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mac_1103

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So we had our first 90 degree day of the year on Monday, and of course one of my heat pumps conked out. The 240V/40A breaker for the compressor had tripped and could not be reset unless I turned the system off. Then it would trip again as soon as I put it back in cool mode and set the temperature below ambient. Since I can't get a tech out here until next Tuesday, I decided to check the capacitor because a) it's easy, b) they fail often, and c) I've heard that a stalled motor can mimic a dead short. Sure enough it was shot - read zero on the compressor side and low on the fan side. However, replacing the capacitor did not solve the problem. I'm thinking this might mean that the compressor motor is dead, but maybe that's just because I'm a pretty pessimistic guy. Any suggestions for other relatively simple diagnostics I could do while I'm waiting for a pro to get here?

BTW, the unit is almost 17 years old, so it wouldn't be the end of the world if I had to replace it.
 
If you're certain the thermostat is calling for cold I'd go straight to the compressor power leads and see if the juice is making it to the motor. Gotta be a start relay in front of the motor and those are known to fail as well...

Cheers!
 
Start simple, and just verify all the wires are intact and not grounding out somewhere.

Can you be in two places at once? If your breaker trips instantly when the compressor is called to run, there's a dead short somewhere. If there is a delay before tripping, your compressor is simply locked up (or bad capacitor, but you already covered that).
 
So we had our first 90 degree day of the year on Monday, and of course one of my heat pumps conked out. The 240V/40A breaker for the compressor had tripped and could not be reset unless I turned the system off. Then it would trip again as soon as I put it back in cool mode and set the temperature below ambient. Since I can't get a tech out here until next Tuesday, I decided to check the capacitor because a) it's easy, b) they fail often, and c) I've heard that a stalled motor can mimic a dead short. Sure enough it was shot - read zero on the compressor side and low on the fan side. However, replacing the capacitor did not solve the problem. I'm thinking this might mean that the compressor motor is dead, but maybe that's just because I'm a pretty pessimistic guy. Any suggestions for other relatively simple diagnostics I could do while I'm waiting for a pro to get here?

BTW, the unit is almost 17 years old, so it wouldn't be the end of the world if I had to replace it.
Does the fan spin?

Capacitors degrading over time is pretty much how they are designed (those caps are rated for 2000 hrs run time!). They are needed only to get the motor going (I/V phase thing). Once a motor is spinning, they have no purpose.

Regarding stalled motors, they do suck in many times more current as one that's running because there's no Back EMF that acts to limit the current through the windings, but BEMF only occurs once the motor is spinning. When motor is stalled, it's only the wire resistance that's limiting current, and they aren't designed for that. A motor can definitely not start with a bad start cap, so yea you can have a bad cap AND a bad motor winding - one leads to the other.

What setting was your meter set to when you checked the cap? If voltage, was it on AC or DC? Would be best to check it with a capacitance meter.
 
Can you be in two places at once?
Is that a rhetorical question? :)
If your breaker trips instantly when the compressor is called to run, there's a dead short somewhere. If there is a delay before tripping, your compressor is simply locked up (or bad capacitor, but you already covered that).
Yes, it trips instantly - with the breaker off I set the thermostat to call for cool and then go to the panel and try to reset the breaker; it trips before I can move my hand away. So yeah, I figure there has to be a dead short to ground somewhere.
Would be best to check it with a capacitance meter.
That's what I used.
 
Capacitors degrading over time is pretty much how they are designed (those caps are rated for 2000 hrs run time!).

Never quite sure how pedantic stuff like what I'm about to say come across.

Bear in mind capacitor life rating is generally given at rated temp. 85C is a fairly common rating. So, 2000 hr @ 85C operation. As a high level generalization, life doubles for every 10C lower temp so caps (properly applied anyway) typically last much longer than what the printed rating might suggest.
 
Never quite sure how pedantic stuff like what I'm about to say come across.

Bear in mind capacitor life rating is generally given at rated temp. 85C is a fairly common rating. So, 2000 hr @ 85C operation. As a high level generalization, life doubles for every 10C lower temp so caps (properly applied anyway) typically last much longer than what the printed rating might suggest.
That's good info!
 
