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Thread: Dehumidifier in Basement

  1. #1
    DIY Senior Member rockycmt's Avatar
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    Default Dehumidifier in Basement

    My friend put in a aprilaire 1730 in his basement (about 1000 sqr feet) and it made a world of a difference to the air quality of the rooms in the basement. I want to do the same. He has it as a standalone setup not tied into his HVAC. I plan to hang it from the ceiling near my haundry sink to have it self drain.

    Questions.
    Where is the best place to order this online?
    Should I get the 1750 for a few $s more?

  2. #2
    In the trades Dana's Avatar
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    Basement humidity in the NE is predominantly an air-infiltration problem, and dehumidification is usually only necessary in basements when the outdoor dew points are ~55F or higher (since 55F dew point air pulled into a 68F basement raises the relative humidity of the basement to ~65%.)

    If you air-seal the band joist & foundation sill with an inch or two of closed cell spray foam (or careful application of caulk & 1-part foam such as GreatStuff) and tighten up all exterior windows, doors & flues, the house pulls in less outdoor air into the basements and humidity loads for a whole basement are easily handled by a mid-sized room-dehumidifier. Many/most have provisions designed in for a hose attachment to allow draining to a sump or sink rather than relying on manual dumping. I have 1500' of basement, and a ~500W room dehumidifier that keeps the whole basement under 60% RH with PLENTY of margin, even with outdoor dew points well into the 70s. Mine is set up to drain into an existing sump. (The slab is near the average water table- even during the dry season the groundwater water is less than 2' below the slab, so we have several sumps with pumps to keep the slab dry during the the springtime rainy seasons when the groundwater rises a bit.)

    If you want to go one better (it's a bigger project, but with bigger payoff), insulating the basement walls to R15+ (1-2" of extruded polystyrene on the foundation walls + 2x4 studwall snugged up against it with UNFACED R13 batts, no interior vapor retarder/barrier of any type), the temp of the basement will come up several degrees, which lowers the relative humidity. (Depending on where you are in NY you may need more than 1" of XPS to prevent wintertime moisture accumulation in the studwall, but 2" would be more than enough everywhere.) If the basement walls are already finished sans-insulation, there may be other options, but the options depends on the particulars.

    To keep mold at bay you need only keep it below ~60% relative humidity, but the lower you set it, the greater the duty cycle when outdoor dew points are high- it can add up to quite a bit of electricity in a season if you're going for a bone-dry 40%RH.

    True, an Aprilaire 1730 will run at higher efficiency, than the best of the room-versions but they're expensive enough to consider applying that money to a mini-split heat pump with a dehumidifcation mode for only ~2x the installed cost of the Aprilaire that could also heat & air condition the basement at high efficiency. (IIRC Samsung, Sanyo and several others have an explicit dehumidify-mode, as well as sensible-cooling & space heating modes of operation.) The better mini-splits are WAY more efficient than window-shakers in cooling mode, and even at 20cent/kwh electricity cost less than half as much to operate than a state-of-the art oil or propane burner (about the same operating cost as $1.50/therm natural gas in a 90% AFUE burner when in heating mode when it's 20F outside, cheaper when it's 30F+.) A mini-split with the interior unit mounted over the sink (either a ceiling or wall-type mount) would be as easy or easier than an Aprilaire.

    Enough derailment- I don't know if online is better than local, but if you can find one through local distribution the support might be better than a random online vendor.

  3. #3
    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by Dana View Post
    Basement humidity in the NE is predominantly an air-infiltration problem, and dehumidification is usually only necessary in basements when the outdoor dew points are ~55F or higher (since 55F dew point air pulled into a 68F basement raises the relative humidity of the basement to ~65%.)
    Here in the midwest the dew point is rarely that low in summer. Typical is about 65-68 F ambient dewpoint.

    With high ceilings, multi-level and open stairwell from the basement it is tough to keep humidity in the basement in check. The neighboors all comment on having the same issues with temp stratification. Cool air sinks to the basement which is sitting on/surrounded by cool earth. In our case the delta from the upper floors to the basement runs about 8 F in summer. And that pushes 50% humidity air from the upper levels to around 60+% in the basement.

    If I were to design the AC system for a home like this one from scratch (3 effective levels with open floor plan) I would set up the AC to work primarily as a dehumidifier in the basement...then recirculate/exchange air from there with other levels to do the temp control on those levels with only supplemental AC required for the other levels during peak demand (ambient north of 100 F).

