Does a hot air system remove the
moisture from the air circulating through
it?
If this question means that it reduces
the actual moisture content in the air as it
passes over the heat exchanger, the answer
is “no”.
When an air sample is heated, the
amount of moisture in the air sample
remains the same. This is referred to as the
absolute humidity and is measured in
grains per pound of air or as grains per some
unit volume. It takes 7,000 grains of
moisture to make 1 pound of water.
When water vapor is heated, what
do we get? You guessed it, water vapor.
There is no change of water quantity;
therefore the amount of water vapor in the
air sample entering the heat exchanger will
be the same amount in the supply air to the
conditioned space.
You may wonder if what I am
saying is true, why is there an increase in
static electricity shocks, cracking furniture,
dry skin and nosebleeds, all sign of low
humidity levels during the heating season?
The answer for this is that while the
absolute humidity doesn’t change, the
relative humidity does.
Relative Humidity is defined as the
ratio of water-vapor pressure—amount
currently in the air to the saturation-vapor
pressure—the amount the air can hold—at a
given air temperature. What?
If you were holding a sponge that
was completely saturated with water,
meaning if you added one more drop of
water it would drip from the bottom of the
sponge, it can be said the sponge is now
holding the maximum amount of water that
it can and its relative humidity is 100%.
Now say the sponge just suddenly
doubled in size. The size of the sponge, its
ability to hold water and the relative
humidity have changed but the amount of
water in the sponge is the same.
When you doubled the size of the
sponge, you are also doubling the amount of
water it can hold. Since you didn’t add
anymore water, the relative humidity is now
only 50%.When air is heat it expands and
increases its ability to hold moisture. This
causes the relative humidity to drop.