Just throwing it out there, occasionally breakers go bad. I've only ever had it happen on GFCI breakers however.
 
Just throwing it out there, occasionally breakers go bad. I've only ever had it happen on GFCI breakers however.
Very true. I had a new GFCI installed for a new outlet for my brew system. It tripped instantly, and while I didn't think the system was at fault, but it seemed odd that a brand new GFCI would be bad. And yet, it was. Everything worked fine after that faulty GFCI was replaced.
 
Reset the breaker with the thermostat off, then (carefully) manually press in the contactor on the evap unit. That would eliminate a coil short. If the unit starts up, it's the contactor coil. Also disconnect the fan and try to run it. That will eliminate the fan motor circuit.
 
Bear in mind capacitor life rating is generally given at rated temp. 85C is a fairly common rating. So, 2000 hr @ 85C operation. As a high level generalization, life doubles for every 10C lower temp so caps (properly applied anyway) typically last much longer than what the printed rating might suggest.
Well, this one lasted 13 years FWIW. The original one on the unit only lasted 4 years.
Just throwing it out there, occasionally breakers go bad.
Yeah, but I don't have a spare 240V/40A double pole breaker lying around. :(
Reset the breaker with the thermostat off, then (carefully) manually press in the contactor on the evap unit.
So pull the contactor, reset the breaker, set the the thermostat back to cooling and then re-insert the contactor?
 
Well, this one lasted 13 years FWIW. The original one on the unit only lasted 4 years.

That's statistics or perhaps part quality for ya. A bit different thing than the generalities of how capacitor life is rated.

The cap in our central AC is original, which makes it about 20 years old. Worked last year, I'll find out in a month or two if it still works this year.
 
...

Yeah, but I don't have a spare 240V/40A double pole breaker lying around. :(

...
I have no idea whether there is a bench test for that either. Sometimes electric stoves use that size but I don't want to send you down a rabbit hole. In my situation I knew the branch wiring for that specific room and the loads so eventually it made sense it was the breaker itself. But if nothing else fixes it something to consider.
 
Very true. I had a new GFCI installed for a new outlet for my brew system. It tripped instantly, and while I didn't think the system was at fault, but it seemed odd that a brand new GFCI would be bad. And yet, it was. Everything worked fine after that faulty GFCI was replaced.
Had this happen with my ac unit. The new gfci tripped and failed within a day of install.
Course my ac unit is cursed- had 2 pumps die a year apart. First was only 2 years old.
 
Well, this one lasted 13 years FWIW. The original one on the unit only lasted 4 years.

Yeah, but I don't have a spare 240V/40A double pole breaker lying around. :(

So pull the contactor, reset the breaker, set the the thermostat back to cooling and then re-insert the contactor?
No, don't pull the contactor out. Just manually actuate it in the same fashion the coil would when the thermostat calls for cooling. Most of them can be pushed into to the closed position.
 
No, don't pull the contactor out. Just manually actuate it in the same fashion the coil would when the thermostat calls for cooling. Most of them can be pushed into to the closed position.
This is how I got familiar with various 480V systems at work.

Just stick your finger in there and don't stare at the arc flash.
 
Doesn't the contactor coil typically run from 24V supply, current limited transformer part of the control system thermostat, pressure switches, etc.? Probably not likely that itself would be tripping a 40A breaker.
 
Doesn't the contactor coil typically run from 24V supply, current limited transformer part of the control system thermostat, pressure switches, etc.? Probably not likely that itself would be tripping a 40A breaker.
It's not the 24v side of the contactor that's tripping the breaker. It's the 230v side closing to run power to the compressor.

I'm not an HVAC expert, but I've seen one on YouTube.
 
It's not the 24v side of the contactor that's tripping the breaker. It's the 230v side closing to run power to the compressor.

I understand what the contactor does.

Previous posts are suggesting to manually actuate the contactor. The only thing that really eliminates is the coil, which I'm suggesting probably isn't the problem that's causing a 40A breaker to trip out.
 
Yeah, the coil wouldn't trip the 240 breaker. I'm throwing stuff at the wall here.

