# FA4BNF024 HVAC Unit - Blows 5 amp low voltage fuse intermitently



## ArmyChief (Jan 25, 2012)

Dear fellow DIY techs,

I have a Bryant FA4BNF024 HVAC handler that blows the 5 amp low voltage fuse intermittently. Last time it blew, was May of last year. Now it has blown again. I replaced the fuse yesterday morning and it took a whole day to blow again. I have a degree in electronics and am home for the next week, so I thought I'd try and tackle this situation. Now, by observing the fuse itself, it is definitely a high current situation...no metal fatigue or slight over-current. The fuse is blown black..wide open. I Google'd the model number and found this handler has been sold under MANY names...thankfully I found the service manual. I placed a fused jumper in place of the fuse and a current probe, then waited to see if I could isolate the problem area. Wouldn't you know it....3 hours and no-blow. I ran the thermostat up to 90 to make the aux heat turn on..still nothing (yes, I have a heat pump..sorry did not say initially). Now, my Totaline touch-screen programable thermostat for the unit has been flakey in the past..goes blank, resets..even blacks out sometimes when removing the bezel. However, I cannot get ti to act up now. My thoughts were maybe the thermostat was blowing the fuse. I checked all wiring outside at compressor unit and at thermostat...no nicks, rubs or bare wiring.

Any guidance as to how to split the circuit, maybe install more fused wiring in each leg until I can determine for a fact what area or component is bad?

Last time it blew in May, I was deployed to Iraq...the young service tech my wife called just replaced the fuse and last..no testing. What I don;t want is someone coming out and "shooting from the hip" with my $$$. So, that is why I am attempting this....plus..it gives me a chance to learn how the control side of home HVAC works.

Thanks in advance,:thumbsup:

Doug


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## biggles (Jan 1, 2008)

focus on the items that have coils within them... definitly that compressor contactor outside in the condenser and the run out of the stat wires from the TR in the air handler.if your reversing valve is also 24Vs out there that and the contacor are the loads pulling on the TR thru that fuse.question.did you upgrade the present stat with the exsisting transformer typical is 40VA-50VA as installed? does the air handler have a control board in it,does the elctric backuo heat have a relay these are all loads.the sta won't blow the fuse or it would never control what the stat is calling in either heat or cool is the hit.might want to install a mini circuit breaker on the TR-R hot to replace the fuse.wire run outs like stat wire to the condenser draw on that VA from the TR.when they first close there is a amp spike but for milli seconds.shut the disconnect outside on the condenser and check the action of the contactor should be free in/out ny hang up it will blow a fuse trying to close....spritz a little WD40 on the slide bar ...but definitly consider a 3A-5A CB,or an upgrade on the TR to 60VA(if stat was upgraded) with a CB mounted on the 24V side #3TZ69 at local Grainger


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## ArmyChief (Jan 25, 2012)

Biggles,

Thanks for the quick reply. The handler and thermostat, one on wall or course, are original with the house..approx 2006. So, I can go look at transformer (what you call TR I think)..to see if it is marked with VA rating.

Air handler does have control board. Thermostat is not battery operated Nd is powered from the 24 VAC from transformer..through the control board which has the 5 amp fuse on it.

I've read contactors typically go bad in that the silver wears off and the metal fuses together...causing compressor to run all the time. I do know e outside unit comes on and off. With that and aux hear..I measured no more than 1 amp...even with glitch detect on. Like I said above...this fuse, when it blows...it is obliterated inside...burnt black.

I have stripped back the low voltage wiring outside going into the compressor assembly. Can I install a fuse in one of those wires to determine if a component (not sure what components inside the compressor/fan assembly outside draw from the 24 VAC). I see what appears to be a pressure transducer on the discharge line, a startup capacitor..what does the contactor look like?...what end item would it be connected too? I do not understand the WD40 reference..can you expand what it is testing and why?

I don't care how long it takes, since this is a upstairs unit, currently not occupied...I just want a good diagnosis and repair :yes:

Thanks again


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## Tator1076 (Dec 22, 2009)

You may have a bad t-stat. If you are 100% not seeing any chew up wire's then replace t-stat. I will start from there first. P.s. thank you for serving us over sea keeping us all safe here in the great U.S.A!!!!!!


