Dr. Kool‘s

Where Good Neighbors and Good Servicemen Meet


College Station and Bryan are not that far from the Gulf of Mexico. Warm water in the Gulf generates considerable humidity. Wind from the South invariably brings it our way.

Relative Humidity

Warm air, above warm water, soaks up moisture like a sponge. The higher the temperature, the more moisture the air can hold. The term relative humidity applies to a particular temperature, and tells you two things: (1) How much moisture does the air hold, and (2) How much more can it hold. For example if the relative humidity is 60%, there is a 40% reserve capacity. As the air continues to absorb moisture, if it exceeds 100% relative humidity, it will start to rain.

Two Ways to Dehumidify

If you raise the temperature without changing the moisture content, the relative humidity will fall. In the winter, you lower the relative humidity each time your furnace fires up. You haven't removed any moisture, but the air will feel more dry. You can also dehumidify by running your air conditioner, and that actually removes moisture from the air.

In rainy weather, your windshield may start to fog up. If you'll run your automotive AC and heater at the same time, you'll clear the fog from your windows. You are dehumidifying—both ways at the same time, which is very effective!

Raining In the Attic

Water was dripping from the ceiling, and I responded to the AC call. I expected to flush a stopped up condensate drain, but I was in for a surprise. I couldn't find where the water was coming from, but I did notice the air distribution plenum had broken open. Once I repaired the plenum, the dripping stopped.

Here's what was happening: cold air from the broken plenum was meeting the moisture laden air from the hot attic. As the temperature of the warm air fell, the relative humidity rose until it exceeded 100%. It was raining in the attic.

Household Humidity in the Spring and Fall

A customer called and complained it was too humid in her house. I explained that it was spring time, and she was neither heating nor cooling. Since she can't run her furnace and air conditioner at the same time, there is only one other option—a dehumidifier. This is a portable device, I used to run in Spring and Fall. I'll tell you later why I no longer use it.

Controlling Household Humidity

In the summer, humidity is removed using an AC. Many contractors undersize condenser units so they run excessively, and for the express purpose of removing more moisture. AC manufacturers are pushing variable speed blowers, and one of the reasons is to control humidity.

A Better Approach

Dehumidification can add a serious load to your AC. Up to 1/3 of your energy consumption can be for that purpose. When the moisture is kept out of the house, your comfort level is improved, and you save, big time, on your utility bill.

Old inefficient windows allow outdoor air to infiltrate your living space. New efficient windows will make your home more comfortable by reducing humidity.

A Personal Experience

At the start of 2006, we replaced our inefficient aluminum windows with top quality double pane, low e vinyl ones. We have been extremely pleased with the results. I no longer use my portable dehumidifier because we have little moisture in the house. Sometimes there's a localized problem caused by showering. Then I run a small fan which dries out towels, etc. and moves the moisture to other parts of the house that are more dry.


Now that you better understand relative humidity and dehumidification, you're prepared to deal with it. You might even choose to replace your old inefficient windows with new double pane, low e ones. The company I used is one of our Preferred Kontractors.

I recommend you keep the moisture out, and use your AC for temperature control. And for heaven's sake, when you install a new AC system, find a contractor who won't (1) Undersize your condenser, or (2) Install expensive-to-maintain extras.

What you want is high efficiency equipment that does not include multi-speed compressors, variable speed blowers, zoning, or a heat pump.

Be Kool and Be Dry,

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Dr. Kool

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