I live in NY.
I am wondering if it is better to leave a central a/c unit running during
the day when no one is home, either at the normal temperature or at a few
degrees warmer -or- use a set-back thermometer when leaving for work in the
morning and have the A/C unit start up again an hour or 2 before coming home
later in the afternoon?
I use a set-back thermometer and it takes my unit 3 hours of continuous
operation to bring the temperature down 8 degrees.
Some people have told me that it makes more sense, energy wise and cost
wise, to not set the thermostat warmer in the morning and have the unit
struggle for hours on end in the afternoon trying to get back to the
original comfortable temperature. Instead they say to leave the A/C on as
it would probably cost the same if not less to periodically cool an already
cool house instead of cooling a house that is not cool at all.
Con Edison says to turn off the A/C when no one is home but I think they
refer to window units (as they also say to turn the AC back on again via
auto-timer a half hour before returning - a half hour would do nothing for
Starting and stopping are not as efficient as running continuously. AC
units are designed to run continuously. Also the house will gain less heat
during the day as it is warmer so you will be removing less total heat.
When the house is unoccupied it will have no moisture source vs. when you
While its true that starting a compressor takes more energy than running it,
that surge of extra use only lasts for a second or two. Most residential
compressors are sized to meet the max load and it most definitely costs more to
run them continuously than to run as required.
High efficiency ACs have two small compressors. They run only one unless higher
cooling is required. This is the only time running continuously or near
continuously makes sense, but most people don't have these types of AC.
Again, we're not talking about whether the compressor can handle it, we're
talking what is the most efficient. See above.
Excuse me? That statement makes no sense. The issue is cooling loss from inside
the house. As the interior temps rise, there's less cooling loss. By not running
the AC during the day, you aren't losing cool air through windows, doors and
insulation gaps which gets replaced with warm air, which has to be cooled again.
I THINK that he really means TWO speed compressors. Heat pumps have
this feature too.
The only two compressor things I am aware of are ultra high end
refrigerator/freezers. Viking and SubZero have a separate compressor
for Refrigeration and for Freezing
Your answer is confusing. "less efficient to stop and start".
Well, most AC I've seen stop and start, unless you crank em down to
"house will gain less heat during the day when it's warmer". Well,
when it's warmer out, the house will gain more heat, compared to night
when it's cooler.
"no moisture source". Such as through the cellar floor, or around
doors and windows?
Also you didn't say if you suggested leave on, or setback.
Christopher A. Young
You can\'t shout down a troll.
My newsreader chops off stuff.
I agree that starting/stopping a central AC is NOT a problem, unless the
interval between a stop and a restart is too short, i.e. hard start
On balance, a house with adequate insulation and adequate air leaks
plugged will need little more than a bit of cooling and a bit of
dehumidification, depending on where the house is located. The rub is
'adequate' insulation and 'adequate' plugging of air leaks. Adequate in
one climate is inadequate in another. The concerns of someone in
Calgary Canada are quite different from those who live in Mobile
Alabama. With adequate insulation and sealing even on the Gulf Coast,
peak summertime AC expense for a 2000 sq ft home CAN be less than $100 a
Another issue is that with 'adequate' sealing of air leaks, odors can
accumulate in the home, so a controlled air exchange with the outside
air then becomes necessary. AprilAire's Energy Recovery Ventilator is an
example of such a product. www.ourcoolhouse.com shows one of these in
operation with minute by minute readings of temperatures and power
In your case it is a toss-up. All of my units are so oversize, it is
like when you check into a motel and in an hour it is livable. So in my
case a set-back works well. I have one thermostat that learns, if it
runs too much to make its goal, it will start earlier the next day
regardless of the set time for run up to normal. That one is a
Honeywell I believe.
I have one of those "learning" thermostats that will turn on the air (or
heat) ahead of time. So if I program it to be 70 degrees at 5pm, it will
turn on ahead of time so that at 5 pm it is 70 degrees. However its been so
humid here lately that I have been running it constantly.
I fear your installation is one that is not easy to say. I would guess
that there will not be a great deal of difference in the results either way,
some days you may win and some days you may loose. If it were me, I would
(and do) use a setback thermostat. Even setting it back a few degrees will
help Turning it off or allowing too much of a setback could cause it to run
for a long time during the hottest part of the day when it is less efficient
wiping out all or some of the savings of setting it back.
On Mon, 17 Jul 2006 20:04:18 -0400, "Walter Cohen"
CAn they explain that?