I have had breakers tripping on more than one unit due to very dirty copper pads on the contactors. When that was happening, I had every intension to replace the $20 part, but an emery board polishing kept the unit working for the rest of the weekend in a pinch. Obviously pull the disconnect before you do that. The reason I thought about manually pushing in the contactor is when I did that, it was enough pressure to keep the breaker from tripping but when it was actuated via the coil, it must have been less pressure and would trip within 2 minutes of running.

One of the most recent breaker tips I've had on a evap unit was a bad capacitor on the fan. It would have been easy to see if someone could turn the cooling on while I was standing there because the fan wouldn't turn, but I disconnected the fan hot wire and the unit would power up fine. Obviously you can't run it like that more than a minute.
 
OP said he tested the caps and at least one read zero F, which means it was shorted. If replacing that did not get the motor (compressor) running, then the bad cap likely dominoed into a compressor failure.
I know almost nothing about these things, but at this point I am definitely leaning toward the compressor itself being the issue. Although I was naively thinking that it was the other way around - that the shorted compressor motor had fried the capacitor. But yes, we will find out on Tuesday.
is your AC disconnect fused?
It does not appear to be, but I will take another look.
 
bad cap likely dominoed into a compressor failure.
One would hope the overload would save it - OP, have you checked for a resetable overload for the compressor? Or maybe fuses? (Not familiar with typical residential unit wiring.) edit: Tripped overload would trip breaker, so probably not the case.

But an aging stalled compressor is definitely a chance for failure.

OP: on the upside, the energy savings on modern heat pumps vs a 20yo system could be significant. Is it R-22 or R-134a?
 
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It's R-134a; 14 SEER. Upgraded from R-22 when the other zone failed in 2007. And that was a big energy efficiency upgrade. The other unit is 16 SEER because we also replaced the air handler for that zone at that time.
have you checked for a resetable overload for the compressor? Or maybe fuses?
Where would a re-setable overload be located? I haven't found any fuses but quite possibly because I'm not looking in the right places.
 
I know almost nothing about these things, but at this point I am definitely leaning toward the compressor itself being the issue. Although I was naively thinking that it was the other way around - that the shorted compressor motor had fried the capacitor. But yes, we will find out on Tuesday.

It does not appear to be, but I will take another look.
If you still have the caps, do a continuity test on them. Zero on a capacitance meter means it's shorted. On the other hand, if it read 0L, that means overload, which means infinite capacitance, which means OPEN ckt, which is what I would have expected. A cap that fails open will result in a motor that fails to start turning, aka stalled, which can result in overcurrent and failed windings, which results in a breaker that is tripping. LOL I've had a whole pot of coffee already today.
 
Where would a re-setable overload be located? I haven't found any fuses but quite possibly because I'm not looking in the right places.
Some compressors have a little reset button on the compressor itself.

But upon further thought, a tripped overload would not trip the breaker, so probably not the problem.
 
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If you still have the caps, do a continuity test on them. Zero on a capacitance meter means it's shorted. On the other hand, if it read 0L, that means overload, which means infinite capacitance, which means OPEN ckt, which is what I would have expected. A cap that fails open will result in a motor that fails to start turning, aka stalled, which can result in overcurrent and failed windings, which results in a breaker that is tripping. LOL I've had a whole pot of coffee already today.
Zero, not 0L and passed continuity. Have another cup. :D
 
One could probably pretty easily disconnect the compressor and fan from the contactor and see if the breaker still trips. Guessing it won't then, but?

If no longer trips and depending how the connections are and or how much you care to temporarily rig up, one might then reconnect fan and compressor, one at a time, and isolate which is causing the breaker to trip.
 
Should have noticed this sooner, but the thing has a diagnostic module. It's flashing code 6, which is "Open start circuit." Of course, there are probably at least a dozen reasons why the start circuit might be open...
 
We use heat pumps, so when it's cold we also get heat out of them.
This is a heat pump. Don't know about you, but up here you need a solid supplemental heat system. Which in this case is an oil burner that is as old as the house (i.e., 1967).

I think we'll be able to keep the inside coil and just replace the outside unit.
 
You probably can, but I think the internal coil often isn't terribly expensive and on a older install might be wise to do. Possible too, it might be a warranty stipulation too if you're buying a new outside unit.
 
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