Take a jumper wire and place it from the R (red wire) to the G (green wire) on the furnace and see if the fuse still blows.

Matthew :

If it doesn't blow then you know it is nothing internal to the furnace.

Matthew :

Then, take and hook the thermostat wire back up but this time leave the outdoor unit wire loose. If it doesn't blow with the outdoor unit off line, then you know the problem is in the outdoor unit or within the wire leading to the outdoor unit.

Matthew :

If this is the case, then hook the outdoor unit wire back up to the furnace; then go outside and unhook the wire from the outdoor unit. Fire the system up again and if it still doesn't blow the fuse, then the problem is the outdoor unit contact/24v connections. If the fuse blows, then you know the wire to the outdoor unit is bare and touching something somewhere from point A (furnace) to point B (outdoor unit).

Matthew :

If the fuse still blew when you had the outdoor unit off line then you know the problem is the thermostat or the wire to the thermostat. So same situation, take the wires loose from the thermostat's sub base and twist the red and green (or yellow) wires together to see if the fuse still blows. If it doesn't blow, then the problem is the thermostat. If it does still blow, the problem is the wire itself.

Matthew :

Make sense?

Matthew :

If the fuse blows when you have no low voltage wires hooked to the furnace then you have an internal problem like a bad limit, gas valve connection or control board. It isn't the HSI, inducer, or blower motor as these are all high voltage parts, not low voltage.

Matthew :

have lots of fuses on hand and just start testing one thing at a time

Customer :

There is no outdoor unit on the furnace; however there is a humidifier in the system on the low volt side. I will disconnect it also and see what happens.

Matthew :

If they tied the humidifier into the furnace controls (using the furnace transformer) then that is possible. Most generally the humidifier should have its own 24v transformer and the switching device is a current sensing relay or a sail switch. If they used the C and W terminals of the furnace as the on off for the humidifier, then yes, it could cause the fuse on the furnace to blow, provided there is a problem with the humidifier or the wires to it.

Matthew :

If you are using only heat, then when you unhook all the low voltage wires from the furnace, jump the red and white wires that will fire up heat.

Matthew :

same at thermostat, when you take the wires off the thermostat to test.

Matthew :

you can simply jump the wires to each limit together thus bypassing the limits in the furnace to see if they are the problems.

Customer :

Thanks Mathew! After I get more fuses and perform the tests and if I still have a problem, can I contact you for more answers if I click on accept at this time?

Matthew :

sure I will be off and on all day so I will try to check in when I can.

Matthew and 3 other HVAC Specialists are ready to help you