Absolute Humidity Versus Relative Humidity

Absolute and Relative Humidity

At a recent seminar a question was raised about the widely-accepted statement that forced air heating system dry out the air as it passes through the heat exchanger.

Is this, in fact, true?

Well, the answer is that it depends. On What? Glad you asked.

If, by drying out the air you are referring to an actual reduction in the moisture content of the air, the answer is no.

In a nutshell... When air is heated, the amount of moisture contained in an air sample (as water vapor) remains the same. After all, if water is heated, what do we get? WATER VAPOR! So, if water vapor turns to water vapor (no change) the amount of water vapor in the air as it enters the heat exchanger is the same as the amount of water vapor in the air being supplied to the occupied space. The actual amount of water vapor in an air sample is referred to as 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.

So far so good? Great!

So.... what does happen to the air and why do we get that feeling of dryness? Why does my furniture crack? Why do I get nosebleeds in my house?

Although the ABSOLUTE humidity does not change, the RELATIVE humidity does. Okay, time for some quick definitions. As we mentioned, the actual amount of moisture in an air sample is referred to as absolute humidity. Relative humidity is a little more involved.

Relative humidity is the ratio of the amount of moisture in an air sample (multiplied by 100) divided by the air sample's ability to hold moisture (at saturation). For example, if an air sample has the ability to hold 80 grains of moisture but is only holding 40 grains, the relative humidity is (40/80) x 100 = 50%.

Okay, okay... But what does this have to do with that dry feeling on my skin? Hold on... we're getting there.

Now, as an air sample is heated, the air's ability to hold moisure increases. Think of a kitchen sponge that is completely saturated with water. By saturated I mean that, if you placed one more drop of water on that sponge, it would drip from the bottom. Completely saturated. Now, the amount of water in the sponge is well, the amount of water in the sponge and the total amount of water the sponge can hold is the same as the amount of water in the sponge since we said that the sponge is completely saturated. The relative humidity is, therefore, 100%.


Now, wave your magic wand over the sponge to double its size. Now, the waving of the magic wand does not increase the amount of water in the sponge, but only the size (and water holding capability) of the sponge. Now the relative humidity in the sponge is only 50%, since the sponge's ability to hold water has doubled, but the amount of water in the sponge remained the same.

We proceed.

As air is heated in a forced air heating system, the air's ability to hold moisture increases while the actual amount of moisture in the air sample remains the same. This causes the relative humidity to drop. Since the air can now hold more moisture and our bodies contain a whole lot of moisture, the moisture from our bodies is released to the air. The same hold true with moisture in our furniture, floors, paintings, etc.

So, we use humidifiers on forced air heating systems NOT to replace the moisture that the heating system took away, but to increase the moisture content in an attempt to increase the relative humidity.

Something to think about for all of you chill heads to think about. When air passes over the evaporator coil, have you ever notices the condensate dripping from the coil? Of course you have! Well, as we mentioned before, as air is heated, its ability to hold moisture increases, right. Well, if that's true, when air is cooled, its ability to hold moisture decreases! This moisture is what you and I know to be condensate.

Have fun!