by Andrew Rudin
Mold, mildew, and rot are causing a lot of problems these days. The company that carries the commercial liability insurance for my energy surveying work just sent me an exclusion of coverage on any work I do dealing with mold and mildew. That caught my attention. Why all the concern? There are several reasons. Newer houses are tighter, trapping more humidity. Builders have substituted oriented strand board (OSB) for plywood. In OSB, the cellular structures of the wood have been ripped open, leaving them vulnerable to mold. Plywood has layers of resin between layers of wood with relatively intact cells; these intact cells act as mold barriers. And more people have developed allergies to molds and mildews. Thus control of mold and mildew—and therefore of humidity—is of greater concern than ever.
One way to control home humidity is by using a residential dehumidifier. A dehumidifier is like an air conditioning system that first cools the air to condense water with an evaporator coil. Then, unlike the air conditioner, the dehumidifier returns the heat back to the air through a separate heating condenser coil. The net effect is that the dehumidifier adds heat—all the heat removed from the air is returned and the compressor heat is added back in. This is in contrast to air conditioning, which dehumidifies but also cools, since the heat rejection coil—the condenser—is outside.
The State of the Dehumidifier
In 2001, some 805,700 dehumidifiers were sold in the United States, according to Appliance magazine’s 49th Annual Report, and that is down a bit from the yearly average of about a million. More than twice as many freestanding humidifiers, compared to dehumidifiers, are sold each year. In September 1999, Appliance magazine estimated that the average life of a dehumidifier was 11 years, with a low of 9 and a high of 13 years. Based on housing data from the 2001 Statistical Abstract of the United States, about 1 of 100 occupied residences in America purchases a dehumidifier each year.
My library of a couple of thousand energy books is stored in our basement, which is where most residential dehumidifiers are kept, since below-grade moisture keeps basements more damp than the rest of the house. The basement in our 1917 home is relatively dry, but not dry enough to protect my complete set of Home Energy magazines, for example. So we purchased a dehumidifier when we moved in ten years ago. Since then, we have experimented with ways to lower its use of electricity. Our grid-tied 2.7 kW solar electric system generates more electricity than we use (see cover photo). Wanting to preserve this dynamic, we have become very conscious of minimizing our electric use.
The dehumidifier is an electric load from hell
because it uses a lot of electricity and is often used along with air
conditioning. Running a dehumidifier and an air conditioner
in the same space at the same time is counterproductive; one appliance
adds heat and the other removes it. Fortunately for us, we dehumidify
our basement and cool only the third floor bedroom. Also fortunately,
both of these electric loads coincide with abundant summer sunshine,
with which we produce electricity.
We have used hygrometers, Hobo data loggers, and a circular chart recorder in the basement to monitor relative humidity (RH) and temperature. The dehumidifier has a dedicated submeter to measure its use of electricity.
White Westinghouse manufactured our dehumidifier. It is a model MED25P2 , rated at 5 amps and 25 pints (11.9 liters) of water removed per 24 hours. The operating wattage, measured by a Watts-Up meter, is just about 400 watts. If our dehumidifier ran constantly, it would consume about 292 kWh per month—more electricity than we use for everything else combined.
In general, there are not a lot of reliable energy data on the electricity used by dehumidifiers. Utilities and state energy offices have electric use ratings for dehumidifiers; they vary from 31 to 180 kWh per month. Since the amount of moisture in the air and the settings of dehumidistat controls vary a great deal, these estimates of electric use are not very meaningful or helpful. Similarly, although our dehumidifier is submetered, the data on its electric use are of limited value. We changed more than one factor at a time, and the amount of moisture in the air varied each summer. So this article does not provide hard data that could come only from a testing laboratory with carefully controlled conditions. Rather, it is a summary of the factors involved with dehumidifier use.
