The Pros and Cons of Running a Dehumidifier
Will a dehumidifier overheat your superinsulated house in the summertime?
Superinsulation is the most effective weapon we have against wintertime heat losses. R-values of 60 or more in the roof and 40 in exterior walls can slow the movement of heat to a crawl, keeping energy costs far below what they’d be in a conventionally built house.
Yet Harry Seidel puts his finger on a potential problem. During the summer, any heat generated inside the house will have just as much trouble getting out of the house.
In a Q&A post at GreenBuildingAdvisor, Seidel wonders about the impact of running a Therma-Stor dehumidifier in a basement utility room of a superinsulated house in New Hampshire. The dehumidifier solves one problem but may create another.
“My question is in regard to the potential for the slab assembly in the house mentioned above to absorb and mitigate the heat generated?” he writes. “Given the sub-slab insulation [R-19], will this cause the basement to heat up? I could be wrong, but was told that 1 pint of water removed equates to 1,000 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. of heat.”
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“In N.H. and much of [New England] we have relatively high humidity in the summer months (trust me, it's a problem) and a fresh-air delivery system (Renewaire ERV), although able to reduce moisture somewhat, achieves this by simply exchanging ambient exterior air for interior air,” he adds. “In our summers this will be useless and the house's design is tight enough that moisture will prevail. It will build up and be a menace.”
Among the options he’s considering is the elimination of insulation below the utility-room slab to create a more effective heat sinkWhere heat is dumped by an air conditioner or by a heat pump used in cooling mode; usually the outdoor air or ground. See air-source heat pump and ground-source heat pump. for the dehumidifier.
Is there another approach that would be more effective? Or is Seidel anticipating more trouble than he’ll actually have? That’s the topic of this week’s Q&A Spotlight.
Your concerns are valid
Dick Russell thinks Seidel has a point. “The concern about where the dehumidification heat will go is valid,” Russell writes. “It won't leave the superinsulated structure too easily in the summer, and thus the temperature must rise, however slowly, due to the thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. of the slab and foundation walls.”
Russell also lives in New Hampshire and did some calculations for his own superinsulated house, leading him to believe that a dehumidifier extracting a couple of pints of moisture per hour would not make a substantial difference on interior temperatures. “At least no more than what a handful of light bulbs and a TV would do,” he says.
Russell’s house has a ground-source heat pumpHome heating and cooling system that relies on the mass of the earth as the heat source and heat sink. Temperatures underground are relatively constant. Using a ground-source heat pump, heat from fluid circulated through an underground loop is transferred to and/or from the home through a heat exchanger. The energy performance of ground-source heat pumps is usually better than that of air-source heat pumps; ground-source heat pumps also perform better over a wider range of above-ground temperatures., which can both heat and cool. But the system is “grossly oversized” for summer air conditioning, so there are times when the house is comfortably cool but uncomfortably humid.
He tells Seidel that leaving the door to the basement open while the dehumidifier is running will have the effect of lowering humidity levels throughout the house. Circulating air mechanically would help.
Don’t get rid of the insulation
Philipp Gross, another cold-climate inhabitant, advises Seidel not to get rid of the insulation below the slab in the utility room. He estimates heat losses in winter through an uninsulated 4-in. concrete slab would be considerable — 10,000 BTH (per hour, we’re assuming). “That is definitely more energy and money than the potential cooling demand caused by your dehumidifier,” he writes.
If summer heat gains from the dehumidifier are a concern, he adds, why not go with a minisplit air conditioner? “I do know that under some circumstances ‘only dehumidification’ is necessary and may be so in your climate,” he writes. “If you are worried about this look into products that can do both. The Daikin Quaternity is one example. I have never used it but it sure looks promising. (No idea about the costs.)”
Actually, it’s not that important
Tim O’Brien has a different point of view. If the slab is more than a couple of feet below ground level (which it is), the sub-slab temperature should be “nearly constant” no matter what the season, he says — a plus in summer and a drawback in winter.
But he adds this: “Insulating sub-slab will provide a lower return on investment than insulating your basement walls and your above-ground enclosure. I definitely recommend insulating under your slab if you are placing hydronic [tubing] in the slab, as the energy loss from a heated slab is considerable.
“Insulating below the slab will slightly raise the slab temperature which will reduce concerns from condensation during the cooling season; in other words, an insulated slab can handle slightly higher dew point air above it than an uninsulated slab without forming condensation on top of the slab. The insulated slab will also provide more comfort if the floor surface is finished. This is my way of saying that the insulated slab decision is likely one of the least important decisions you will make regarding your building enclosure.”
