Last week, after reviewing a little physics regarding condensation and latent heat, I described how air conditioners remove unwanted humidity. This week I’ll examine how dehumidifiers work in removing moisture and when it makes sense to use them.
Like air conditioners, dehumidifiers remove moisture by condensing water vapor out of the indoor air. While an air conditioner dumps the warm air that’s produced through that condensation process (latent heat) outdoors, a dehumidifier doesn’t get rid of that heat. Instead, releases the warm air into the space where it’s is located.
In other words, while there’s a cooling process involved with dehumidifier operation (chilled coils on which condensate collects), that cooling of the air is offset by the heat of vaporization released by the condensation process. Due to the use of a blower and pump in the system, a dehumidifier actually warms a space somewhat, though by reducing relative humidity levels in a house, it may help you feel more comfortable.
When a dehumidifier makes sense
Dehumidifiers are optimized to remove moisture rather than to cool air, so they work better at that function than air conditioners. It makes sense to install a dehumidifier when the relative humidity gets high enough to cause significant problems—like growing mold in the home. Most experts suggest that indoor relative humidity levels should be kept below 50 or 60 percent.
Dehumidifiers are effective at removing moisture when cooling isn’t also called for—such as during spring and fall when there might be high humidity but cool enough temperatures that air conditioning isn’t warranted. They can also make sense in highly energy-efficient homes with good cooling-load-avoidance strategies, such as shade trees on the east and west, awnings or overhangs above windows, and very energy-efficient lights and appliances. In these spaces, it may be important to get rid of excess moisture, while cooling isn’t needed.
That said, the most energy-conserving choice is to avoid using a dehumidifier (or air conditioner) by controlling unwanted moisture sources (see my column from two weeks ago). A lot of materials in a home, such as wood, naturally absorb moisture during the summer months and then release that moisture in the drier winter months; that sort of moisture cycling is acceptable in most houses.
Demumidifiers have either a plastic bucket that has to be emptied when full (an automatic shut-off prevents overflow), or a drain line for dumping condensate into a floor drain or sump. Look for a model with a humidistat that automatically turns it on and off, depending on the relative humidity.
Dehumidifiers are rated by their moisture-removal capacity (usually in pints per day) and their Energy Factor in liters of water removed per kilowatt-hour (kWh) of electricity consumption. The Energy Factor is typically higher for larger dehumidifiers that have greater moisture-removal capacity. To carry an Energy Star label, dehumidifiers must meet the following Energy Factor requirements (the odd mix of English pints and metric liters in the standards may be part of a plot to keep us confused!):
Minimum Energy Factor (l/kWh) based on water-removal capacity:
- Less than 25 pints/day = 1.20
- 25 to 35 pints/day = 1.40
- 35 to 45 pints/day = 1.50
- 45 to 54 pints/day = 1.60
- 54 to 75 pints/day = 1.80
- 75 to 185 pints/day = 2.50
The most energy-efficient dehumidifiers today are made by Therma-Stor Products, of Madison, Wisconsin. The company offers a wide range of both free-standing and ducted dehumidifiers that are typically integrated into forced-air distributions systems. Standard stand-alone dehumidifiers typically cost $150 to $300, while top-efficiency models, such as most of those from Therma-Stor, will cost over $1,000.
Installing and using a dehumidifier
Stand-alone dehumidifiers are typically installed in the basement or crawl space, though they may also be located in a utility closet in the living space. As you think about placement, be aware that dehumidifiers have fans, which homeowners may find annoying. Also, for efficient operation, be sure that air can freely circulate around a dehumidifier.
If you have an older model without a humidistat to automatically turn it on and off, buy a digital hydrometer (relative humidity meter) and turn on the dehumidifier when the relative humidity gets to an uncomfortable level—say, around 60% relative humidity. Even with a humidistat, it’s not a bad idea to buy a hygrometer to make sure that the dehumidifier’s humidistat is working properly; these can be significantly mis-calibrated.
If the outdoor air is fairly humid and you decide to turn on a dehumidifier, it usually makes sense to close up the house — so your dehumidifier won’t have to work so hard just to keep up with incoming humid air. This has to be balanced with the benefit of cool, outdoor airflow through the house, though. It will probably take some practice to balance the use of dehumidification, ventilation, and mechanical air conditioning. Follow manufacturers’ instructions on keeping a dehumidifiers clean to ensure efficient operation.
