Help me fix this (air handler with heat strip) - blows fuses

[oldnslo]

I thought we had a help me fix it sticky, but it was probably the old site where we nuked a number of child threads. Anyways....

(If someone cares to connect on the phone regarding this, please PM me your number)

It's my 30 y/o Bryant attic mounted air handler. Just started by blowing a fuse. Otherwise had no issues for 30 years.

1. Replaced the fuse, system ran and heated but short cycled > blew fuse > replaced
2. Tested cap > ok
3. Replaced fan motor on outside advice (single speed) > old one spun ok, didnt smell, test results didnt match Youtube.
4. System ran and heated but short cycled > blew fuse
5. Tested heat strip, visually inspected, replaced both heat thermostats because of inconsistent connectivity across terminals
6. Unable to fully test heat sequencer (27v in, couldn't get it engaged) > replaced
7. System ran and heated but short cycled - at this point I stopped before blowing another $10 fuse
8. Bypassed the wall T-stat, jumping red to white - system ran and heated - DIDNT short cycle - ran for 20 mins - blew fuse
9. I've got a full 240v to the unit, and the transformer and 27v out and down to the wall T-stat.

Argh! HVAC folks, what am I missing? I normally dont throw parts at problems without serious diagnosis, but this (seemingly) simple "too many amps causes my fuses to go bye-bye" is getting to me!

I've got a Greenleee clamp amp probe, DVOM, etc.

What kind of conditions are present to surge past 60 amps? I'm thinking I need to pull the breaker and look for bad news at the terminal connection.

What say ye?

[bonneyman]

Yeah, could be bad disconnect where the fuses plug in. If you had a direct short the fuse would blow immediately.

You're probably able to check the internal guts. Turn off power at main circuit breaker panel so you can work on attic disconnect safely. Pull form cover off, inspect everything in there for any signs of heat stress. (Discolored copper components, melted or burned looking wire insulation, etc). Check tightness of screws holding the supply wires into the terminal block. Loose connections - or those that become loose due to long use (i.e. repeated don/off cycles cause heat and cooling which does tend to loosen connectors over time).

Put an amp clamp on the thing and turn it on. See if amps stays real close to the max rating or if it's normal then spikes after the short run time.

Just some things off the top of my head.

[oldnslo]

Thanks. All those visual checks were done early on.

I'm back from the visual inspection of the main breaker, no issues, all tight, right volts.

Back from the attic, the fan is running within normal amp range both startup and run. Goes from 4.8a start to 2 or 2.8a run (depending on the leg).

The heat strip is delayed a good 20 secs before coming on, then the one circuit (there are 2), ramps up to 21.5a, then the other kicks in 20 or so seconds later (same reading) and combined they draw 42.7.

Thus, I have not detected a huge amp draw. Stumped.

[Jamesyarbrough]

ok what kind of system? heat pump or ac and electric heat? what fuse is blowing? where is it at? I side or outside?

[Jamesyarbrough]

is this a fused heat kit or a separate fused disconnect? are there any fuses or breakers on the face of the airhandler?

[oldnslo]

Ground mounted AC with attic air handler having a 10kw heat strip. Dual fused (60a slow blow) disconnect 3 feet from the air handler. Circuit breakers on the main box outside.

I have it diagnosed this far as being the fuses are blowing from heat not amps (my theory). In using a infrared digital thermometer one of the 4 terminal ends (each end of the fuse) start with an ambient 87d (Phx, inside roof temp) and ONE terminal will rise to 117d before I shut it down (to avoid another fuse blow), this takes about 10 mins or so. Took the disconnect box out, ensured the connections were tight (pullout to box) and reinstalled. Same results.

I get heat, and fan, so life is good until the terminal lug (probably transmits heat to the slow blow fuse) and then silence. I noticed the 3' run of wiring from the disconnect to the air handler is "warm", which concerns me (not hot), but I'm thinking this should not be warm to the touch.

My question of the day would be, if the amps are in check, why the heat rise? Using Ohms law, I guess I need to start deeper resistance (ohm) diagnostics downstream of the disconnect box.

[walrus]

I'm in MAine so I know diddly squat abut AC but is there anything else that turns on that you aren't seeing on amp probe? Can you check amperage at main panel before fuse blows?  What kind of fuse is it? Is just one leg blowing every time?

[Jamesyarbrough]

I think ur on the right track. if it were amps it would trip the breaker first. it's heat. what size wire from the disconnect to the air handler? should be 6 awg or bigger.  it's gotta be a loose connection causing it to heat up. if it were me id change the disconnect. get a cheap non fused pull out type and see what happens

[oldnslo]

Climbing back down from a morning jaunt up there, I'm close to giving up.