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## Technow (Nov 12, 2010)

I had an FA4B that I had the same problem with.....could not find a short in any of the wires, or circuit boards or defrost boards.....The only thing I could find wrong was there was never a ground wire run to airhandler. I ran the ground wire to the airhandler and have never had another fuse blow......:surrender::surrender:


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## biggles (Jan 1, 2008)

the WD40 refeence was to give the contactor within the condenser a shot right on the pull in on the contactor.the compressor and fan are on the same contactor.the low pressure maybe hi side have sensors on them with wires that is all 24Vs either breaks the compressor when running in heat or cooling...TR is the transformer just didn't want to type it out...:wink: good point on the solder breaking down over the years even vibration from the fan.see those black relays on the control board they are your relays with 24V coils within.fusing the condenser isn't going to help you need it right at the TR as you have.the board is probably the biggest VA(voltamps) draw on that TR and it should have a VA rating on the TR tag.that nuke of the fuse is sounding like a dead short,and not a load break could be a wire rub out R (HOT) to metal.the trick is to get a routine going and slowly eliminate items..check the back of the control module for discolered and or the resister on the relay side.biggy there also is the reversing valve if it is 24Vs?..did you notice a TR in the condenser.the biggest give away if your blowing fuses in heating and cooling seasons either something in the condenser or the stat wire run going out there.keep this site http://www.bestbuyheatingandairconditioning.com/ handy and local Grainger for items


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## hvac5646 (May 1, 2011)

My two cents FWIW, cheek all low voltage wiring for the the slightest chafe.

I have found chaffing where conductors pass thru the wall. They can also be harder than the dickens to locate on conductors that are in the open.

like trying to trace the wiring diagram on an Army ANGRC/26Delta


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## ArmyChief (Jan 25, 2012)

@tator
I'm still thinking possible T-Stat...however, don't like guessing. Will keep it on the plausible list :thumbsup:

@biggles
Where is the reversing valve and what does it look like? Is it a electromechanical device...you say operated with the 24 VAC...by a relay?

Since there is a fuse on control board which is in series with SEC2 winding of Transformer...why can I not isolate the 24 VAC going out side to determine if issue is in handler to T-Stat or handler to outside unit?

Thanks again everyone


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## biggles (Jan 1, 2008)

the 24V going outside might not exsist if there is a TR within the condenser...goggle reversing valve for pix or youtube it.reversing valve swings the discharge superheated freon from the condenser outside to the indoor air handler coil that force is coil controlled from the stat check this youtube... http://www.youtube.com/watch?v=trBVt_SZ3o0


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## Tator1076 (Dec 22, 2009)

Is it running fine in Emergency heat?


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## Tator1076 (Dec 22, 2009)

FYI you can get a t-stat from lowes and if don't work you can always take it back.


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## ArmyChief (Jan 25, 2012)

Emergency heat works fine..and it has not blown all day 

Luckily this is not the main unit...so time will tell.

Thanks,


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## hvac5646 (May 1, 2011)

TIME TO CALL IN THE BIG GUNS....


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## ArmyChief (Jan 25, 2012)

Nah...you guys would fix the problem...but there'd be nothing left of the house when you were done !..lol :laughing:


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## Tator1076 (Dec 22, 2009)

Do you have a humidifier on this system? I still go with replace t-stat. Did you check t-stat from indoor unit to outdoor unit because I did find other day another company cable tie the t stat wire to bare suction line and wires had melted to cooper over time causing the short. Note did you had new siding put on the house maybe a nail hit it. But most case that will be 100% short


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## hvac5646 (May 1, 2011)

ArmyChief said:


> Nah...you guys would fix the problem...but there'd be nothing left of the house when you were done !..lol :laughing:


Jjust funnin' 'ya Chief. I'm former army. Balkan War.


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## biggles (Jan 1, 2008)

blowing that fuse as it does the item that is caising it has to be a wire shorting directly to ground.the stat,control board,relay,contact would constanly do it and sooner or later itself would cook.did you have that second transformer out in the condenser for the reversing valve.do you have a C wire terminal for the stat so you see 24Vs constant even with the stat off the subbase?


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## ArmyChief (Jan 25, 2012)

Tator1076 said:


> Do you have a humidifier on this system?


No sir



Tator1076 said:


> Did you check t-stat from indoor unit to outdoor unit because I did find other day another company cable tie the t stat wire to bare suction line and wires had melted to cooper over time causing the short.


Checked wiring..all appears good. No new work to home. Has been running two days now..no blow. Does anyone know if these Bryant Outside units use C/O valves that are in the heat or cool as default (no power applied)

Thanks,


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## biggles (Jan 1, 2008)

easy to find out next time running heat just go out to the condenser,and slip the wire off either side of the reversing valve coil.if it stays heating NO "whoosh" then it defualts in the heating mode no power.if it does swing when the wire is pulled then cooling is the default


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## ArmyChief (Jan 25, 2012)

biggles said:


> blowing that fuse as it does the item that is caising it has to be a wire shorting directly to ground.the stat,control board,relay,contact would constanly do it and sooner or later itself would cook.did you have that second transformer out in the condenser for the reversing valve.do you have a C wire terminal for the stat so you see 24Vs constant even with the stat off the subbase?


Did not check for transformer..but I don't see it on the service manual schematic. I will go look later...raining now.

Will look for a C terminal on stat and measure voltage.

Thanks :thumbup:


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## ArmyChief (Jan 25, 2012)

hvac5646 said:


> Jjust funnin' 'ya Chief. I'm former army. Balkan War.