Dehumidifiers use quite a lot of energy — many consume over 600 watts while they are operating — which can be for long periods of time in the summer months. Use of a dehumidifier can easily be the largest electricity user in a home during the months it is used (especially when air conditioning is not being used), so avoiding or minimizing the need for dehumidification should be a high priority. An engineer friend in Keene, New Hampshire, tells me that during the two months of the year he uses a dehumidifier, it increases his electric consumption by 63%, and the 250 kilowatt-hours he uses each month requires roughly $3,000 worth of photovoltaic panels to produce.
In addition to this Energy Solutions blog, Alex writes the weekly blog on BuildingGreen.com: “Alex’s Cool Product of the Week,” which profiles an interesting new green building product each week. Last week’s blog was on Wasco’s new triple-glazed skylight that meets the 30-30 rule for the federal tax credit. You can sign up to receive notices of these blogs by e-mail—on the BuildingGreen.com blog page enter your e-mail address in the upper right corner.
Alex is founder of BuildingGreen, LLC and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.
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ERV's and Dehumidifiers
For very tight and highly insulated buildings, high humidity is a problem from my understanding. Since we also have to introduce outside air to that type of building, typically through an ERV or HRV , would placing the dehumidifier downstream of the fresh air be the best practice? I know an ERV can do some humidity control, but what I've read seems to indicate it is quite limited and can't handle the massive amount of humidity present the hot-humid and some mixed climates. Therefore conceptually it seems to make sense to have a dehumidifier connected to the dedicated ERV/HRV ductwork.
The Ultra-Aire 90 has the capability of bringing in outside air and dehumidifying it, but from what I see has no exhaust to the outdoors leading to only building pressurization and no recovery of heat from any exhausted air. I envision a more ideal component that provides outdoor air, with heat exchange, and active dehumidification on the supply air side. Does this exists?
Response to Brad
There are many sources of moisture that contribute to high indoor humidity levels. During the summer in hot humid climates, exterior air (entering the home through random cracks or due to mechanical ventilation) is only one source; others include cooking, showers, watering plants, and housecleaning chores.
There is no need for a dehumidifier to be connected with ventilation ductwork. A dehumidifier can be located almost anywhere, as long as it is within the envelope of the house. It can be connected to an HVAC system if you want, but it doesn't have to be. It handles dehumidification just fine even if it only sees recirculating indoor air rather than outdoor air introduced for ventilation.
You are only considering one source of moisture.
ERV in mixed humid climate
Do you think it makes sense to use a water coil radiator with an underground PEX loop for frost control and dehumidification in a mixed humid climate? This would temper the outside air before hitting the ERV.
Also, are there any objective analyses of which is better in a mixed humid climate - ERV or HRV?
Response to Greg Duncan
Concerning the choice between an HRV or an ERV, check out this article:
HRV or ERV?
I have heard of several experimental homes that use a PEX ground loop to precool outdoor ventilation air. I don't know of any studies of their effectiveness. You'll have to make the loop as long as possible, since you will be basically adding heat to your soil all summer long. You are using a pump to raise the temperature of the soil outside your house. As a result, the loop will become less effective as the season progresses. If the loop is not well designed, the electrical load required to circulate the fluid in the PEX load may not be justified.
If you are using the coil for dehumidification, be sure to include a drip pan, a trap, and a drain.
Thanks, Martin. That's a great article on ERV vs HRV.
PHPP and sub-soil ground loops
In PHPP if you model adding a 50% efficient sub-soil ground loop to a small single family home in Boston that meets Passivhaus air tightness with an Ultimate Air ReCouperator as the ERV, the ventilation heat losses drop from ~2,800 kBTU/yr to ~2,400 kBTU/yr. 400 kBTU is 117 kWH, even with electric resistance heat that's only about $18 / yr.
WIth a less efficient ERV it saves more energy (70% = 700 kBTU/yr), but buying a better ERV would be cheaper than installing a ground loop, I would expect.
If you had a small 10 watt pump doing the work you wouldn't lose too much energy in pumping, but it's not a huge effect by the numbers.
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