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Is there an energy efficiency penalty for an oversized dehumidifier?

Alan B | Posted in Energy Efficiency and Durability on

If i buy one that is oversized for my basement it will presumably run fewer hours a day at higher wattage, is there any penalty or lower effectiveness on this?
The one i am considering is 1.85L/kWh energy star rated compared to another 1.85L/kWh unit.

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  1. Alan B | | #1

    I can choose between 30pint/day, 50pint/day and 70 pint/day units from the same manufacturer, all are energy star rated

  2. Charlie Sullivan | | #2

    There's a bit of a penalty for cycling--when it sits idle, the evaporator goes from cold up to room temperature, and so there is energy expended to bring it back down before the dehumidification can start again. If you were comparing a higher-efficiency large unit to a low-efficiency small unit, I'd still opt for the large unit, but given that all are rated the same, I'd expect the smallest one to consume slightly lower energy because it would cycle less.

    Perhaps another advantage of the small one is that if it isn't quite powerful enough to keep the humidity down where you want it, you'd have an incentive to find how the moisture is getting in (air leaks or moisture coming through the concrete) and reduce the source. If you get both low enough you should be able to maintain low humidity with even the small dehumidifier running very little.

  3. Alan B | | #3

    @ Charlie
    You make some very good points, the crawlspace is responsible for most of the air leakage and humidity entering the house (realized from the energy audit blower door and noticing humidity tracks outdoor humidity/temperature in a predictable manner).
    But after crawling in there and getting very dirty i can't find actual gaps the air comes through though the floor above does get ice cold in winter confirming it is the crawlspace, and the rest of the basement is brick walls and concrete floor, which are not appreciably damaged but do suffer from mild efflorescence.
    I was planning on sticking with the large dehumidifier, but i would prefer the smaller one, i may now reconsider, but i cannot afford an engineer to determine if and how deep i can dig, and to dig out the crawlspace, and its too uneven for plastic and not dug out enough to easily access.far parts of it.
    According to the EPA at 700 sq/ft (2/3 basement, 1/3 crawlspace) should need 23 pints at full load, however i cannot be sure my house fits their models
    I am planning on going tomorrow and getting it so i need to decide by then

  4. Charlie Sullivan | | #4

    Given that all the sources of moisture are unintentional, it's very hard to estimate how big they are. The EPA sizing table is just a rough guide.

    Since the efficiency penalty for a larger size is small, and the uncertainty in the size needed is large, I'm now thinking I'd opt for at least the 50 pint model.

  5. Alan B | | #5

    That was my conclusion as well, i bought the 70 pint on sale a few weeks ago, it was out of stock, so they are supposed to call me when it comes in, the 30 pint is on sale this week for 10% less then the 70 pint, i could easily get either one, if i had an old dehumidifier and had numbers to work with i could say if the 30 pint would suffice because i would prefer it if it does.
    The 50 pint is the most expensive option ironically!
    At this point the best option seems to be to stay at 70 pint :(

  6. Alan B | | #6

    For anyone who comes across this thread looking for advice, learn from my mistake, there is an energy penalty to over sizing the dehumidifier, i bought the 70 pint (choice between 30 and 70) and after 7kW it has taken out about 4L of water, even though its rated at 1.85L/kW.
    Basically it seems to come down to this, the fan uses 77W on low and 81W on high, so most of the energy seems to go to the fan (which would likely be less wattage on a smaller unit) and the rating is likely only when the compressor is running with a large humidity difference. So a smaller unit likely uses a smaller fan, and runs on a higher duty cycle, both leading to greater efficiency

  7. Charlie Sullivan | | #7

    Thanks for reporting back, and I'm sorry we didn't manage to steer you clear of an unfortunate choice. Can you return it?

    Not all dehumidifiers keep the fan on when the compressor cycles off. It might be hard to figure that our from the box on online catalog page, but some even allow you to switch whether the fan stays on or not.

    Even without the constant-on fan issue, you can't expect to get the rated L/kWh in practice, because it depends on the humidity and temperature you are operating in. They are rated at 80 F, 80% humidity (if I remember right), which is an unrealistic choice--if it's that hot, you'd want to be air conditioning instead, and presumably you want to get the humidity lower than 80% in steady-state.

    The fan power consumption sounds like a typical low-efficiency single-phase induction motor--way more energy use than it should have and using almost the same power on low as on high. An ECM fan motor might use 40 W on high and 10 W on low for the same airflow. Too bad the energy star threshold is set low enough that it doesn't force that upgrade.

  8. Alan B | | #8

    I can return it, but now the sales are gone, its almost $100 more for the smaller unit :(

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