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
Thanks for your help.
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
They should make a digital RM TH that has an on/off temp differential
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
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.
- udarrell - Darrell
Air Conditioning\'s Affordable Path to the "Human Comfort Zone Goal"
yeah, what darrel said plus, my thermastat has a 1 or 2 degree
differential setting. The 2 setting makes it run longer between
This time of year, in the hottest part of the day, a well designed
system should be running just about all the time.
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
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.
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.
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 e-mail address in our reply-to line is reversed in an attempt to
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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.
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.
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.
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
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.
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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