Blown fuse?

[DrJimmy]

Hi, my Google travels have led me here and I hope some of you wise ones can help. First of all I am not an HVAC guy by any stretch, but I do happen to be an electronics tech.

I have a Carrier 9200 furnace that's acting funny. Tuesday I came home from work and noticed the upstairs was hot, checked the t-stat and it showed that the a/c was on, but got nothing. Checked to see that the outside condenser fan was not running, the breaker for which was not tripped.

Called our LP provider for a service visit, next morning he came by checked everything out and discovered a blown 3A fuse on the fan pcb. He replaced it (even leaving a spare one) and all was well when my wife left the house (I wasn't home).

Came home after work, and nada, same thing as the day before. Went and checked the fuse and noticed he replaced it with a 5A fuse. This fuse was not blown, but still no condenser fan and the t-stat definitely showed a/c as being on.

Here's my educated guess as to what happened: The initial 3A fuse blew due to a possible power surge, the 5A fuse worked for a while but after a few cycles the extra current draw took out the CPU on the fan board (the fuse is tied to that chip).

Does this sound reasonable? They want to replace the fan pcb which is quite expensive.

Any ideas/insight/recommendations would be great!

Thanks for your time.

[beenthere]

You may need a better service company.
They need to eliminate the other possible causes before just replacing the board. Have they checked all the other componets.

Parts changers can cost you a lot of money until they change the right part.

[DrJimmy]

You may need a better service company.
They need to eliminate the other possible causes before just replacing the board. Have they checked all the other componets.

Parts changers can cost you a lot of money until they change the right part.

I'm not too hip on what other parts may need to be checked (and how to go about checking them), any suggestions?

The tech did spend a fair amount of time checking stuff, but again I wasn't home (if I was, I probably would've been right over his shoulder...).

Thanks.

[beenthere]

Not allowed to go into detail on this site.

Although it could be the board.
There are several other things that can cause it too.
Using a 5 amp fuse in place of a 3 amp is not a good thing to do. It can cause more damage yet.

[DrJimmy]

Not allowed to go into detail on this site.

Although it could be the board.
There are several other things that can cause it too.
Using a 5 amp fuse in place of a 3 amp is not a good thing to do. It can cause more damage yet.

Understood, sorry for any infraction

May as well just put a piece of aluminium foil across the fuse holder, eh?

Thank you for the assistance, much appreciated.

[bigdummy39]

Dr. Jimmy.......You need to find out why the 3 amp fuse blew in the first place. Your tech should have checked for the reason first before just changing the fuse......and replacing it with a 5 amp fuse WAS NOT the right thing to do. Before your tech goes and replaces an expensive board have him check to see if it's something simple like wire rub-through on a 24V wire somewhere.

[rojacman]

Dr. Jimmy.......You need to find out why the 3 amp fuse blew in the first place. Your tech should have checked for the reason first before just changing the fuse......and replacing it with a 5 amp fuse WAS NOT the right thing to do. Before your tech goes and replaces an expensive board have him check to see if it's something simple like wire rub-through on a 24V wire somewhere.

I just made repairs to a system that had animals chewing through hi exposed lv wire to his outdoor condenser. These wires should always be run through the whip , with the power and GROUND wires using # 16 thhn or equiv. The wires were shorting out , that cleared up the problem..........Jack

[DrJimmy]

I just made repairs to a system that had animals chewing through hi exposed lv wire to his outdoor condenser. These wires should always be run through the whip , with the power and GROUND wires using # 16 thhn or equiv. The wires were shorting out , that cleared up the problem..........Jack

Wow. Our tech found that someone had installed very thin wire in place of normal thermostat wire outside to the cond. assy. (it looked almost like telephone wire it was so thin), looked like a mouse ate through the insulation causing it to short and take out a 24v x-former. Problem solved!
Thanks for all the help guys!

[beenthere]

Much cheaper then just installing a board, and still having to find the short.