Hi all! I've been reading the posts regarding fuses blowing in the secondary voltage circuits on Bryant furnace/blowers. I blew the 3 amp fuse in this circuit two weeks ago on my gas Bryant furnace. Repairman diagnosed the problem using the flashing led light (2 fast, 4 slow). He replaced the fuse and the problem was solved... for about a day. Then the fuse blew again overnight. I replaced it myself the second time but now it has been blowing every few days or so and this morning it blew and now every time I replace it, the new one blows immediately after the front panel is replaced and the system is turned on. I've checked the wiring that I can access (mainly the red and blue wires to and from the transformer) and everything appears to be normal. I have a new thermostat on order and will try that when it is delivered but could the transformer be bad? Do transformers ever have problems? The system is 8.5 years old and have never had any problems. I am a bit curious that it never had a problem, then it blew a fuse... then 3 days later again, then every few days and now every time. A definite progression/accelerating trend. I am a bit of an electrical noob. I can replace fuses and thermostates and check wiring but beyond that I am lost. Any help would be greatly appreciated!! Thanks in advance!!

Now that the intermittant is not an intermittant anymore, you have a couple of choices:

1. inspect wiring
2. Disconnect everything at the t-stat. Put furnace together.
If it works - not likely tstat. If it doesn't likely wiring.
3. Disconnect at furnace (determine if wiring)

There is a post about a light bulb trick that can allow you to trace a short. You basically replace the secondary fuse with two automotive light bulbs in series that draw slightly less than (40 W/24) amps. Disconnect wires until the light goes out.

Awesome! Thanks for the fast response! Will try those suggestions. I've had someone suggest that I put a slow blow (delayed) fuse in just in case it's a momentary amperage surge but I don' really like that option. I figure that the 3 amp fuse is there to protect stuff and allowing more than 3 amps to go through for even a few seconds longer probably isn't too prudent. I'll try your suggestions and report back... Thanks again!

Here is the thread.

Hi,
When my wife found your posting, I was surprised to see how similar our situations are. My gas furnace, Bryant model 373LAV, a horizontal unit in the attic started with a blown fuse 3 nights ago. I first thought it was a bad thermostat, so I switched a new thermostat replacing the builder stalled Honeywell T8000 with a digital one. Nothing improved. Then I found the blown fuse and switched it out. Heater worked for a few hours after that, but didn't last through the night. I then replaced the fuse again and just monitored the operation. It seems that after the flame goes on for 20-30 seconds, the fan kicks on, and that's when the gas shuts off and the whole cycle starts again. I suspected low air flow, so I inspected the filter and it was dirty. But after replacing the filter and another fuse, the heater worked for a few more hours, then it burns out the fuse again.
I too was tempted to put in a 5 amp fuse, but am hesitant for the same reasons that it might cause more damage to the unit. I suspected wiring touching each other at the thermostat end when I replaced it. I went back and inspected the wires carefully to make sure they are not in contact, and I actually put the old thermostat back.

I think my next step is to call the tech support for Bryant and see how far I can get. If you get some resolution to your problem, can you email me and let me know? Or post your solution? I will certainly do the same. Thanks.

I can't find my post, but the idea is to get two 12 V light bulbs wired in series that draw about 40 VA/24 Amps. I just forget the lamp numbers. I can find them I guess.

Put this where your fuse goes. When the system shorts the ight lights. It's a current dependent resistor.

Then the next step is to wiggle wires to see if the light lights.

I'm not sure what your thermostat wires are or if it's a heat pump.

Basically shorts at the thermostat end except for the common wire might be rare.

Short going to the outside unit is definitely a possibility.

Give me your thermostat wires that are used, I'm particularly interested in the use of the common wire.

I'm also interested as to how long it takes to happen and what modes it happens in.

Does it happen in fan only, heat only or heat and cool?

We can go from there.

Remove all t-stat wires from low volt terminal board and use jumper wire to see if short occurs while cycling unit with jumper.

Hi guys, thank you very much for your input. I couldn't really find any obvious wire problem, and frankly just don't have enough time/energy to climb into the attic (where the furnace is) to try the various thing. I called a heating/cooling tech, he came out today and informed me that it was the transformer. He said something about the fuse was to protect the secondary circuitry and it's the primary circuitry that was shorting out the fuse. He will come back on Monday to replace the transformer. I just hope it won't cost me an arm and a leg. And so, thank you all very much for your input.

If he tells you he blew another transformer and wants to charge you for two of the watch out. LOL It has happened before.

Sorry, need to use jumper with inline 3 amp fuse when doing the low volt test, or a mini 3 amp breaker., thanks 1000.

If true, a primary to secondary intermittant is a hard one to find unless he got lucky or was able to use a megger on the transformer which would have found it easily.

Did the new transformer take care of the problem? I have the same symptoms!

HI, so did the transformer fix your problem?

I'm still replacing fuses once a week. Was waiting to see if it helped "KISS" before I replaced the transformer.
Sorry -
Jeff

A low voltage short can come from anywhere. Its common for low voltage wire to rub into the refrigerant lines on the A/C and short out and be intermittent as the system vibrates. Is your short after you run the heat or the A/C?

Seems random, don't usually notice it until the house gets cold and I go looking for the problem. I'll check for some abraded wires though and see if I can find anything. Thanks

J

So it sounds like you notice it when your trying to use your heat? If so you may have a gas valve going bad. Not knowing What type of system you have makes it hard to answer. Not to discourage you from giving me more info but its like you trying to guess what number I'm thinking of from 1 to 1000

No problem. I'll cycle it a couple times and see if I can force the issue and find out when it occurs. I also have a scope I can look at the transformer output with and compare to one of my other zones that are working to see if the output voltage is clean and on target. Appreciate your help.

If you have a zone system it's a whole new ball game of what to start checking for the short. Any damper, limit or thermostat in heat or cooling mode could have the short.

Did replacing the transformer fix the problem? "rclee71"