In August 2001, Energy Star began listing dehumidifiers in a more meaningful way—based on the electricity used to remove water from the air. In their June 13, 2002, listing of residential dehumidifiers, the average rating was 3.2 pints (1.53 liters) per kWh. The lowest rating was 2.7 pints (1.3 liters) per kWh, and the highest was 5.72 pints (2.75 liters). The ratings are made in air that is 80°F dry bulb and 60% RH (70°F wet bulb), following ANSI/AHAM-DH1 or Can/CSA-C749-94. (The U.S. and Canadian testing standards are about the same.) With 400 watts operating continuously, our dehumidifier would be roughly rated at 2.6 pints (1.24 liters) per kWh, worse than the lowest of the 2002 models.
How to Lower Dehumidifier Electric Use
We tried several strategies to lower our dehumidifier’s use of electricity. All were helpful, but one was especially effective.
Use the smallest practical dehumidifier. Other than practical experience, I know of only one source for sizing dehumidifiers (see Table 1). In June 2002, the Web site for The Home Depot listed three dehumidifiers, all branded Hampton Bay—30 pints per day for $157, 40 pints for $178, and 50 pints for $197. That’s $5.23, $4.45, and $3.94 per pint capacity respectively, and none of these dehumidifiers was Energy Star rated.
Reduce the amount of moisture entering the space. We knew that some moisture was entering the basement through the floor, which was installed over dense clay without a vapor retarder. We could tell because the bottoms of the rubber floor mats were continually damp. The walls are also a source of moisture, but they are laid-up stone parged with a cement coating that is peeling off in places. To lower the rate of moisture entering through the floor, we painted it with a semipermeable latex paint designed for swimming pool decks. The effect on electric use was hardly noticeable, probably because most of the moisture comes through the walls.
We sealed up some openings for outdoor air with rigid foam insulation board and caulk. We replaced a leaky wooden outside door to the basement with a metal-clad door with a magnetic weather seal. We covered an open sump pit. We also sealed a hole in the vent pipe of the gas clothes dryer, repaired a leaking faucet, and covered the open container on the dehumidifier that collects the condensate. We hooked up a garden hose to that container to carry the water to a basement drain.
We made sure that our gutters and downspouts channel water away from our
basement by piping the rainwater runoff underground into our garden. We
graded so that water runs away from our foundation by filling in with
concrete a small garden patch next to the house foundation. We could
place heavy-duty plastic over the basement floor, but we feared that
mold and mildew would build up underneath. The basic principle: If less
water comes into our basement, we won’t have to dehumidify as much.
We also open the windows on days that are cool and dry. In Philadelphia, this seems to work up to the middle of July. That’s when we plug the dehumidifier in. When cooler air enters the basement, the RH rises if the absolute humidity remains the same, so we have to be careful about introducing outside air.
Maintain better control of operation. The humidity control on a dehumidifier does not have settings in percentages of RH. The settings are simply numbered 1 through 10. Based on my research about mold, mildew, and books, we aimed to maintain 60% –70% RH. Judging by data logger recordings of temperature and RH, however, the dehumidifier’s control seemed inexact. We purchased and installed a line voltage, wall-mounted Honeywell humidistat that is more precise, has a narrower dead band, and allows control of both the dehumidifier and some fans. The RH settings are marked on the face of the Honeywell humidistat.
If you intend to use a dehumidifier’s humidity control, you should probably calibrate it with the help of a hygrometer. Otherwise, the temptation is to set it to the driest setting and then faint with the next electric bill. With your dehumidifier running and its humidity control set in the middle of its range, place a good hygrometer in the middle of your basement, about 1ft off the floor. Wait 24 hours and then read it. If the hygrometer reads lower than 70%, turn the dehumidifier control to a lower setting (selecting higher RH) so that it runs less. If the hygrometer reads higher than 70%, turn the dehumidifier control up to a higher setting (selecting lower RH) so that the dehumidifier runs more often. Wait 24 hours and read the hygrometer again. Continue making adjustments, waiting 24 hours between them, until you achieve close to 70% RH. Use a permanent ink pen to mark this position of the control dial on your dehumidifier. If you want a drier basement, calibrate to 60% RH.
