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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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Furnace blows 3 amp fuse on control board after several years

Resolved Question:

Furnace blows 3 amp fuse on control board after several years of opperating just fine. Can you help? Thank You, "Obie".
Submitted: 3 years ago.
Category: HVAC
Expert:  Matthew replied 3 years ago.

Matthew :

Hello, My name is XXXXX XXXXX 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 in 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 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 sub base and twist the red and green(or yellow) wires together and 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 tot he 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 howerer there is a humidifier in the system on the low volt side. I will disconect 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 it's 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 if there is a problem with the humidifier or the wires to it.

Matthew :

If you are using heat only then when you unhook all the low voltage wires from the furnace jump red and white 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 problem.

Customer:

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

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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