The specific dehumidifier Seidel is considering are highly efficient, he says, so the additional heat it generates should be less than would be the case with most other models.
That said, predicting inside humidity and temperature conditions is complex, to say the least. “Over the long term, your basement air will be warmer with a dehumidifier than without,” he says. “How much warmer will depend on a number of variables — your desired indoor humidity (dew point), the outdoor dew point, the rate at which outdoor air enters your house, and the amount of moisture created by the house occupants. All of these variables will affect the required running time of a dehumidifier — the dehumidifier is only adding heat when it runs.”
O’Brien suggests planning for both a dehumidifier and a ductless minisplit even if they aren’t installed right away.
Our expert's opinion
We asked Peter Yost, GBA's techical director, for his thoughts. Good timing, too, since he's been delving into this very problem of late. Here's his take:
There are a bunch of questions wrapped up in this very interesting discussion on dehumidification.
1. What’s the difference between dehumidifiers and air conditioners?
Dehumidifiers have both sets of coils inside the conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. ; air conditioners have the evaporative coil inside, the condensing coil outside. The latter has to be outside to dump the heat. There are other differences between the two, with one of the most important being the speed with which air passes over the coils: typically, air moves much more slowly over dehumidifier coils, giving the air more time to drop its latent loadCooling load that results when moisture in the air changes from a vapor to a liquid (condensation). Latent load puts additional demand on cooling systems in hot-humid climates. as condensed water.
2. How much heat does a dehumidifier generate as it removes water from the air?
There is a metric for this: the Energy Factor or how many liters of water are removed per kilowatt-hour (kWh) of energy spent. Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. labels dehumidifiers based on Energy Factors and the capacity of the unit, as follows:
≦ 25 pints: 1.2 liters/kWh through to
≧ 75 to ≦ 185 pints: ≧ 2.50 l/kWh
NOTE: the larger the capacity, the larger the coil; larger coils are generally more efficient than smaller coils. See the Energy Star DEH Product List and Criteria.
Since there are about 3,413 BTUs per kWh and about 2.11 pints to a liter, yes, quite a bit of heat is added to the space as the dehumidifier does its work.
As it turns out, I am right in the middle of trying to figure out how efficient my own 6-year-old 50-pint Energy Star Whirlpool dehumidifier in my basement is (mainly because it has seemed to be running less efficiently this summer). I can share that last week, my basement dehumidifier ran for about 6 hours using about 5 kWh, pulling the interior relative humidity down to 66% from about 76%, and raising the interior temperature of our basement about 1 to 1.5 degrees F (from about 71 to about 72 degrees F). Note that my basement is about 1,000 square feet, with about R-10 on the walls and NO insulation under the slab.
I calculated the current actual Energy Factor (using a Kill-a-Watt meter to measure the electricity used and just measuring the volume of water) at about 1.0 l/kWh. That is not very energy-efficient at all (more on this in a later blog), but I think all this adds up to this conclusion: if you have a superinsulated house and a really efficient dehumidifier, the temperature rise in your basement as you dehumidify won’t be all that noticeable.
There are a lot of variables (the size of your basement, how much you want to pull the interior RH down, the actual Energy Factor of your dehumidifier, etc.) that will affect this exchange, not surprisingly, but I recommend that you don’t change your slab or wall insulation package based on your dehumidification needs.
In our own home (also in New England), we only run the dehumidifier when high humidity forces us to close all the basement windows. So if the basement air temperature sneaks up a bit, it won’t be long before we, and you, can be popping open all those windows, keeping your basement cool and dry au naturel.
3. What about ducting the stand-alone dehumidifier to keep the heat in the utility room?
It goes without saying that you never want to confine the dehumidifier’s operation — you keep the heat in, but you also limit the volume of air being dehumidified. Most stand-alone dehumidifiers are NOT set up to be ducted — their fans are not designed for this sort of pressure resistance or constrained air flow (although Therma-Stor does make some very high efficiency DEH units that can be ducted).
4. Don’t ERVs dehumidify in the summer?
No. While ERVs can help to maintain relative humidity differentials between outside and inside air, they cannot actually actively lower interior relative humidity. (For more on this topic, see Martin’s blog on ERVs and HRVs.)
The bottom line: Taking the edge off the humidity levels in the basement of superinsulated home with a high-efficiency dehumidifier will not raise the interior air temperature significantly. Don’t change your slab insulation details to try and heat sink that slight temperature increase out of your basement.
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