Basement Dehumidifiers

Basement Dehumidifier

Dehumidifiers use a lot of power, in the range of 600 to 850 watts.  If it’s running 24/day, 365 days a year that’s 5,256 to 7,446 kWh per year or about $950 to $1,200 per year at eighteen cents per kWh.  That’s like $100 per month!  But wait, you say, they don’t run 24/365.  They do if the homeowner has cranked the control all the way to Continuous or doesn’t know how to set it.  I have been in people’s homes where I have saved them about half their electricity bill by simply turning down the dehumidifier.

I put a data logger down in my basement this summer when it was really hot, humid nasty outside, running with an outdoor dew point above 70 F.  The temperature in the basement averaged about 70 F and relative humidity averaged about 70% RH.  The dew point cruised at about 61 F.  My data logger was recording the air temperature, humidity and dew point.  That doesn’t mean that there weren’t surfaces in the basement that weren’t below 61 F.  Most of the mass of the basement has been there long enough to reach an even temperature, however.  So there may be dark, damp corners, but for the most part, the entire basement and all the stuff in the basement was above the dew point.

Many of the new IR cameras have a dew point screen that can be used to figure this out.  Or you could put in a data logger or hygrometer and figure it out.  There are so many dehumidifiers running in so many places that we could make a major impact on energy consumption just be getting them set up and working properly.  (There is a nice little dew point calculator at: http://www.decatur.de/javascript/dew/index.html .)  There are a bunch of places that sell hygrometers that will provide the temperature and humidity.  You can plug those numbers into this calculator and get the dew point.

The dehumidistat controls have vague settings from OFF to NORMAL to

Dehumidifier Control Panel

DRYEST to CONTINUOUS.  What does NORMAL mean?  There is a difference in using the dehumidifier to maintain a comfortable humidity in the living space and the right humidity to keep mold from growing in the basement.  For the basement application, the dehumidifier should be set to the lowest possible setting to meet the need.  It should be set so that the RH is below the dew point.  It should definitely be cycling on and off.

Many of the product performance numbers are based on operation at 80 F and 60% RH which is a pretty high temperature for a typical basement (or even a house).  Some of the manufacturers rate their dehumidifiers at 100% RH which is not a condition you would ever want to see in a basement!  One measure of the efficiency of these machines is how many pints (Energy Star rates them in Liters) it can remove per kWh.  If you take the stated Water Removal Capacity (in pints) and divide it by 24, divide that by power consumption in watts and multiply the whole thing by 1000, you’ll get the pints per kWh.  In the handful of units that I looked at they ran from a low of 3.47 (1.8 L/kWh) to 6.49 (3.07 L/kWh).  There is an interesting little closet sized unit that came in at 10.65 (5.04 L/kWh)!  Something seems a bit off with that one.

Why is the dehumidifier in the basement?  If it is to keep the mildew off the suitcases stored in there, it just has to keep the RH low enough to prevent condensation.  If it is to remove standing moisture in the basement, then you probably need a pump instead of a dehumidifier!  If the house having an energy audit has a dehumidifier, it should be included in the audit.  It is more of an energy load than a whole lot of light bulbs.

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