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KingPin461 03-19-2013 08:30 PM

5 Amp fuse blowing
Good evening guys, having a low voltage issue with my A/C unit. A bit about the system, it's a 3 ton Aire Flo (Lennox rebadge I think) split system with a gas furnace (no heat pump) Everytime I have the tstat on cool/auto, it immediately blows the 5 amp fuse on the control board in the air handler.

I initially thought the tstat was the issue (digital programmable Honeywell that has been problem free for +5yrs) so I switched it back to the original mercury Honeywell that came with the house and the fuse still blew. I checked the low voltage wires going the condenser outside, disconnected all power, removed wires from the contactor and check for continuity back to the air handler and everything checks out fine. No nicks, cuts, or rubbing against any metal outside causing a short. I replaced the contactor with a new one, thinking the low voltage coil was bad, but still blew a fuse.

Can the capacitor have an internal short causing the contactor to send excessive voltage through the low voltage coil causing the fuse to blow?

Besides the capacitor, the only other things I can think of now, are the transformer in the air handler or the control board itself. Any suggestions on how to properly test the transformer? It has three wires coming out and 2 (if I remember correctly go to the control board) I did try running just the fan (system OFF, fan ON) and the blower motor fired up, then I had my wife put it in Heat/Auto crank the temperature up and the furnace ignited just fine.

The problem occurs only when in Cool/Auto, as soon as the tstat clicks, it blows the fuse. The condenser fan/compressor never even budge. Any and all suggestions are much appreciated....thanks fellas

chitownken 03-19-2013 09:03 PM

Does it blow the fuse it you turn the fan to ON but leave the system set on HEAT?

HVACTECH96 03-19-2013 09:08 PM

Try removing Low voltage wire for condenser ,inside the furnace.R &Y.If fuse still blows , problem is inside the house.Usually find a short where low voltage wire exit the furnace cabinet, and has been rubbing on metal edge.If fuse doesnt blow the the problem is between furnace and condenser.Bare spots where critters have chewed wires is common.

jfrotten 03-20-2013 01:15 PM

You're over-thinking it. You have a low voltage short somewhere. Take the thermostat off of the wall. Take the red wire and touch it to each of the other wires one at a time for a few seconds and see which one blows the fuse. DON'T TOUCH RED TO BLUE. This WILL blow your fuse because blue is your common. Once you know which wire you're dealing with you usually have one or two spare wires like brown or black depending on your setup. Swap out the shorted wire and you'll be back in business.

hvac instructor 03-20-2013 09:07 PM

check the actual contactor. i had one that the coil was shorted to the mounting base. everything was fine untill the contactor pulled in.check for short on coil to the mounting base. i saved that contactor and use it in class.

KingPin461 03-27-2013 02:58 PM

Hey guys, sorry for the late reply...been outta town for a few days. I was able to fix the problem...:thumbup:

I decided to disconnect the low voltage wires for the condenser from the air handler control board to see if the problem was inside the house or outside. When I set it on Cool, the fuse didn't blow this time.

I reconnected the wires at the board and disconnected them outside at the condenser to isolate the contactor. This time, the fuse did blow again.

I ended up running a brand new line from the air handler to the condenser outside....problem gone. Wanted to thank all of you for your suggestions and input!

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