[Nocturnal]

I have a Bryant 373LAV horizontal furnace and the 3 amp secondary voltage fuse keeps blowing. I swapped the thermostat with the same problem.

[hvac1000]

Check all low voltage wires and components for shorts. Usually the thermostat itself will not cause this type of problem.

[JimmyTheAirGuy]

I have a Bryant 373LAV horizontal furnace and the 3 amp secondary voltage fuse keeps blowing. I swapped the thermostat with the same problem.

Dear Nocturnal,
You've obviously have a shrunt or a short in your low voltage circuit.
What you will need to do is get an amp meter that can read milliamps and go through each cycle to determine your cause of failure. For example in the heating mode read the milliamps on your ignition, then if you have a humidifier read the amps on that electronic air cleaner etc. If you have air conditioning read the miliamps on your compressor contactor coil, and the indoor blower relay.
Meanwhile look for cuts and srapes in your wiring.

Goodluck and GOD bless
Jimmy The Air Guy

[Nocturnal]

Dear Nocturnal,
You've obviously have a shrunt or a short in your low voltage circuit.
What you will need to do is get an amp meter that can read milliamps and go thru each cycle to determine your cause of failure. for example in the heating mode read the milliamps on your ignition, then if you have a humidifier read the amps on that electronic air cleaner etc. If you have air conditioning read the miliamps on your compressor contactor coil, and the indoor blower relay.
Meanwhile look for cuts and srapes in your wiring.

Goodluck and GOD bless
Jimmy The Air Guy

I also swapped the control board with the same result. I am not sure but I think the problem only occurs when there is a call for cooling. My thermostat is set auto switch between heat and cool and while it was heating it ran overnight, but during the next day when the outside temps got warmer and the system called for cooling the outside unit wasn't on and the fuse was blown in the control board. My next area of interest would be the contactor, but I don't want to easter egg this anymore. What should the milliamps read? Any other suggestions? Thank you for you help...

[hvac1000]

You will have to check to see if the milliamp rate is listed on the contactor itself but before I would try and hunt that down I would check the low voltage wiring from the inside unit to the outside unit for scrapes or cuts to the wire. I would also manually put the thermostat unit in the A/C mode and see if that actually blows the fuse and confirm what you are thinking. In other words truly start to isloate the problem. If manually turning on the A/C does cause the fuse to blow then you know it is in the wiring to the unit or in the unit itself. Then you can disconnect the low voltage wire in the outside unit and see if it is the wire or the contactor in the outside unit.

I have always found that elimination is the best way to find these kinds of problems.

You still have to eliminate the yellow and green wires from the thermostat to the inside unit unless you have already done that. That is the cooling command wire Yellow and the indoor fan command wire Green. These two are activated by the thermostat when it goes into cooling.

Next question. Are these the original thermostats or have they been upgraded lately? This could also be a problem.

[Nocturnal]

Got the system fixed. I went to the outside unit, carefully inspected the low voltage wire, opened the high voltage breaker and removed the service panel, and I noticed that one of the low voltage wires going to the contactor was shorted to the wires that were cut off, (not used). There definitely looked like there was some arching going on there. After cutting out the bad section, taping off the unused wires and reattaching the wire, I performed a stress test on the system and it works great. I really have to attribute this problem to sloppy installation by the new construction contractor. I don't think I would have had this problem if the unused wires would have been capped off properly. Or at least the copper ends separated enough from the live wires.
Anyway, I could not have accomplished the repair without the help from you guys. The expertise and insight that you provided does great credit to yourselves and your craft.
Thank you very much.

[hvac1000]

Glad you got it going.

[JimmyTheAirGuy]

Atta Boy Nocturnal, Glad I could assist you.