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Community and Q&A

Can I dehumidify without adding heat?

Luke Brechtelsbauer | Posted in Energy Efficiency and Durability on

I live in New Orleans and I have a 6x8ft room that I keep an antique musical instrument in…and some more modern ones I practice on. I try to keep the temperature between 65 and 75 and the humidity between 40 and 60%.

I have a window AC that can cool the room just fine, but the dehumidifier puts out so much heat that this room uses more electricity than everything else I do.

Is there a way to keep the dehumidifier and AC from fighting?

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Replies

  1. Charlie Sullivan | | #1

    The first choice should be to run the A/C to dehumidify. Then only run the dehumidifier if it gets too cool. Is that what you are already doing?

  2. GBA Editor
    Martin Holladay | | #2

    Luke,
    Charlie's right. Ordinarily, an air conditioner does two things: it cools and it dehumidifies.

    If the air conditioner doesn't lower the humidity enough to satisfy your expectations, then you have to use the dehumidifier.

    All of this takes energy, of course. There are several ways to lower energy use:

    1. Choose an air conditioner with a high EER rating.

    2. Choose an Energy Star dehumidifier.

    3. Use shading devices or window films to reduce the amount of solar heat gain that comes through your window glass.

    4. Pay attention to air sealing. You want to seal air leaks in the room being conditioned -- air leaks in the partitions as well as air leaks to the outdoors.

    5. Install weatherstripping on the door that separates this room from the rest of your house.

  3. D Dorsett | | #3

    The AC and dehumidifier are not "fighting", they are working together. The dehumidifier is taking the latent heat load and converting it into a sensible heat load. If the AC was managing the latent load better on it's own the dehumidifier would run less. But since it also takes energy to do that latent to sensible conversion, it's adding some more sensible heat into the room than the latent heat it is removing.

    Unless there is a replenishing source of humidity the latent load should whittle down fairly rapidly. On the gulf coast ventilation air is a big source of that humidity for at least 7 months out of the year. The dew point of 75F 50% relative humidity air is about 55F, and the mean outdoor dew point in New Orleans is above that from about half-past March well into November.

    I would re-order Martin's list:

    1: The number one thing to focus on is air tightness, since the biggest humidity source is the oudoor air. The more air-tight you can make that room, the easier it will be to control the humidity. (This would be Martin's #4 & #5)

    2: The SMALLEST BTU/hr window air conditioner you can find will remove more moisture than a high EER unit of larger size, since it will be running longer duty cycles The cooling load of a 50 square foot room is miniscule, with or without window films, and adding window films would only lower the duty cycle of the AC unit, forcing the dehumidifier to pick up the load. Air tightness of the window unit also counts. If it's in the budget, a cheap modulating ductless mini-split AC unit may be preferable, since it's air tightness is inherently higher than a window unit, and it's minimum-speed output is usually much lower than a half-ton window unit, so it will run much longer cycles for better latent cooling, and higher efficiency.

    3: An Energy Star dehumidifier would put out somewhat less "extra" sensible heat than the latent heat is removing, but that will have a comparatively small effect on power use relative to #1 and #2.

  4. GBA Editor
    Martin Holladay | | #4

    Thanks, Dana. My listed remedies were not intended to be in order of importance -- the numbering system was to allow people like you to reference my points. Thanks for the useful clarification.

  5. Luke Brechtelsbauer | | #5

    As of two days ago, I am only running the A/C because the dehumidifier started covering its backside with a giant layer of ice. It keeps the room at 70 without running a whole lot... but humidity is 70% or more (it is 80% or more outside right now)

    My main question was if there is some other ducting arrangement where the warm moist air can be ducted outside or something.

    I have added some weatherstripping, but I need to work on sealing some more things. The room has lots of single pane glass...I but no solar gain.. I am just amazed at how much power it uses.

    It seems like Charlie is suggesting to use the AC to hold the minimum temperature and then see what the RH% does? I have been keeping it around 70F and there is still definitely too much moisture.

  6. GBA Editor
    Martin Holladay | | #6

    Luke,
    How old is your dehumidifier? It may be defective.

  7. D Dorsett | | #7

    The outdoor relative humidity is irrelevant, since it's relative to the outdoor temperature. The outdoor dew point is the measure of it's absolute (not relative) humidity.

    The warm moist air is what's coming in, creating the latent load. The warm air coming out of the dehumidifier is much drier than the room air. Pulling up a dew point graph on weatherspark.com, the outdoor dew point in New Orleans as of this minute is about 68F which is close to the outdoor temperature (it's over 95% RH outdoors at their weatherstation), which at a 70F indoor temp would translates to about 93% relative humidity. That's a heluva latent load, despite having effectively zero sensible load.

    https://weatherspark.com/#!dashboard;q=new%20orleans%20la%2C%20USA

    If the dehumidifier is icing up it's either low on refrigerant, has corroded coils, or sensors that are off. It's efficiency when iced up is EXTREMELY low, which adds quite a bit to the "extra" sensible cooling load. You probably need to replace it (with an Energy Star version, to minimize the additional sensible load.)

    Air seal those single pane windows first, but also look at floors, ceilings and partition wall leaks, such as electrical & plumbing pentrations, etc. A window fan pressurizing the room might help find most of the other leaks, but the window is by far the main suspect.

  8. Charlie Sullivan | | #8

    I agree that it sounds like your dehumidifier has a problem. If you are shopping for a new one, in addition to energy star, I'd look for one with a humidistat--that can be set to run just enough to maintain the humidity you set. To reduce energy use, you will probably want it to turn off its fan when it doesn't need to run. The fans tend to use cheap fan motors and draw more power than they should, so having that run continuously is an unnecessary power draw. Mine can be set to run the fan continuously or not; some don't give you the choice. It may be hard to tell from the packaging whether the fan runs continuously or not.

  9. Charlie Sullivan | | #9

    Also, on the question, "My main question was if there is some other ducting arrangement where the warm moist air can be ducted outside or something."

    As Dana says, the air coming out is dry. But it is hotter than you want. The alternative in which that hot air stream is outside is the air conditioner. You could imagine a hybrid, in which some of the heat was kept inside and some sent outside, so that the net was zero heating and zero cooling. I don't think anyone make such a thing for residential scale use. So what you have to do mix the operation of the A/C and the dehumidifier to get the right net temperature that you want. Think of the A/C as a dehumidier that dumps all its heat outside. If you want to dump 50% of the heat outside, you mix the operation of the dehumidifier and the A/C 50:50.

    Probably not practical for you, but to satisfy your thought that something like that should exist, there are alternative types of dehumidifiers that do work by blowing warm hot air outside instead of condensing the water vapor. Unfortunately, the smallest commercially available unit like that that I know of is a $3k whole-house unit. It can run on solar heat, so it's electricity draw could be very low, at least in theory, but by the time you get a plumber to set of the solar water heating system for it, it will be more expensive than an antique musical instrument.

    http://www.novelaire.com/dehumidifiers/residential-dehumidifiers/hot-water-dehnumidifier.html

  10. Luke Brechtelsbauer | | #10

    Thank you everyone for your thoughtfulness.

    I tried turing the AC up to cool the room to close to 65. There is still too much moisture (upper 70's and lower 80 %s)

    I am chucking out the dehumidifier with the ice problem.

    Many window air conditioners have a 'cool' setting and a 'dry' setting. I haven't found one that will cycle automatically between the two, using both a thermostat and a humidistat.. but that sounds like what I am wishing for.

    I will also work on sealing gaps to an adjacent closet and I can double up the glass panes to the outside at some point.

  11. GBA Editor
    Martin Holladay | | #11

    Luke,
    One more thing to consider: you may have a bad hygrometer. What type of device are you using to measure the indoor relative humidity?

    Cheap hygrometers are notoriously inaccurate. Buy two or three new ones -- not necessarily the cheapest ones you can find -- and see if they agree.

  12. Luke Brechtelsbauer | | #12

    I have three super cheap ones from ebay in different parts of the room. they are two different brands and they agree pretty well. They seem to fluctuate in ways that make sense, but yes, they are cheapos.

    I can independently verify that humidity is too high now by a broken harp string.

  13. GBA Editor
    Martin Holladay | | #13

    Luke,
    Ah, yes -- the broken harp string test. OK -- I believe you.

  14. Expert Member
    Dana Dorsett | | #14

    What is the wall construction type?

    What's under the floor?

    What's above the ceiling?

    How are you cooling & heating the rest of the building / house?

  15. Luke Brechtelsbauer | | #15

    I have 2x6 joists above and below, with R-19 batts in them. Under the house is 1/2" rigid foam, with foil on both sides. The exterior walls are 2x4 with R-13. One issue is the interior wall, which is uninsulated to the rest of the house, which I usually keep open and breezy to the outdoors in summer (no A/C there). I will be insulating that wall shortly and adding a vapor barrier. The fourth wall is all 1/4" single pane glass so we can look at the harps all day long.

    Yesterday I swapped my old analog 5k BTU window unit for a new digitally controlled one. It has a 'dry' mode which is keeping the RH in the 60's at 65F... so a definite improvement... I also went from a CEER of 8 to 11.6 or something like that.

    Now I am wondering if it is more efficient to run a dehumidifier and keep the room warmer than to keep it so cool. Also keeping it warmer would be less of a shock to the instruments when they go out on business.

    I am still shopping for a dehumidifier.

    I have a small compressorless "peltier-effect" dehumidifier which does absolutely nothing. don't get one of those.

  16. Expert Member
    Dana Dorsett | | #16

    As long as the partition walls are air tight you don't need to add vapor barriers- latex paint is good enough.

    If there is access to the foil-faced polyiso under the floor, tape the seams with a decent foil tape, and use can-foam to seal any electrical & plumbing penetrations. R19s are not very air retardent, and it may be worth cutting in some air-barriers along the joist bays at the partition walls to prevent air leaks elsewhere from having a path to that moisture-critical room.

    Air sealing the ceiling, including the electrical boxes behind any lighting fixtures etc. will reduce stack effect infiltration drives.

    Simply raising the temp of 65%RH 65F air to 70F results in 55% RH air. If you're raising the temp by converting the humidity into sensible heat with a dehumidifier you'd be under 50%. It sounds like with your better window AC plus an Energy Star dehumidifier you'd have no problem keeping up with the load, but it's still worth reducing the load to the extent possible by air sealing.

    Peltier effect dehumidifiers can work, but at extremely low efficiency compared to compressor based units. Most standalone PE dehumidifiers have extremely low capacity too, which is why you don't see much effect. They are appropriate for a limited number of applications, where the vibration & noise of compressor based solutions would be unacceptable and the humidity loads are modest.

  17. Joe Suhrada | | #17

    So it is a harp. I was wondering.

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