On Mon, 17 Jul 2006 20:04:18 -0400, "Walter Cohen"
wrote:

Hello.
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,

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.

to not set the thermostat warmer in the morning and have the unit
struggle for hours on end

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.

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.

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.

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
me)

They are not just talking about window units.

And why would window units be different from central AC?

Thoughts?

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.


Thanks,
Walter