Goodman GMT-070-4 Furnace blows 3A Fuse
I have a 4-year old Goodman GMT-070-4 gas furnace coupled with a U-47 coil and CLJ36 condening unit. The system has worked well in both heating and cooling modes until this winter. (Well, what passes for winter in Phoenix)
Upon starting the system in the heating mode it ran for about 30 minutes and then blew the 3A control fuse. Since this had happened once, about a year ago, I installed a new fuse from the box I had on hand.
The furnace started up, ran for about 20 minutes and blew the fuse again.
I checked the thermostat wires on both ends and relocated them in the thermostat itself thinking perhaps they were too close. I installed another new fuse, set the t-stat to off, and measured the control voltage at the fuse: 28.05 V.
I then started the indoor fan by placing the t-stat switch in run. The voltage dropped slightly to 27.95V and I let the fan run for about 25 minutes. I then set the t-stat for heat, the system started reducing the control voltage incrementally to about 25.6V as the various relays and sensors kicked in. The system ran for about 10 minutes and, you guessed it, blew the fuse. So, it appears that the fault is worsening either due to:
1) Repeated electrical stress, or
2) The temperature of the furnace increasing from ambient as I attempt to get it to continue running.
A visual check of the control board does not reveal anything out of the ordinary.
Has anyone encountered this failure mode before or have any ideas and/or suggestions?
If your blowing a fuse, you have a low voltage short circuit, which can be a real pain to find if you aren't experienced with this. Call a pro and cut the repair short.
If it runs for a time as you discribe, I doubt you have a short. I had a similar problem a couple summers ago. A 3 amp fuse kept blowing when a home owner would set the 'stat for cool. I was dumbfounded at first, but after I ran thru the routine of "process of elimination", it was determined the remote wire to the condenser was too long. After installing a 5 amp fuse, this problem never occurred again.
Try this. Don't worry, the transformer can handle it.
Eureka! I found it!!
In searching other forums (if I may mention that) I came across the following:
Q: "I have a 3 ton Goodman gas heat/elec air unit. When I turn on the heat in house after about 20-30 minutes it blows a 5 amp fuse. Had a service tech to the house it blew a 5 amp and a 3 amp fuse withing 45 minutes. He said what we have is known as a "ghost" he then pitched us for a new system, though he said our system is fine. It is now set at 66 degrees, with a 3 amp fuse in it, and it has not blown...yet...what can be causing system to trip the fuses."
A: "Look for where the limit switch enters the furnace. Remove the screws and remove the limit. Pay attention to how it sits in the furnace and bend it slightly away from the heat exchanger. I have found a few over the years that when they would heat up, the fan would push them and they would touch the heat exchanger and short out. It seam to only happen when their quite hot. I guess the metal weakens or something when hot"
This answer is courtesy of XENOS, webmaster on HVACMechanic.com.
Well, I pulled the limit switch on my furnace and found a very nice (if you can call it that) arc site on the top of the switch. See inside of left lead in photo. Apparently my limit switch was doing exactly as Xenos described.
Looking in the mounting hole I could see that the heat exchanger was directly above the switch by about 1 or 2 inches while there was probably 4 to 5 inches clearance below it. As Xenos suggested I bent the 7" leads so that the limit switch is about one inch lower than before. After reinstalling and buttoning up the furnace nows runs as long as it takes to heat the house!
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