27 years and going strong....it gets harder and harder to pass the APFT though


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## biggles (Jan 1, 2008)

no rush....but it will get narrowed down,is the stat wires out to the condenser just wire tied to the suction armaflex or thru a tinwall piping..bx with the line power?where are you located


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## ArmyChief (Jan 25, 2012)

biggles said:


> no rush....but it will get narrowed down,is the stat wires out to the condenser just wire tied to the suction armaflex or thru a tinwall piping..bx with the line power?where are you located


The low voltage wiring comes out alongside the suction line (wrapped in foam)...there are about 5-7 wires in the casing. That goes through a through hole in the bottom of the condenser unit up inside and connects to a few devices.

Located in Chattanooga, TN


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## biggles (Jan 1, 2008)

so the brown stat pull is outside that black armaflex on the suction piping...just check right whre the stat wire goes into the condenser opening vibration wear..maybe..just wondering


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## ArmyChief (Jan 25, 2012)

biggles said:


> so the brown stat pull is outside that black armaflex on the suction piping...just check right whre the stat wire goes into the condenser opening vibration wear..maybe..just wondering


I did find a few nicks in the outer jacket (jacket is really stiff and brittle..thinking cause of weather and/or UV). I believe the lawn maintenance guy might have nicked it while weed whacking. I cut back the jacket and found the yellow and white wire had exposed copper...but I did not see anything touching. So, I wrapped them up good with electrical tape. Then wire tied the whole harness to the suction line every 6 inches or so. Did that the first day...so far, no blow...maybe moving the harness, like ou stated...did something. One can only hope


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## bobinphx (Nov 25, 2011)

my training is that wd-40 should not be used on contactors. The oil evaporates and becomes a sticky sludge. Additionally wd-40 is flamable. open frame contactors spark and this could spell trouble. 

If the contactor is sticky, replace it. Chances are the points are pitted anyway. Of course you could file the points and then clean the slid points if you want, but I would replace it if its at all sticky.


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## ArmyChief (Jan 25, 2012)

bobinphx said:


> my training is that wd-40 should not be used on contactors. The oil evaporates and becomes a sticky sludge. Additionally wd-40 is flamable. open frame contactors spark and this could spell trouble.
> 
> If the contactor is sticky, replace it. Chances are the points are pitted anyway. Of course you could file the points and then clean the slid points if you want, but I would replace it if its at all sticky.


Bob, I think I agree...unless the contractor is terribly expensive, just replace every 5 years or so. What's your thoughts on replacing certian items every X years?

Also, any good Internet mail order sites for private label, discounted parts? I know things like run/start capacitors fail and you don't need the name brand replacements.

Thanks in advance :thumbup:


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## bobinphx (Nov 25, 2011)

I dont know any parts houses on the internet. I have never tried to find one. 

I do know that you will need to replace the part with one that has the exact electical charicatistics. In other words, make sure the points voltage and amperage ratings are the same (or better) and since you are having trouble with the 24vac fuse blowing, make sure that the coil on the contactor is at or below the amperage of the old one.... 

I think a google seach and some research is in your future!!!!!!! 

Additionally, I dont normally replace working parts... but I do try and protect the parts as I see fit. IE, I cover all wires that could be exposed to sunlight. I oil motors (where possible) a time or two per year. I keep an eye on water and corrosion too... I keep an eye on the various pressures and temps. Like static pressure and delta t of in comming and out going air... but then again, I worry a lot... LOL


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## hvac5646 (May 1, 2011)

biggles said:


> so the brown stat pull is outside that black armaflex on the suction piping...just check right whre the stat wire goes into the condenser opening vibration wear..maybe..just wondering


:thumbsup::thumbsup::thumbsup::thumbsup::thumbsup: that where i find most of my shorts cause some jack leg didn't install the right non chaffing grommet.


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## biggles (Jan 1, 2008)

and when it rains and the unit isn't running all it takes is some water to rundown or on that brown stat wire,and when it calls:huh:..never able to find a ground or shorts


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## ArmyChief (Jan 25, 2012)

That's why I'm thinking it might be fixed (fingers crossed). Three days now and still going...even through all the rain and wind. We'll see...I have a plan ready to isolate and determine leg


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## twelvevoltman (Nov 28, 2011)

If it's not hard to get to, I would replace the stat wire running between the indoor and outdoor units. You can get it at Lowe's.


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## biggles (Jan 1, 2008)

hate to ask again but is the bell wrapped stat wire wire tied to the armaflex or on the pipe with the armaflex around it....


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## bobinphx (Nov 25, 2011)

we had something similar with our thermostat wiring. it was raining out and water started to drip out of the thermostat!!! turns out the sun had eaten up the jacket of the thermostat cable and it was acting like a funnel and pipe!!! it ran water right down into the back of the thermostat!! Wire was replace the next day!!!.


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