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  1. #1
    Join Date
    Jan 2002
    Posts
    40

    Crawl space ventilation fan w / dehumidistat

    I am trying to help someone with an under vented crawl space ( Atlanta GA area) . No more vents can be added due to the HO's architectual considerations, so it looks like a small exhaust fan with a a dehumistat . I would like some advice on what to do but don't want to take it to the $/complexity extreme like the stuff I saw at http://www.atmox.com . - thanks

    [Edited by rkpatt on 10-12-2004 at 10:53 AM]

  2. #2
    Some considerations....
    If you utilize the ventilation fan to blow fresh air into the crawl space, you will increase the crawls static pressure, possiably forcing crawlspace smells and other airborne particulate into the first floor... They may call back with the report of an odor... If you utilize the ventilation fan to pull stale air out of the crawl space, you may cause a slight decrease in static pressure, causing the migration of living space into the crawlspace, creating negative pressure problems in the living space... While they won't call about this, they might if they had other factors causing negative pressure... The best fixes I've found are sump pumps to alleviate pooled moisture, laying down heavy plastic sheeting and securing it to the crawl space floor so that it cannot come up and that it'll keep a majority of the ground moisture under the sheeting (I've seen this done, and if done properly, works VERY well)... and if you must use a ventilation fan, use 2, one introducing fresh air into the crawl and one removing stale air from the crawl to maintain static pressures, but be forwarned, if the duct work is in the crawl space and you run the fans during the summers, the ducts WILL sweat, and if they leak at all, you'll get odors...

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,538

    Re: Crawl space ventilation fan w / dehumidistat

    Originally posted by rkpatt
    I am trying to help someone with an under vented crawl space ( Atlanta GA area) . No more vents can be added due to the HO's architectual considerations, so it looks like a small exhaust fan with a a hehumistat . I would like some advice on what to do but don't want to take it to the $/complexity extreme like the stuff I saw at http://www.atmox.com . - thanks
    Here's the latest way to handle a crawlspace. This is proved by several building researchers. After stopping the evaporation of moisture from the soil by covering the soil with heavy plastic, close all the outside vents. This stops the infiltration of moist outside air. Moving more moist air into the crawl only makes the space more damp. Operating a fan causes negative pressure that sucks air from the house causing more moisture to enter the home also. After closing the vents, add enough dehumidification to maintain <50%RH. It takes about 1 pint of dehumidification per 25 sq.ft. of crawlspace. If you want to do it right get a high efficient, low temperature dehumidifier like Santa Fe.

  4. #4
    Join Date
    Jan 2002
    Posts
    40

    Re: Re: Crawl space ventilation fan w / dehumidistat

    I have explored the dehumidifier idea too but equipnent and operating costs are high . Also yikes, what happens it fails with vents closed - Most HO's wouldn't check it on a regular basis .



    --snip -[/QUOTE]

    Here's the latest way to handle a crawlspace. This is proved by several building researchers. After stopping the evaporation of moisture from the soil by covering the soil with heavy plastic, close all the outside vents. This stops the infiltration of moist outside air. Moving more moist air into the crawl only makes the space more damp. Operating a fan causes negative pressure that sucks air from the house causing more moisture to enter the home also. After closing the vents, add enough dehumidification to maintain <50%RH. It takes about 1 pint of dehumidification per 25 sq.ft. of crawlspace. If you want to do it right get a high efficient, low temperature dehumidifier like Santa Fe. [/B][/QUOTE]

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,538

    Re: Re: Re: Crawl space ventilation fan w / dehumidistat

    Originally posted by rkpatt
    [B]I have explored the dehumidifier idea too but equipnent and operating costs are high . Also yikes, what happens it fails with vents closed - Most HO's wouldn't check it on a regular basis .

