Carrier home furnace keeps blowing main 6-1/4 amp fuse


  #1  
Old 10-23-10, 05:00 PM
J
Member
Thread Starter
Join Date: Oct 2010
Location: USA
Posts: 6
Upvotes: 0
Received 0 Upvotes on 0 Posts
Carrier home furnace keeps blowing main 6-1/4 amp fuse

Been running fine all year for air conditioning. I put it on 'heat', and it ran fine for a few nights, then it popped the fuse twice in a matter of minutes. It doesn't make the fuse window black, like you'd get from a 'short'. It just melts it clean. I don't have an ammeter to measure the current flow.

It is a gas unit. About 95k BTU. Carrier. Model number 395BAW048080 1986

I pulled off the door and pulled out the blower. It turned freely (there is no belt on this model), but I still cleaned out all the dust from the vanes and the motor windings. Put it back together. Filter is clean. Blew the fuse again within a few minutes.

All the terminations of power, high voltage and control voltage, look good and normal. Nothing burnt.

Any ideas appreciated
 
  #2  
Old 10-23-10, 09:06 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
Is this the fuse in the fuse box, or a fuse on the circuit board of the furnace?


Usually I'd expect a 15 amp circuit breaker in a breaker box or a 15 amp fuse in a fuse box.

The fuse on the circuit board is typically 3 amps.


So what you say doesn't add up for me so far.
 
  #3  
Old 10-24-10, 11:16 AM
E
Banned. Rule And/Or Policy Violation
Join Date: May 2007
Location: Wisconsin
Posts: 7,826
Upvotes: 0
Received 1 Upvote on 1 Post
I am not familiar with a 6 1/4 fuse application either, but my guess would have it that your actual furnace switch has that fuse in it, under a hinged compartment, to probably not so much help protect the 15 amp wiring (protected by your panel box fuse) but to help more in actual protecting the furnace equipment. Actually a good idea, IMO. I've often wondered why there are not more such fuses for appliances in your house to possibly prevent more fires when in those appliances.

When the fuse blows, do you hear anything run first, that might be the cause of it? -like the inducer or blower motor?
 
  #4  
Old 10-24-10, 08:30 PM
J
Member
Thread Starter
Join Date: Oct 2010
Location: USA
Posts: 6
Upvotes: 0
Received 0 Upvotes on 0 Posts
It is the main fuse, as I referred to it in my title, under the hinged cover for the main power for the furnace. I've seen lots of these 6-1/4 ampers before. They are mentioned in this forum too. 15amp is usually only at the power panel for the breaker rating. I haven't seen any circuit board fuses over 3amp.

I did, however, increase it to an 8 amp fuse today since the nameplate on the furnace says '8.5amp max'. It ran fine for a test hour with the windows open, since it is too warm to run the heater today, so maybe it was just a fluke?

The first fuse blew with just the blower running. The second fuse blew with the heater on and blowing. After reading further online, perhaps I replaced the fuse too soon as several troubleshooting areas recommend to wait up to 30 minutes for the furnace to reset before trying to run them again. Maybe if I would have waited 30 minutes, the problem would have been gone?
 
  #5  
Old 10-25-10, 07:07 AM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
Based on your last comment, I'm supposing you have a circuit for the furnace coming from your breaker box controlled by a 15 or 20 amp breaker.

You then have a second fuse controlling power to the furnace only now with an 8 amp fuse controlling it.

Is that correct?


I've seen that kind of arrangement often enough, but it's relatively rare. Usually the furnace is the only appliance on a circuit and it's controlled by a 15 amp breaker at the breaker box.

I'm not an electrician. My limited understanding is that fuses and breakers are typically sized by the current carrying capacity of the wire, not the size of the appliance.

If you have 14 gauge wire, I would expect a 15 amp breaker and fuse for the furnace.

While it might seem logical to size the fuse according to the current needs of the furnace, it may well be that that's a mistake and causing the problem.

Personally, I think you need the opinion of an electrician on how that fuse issue should be handled. My opinion is that you are better off without that fuse there, and if you keep it it should be sized according to the wire gauge, not the furnace. But I might be mistaken.
'
 
  #6  
Old 10-25-10, 07:41 PM
J
Member
Thread Starter
Join Date: Oct 2010
Location: USA
Posts: 6
Upvotes: 0
Received 0 Upvotes on 0 Posts
There are many furnaces in my area set up this way. An electrician once told me the answer to the 15amp concern: the 15amp breaker on the panel protects the wire. Any fuse/breaker at the furnace protects the furnace. 15amps is too high of a trip for an 8.5 amp rated furnace. 6-1/4 is prudent, but maybe too conservative for a 25 ur old furnace. I'm thinking the 8amp furnace main fuse may be appropriate. 6-1/4 amp fuse has worked fine for 25 years -- I don't think that is the problem here .

Does anyone have any ideas as to what else could be causing the blown fuse in addition to any of the ideas I've presented?
 