Your home loses cold (admits warmth), right? If it didn't you would
only have to cool it off once and you wouldn't have to run the AC
again until you opened the door to leave. So when would the warmth
on the outside most affect the cold on the inside? When it's the most
cold on the inside, right? If it were the same temperature on the
outside and inside, you wouldn't lose any cold, right, and the bigger
the difference in temperature, the more you lose.
The AC does not struggle. You're anthropomorphizing the AC. It just
does what it normally does, at the rate it normally does it.
For you it would be harder to somehow cool off your house if it were
hot in the house when you were doing it, because you get sweaty, but
it doesn't. If anything, it's easier for the evaporator to vaporize
the freon when there is plenty of heat in the house (I don't really
think it is easier. I think it is the same.)
OK cars sometimes struggle and they are not animals. But it's not
like a car that doesn't run well struggling to get up a hill**. The
AC isn't trying to go from 85 to 70 in one minute, and spinning it's
compressor harder than it ever did before, saying "Yes, I can. Yes, I
can." like a car might try to climb 2000 feet in one minute. The AC
just does what it does at the rate it does it.
**By which I mean backfiring, engine missing, faltering. A) I don't
think that happens anymore since there is computer controlled
ignition, and B) when it did happen it was because the car was being
told to do more than it could do. All that would be necessary is to
downshift to a lower gear, or if there were no lower gear, there
should have been, and the car wouldn't struggle, but it wouldn't get
to the top of the hill in the one minute allowed. ACs don't have 1st,
2nd, 3rd gear because there is afaik nothing parallel to that in the
AC world, and because they don't need them. They just cool as much as
they are designed to cool. It's like one workman who is told to
shovel the sand out of a child's sandbox, and another who is told to
shovel the sand out of an Olympic size swimming pool (open at one
end.) If the second guy goes at the same pace as the first guy, he
won't struggle anymore than the first guy does. That's what your AC
does. When the second guy feels frustrated, it's only because he can
think and he knows he won't be finished for a long long time. The AC
doesn't know from that.
Let's extend that theory. Say you went away for a year. Doesn't
their theory say that it would cost the same if not less to
periodically cool an already cool house instead of cooling a house
that is not cool at all.
I don't think these people can explain why they are right. I think
they are just speaking intuitively and their intuition is wrong.
They are not just talking about window units.
And why would window units be different from central AC?
Do you live in a house or apartment. How much insulation do you have
in your ceiling and elsewhere? These have maybe nothing to do with
your question, but they are 3 of my thoughts.
I believe you and Griswold got it right; so many don't.
The only thing I would add, which complicates things, is a
consideration of what time you turn the AC back on when you come home.
If you turn it on during the blazing hottest part of the day, the AC
has lots of work to do when it is least efficient--working against the
Well, sort of. A/C compressors are nothing more than heat transfer devices. As
someone else posted, they aren't working harder of easier based on what the
outdoor temps are. They consume the same amount of electricity.
A little refresher course: I'm making up the numbers, but the principles hold.
An AC compressor takes a freon like gas and compresses it. Basic thermodynamics
states that when you compress a gas it heats up. Lets say its 95 outside. By
compressing the gas it goes up 15 degress to 110. If it were 70 outside, it
would go up to 85. The 15 degrees from compressing doesn't change - that's basic
We run the compressed gas through an outdoor radiator and blow a fan across the
radiator. That cools the compressed gas back down to outdoor temp - say 95.
Bring the gas inside and through a heat exchanger, only this time we let the gas
Basic thermo says the gas gets colder, lets say by the same amount, so the heat
exchanger temp is now 75. By blowing the 90 degree house air through a 75 degree
exchanger, the house gets cooler and the cycle repeats.
Will the house cool faster if the heat exchanger is 50 degrees (ie outdoor
temp/compressed gas is 65)? Sure. Is the compressor running less? Probably. Is
the AC working easier? No. Its working the same.
Will you run the compressor more by leaving it on all day compared to turning
the thermo up while you are gone? Absolutely, because the house isn't perfectly
Will the house take a while to cool down? Sure - there's a certain amount of
thermal mass involved besides the air that has to be cooled. But that same mass
also acts as a brake to keep the house from heating up.
Sorry, not true. The compressor is a pump, pumping a fluid against some
head pressure. The simple fact, readily measured, is that this
pressure depends on outside temp. As Joe Meehan alluded to. Distilled:
high-side gauge pressure is proportional to ambient temp. Higher
pressure demands more power output of pump, thus higher power draw.
With a window unit and a "Kill-A-Watt" you can see this directly.
Excuse my ignoring the bogus physics/thermo. Rather, consider that an
a/c is pumping heat from one reservoir to another, and the cost of
doing that depends on the "vertical" (read: temperature) difference.
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