One confusing ergonomic aspect of dehumidifier controls is that you pick a higher RH setting by dialing a lower number. Clockwise turns lower RH. On round thermostats and valves, higher settings give you more heat, water, and so on.
If a dehumidifier is too cold in a space, its cold coils not only will condense the water out of the air, they will freeze it. (I am not aware of any defrost cycles on common residential models.) The resulting ice buildup will make the dehumidifier much less able to dehumidify air, and its compressor may become damaged from overuse. Some sources of information show that air temperature at 65°F or lower will start the formation of ice.
Seek the highest tolerable RH. Research shows that the appropriate temperature and RH settings for libraries are a flat 70°F and 30%–50% RH. These settings are designed to protect valuable books. I am not saying that my copies of Home Energy are not valuable—but 50% RH and 70°F are too strict for my basement library. To set our dehumidifier, we gauge by smell, feel, and any appearance of mold or mildew. We picked 60%–70% RH. In playing with the settings, I got the impression that electric use has an exponential relationship to RH, but I did not collect any data to prove that.
Move the air. We experimented with two window box fans that would operate any time the dehumidifier operates. The humidistat controls the dehumidifier and fans through an electric submeter and a strip of electric outlets. The fans are aimed away from the dehumidifier so that the drier air is circulated into the corners of the basement. The cycles of dehumidifier operation are lengthened. The goal is to warm the wall and floor surfaces and stagnant, cooler corners and nooks uniformly, in order to make condensation on cold surfaces less likely. Because we are moving so much air each time the humidistat turns on the fans and dehumidifier, I am gambling that the higher RH settings are fine. Each of the two box fans uses about 90 watts on the low setting, which is the setting I use. The fan built into the dehumidifier is not strong enough to circulate air thoroughly within the enclosure. In terms of preserving the books, warming surfaces more uniformly, and moving drier air into dead spaces, I think the fans earn their keep.
Relocate items to be kept dry. All the books and magazines are stored off the floor on shelves. We installed the shelves away from the walls for better air circulation.
These six strategies together made the operation of the dehumidifier much more effective. There was less musty odor and the air seemed dryer. Still, we kept looking for new ways to lower the electric use. We thought of adding desiccant dehumidification, air conditioning the basement, replacing the concrete floor and installing a vapor retarder under the new floor, and replacing the cement parging on the walls. None of these strategies seemed right. Then we had a visit from a nonengineer friend, who simply recommended the following:
Lessen the volume of what needs to be dehumidified. After our friend left, we installed wood blocking between the first-floor joists. Then we draped 6-mil polyethylene to the floor to make a barrier around the part of our basement that we wanted to dehumidify. We arranged multiple overlapping sheets of plastic so that we could still get into the library section. The dehumidifier, humidistat, hygrometer, two box fans, and electric meter are all inside this temporary enclosure. The volume of what we dehumidify has decreased to a quarter of the original. The enclosure is not very tight, and we may try to improve it, but the tighter it is, the more difficulty I have in using the library. So it’s a trade-off.
Magazines and Energy Saved
In the three years from 1998 through 2000, our average dehumidifier electric use was 237 kWh per summer. In 2001, with the polyethylene barrier and the RH raised from about 60% to 70%, we reduced electric use to 38 kWh—a decrease of 84%. This is not surprising, since we reduced the volume that is dehumidified by three-quarters. It’s not clear how much of the reduction in electricity use was due to raising the RH level. But now our copies of Home Energy will be better protected from mold and mildew using minimal dehumidifier electricity.
Bear in mind that if we had severe mold and mildew in our basement, we would have to dehumidify the entire space. We are lucky just to need dry books and magazines.
Andrew Rudin is the project coordinator of the Interfaith Coalition on Energy, an ecumenical group whose mission is to reduce energy costs for religious congregations in and around Philadelphia, Pennsylvania.