  #7  
Old 10-25-10, 08:39 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
The usual troubleshooting method is to disconnect circuits in the furnace and add them back until you identify the one causing the fuse to blow. Add back just the motor, just the inducer motor, just the transformer and low voltage circuits for example.

(Also--- check the transformer ---is it getting hot indicating it's overheating?)


I don't really understand the logic of the electrician's explanation, but perhaps that proves I'm not an electrician.

Are there any other outlets or appliances on the circuit with the furnace? Having a separate fuse in that case might make some sense to me if that's the case.
 
  #8  
Old 10-25-10, 10:36 PM
F
Member
Join Date: Mar 2006
Location: NE Wis / Paris France{ In France for now }
Posts: 4,364
Upvotes: 0
Received 2 Upvotes on 2 Posts
I am from Electrical topic area and I will be more than glad to help you on this matter.

After I read the whole thing so it kinda dail down to couple items due you say the Air condting unit run just fine so more on gaz side there possiblty you may have shorted gaz valve or other items related to this side of it.

Is your furnace is constant polit or have interment polit unit or hot glow igntion ?? if latter two sound like something is shorting out just engough load to blow the fuse.

You will need a clamp on ampmeter to check the current drawage on those items I mention above.

I will not worry about the transfomer itself due it working fine so just contrated on the valve and polit system.

However if gaz valve is bad I rather defered to the HVAC contractor to deal with it for safety reason.

Merci.
Marc
 
  #9  
Old 10-26-10, 10:08 AM
J
Member
Thread Starter
Join Date: Oct 2010
Location: USA
Posts: 6
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks French277V. It is an intermittent pilot unit. I'll get an ammeter and check it out. Do you know how much electrical current the pilot unit should be drawing?

Carrier. Model number 395BAW048080 1986

Thanks again.
 
  #10  
Old 10-26-10, 12:05 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
Your furnace has a low voltage (24 VAC) transformer that controls the spark ignition system, gas valve and a lot of the controls, and I believe that's protected by a 3 amp (24VAC) fuse on the circuit board. So none of that is likely to be the problem.

That mainly leaves the inducer motor fan and the fan circulating air about the house and the 120 VAC wiring to the furnace, transformer and fans that's likely to be a problem.

If you have an AC ammeter to check out where the current is going that's certainly the best method. Otherwise, disconnecting those 120 VAC components and reconnect them to see when the fuse blows would be an alternative method of identifying where the problem is.
 
  #11  
Old 10-26-10, 12:10 PM
E
Banned. Rule And/Or Policy Violation
Join Date: May 2007
Location: Wisconsin
Posts: 7,826
Upvotes: 0
Received 1 Upvote on 1 Post
Originally Posted by jzer10
It is the main fuse, as I referred to it in my title, under the hinged cover for the main power for the furnace. I've seen lots of these 6-1/4 ampers before. They are mentioned in this forum too. 15amp is usually only at the power panel for the breaker rating.
Just so we are clear, that is what I said. So you have what I was talking about.
 
  #12  
Old 10-26-10, 12:15 PM
E
Banned. Rule And/Or Policy Violation
Join Date: May 2007
Location: Wisconsin
Posts: 7,826
Upvotes: 0
Received 1 Upvote on 1 Post
Originally Posted by jzer10
There are many furnaces in my area set up this way. An electrician once told me the answer to the 15amp concern: the 15amp breaker on the panel protects the wire. Any fuse/breaker at the furnace protects the furnace. 15amps is too high of a trip for an 8.5 amp rated furnace. 6-1/4 is prudent, but maybe too conservative for a 25 ur old furnace. I'm thinking the 8amp furnace main fuse may be appropriate. 6-1/4 amp fuse has worked fine for 25 years -- I don't think that is the problem here .

Does anyone have any ideas as to what else could be causing the blown fuse in addition to any of the ideas I've presented?
Is this a motor type time delay fuse? We don't know the normal amp draw of the furnace parts. To me, considering the higher starting amps, it seems that size fuse may be cutting things awfully close, IMO.
 
  #13  
Old 10-26-10, 12:24 PM
J
Member
Thread Starter
Join Date: Oct 2010
Location: USA
Posts: 6
Upvotes: 0
Received 0 Upvotes on 0 Posts
sorry for not mentioning earlier. It's a time delay type T fuse
 
  #14  
Old 11-01-10, 01:19 AM
F
Member
Join Date: Mar 2006
Location: NE Wis / Paris France{ In France for now }
Posts: 4,364
Upvotes: 0
Received 2 Upvotes on 2 Posts
I know it been few days so just want to follow up if you got this issue fixed or what ??

Merci.
Marc

P.S. Type T fuse is correct useage for motor loads.
 
  #15  
Old 11-01-10, 07:04 AM
J
Member
Thread Starter
Join Date: Oct 2010
Location: USA
Posts: 6
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks for asking. With the 8T fuse in there, I haven't had any problems, but it also hasn't been cold, so the furnace hasn't run much more than an hour total in the last week. Maybe I should open all the windows again and run it for a while.

I borrowed an amp probe, so I can check the amps going to the igniter now.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: