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The Pros and Cons of Running a Dehumidifier

Will a dehumidifier overheat your superinsulated house in the summertime?

Posted on Sep 6 2011 by Scott Gibson

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.”

“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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Image Credits:

  1. Therma-Stor
1.
Tue, 09/06/2011 - 08:19

Vented portable AC?
by Michael Chandler, GBA Advisor

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I've been pretty happy with the vented portable AC unit I bought at Wall Mart.

It exhausts heat and humidity through a 4" dryer vent when it runs so I don't need to worry about water freezing in a drain hose or getting gravity drainage (the vent is about seven feet off the floor)


2.
Tue, 09/06/2011 - 08:44

Damn units
by Philipp Gross

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Sorry, I was thinking in kWh and wrote BTU. I estimated 10,000 kWh/YR more heat loss through a uninsulated slab, which is about 34,000 kBTU/YR. Hope I didn`t cause to much confusion.... . Anyways at least the conclusion was right. Insulate Slabs!


3.
Tue, 09/06/2011 - 12:02

You can't dry any basement or house with humid outdoor air
by Dana Dorsett

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The whole notion that you can use ventilation with outdoor air to dry a New England basement is a bit wrong-headed, given that summertime dew point averages are north of 60F. Most healthy-air guides put an upper bound of 50% relative humidity (to keep dust mite reproduction under control) or 60% (to keep mold & fungus under control). ASHRAE specifies a 65% maximum (for comfort). Pulling 60F+ dew point air into a 70F basement raises the basement dew point to 70%+, and the mold risk skyrockets. Pulling 60F dew point air into a 75F first floor it's already at the 60%RH limit. It takes sub-55F dew point ventilation air to achieve 50% RH in a 75F room, or sub-51F dew point air to get 50%RH in a 70F basement.

The number of days/hours of sub-60% outdoor dew points between June15 & September 15 in southern New England are half at best, and sub-55F less than a quarter. Outdoor air infiltration is usually one of the largest SOURCES of indoor moisture in this region,usually exceeding the occupant-activity sources in typical, not-so air-tight houses. Keep the windows closed and running the AC + dehumidifier would generally result in lower energy use. To gain any drying advantage with outdoor air in New England requires watching the outdoor dew points like a hawk and scrambling to open & close windows often.


4.
Tue, 09/06/2011 - 12:20

Response to Dana
by Martin Holladay, GBA Advisor

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Dana,
You're right. That's why Peter Yost wrote, "Don’t ERVs dehumidify in the summer? No."


5.
Tue, 09/06/2011 - 13:24

Vented dehumidifier
by Eric Sandeen

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@Michael Chandler - I had wondered about those. Is it efficient? I think I had understood that they must then boil the water out the vent...


6.
Tue, 09/06/2011 - 14:50

Response to Martin
by Dana Dorsett

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'Twas Peter Yost who also wrote:

"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."

...which is what prompted my post.

BTW: As I understand it smaller standalone dehumidifiers typically have an EF less than 1.5, whereas bigger ones can have an EF north of 2, approaching that of whole-house dehumidifiers (the higher efficiency due largely to the larger chilling coil). In a small to mid-sized fairly tight house where the occupants and their activities are the primary humidity source, it may be difficult to rationalize the price differential between whole house vs a larger standalone "room" dehumidifier. Most EnergyStar 65pint+ models will have a EF of 1.8 or better, whereas typical 30-35 pint models are under 1.2-1.4 (even with the Energy Star label.)

Daikin Quaternity isn't the only mini-split out there with a dehumidify mode, there are quite a few cooling-only minisplits that with dehumidify modes as well as a few heating & cooling mini-splits, but it may be the only one with true dehumidistat control. There is also at least one dehumidify-only mini-split targeted at the indoor-pool market (DryMAX).


7.
Tue, 09/06/2011 - 15:32

Edited Tue, 09/06/2011 - 15:33.

You can't dry basement with humid outdoor air - Dana
by Peter Yost

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Hi Dana -

You are absolutely right--just a question, I think, of what each of us means by how often and how long we keep our basement windows open and what interior RH we will tolerate in the basement.

I have not been keeping a record of our DEH run time this summer--which I had. I bet we run our DEH in the basement less than 25% of the time overall. And yes, we do open and close our basement hopper windows as a habit, just as we do on the first and second floor above grade floors. Has never seemed that big a deal. And also, we have found that the contents in our basement--including cardboard boxes and the like--will tolerate just about 65% RH but no higher before we start to get any real moisture or mold issues. So we close up the windows when it gets above 65% and then we set the humidistat on the DEH unit at about 65% (actually, the DEH unit just has a qualitative dial--I use a stand-alone hygrometer to gauge the setting on the DEH unit).

And thanks for the info on other equipment options for DEH - the smaller stand-alone DEH units, even Energy Star-rated, as you pointed out, are pretty lousy on efficiency, but we just don't seem to run ours enough at this point to justify an upgrade.

And note: we have stained concrete for our basement floor as the finished surface, except for a rug, which we roll-up and air out/UV treat (the sun) at least once a year. The carpet has absolutely no odor and we keep it well vacuumed (http://www.greenbuildingadvisor.com/blogs/dept/green-communities/carpet-...).


8.
Tue, 09/06/2011 - 17:21

The benefits of a tight house
by Dana Dorsett

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It doesn't take much dehumidifier to keep the interior RH from drifting high even during the summer months as long as you're not over-ventilating. Most "whole house" versions are overkill for a tight house in a climate with only moderate latent loads. And the smaller the load, the less important the overall efficiency is.

Spending coupla grand for a whole house dehumidifier to double the efficiency over a 30-pint pipsquick to save $8.11/year isn't likely to be the best expenditure. For that kind of capital expenditure a smaller mini-split with a dehumidify mode that can also be used as high-efficiency auxilliary (or even primary shoulder-season) heating or cooling as well makes more sense.

Spending an extra $50 for a 70-pint unit that's 50% more efficient than the 30-pint version probably makes financial sense though, and could keep up with a pretty significant ventilation rates in New England weather.

Keeping the basement as high as 65% RH with a rug on an uninsulated slab can be pushing your luck a bit though, especially if your subsoil temps are in the 40s or lower, but even R3 of slab/floor insulation can make quite a bit of difference.


9.
Thu, 09/08/2011 - 12:56

What about using a heatpump water heater
by Mike Strecker

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I not an expert but was wondering if using one of the new style water heaters that use a air source heat pump build into them to heat the water would help. The heat produced would go to heat the water not the house and it runs more when hot water is used in the house (like showers). While it runs it also dehumidifies. it would not run constantly but may help lower the need for a full time dehumidifier and you get lower cost heated water in return. Just a thought!


10.
Thu, 09/08/2011 - 18:41

Mike, northern cellars, mine
by aj builder, Upstate NY Zone 6a

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Mike, northern cellars, mine at least are humid because they are cool in the summer. So a dehumidifier in a cellar does everything right. It warms the space and lowers the humidity. The heat pump water heater would work nice upstairs where it is warm and humid. But only for the months that we need cooling. Where I live I used to think I needed cooling but really prefer open windows and ceiling fans which lasts for four months or so. And screened sleeping porches are the best by the way.


11.
Tue, 08/06/2013 - 11:19

basement humidity
by Eric Carlson

Helpful? 0

We utilize a small fan in the basement, that vents out at ceiling level to the outside , with the intake in a connecting shaft at floor level. It lowers our humidity quite considerably.


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