    If you air-seal the band joist & foundation sill with an inch or two of closed cell spray foam (or careful application of caulk & 1-part foam such as GreatStuff) and tighten up all exterior windows, doors & flues, the house pulls in less outdoor air into the basements and humidity loads for a whole basement are easily handled by a mid-sized room-dehumidifier. Many/most have provisions designed in for a hose attachment to allow draining to a sump or sink rather than relying on manual dumping. I have 1500' of basement, and a ~500W room dehumidifier that keeps the whole basement under 60% RH with PLENTY of margin, even with outdoor dew points well into the 70s. Mine is set up to drain into an existing sump.
    Two things I would like to do eventually: tightening the walkout basement more, and adding a dehumidifier. The window installation in the walkout that the previous owner had done a few years ago was typical average tradesman level crappy work. Probably need to pull them out and reinstall to get it right because they have been a bitch to seal (rain blows through the casing at times.) What really irks me from the original house build is that there is no insulation at the header or band joists (builder was apparently allergic to insulation.) Nor is there any insulation in the plumbing/vent cavities in exterior walls that I've found so far. Putting it in after the fact is not so easy without remudding/resurfacing, and repainting all affected walls.

    If you want to go one better (it's a bigger project, but with bigger payoff), insulating the basement walls to R15+ (1-2" of extruded polystyrene on the foundation walls + 2x4 studwall snugged up against it with UNFACED R13 batts, no interior vapor retarder/barrier of any type), the temp of the basement will come up several degrees, which lowers the relative humidity. (Depending on where you are in NY you may need more than 1" of XPS to prevent wintertime moisture accumulation in the studwall, but 2" would be more than enough everywhere.) If the basement walls are already finished sans-insulation, there may be other options, but the options depends on the particulars.
    I did this in one of the rooms already that is north facing, used 2" XPS to fill each existing stud cavity snugly, not the batts as I feared moisture from infiltration. Made the room much more comfortable in deepest winter, as did adding an additional register so that it is now easy to set temps there, I can make it downright toasty now if I want just by manipulating the new register.

    I need to insulate one other semi-full basement room this way (empty stud cavities behind painted/finished drywall at present...did I mention the builder's insulation allergy???), but am planning to convert it into a large 4th bedroom by cutting out a window for full egress, adding insulation/sound deadening to ceiling/living room floor, adding an air return and 2nd register, and enlarging a closet and adding a bedroom door (these latter two are tricky with the current stair arrangement.) I figure I can do the full project for a few grand and add about 15-20 k to resale while making the space much more usable/comfortable for guests and for the kids as a playroom.

    The better mini-splits are WAY more efficient than window-shakers in cooling mode, and even at 20cent/kwh electricity cost less than half as much to operate than a state-of-the art oil or propane burner (about the same operating cost as $1.50/therm natural gas in a 90% AFUE burner when in heating mode when it's 20F outside, cheaper when it's 30F+.)
    Our gas pricing has been about half that for the past three years. Electric here is less than you noted but keeps rising and is at about 14 cent/kwh. (With utilities cost it is always a matter of locale when trying to figure this stuff.) I considered a heat pump briefly for the whole house but it looked like a wash compared to nat. gas. I went with the nat. gas condensing at 95% AFUE. There were two major negatives to heat pump in my locale: 1. It rarely works well comfort-wise in existing homes (talking with other homeowners who had them, they warned me away) because of the lower delta T of the heated air relative to a furnace and the typical undersized ducts. On low temp days, 0...-5...-10...-15 F, strip heaters would eat our lunch cost-wise---we had some strip heaters in a home as a kid during a very cold winter and I remember electric bills that exceeded anything I've had in the 30 years since. We shut them off and used a pot belly stove and fan to heat ~2000 sq. feet after that. 2. A nat. gas furnace can be operated in a blizzard or ice storm when power is out for days as long as an aux. generator is available to run the blower/board circuit. (In the old days we used our wood fireplace and cooked using a wood cookstove along with kerosene lamps and were quite comfortable without a generator...milking the herd by hand twice a day was a pain though...) Perhaps the same could be accomplished using nat gas back up for a heat pump with the blower/board connected to aux. generator panel. I wasn't sure on this latter point.

  4. #4
    In the trades Dana's Avatar
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    Most places in the midwest have average July dew points in the 60s, much like the NE. In the gulf states low-70s dew point averages are common- even more reason to make houses air-tight!

    Split-system air source pumps are cost-competitive with condensing gas in most of US zones 5 or lower, but in some places they'll beat them. Ducted air-source heat pumps only rarely beat gas on operating cost and as a retrofit to a ducted system they SUCK on comfort (exit temps too low). I wouldn't advise using a mini-split as a primary heat source in zone 6, but as a separately controlled heating/cooling zone for a basement, why not? Some have "dehumidify" modes, which make them into high-efficiency dehumidifiers when the sensible cooling load is low. In heating mode those with continuously variable blowers on the indoor units never give you that wind-chill blast of tepid-air the way ducted heat pump systems can. So long as they're actually keeping up with the load comfort is usually quite high. SFAIK no mini-splits use heating strips for aux heat, but some of the bigger multi-split systems do.


    For future basement- insulation project, ponder this:

    http://www.buildingscience.com/docum...study-analysis

    Note that the moisture analysis uses Minneapolis weather data. Note also the locations of capillary breaks at the footing & foundation sill, especially if you're going more than 2" on interior XPS or using foam with low-perm facers (foil faced iso), or you may drive ground moisture into the foundation sills by going too-low perm on the basement insulation.

