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Re: [Phys-l] cooling hints



Now this makes a bit of sense!

But what if the air entering the AC is 90F. I would assume the air coming
out of the vents would be much lower that 15F below 90, but probably not
55F. So here is a case where the actual fact has been translated to a
different context where it is not appropriate. In other words under normal
operation the air is only chilled about 15F. Normal operation would be
input at slightly above comfortable room temperature. As I remember one AC
man claimed the air coming out of the vents should be around 60F anytime the
system is on. They often use a thermometer or an educated hand to check
this.

Of course this also depends on other factors such as the length of the air
ducts. Car AC systems may chill the air even more, as I can put my hand
over the vent and it feels extremely cold. Of course some of this is due to
the fact that the air is moving.

Actually this "misconception" shows how some student misconceptions are
born. They take observations in one place and then inappriately use them
elsewhere.

All this is good, but I wish it would tell me how to halve my electric bill.
I have an old very small house with 4 way exposure. This is great when the
climate is nice as we can open the windows or use a window fan. But for the
summer half of the year it is a deficit because it lets in too much thermal
energy. Walls and the attic have isulation installed by me with a great
deal of labor. Replacing windows to decrease insolation would probably not
be cost effective because the house with its poor foundation and shifting
soil is a tear down. Actually Houston seems to have 2 seasons, summer and
January. But then I came from upstate NY which before global warming had 2
seasons, winter and August. But one year we had snow in August.

John M. Clement
Houston, TX


The 15 degree difference does not pertain to indoor versus
outdoor. It
pertains to the indoor air temperature entering the
evaporator unit compared
to the cool air leaving the evaporator unit. This indeed
depends somewhat
on the outside temperature, but not as much as you might
think. It also
depends on the airflow speed. I generally see it run from a
15-degree
difference to a 20-degree difference.

That is, if the temperature of the air in the house is 75F, then the
temperature in the plenum entering the cooling chamber is
75F, and then the
temperature in the air plenum right after the cooling chamber
should be
between 55F and 60F. Note, the temperature at the grills
where the cold air
enters each room will be a bit higher.