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HVAC - help designing dehumidification system

  1. Oct 17, 2010 #1

    I am trying to design a "dehumidification system" for my home; I use quotes because this not really much of system, I am simply deciding what rating of a portable dehumidifier to buy!

    This is my first real world HVAC application and I wanted to know if what I am doing really makes sense.

    A house which has one exterior wall that is partially, about 3 feet, below grade. This below grade wall elevates the humidity inside the house above the desired comfort level (I assume around 40% to 60% relative humidity is ideal). High humidity is most noticeable in winter time when heavy condensation develops on the only 2 aluminum framed windows in the house that haven't been replaced with new vinyl windows yet. My proposed solution is to use a portable dehumidifier to lower the humidity in the rooms that are adjacent to the problem exterior wall. However I am not sure how powerful of a dehumidifier I need. And the manuals and specifications supplied by the manufacturer don't really provide much guidance. The manufactures supply one specification to describe the work: rate of water extracted per day; one of the higher rated residential portable dehumidifies is rated at 70 pints per day (Frigidaire FAD704TDP).

    Proposed Solution:
    To find out how powerful of a dehumidifier is needed I need to figure out the following:
    1.) define how much water is in the air of my home in its current humid level and compare that to a target, more comfortable, humidity level.
    2.) I also need to make some assumption on how much humidity is being added into the home every day. I am not sure how to accomplish this step besides some on-site experiment measuring rate of humidity increase following a room drying cycle with a dehumidifier.
    3.) With 1.) and 2.) I will know how much water needs to be pulled out of the home on a daily basis. However I don't think I want to simply find a dehumidifier that meets this minimum requirement. Instead I want to define the machine that will accomplish the task using the most energy efficient operating cycle. That would mean evaluating energy consumed by the machine during steady-state and start-up conditions compared to its dehumidification rate (pints/day).

    Once I complete all three steps I think I will have clear idea on what size dehumidifier is best!

    Solution Progress:
    So far I have worked only on step 1.) above. And I find the result quite surprising (so i guess i probably made some dumb mistakes)! Even when I assume a worst case scenario of 100% relative humidity the volume of water within the house is quite low compared to the capabilities of the dehumidifiers; only a total of 7.5 pints are suspended in the air of the 7200 ft3 space. I used excel to model this step and I can update these calculations when I work on step 2.). The formula used in the calculations are listed, and the notation I used is pretty obviously labeled. The formatting is a little rough, let me know if anything needs clarification.

    http://lh6.ggpht.com/_cu78o75EUkw/TLtyFFCa3sI/AAAAAAAAAPc/RQQe7P6dG7A/dehumid-calc.jpg [Broken]

    Questions for forum:
    a.) Is my proposed solution, steps 1.) 2.) and 3.), valid? This is my first HVAC work, I wouldn't be surprised if I missed something big!
    b.) Could you please comment if my work in solving step 1.) has any errors?
    c.) Could you please provide some advice on how to evaluate step 2.)?

    Thanks for your help!
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 18, 2010 #2


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    Gold Member

    I have no knowledge in this area but can suggest that you post at

    http://forum.doityourself.com/" [Broken]

    I have received good advice in the past from the professionals there. They have a forum devoted just to humidifiers and de-humidifiers!
    Last edited by a moderator: May 5, 2017
  4. Oct 19, 2010 #3
    Russ would be your best bet at answering this, however the way I'd look it is like this:

    Ignore the water removal rate of the dehumidifier and concentrate on airflow.

    If you have a room that's 20m^3 and the humidifier can process 1m^3 per hour of air, it will take 20 hours to completely process the air in that room.

    You should work out the volume of the volume required for the dehumidifier to cover and then find a system that can cover that. Remember to allow for drafts and take into account that you don't want to remove all of the moisture from the air, that would be just as uncomfortable.

    If your overall volume is 50m^3 and your system can process 1m^3 per hour it's going to take 50 hours to cover everything. (This is assuming no drafts.) This system wouldn't be very effective. You'd want something with around 10m^3 per hour to give you a 'complete air change' roughly every 5 hours.

    I'd also say that portable humidifiers aren't really designed for multi-room use. They are really for one room only so I'm not sure how well they'd fair with being one room and you expecting them to do others as well. The only way this would be effective is if you could control the airflow, which isn't particularly realistic for a private home situation.

    As a side note, don't humidifiers come with instruction on what size room they can handle?
    Last edited: Oct 19, 2010
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