HvacTech2 wrote:

Hi meirman, hope you are having a nice day

On 12-Jul-03 At About 16:06:07, meirman wrote to All
Subject: Why is AC running in the morning when it's cooler outdoors?

m From: meirman

m In alt.home.repair on Sat, 12 Jul 2003 18:47:57 -0400 "HeatMan"
m posted:

m Yes, but I think it is harder, that is, takes longer, takes the
m removal of more heat, to cool humid air than to cool dry air.

This is true

m The maximum amount of water in the air is lower when the air is cooler.

This is where your theory goes wrong. To a certain extent cooling with A/C
does dehumidify. but as the air gets colder it can hold more humidity. Think
of it as condensing the air smaller. when you get too cold the air can hold
more "relative" humidity. the only way to reduce it at this point is to
reheat it and expand the air so the relative humidity decreases.

m So when the air gets cooler, some water has to condense out. But in
m the process of cooling that water vapor from X degrees as a gas to
m the same temperature, X degrees as a liquid, the gas gives up a lot
m of heat, heat that has to be removed by the AC.

m The one big question this leaves is: Is the relative humidity inside
m a a room with AC always 100% (or near there). I don't understand
m why it wouldn't be, but by personal experience I don't think it is
m that high. It doesn't seem humid but maybe that is just because
m it is cool.

m Does anyone know?

m I think the humidistat means that the AC will run even when it is
m below the set temperature, until the humidity is at the desired
m level. But iiuc, it does this by cooling the air some more and
m driving out the water. Maybe there is some arrangement of the cooling
m coils that reduces humidity more efficiently than it lowers temp.
m Does anyone know?

as explained above you need to reheat that air to dehumidify it.

-= HvacTech2 =-

.. "Why is the alphabet in that order? Is it because of that song?"- s.w.

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That's about as twisted an explanation as you can get. It's
much simpler to forget humidity and relative humidity and to
focus on the actual amount of water in air. The key is how
much water the air will hold, and hot air will hold a lot
more water than cold air. So as air is cooled, water
condenses out at some temperature. Relative humidity only
means how much water the air actually holds compared to how
much it is capable of holding at a specific temperature. As
air is cooled the relative humidity increases although the
actual water content remains unchanged until the temperature
is reached where the humidity is 100 percent and then the
actual water content of the air decreases as the excess
water condenses into a liquid.

The answer to meirman's question is that no, the relative
humidity in an air conditioned room is not always 100
percent. If you live in a arid climate the humidity in an
air conditioned room is seldom, if ever, 100 percent. In
our house, the relative humidity is usually about 60
percent. The relative humidity of the air outside is often
close to 30 percent. The reason we don't have 100 percent
humidity is simple, we aren't taking hot air with lot of
water content and cooling it. When we close our house and
start the air conditioning, the temperature is usually 72-76
degrees and we set the temp for 74-75 degrees, therefore the
great mass of air in the house doesn't really change in
temperature or water content, the AC just maintains the
temperature. Only the replacement air throughout the day
starts out hot and ends up cool and that hot air often has a
low humidity so very little water is condensed out of the
air as it is cooled. In contrast, in the east the outside
hot air may have lots of water and cooling the replacement
air from 90 degrees to 72 degrees may result in lot of water
condensing out of the air.