The feed wire from the breaker box to the service box is 6g AL wire, marked XHHW (yes aluminum). I've read how that was problematic with homes built in the 70's (as was mine).

I rechecked the resistance and amps on the 10k heat strip, all is in order. Total draw is fine at 43a (both at the elements, and the feed to the unit itself). The sequencer works as it should with bringing both elements online at the right times and cutting out when powering down.

I'm back to scratching my head at the heating up of the terminal ends on the breakout box, and this morning noticed for the first time the supply side of the box (term ends) are heating up as well. I didn't experience that yesterday. I was more prescriptive this time with measuring the timing of when the terminal ends heated up.

The problematic terminal end (lower right side), which is after the fuse, went from ambient 55F, to 100F after 10 mins. This is a gradual rise (65d after 40 secs and continues upward). The line in side, didn't heat up yesterday but today rose to 116F after 10 mins when I shut the unit off at the wall thermostat. No fuses were harmed in this testing. The one change I made today was to test with 50a slow blow fuses (which were extras I've had) but because I am running less than 50 amps, I didn't feel that would impact the testing.

Online searches found a similar (nearly exact) circumstance as mine, but the root cause was not in the thread. All of the forums for PRO HVAC folks will have nothing to do with DIYer, and I've gone that route only to be shown the door.

I don't know what to test next, other than to share my results with someone more schooled than I, and pay to solve the problem.

[walrus]

Can you unhook one of the heating strips and run the unit for awhile? Aluminum wire that caused issues was small, like #12 .

[oldnslo]

Thanks Walrus.

Outcome = half the amp draw, and a slower buildup of temperature on the same terminal end that seems to get the brunt of the heat.

The temp still climbs from ambient  86F to 92 in 6 mins with the one heater strip disconnected, and retesting with the second strip disconnected ambient went from 92F (didnt full cool down in this time) to 113F in the same 6 mins.

Amp draw was the same on either coil whether out of the circuit, or in the circuit (allowing for the second coil to kick in), and line amperage was 23.7a with either 1 of the 2 coils connected.

Fan on delay was normal, and heater strip on delay was normal.

[slip knot]

From what little I recall from Al wiring is that the connections are really prone to issues. it sounds like you may have some resistance in a connection causing the heat buildup.

[walrus]

Can you follow all the wiring in the air handler. Maybe there is a splice somewhere besides the terminations? Where you are measuring the temp, you have cleaned those connections? I take it no aluminum wiring in this unit, the aluminum is just the supply from main panel.

[oldnslo]

Aluminum 6g only on the supply side as noted before.

Took the fuse box out yesterday, terminals were fine, but crimped slightly to add more pressure in case there was resistance between the blades on the fuse holder, and box. Cut back the ends of the alu supply wires 3/8" of an inch for new material to be in the screw down terminals. Terminals are clean and tight. Mounted things back up again. No change.

No unneeded splices.

Temp is measured on the screw connectors to the incoming and outgoing wires at the fuse box. Infrared temp gun. Multiple readings.

[Jamesyarbrough]

that wire and connections is rated to 140°f or 60°c.  your breaker isn't tripping, just fuses in the disconnect are blowing.

go get a $15 non fused disconnect and see what happens.  if that doesn't fix it bite the bullet and drop $100 on a service call.

[oldnslo]

I see what the non-fused disconnect is now that I look it up, and I appreciate the suggestion, but I'm still seeing that the temp rise on the terminals is problematic and truly should be solved. The disconnect is just going to take that same heat, and who knows what will happen until it gets tripped.

A service call at this point might be my best option. I have spent the weekend on this when I could be burning things up with my torch.

Much appreciated guys.

[Jamesyarbrough]

hold up. thinking back to my days in electrical I recall Al wires being found broken all the time. that was 12 awg but your 6 awg will be multiple strands. it stands to reason that maybe some of those strands broke in one of the wires and that's what's causing the heat up.

btw 6 Al is only rated for like 40 amps. not 60 like copper. that will cause it to run hotter as well which in turn could increase the probability of the partially broken wire theory.



just a thought

[oldnslo]

Folks, its been solved. I know you stayed up late at night waiting on the next post.

James had it. It was the box (aka fuse mount). I bypassed it with the big blue 6g wire nuts, and bingo-bongo, no heat buildup with Mr Digital temp gauge. Trust me when I say, there was NOTHING to indicate that fuse box was at fault (fuse saddle pressure, resistance, ohm reading, no corrosion, etc).

I'm off to find a replacement breaker box to replace it with to get me to square one.

Thanks all!

[ron350]

Glad you got it sorted out.

[slip knot]

Sometimes it takes a group effort.   Glad you got it worked out