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Mon, Jan. 20

Air conditioners vs. evaporative coolers

Q: Hi Randy. We moved here a few months ago and have a question for you. The home we bought has an evaporative cooler with a large supply vent in the hall, and it has central air conditioning that blows air out the vents in every room. There are also plastic vents in the ceilings in some rooms that open up when the evaporative cooler is running. We were wondering why a home would have both air conditioning and an evaporative cooler. Neighbors told us to use the evaporative cooler when it's not humid, and to use the air conditioner when it is humid or very hot. We are not familiar with evaporative coolers; is this good advice?-Tom and Carole in Prescott.

A: I know that a lot of people are not familiar with evaporative coolers. The following paragraph is in the "maintenance" section in my reports. This will explain coolers to you (and save me some typing).

"Evaporative coolers are relatively simple devices, are inexpensive to operate, and work very well in low humidity like we have here most of the time. Coolers have a water line that usually connects at a hose faucet or near a water heater. A cooler consists of a float valve, similar to the one in a toilet tank, that keeps the proper water level in the cooler. A small pump will circulate the water through small water lines and onto the pads. A blower will pull air through these pads and into the home. The water evaporating off the pads is what cools the air. (If you get out of a swimming pool on a hot day, you are cool until you dry off. The water evaporating off your body cools you. This is the same principle). Every spring before use a cooler will need a thorough cleaning and check. Pads, pumps and float valves are maintenance items that occasionally need replacing. They are usually readily available and not major expenses. A cooler and the water line should be drained before winter. To drain a cooler you shut off the water and remove a plug in the bottom of the unit. You should also drain the water line by removing the line from the cooler, and then removing the line at the valve (use a bucket if needed to catch the water). Most people cover rooftop units during the winter months. If your evaporative cooler has one or two supply vents in the home, slightly opening a window in a room will help circulate cool air to that room."

So I hope this explains how coolers work. As far as your neighborly advice, it is correct that evaporative coolers will not work well when it is humid. The water will not evaporate off (and cool) the pads, so you basically get outside temperature and humidity air blowing into the home.

Many evaporative coolers in our area have a single large supply vent, usually in a hall. This obviously will not cool every room as well as a supply vent in every room. A cooler will blow a lot of air into a home, so some air needs to get out for the cooler to work properly. The comment above notes that opening a window in a room without a supply vent will 'pull' cool air into that room. But even if the evaporative cooler has multiple supply vents, you still have to open some windows and let some air out. Imagine blowing air into a plastic bag. Once the bag gets 'full', you cannot blow any more air into it. If you don't let air out of the home, the evaporative cooler cannot blow more cool air into the home.

Those ceiling vents in some rooms you mentioned are intended to allow air out. When the evaporative cooler is operating and the house is being 'pressurized', these vents will open and allow air to escape through the attic. Having several of these 'exhaust vents' in different rooms will improve the distribution and efficiency of the evaporative cooler.

Common sense says that discharging the interior air through the attic should cool the attic a little, which will also help cool the home. I sometimes wonder about blowing humid air into an attic. Proper construction takes measures to keep attics dry, for example, all interior exhaust fans should be routed to the exterior and there should be attic vents on the exterior walls and/or roof. So how wise is it to blow humid air into an attic? However, I see the ceiling 'exhaust' fans frequently in homes with evaporative coolers. I am assuming (yes, I know what that means) that since you only operate an evaporative cooler when the outside humidity is very low you are not causing high humidity levels in the attic.

Operating an evaporative cooler costs much less than operating a central air conditioner. An evaporative cooler has a blower and a small pump. The blower does not use a lot of electricity, and the pump very little. A central air conditioner has a blower (usually in the furnace) and a 240-volt compressor, which uses much more electricity.

An evaporative cooler will not work in high humidity, and will not cool as well as an air conditioner in very hot weather. So having both an evaporative cooler and an air conditioner is a nice feature. In dry, moderately hot weather, you can save money by using the evaporative cooler. But when it gets too hot or humid you have the central air conditioner.

Randy West owns Professional Building Consultants in Prescott. He is state-certified and has performed more than 7,000 home inspections in the Prescott area. West serves on the Home Inspector Rules and Standards Committee for the Arizona Board of Technical Registration. Contact him at or visit

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