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Old 01-17-2010, 02:00 PM   #1
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HVAC fuse


In my attic my HVAC system plugs into an outlet that has a TL 15 amp screw in type fuse.

Over the last 5 years the fuse has blow 3 times. The first time in the heat of summer, I replaced the fuse and a few days later it blew again. Thinking it could be a major problem, I call an HVAC repair person and they charge me $200 to replace the fuse again.

Today after about 2 years the fuse blows again. I'm planning to replace the fuse again, but am wondering what could be causing this?

I have a few vents closed in a few unused rooms, could the back pressure cause the system to draw more amps?

Please let me know if you room more info about my specific system.

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Old 01-17-2010, 02:18 PM   #2
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Is there a "nameplate" on the HVAC unit or label which gives electrical information for the unit? What does it say?

Or do you have the installation instructions for the HVAC unit?

What is the brand/model?

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Old 01-17-2010, 02:19 PM   #3
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One thing that the 15 amp fuse is plenty big however what size motor you have in the air handling unit ??

With squrell cage blower if you close up few dampers it will not affect the motor actally it do drop the power useage a little but not much unless you have mulit speed motor some case it may raise new issue.

Did the motor make a funny noise during start up and running mode ??

If you can able get into the motor itself do they have oil ports which you can add little more oil { one warning not TOO much otherwise you will have oil everywhere } and feel the squrell cage if feel moving back and forth alot or feel really loose then sound like bearing on the motor is going out.

Merci,Marc
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Old 01-17-2010, 03:30 PM   #4
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HVAC fuse


Quote:
Originally Posted by ejconcepcion View Post
Over the last 5 years the fuse has blow 3 times.
Today after about 2 years the fuse blows again.
7yrs x 12 mos/yr = 84 mos
84/4 = 21 months avg. fuse life.

Sounds like very occasionally there are transient overloads. Does the fuse element look just cracked, or is it splattered all over the glass?

You need to see if the 'clearing time' for that fuse*
http://books.google.com/books?id=a98...20slow&f=false
matches what is necessary for the HVAC system*.

*Ask the manufacturer.

If it does, the HVAC equip. could be failing or marginal or your house voltage could be high, continuously or occasionally.

Last edited by Yoyizit; 01-17-2010 at 03:38 PM.
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Old 01-17-2010, 04:02 PM   #5
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Thanks for the feedback. This site is a great resource and has an incredible number of helpful people.

My HVAC system is a Carrier Model 58ST, on the unit there is a label that reads max AMPS 9.8. I removed the side panel and checked that the blower spins smoothly and quietly, I didn't see any simple way to oil the motor.

The fuse window is completely blacked out so I can't see inside. I replaced the fuse with another and it's currently working normally. The blower sounds normal and is still very quiet.

If this is typical for a system like this, I'll be fine with replacing the fuse every so often.
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Old 01-17-2010, 04:20 PM   #6
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Quote:
Originally Posted by ejconcepcion View Post
The fuse window is completely blacked out so I can't see inside.

If this is typical for a system like this, I'll be fine with replacing the fuse every so often.
Take apart the blacked out fuse by wrapping it in many layers of newspaper and hitting it with a hammer or crushing it in a vise or grinding off the metal cap that holds the window.
Look for the element remains.
At this point it's the only clue.

Nah.
Borrow a clamp-on ammeter and see what the current through that fuse is. This problem may be stable for now, but it isn't normal, which means the problem is unknown. It may not even be in the HVAC unit.
This symptom may be like the canary in the coal mine.

Last edited by Yoyizit; 01-17-2010 at 04:23 PM.
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Old 01-17-2010, 04:23 PM   #7
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Could be loose conection in fuse holder.
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Old 01-17-2010, 04:46 PM   #8
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You mention blacked out window that mean something got shorted that is the only way you will know it have short if the window is clear but no power that mean you have overload there.

Just make sure you use the time delay fuse not the cheap one time fuse.

The time delay fuse work the best with motor loads so they can pass the harmless surge without blowing the fuse while the cheap one time fuse it will blow out during the surge of motor start up.

Merci,Marc

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