    BTW: The whole-wall R of 2" XPS jammed in the cavities is about R7-7.5, after the (~R1.9) thermal bridging of the studs is factor in. But R7 is WAY better than R0, eh?

  5. #5
    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by Dana View Post
    Most places in the midwest have average July dew points in the 60s, much like the NE. In the gulf states low-70s dew point averages are common- even more reason to make houses air-tight!
    We've been running the low 70's dew points for daily averages much of the past week.

    Thanks for the info in the study.

    BTW: The whole-wall R of 2" XPS jammed in the cavities is about R7-7.5, after the (~R1.9) thermal bridging of the studs is factor in. But R7 is WAY better than R0, eh?
    Well, it was operating more like R1-R2 once the various interfaces were factored in. But, yes, the XPS is doing a pretty good job by comparison. Tearing out the existing studs and replacing was a much bigger project and would have required new trim as well. That is the smallest bedroom as well, so losing perimeter is/was unattractive. If I went that far I would remove part of the hall and move the door out several feet and reconfigure the closet to enlarge the room.

  6. #6
    In the trades Dana's Avatar
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    Southern New England hits the low to mid 70s for dew points for days on end several times a summer too, but at least that's not the AVERAGE (thankfully!) But with subsoil temps in the low 50s and outdoor air leakage and uninsulated foundations/slabs those conditions end up being hell on basements & crawlspaces mold & mildew point of view.

    By contrast, in wet-rainy Seattle summertime average dew points are in the low 50s, and when there are high humidity problems in basements it's usually a drainage problem. In well drained basements there dehumidifiers aren't needed, and crawlspace ventilation to the outdoors does in fact reduce rather than increase the moisture content of the joists & subfloors. It's quite the opposite in rockycmt's New York.

  7. #7
    Jack of all trades DonL's Avatar
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    Quote Originally Posted by rockycmt View Post
    Questions.
    Where is the best place to order this online?
    Should I get the 1750 for a few $s more?
    Yes, Amazon.com
    Get the 1750 or the best one that you can afford. A better one will just run for a shorter period of time, and save you money.

    Forget all of the Rocket Science, and have a great day.


    DonL
    Last edited by DonL; 07-05-2011 at 11:03 AM.
    Theory only works perfect in a vacuum.

    Cyber Security Protection for Windows C:\ > WWW.WinForce.Net

  8. #8
    In the trades Dana's Avatar
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    Yabbut, a tiny 3/4 ton mini-split with a dehumidify mode will cost about as much, run quieter, and provide more function. (Seriously!)


    There's no payback in oversizing the Aprilaire, and the latent loads of NY won't overwhelm even the smallest units unless you leave all the doors & windows open. The humidity loads of 1000-1500' of basement won't overwhelm a 3/4 ton mini-split with dehumidify mode either. Using a whole house dehumidifer for a basement standalone is using a sledgehammer for a flyswatter.

  9. #9
    Jack of all trades DonL's Avatar
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    I agree with you Dana.

    You are very good at explaining things. I enjoy your posts.

    The original post was a simple question.

    It was very nice of you to explain options.

    Thank You.

    Sometimes people want to get an answer to their simple question.
    They do not want to build a space ship, they just want to ride in it...


    Have a great day.


    DonL
    Theory only works perfect in a vacuum.

    Cyber Security Protection for Windows C:\ > WWW.WinForce.Net

  10. #10
    In the trades Dana's Avatar
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    Can I have a ride in that spaceship too? (Or is the perpetual verbose-mode gonna drive the other passengers crazy? :-) )

  11. #11
    Jack of all trades DonL's Avatar
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    Quote Originally Posted by Dana View Post
    Can I have a ride in that spaceship too? (Or is the perpetual verbose-mode gonna drive the other passengers crazy? :-) )
    lol. You have a valid point.

    They claim that you will be able to ride, In the near future. A company is selling tickets for rides now.

    The Russians will let you ride To OUR Space Station for a hefty price.

    The Company selling tickets for rides, is only for an orbit around the earth, but is much cheaper.


    Enjoy Your Day.


    DonL
    Theory only works perfect in a vacuum.

    Cyber Security Protection for Windows C:\ > WWW.WinForce.Net

  12. #12
    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by DonL View Post
    lol. You have a valid point.

    They claim that you will be able to ride, In the near future. A company is selling tickets for rides now.

    The Russians will let you ride To OUR Space Station for a hefty price.

    The Company selling tickets for rides, is only for an orbit around the earth, but is much cheaper.


    Enjoy Your Day.


    DonL
    or you can wait around for the next Hale Bopp comet visit and ride it free like the previous trip. Oh, it wasn't free for those suckers, was it?
    I am definitely not a pro plumber, but I am a pro crastinator

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