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  1. #1

    Oversizing a dehumidifier?

    What are the disadvantages to oversizing a dehumidifier? Is it even meaningful to say that a dehumidifier is oversized? With A/C, I want longer run times because they lead to better dehumidification. But if a dehumidifier quickly brings the RH down to where it needs to be, what is the problem with that?

    One clear disadvantage is that larger dehumidifier cost more up front. What isn't so clear to me is whether an oversized unit costs more to run.

  2. #2
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,721
    This equipment is inefficient at startup. If it satisfies quickly it does so inefficiently.

    Equipment wear and tear mostly occurs at startup. Often equipment is rated in cycles. Say it's good for 100,000 cycles, if it's oversized you'll be replacing it sooner. Oversized equipment will cycle move over a given period.

    Bigger equipment is often noisier. Bigger equipment leads to bigger spikes. Not good for other equipment such as computers, etc...

    Others will probably offer more reasons...
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  3. #3
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,317
    I'm at a loss as to why one would intentionally oversize any piece of humidity control equipment if the means exist to properly size it.

    Equipment is oversized in a general sense any time it operates at less than design conditions. However, when it is operating at design conditions, and it is oversized, it costs more to operate, even if it remains in a steady state mode of operation (i.e. no cycling).

    Know your load. If you just throw equipment at a problem but don't really grasp the expected range of load it will face over its lifetime, you're doing both yourself and/or your customer, as well as the equipment, a disfavor.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  4. #4
    Okay, that's kind of what I figured. I ended up--kinda due to legacy reasons--with a couple of portable dehumidifiers in my basement, each of which easily has more capacity than is needed for the entire basement. Maybe I'll use them until they break, and then I'll replace them with smaller units.

    With respect to portable dehumidifiers, I'm not sure that larger means noisier. Maybe it does.

    With respect to overall capacity, all I need is one small one. However, I originally had only one large one, and while that unit brought the RH in its own room down below 50%, it had little effect on the adjacent room, let alone the entire basement. So I got another one to place on the other side of the basement, and that worked. Part of the reason for the higher capacity was the hope that keeping the (finished) basement dry would have a significant impact on the humidity in the rest of the house. It didn't. The humidity problem on the other floors has been solved in other ways, so I guess I can stick with two small dehumidifiers, one on each end of the basement, and leave it at that.

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,341
    Dehumidifiers are like a/cs. They take time to load the cooling coil/pan with moisture before moisture goes down the drain. At the end of drying cycle, the moisture on the coil re-evaporates back to the space. Therefore short cycles have the same effective on the dehumidifier as the a/c. A wide deadband on the dehumidistat reduces the on/off cycles. A 3-6% dead band is minimum during lite load conditions. A dehumidifier should be only as big as need for the peak load.
    Operating dehumidifiers on a timer that timesout the dehumidifier when noise is noticeable or during times of the day that the load is light is another way to make the process more efficient. 6-8 hours off during the middle of the day could save energy.
    A strong drying cycle once a day will control mold. Replace to small inefficient res dehus with one properly sized ducted dehumidifier that would circulate dry air throughout the space you are controlling which could be your entire home including fresh make-up ventilation.

    Keep us posted.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  6. #6
    The UAs are too big for this purpose, TB. I don't need to dehumidify the whole house--just the basement. And because the A/C is already doing some of that, I need only 10-30 pints a day...and the 30 tends to occur when we're out of town for a couple days and the A/C is set forward. Heck, with the dehum function on the Evolution, that may not be an issue again. So let's say I need to do 10-20 pints a day.

  7. #7
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,341
    Quote Originally Posted by LooseOnTheLead View Post
    The UAs are too big for this purpose, TB. I don't need to dehumidify the whole house--just the basement. And because the A/C is already doing some of that, I need only 10-30 pints a day...and the 30 tends to occur when we're out of town for a couple days and the A/C is set forward. Heck, with the dehum function on the Evolution, that may not be an issue again. So let's say I need to do 10-20 pints a day.
    Glad to here of your concern for indoor humidity levels and success with humidity control.
    The amount of moisture that needs to be removed from your home is dependent on the amount of outside fresh air infiltration/ventilation, the moisture content of the fresh air, the desired moisture level in your home, and the number of occupants in the home.
    Green grass climates typically have 65-70^F dew point outside. Most homes need a fresh air change in 4-5 hours minimum when occupied. 75^F, 50%RH is a 55^F dew point indoors. Two occupants add about 1 lb. per hour.
    From your moisture removal and number of occupants, you can calculate the amount of fresh air infiltration/ventilation you are getting in your home.
    Typically homes need 3-6 lbs. per hour removed to maintain <50%RH. Homes that need large quantities of dehumidification are getting excess infiltration or have water intrusion.
    I hope you are getting adequate fresh air change to purge indoor pollutants and renew oxygen. Homes naturally infiltrate more fresh air during cold windy weather and very little during calm, warm weather.
    Monitoring the CO2 levels in a home during occupancy also will verify the fresh air change rate of your home during actual conditions. 700-800 ppm CO2 levels are typical for 2 occupants in 2,500 sqft. home with an air change in 4-5 hours.
    This is the intended use of the Ultra-Aire whole house ventilating dehumidifier.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  8. #8
    I'm not even sure there's enough room in our utility closet to fit a ducted dehumidifier. The contractor who replaced our furnace wasn't able to squeeze in a humidifier.

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