Normal A/C on / off cycle durations?

Is there a general rule-of-thumb for how long an air conditioner should run and how long it should stay off? I realize this probably depends on a variety of variables, but is there a rule-of-thumb. I've searched quite a bit and can not seem to find an answer.
If it helps, my house is 1 story, 2400 sq ft. I have the thermostat set at 78. It is 85 degrees outside, humidy 72%.
Right now, my a/c stays on for 8 - 10 minutes and goes off for 7 - 9 minutes.
Thanks for your help.
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glynndaddy wrote:

At those temps and TSTAT setting the unit is short cycling! Each time it starts a run cycle it takes nearly 5 minutes to reach its optimal cooling capacity at those temps and humidity levels.
Where is the RM TH located? Maybe it needs shielding from cool air streams, etc.
They should make a digital RM TH that has an on/off temp differential setting. You could then set it to cycle on at 79 and off even a low as 73-F. Use large floor fans to keep the air circulating as this will add to your comfort. Take a look at the "Human Comfort Zone Chart," on the linked page below.
The way most RM THs are now is that they have a cooling anticipator that aids in keeping the temperature very close to the TH setting. That is the wrong way to go if you want and need longer cycles to reduce humidity levels and increase the operating SEER levels.
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wrote:

yeah, what darrel said plus, my thermastat has a 1 or 2 degree differential setting. The 2 setting makes it run longer between cycles.
This time of year, in the hottest part of the day, a well designed system should be running just about all the time.
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78f 72% humidity is not a comfortable setting. Your thermostat might have a setting to increase cycle time, a longer time is also easier on the equipment.
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m Ransley wrote:

I believe the OP was referring to outdoor humidity, not indoor. Were it 72% humidity indoor I think the OP would be complaining of problems other than short cycle times.
Pete C.
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Thanks to everyone for your help.
Is a thermostat, TSTAT and RM TH all the same thing?
If a properly running systems run for long periods of time, doesn't that translate to high energy bills? What would be a reasonable ratio of on to off time?
I didn't want to take much of anyone's time, but I guess I should disclose that my system is not running right. It was installed in a new house four years ago. I've had to have the system recharged 3 times, the last time a few weeks ago. The previous two times fixed the short cycling, but not this time.
The AC company found a leak in my evaporator coil. I guess it needs to be fixed, but it's going to be expensive. I was hoping that the recharging would buy me some time, but i guess the leak has gotten too big.
Lastly, when they found the problem, the guy did not reseal the evaporator coil housing access panel. I just went up in the attic and noticed there is some cold air coming out. I guess i will get up there with duct tape, but I can't imagine it having that big an effect. There is also air coming out of my drain pipe (because there is no cap on it). I guess I will put some tape on that too.
Thanks everyone.
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NEVER USE DUCT TAPE! Use foil tape as it will last. Duct tape will fail over time.
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replying to glynndaddy, Denco AC - Carrier Dealer - 28 years experience- Licensed in Texas wrote:

A refrigerant leak will have a huge impact on run time. The lower it gets, the longer it will run. Adding more Freon will never solve the problem. A a/c system is like a car tire in that it is air tight. If your car has a low tire it is because it has a leak, adding air does nothing to repair the tire or the leak. The same rule applies to a a/c system. If your unit is 4 years old it should still be under warranty by the manufacturer and a repair would cost you labor and refrigerant, The coil would be replaced by the manufacturer. The leak will only get worse. And in time I running the system low on refrigerant will/can destroy the compressor. As for run time, the length a ac unit runs is determined by variables such as outdoor temperature, heat gains through windows and doors and the amount of insulation in your attic. These same variables determine the length of tine the unit is off. I just a had a conversation with a customer who's old system ran non stop, we installed a new system one ton larger and now it runs 6-7 minutes and is off for 14-16 minutes. So the run time is 18 minutes per hour instead of running 20 hours a day like the old undersized unit.
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JimL wrote:

No matter how hot it is, it can get hotter. I would say a well designed system should be running _most_ of the time; you never want to run out of reserve. If it's at a 50-60% duty cycle, it'll be effective removing the humidity.
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No,No. You DO want to run out of reserve. A well designed system should fall behind on the hottest day of the decade. It won't be able to keep up. Your thermostat will be set at 78 but the temp will creep up to 80, 81, 82...... etc.
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JimL wrote:

With global warming, the next summer it'll be 90, 91, 92 ...
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Thanks to everyone again for your help.
My builder has contacted the AC contractor who installed the system and they've agreed to replace the evaporator coil. I hope they do a good job. Last month my bill was $270 to cool 2400 sq ft to 79 degrees.
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wrote:

I was just looking at warranty period on Goodman coils and the standard warranty is 10 years on the coil with a new system and 5 years otherwise. Lifetime on the heat exchanger.
Good for you on getting a new coil. I suspect a botched install, not a bad coil.
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CJT wrote:

I know from BITTER experience that you do NOT want a AC system that runs from 10:00am to 10:00pm. Power bill DOUBLES. House is livable, but uncomfortable, especially when guests are over.
50% duty cycle, but no more than once an hour, certainly no more than once every 30 minutes. Any faster cycling than this is dangerous for the compressor.
A house that is 'adequately' plugged against air leaks, and has 'adequate' insulation should not have a duty cycle of greater than 50% and really should have a duty cycle of under 25%. Air leak plugs are the new hot button, now that most new homes have at least R38 in the ceiling and R24 in the walls, Course if we do manage to plug those uncontrolled air leaks, then the house accumulates odors (cooking, body odor - human and pets, solvents, cleaning fluids, paint, plastics......) and we must then install a controlled ventilator.
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wrote:

Wrong again.
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JimL wrote:

If it runs constantly just to stay even, it'll take forever to recover after a power failure.
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On Wednesday, July 19, 2006 7:56:34 AM UTC-4, Robert Gammon wrote:

Says who? If an AC cycling more than once every 30 mins was "dangerous", there would be a lot of broken AC's out there.

Really? Even on one of the hottest days of the year?
Air leak plugs are

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On Tuesday, July 18, 2006 12:21:05 PM UTC-4, JimL wrote:

BS. If it's falling behind on the hottest day in a decade, then it's also taking a long time to cool down if it's been off and you come home from a trip and probably not performing very well on a lot of days that aren't the hottest in a decade, but close enough. I want some decent reserve. I agree that you don't want it grossly oversized, because that causes problems too.
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