Generator and A/C Tip

Micrornd

Golden Member
Mar 2, 2013
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I don't know how many here have a whole house genset, but this could also apply to times when the power in your area "blips" or "browns out" sporadically. ;)

OK, here goes.
My genset ran it's normal "loaded" exercise cycle (same as it has for the last year).
When it transfers from "utility" to "genset", my central A/C senses the very brief loss of power and shuts down.
The A/C starts it's 5 min. timer for restarting (which is built into all central A/C units, I'm told).
It follows the same routine when the retransfer from "genset" to "utility" takes place.

But this time, the A/C did not restart after the retransfer to "utility".
So after waiting 10 minutes (for the 5 min. timer) I flipped the A/C breaker off and then back on (since the fancy high efficiency A/C has a logic board, I figured maybe it needed a reboot)
Still no go after 10 more minutes.
Tried that routine once more, with the same negative results.
Every time, the indoor and outdoor fans would run, but the compressor didn't.
Since a single contactor turns on both of the fans and the compressor, so something was screwy.
If the fans ran, then so should the compressor (after the 5 min. delay)

So since it was 95F today and will be 96F tomorrow, I called my A/C guy (who happens to be my nephew).
By this time it's already 7:30pm.
He advised me that he sees this all the time with Generac units.
When they transfer, they somehow confuse the A/C and it thinks it has been "short-cycled" and it locks the compressor out.
I reminded him that I have a Kohler, not a crap Generac and have had it exercising once every month for the last year and never had a single problem.
(His response was "OK, so?") :unamused:

He told me to turn off the breaker, and leave it off for about 2 hrs.
That gives the logic board time to completely discharge any power in it capacitors (we're not talking about the run/start cap, but the caps that filter power on the logic board), and that should release the "short cycle lockout".
Sure enough, when I turned the breaker back on after 2 hrs., the A/C started right up.

He said he uses the 2hr. time frame because it always works (when a "short-cycle lockout" is the problem), and while it may actually take less time, knowing exactly how much less, better safe that sorry.
When the A/C doesn't work people get irritated and if you stand around for an hour and it doesn't work when you turn the breaker back on, only to have you tell them you have to stand around for another 2 hours, well you can imagine the results.
BTW, his service calls are a flat rate, not by the hour, so that's not a factor in the time frame.

A/C's always seem to screw up in the evening or on a weekend, don't they?
Hopefully if this happens to you, this might save you an expensive service call, if you try this first. :)
 

Red Squirrel

No Lifer
May 24, 2003
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A/C units really don't like to be shut down improperly, kinda like computers. They especially don't like to be restarted immediately after being shut down, as there is still a lot of backpressure the compressor needs to start against. Sometimes this will make it stall which is very bad for it. I've seen it happen, it's not a sound you want to hear from an expensive piece of equipment. :eek:

I would make sure to set it up so that if the generator needs to turn on, the thermostat tells the A/C to turn off, and then the generator turns on once the A/C is stopped. I would also ensure that you give at very least 10-15 minutes before it starts again, but I would give more time. Let the generator run it's routine and when it switches back to commercial AC then have it turn A/C back on.
 
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WilliamM2

Platinum Member
Jun 14, 2012
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A/C units really don't like to be shut down improperly, kinda like computers. They especially don't like to be restarted immediately after being shut down, as there is still a lot of backpressure the compressor needs to start against. Sometimes this will make it stall which is very bad for it. I've seen it happen, it's not a sound you want to hear from an expensive piece of equipment. :eek:

I would make sure to set it up so that if the generator needs to turn on, the thermostat tells the A/C to turn off, and then the generator turns on once the A/C is stopped. I would also ensure that you give at very least 10-15 minutes before it starts again, but I would give more time. Let the generator run it's routine and when it switches back to commercial AC then have it turn A/C back on.


I don't know if there is any way to accomplish the A/C shutting off before the generator kicks in. The generator kicks in immediately when sensing a power loss. If it is possible, almost all thermostats have a delay timer built in. If I shut my A/C off, and then turn it right back on, my thermostat will NOT power it back on for 5 minutes, a built in safety feature. 5 minutes is plenty of time to safely restart.
 

Micrornd

Golden Member
Mar 2, 2013
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As I said in the original post, the AC thermostat is set to require a 5 min. restart wait period with any loss of power.
That means whenever the transfer switch changes position, such as utility company to genset or genset to utility company, the AC shuts off and waits 5 min. before attempting to restart.
This is both manufacturer and industry standard.
Additionally mine (and all decent central units) has a "high head pressure" switch, that prevents restarts if head pressure is above what the manufacturer has set as an upper limit.

Guys, that was the point of the post.
Even with everything setup properly and even with everything working properly for over a year, the "fancy" electronics on the logic board made a mistake and locked out the compressor from restarting (and my nephew says this is not uncommon).

The warning was meant so that, since all the symptoms pointed to a bad contactor, bad logic board or bad compressor, no one got ripped off by a bad or unscrupulous AC repair guy, since in almost all cases the first thing they do is try to start the unit, access the situation, then turn off the power and repair.
In a case like mine, turning off the power, for a longer than the generally needed period, was all that was required.
An AC repair guy could have replaced anything, assuming he did it properly, or replaced nothing and the unit still would have restarted and performed perfectly.
And the repair bill would have been either bogus or unnecessary.

All the newer central ACs have this "lockout" feature built into the logic board and it is not resettable by the consumer (some do have a jumper to reset on the logic board).
But simply turning off the unit, as above, will reset it.

That was the point, since the AC is "broken" anyway, before calling for AC repair, turn the power off to it for 2 hours (and you must do this by turning off the breaker, not by the thermostat) and see if that fixes it, saving that expensive service call and "repair" bill.
After all, we all know ACs only break at night, on weekends and only during the middle of summer, eh? ;)
 
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