    --snip -
    Using a unit like Santa Fe, reduces dehumidifing cost by 75%. Residential units remove 2 pints per KW. Santa FE is +5 pints per KW. If the dehumidifiers quits, the humidity level in a sealed crawl is relective of the home above it. This is because no moisture enters from the earth or outside because of plastic on the earth and closed vents. A greater risk is possible flooding. Also suggest a %RH humidity meter with a remote sensor/alarm like Oregon Scientific from Radio Shack for $52.
    Get the meter and try different approaches. The sealed space with adequate dehumidification is the sure way for all weather conditions. If you find something better, let us know.

  6. #6
    Join Date
    Jul 2004
    Posts
    25
    Best active solution I've seen:

    http://www.smartvent.net/crawlspacevent.htm

    Samples outside and crawlspace air to determine appropriate times to run the fan(s). Can add slave fan for added air flow on large foundations. Know a couple folks who have these installed and they've had good results. Will be looking to add this setup to my crawlspace at some future point when I have time to fool with it.

    Short of a dehumidifier, this is the way to go... would not recommend use of any continuous operation fan configuration due to the fact that continuous operation can actually make problems worse by drawing warm humid air into a cool crawlspace where condensation can occur on ductwork (as mentioned by a previous poster). This gizmo has the brains to minimize the chances of that happening. Will also attempt to elevate crawlspace moisture should the crawlspace humidity fall too low in the winter. Pretty neat toy...

    Hope this helps!

  7. #7
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,538
    Remember whatever strategy you use for a crawlspace, +48 hours of +60%RH grows mold. The basic pyhsics are as follows. There are no ventilation strategies that control crawlspace humidity during high outdoor dew points of +60^F . 90^F, 50% RH outdoor air cooled in the crawl to 70^F, is 92% RH. During every night and rainy day, expect high humidity in the crawlspace with a ventilation of warm low relative humidity air on dry days. High humidity in a crawlspace for several summers may make your home unlivable for mold sensitive people. Get assurance of humidity control in writing and track it with a %RH meter. Radio Shack has a neat remote %RH meter for monitoring remote areas. $53 get the remote %RH meter and the reciever/%RH meter for your home, all batteries, no wires. Covering the earth with plastic, closing the vents, and dehumidification is the only way to assure <50%RH during the summer months in a green grass climate.

  8. #8

    !!!

    As already alluded to, fans in the crawl space pull a negative pressure on the living area (worse in some houses than in others). This can cause a rise in humidity in the living area when the AC is not engaged. If it is a central HVAC system this negative pressure can (and most likely will) cause the HVAC system to work overtime. Think of a chimney in reverse and always remember that air replaces air. If fans are used, the crawl space air goes to the outside, the living space air goes to the crawl, the attic space air goes to the living area, and outside (in GA, humid)air goes to the attic.

    I personally agree with the poster about sealing of the crawlspace. I have used his solution numerous times in essentially the same climate you are in. I use a MIL 8 or better plastic over the ground, seal off the crawl space vents and use the thermastor dehumidifier. I have yet to have a call back and the humidity is always in the desired range. Your comment about the HO not servicing the unit is true enough, but we fix the problems, we do not babysit them for eternity. All you can do there is make sure you plant the seed of service importance to the HO.

    Cost of operation of the thermastor unit in my area (north Alabama) is approx $1.27 / day if it runs nonstop. So...38 bucks a month verses replacing floor joist, subflooring, etc. not to mention the cost of mold remediation doesn't seem too outlandish to me.

    One other note...the EPA has been backpeddling on foundation vents of late. The guy who started all of this "open the vents in the summer and close them in the winter" line of thought lived in Arizona not in our hot humid climate. You can go to EPA.gov and it will back up what we have told you.

    Parker

  9. #9
    Join Date
    Jul 2006
    Location
    North Carolina
    Posts
    1
    Quote Originally Posted by rkpatt View Post
    I am trying to help someone with an under vented crawl space ( Atlanta GA area) . No more vents can be added due to the HO's architectual considerations, so it looks like a small exhaust fan with a a dehumistat . I would like some advice on what to do but don't want to take it to the $/complexity extreme like the stuff I saw at http://www.atmox.com . - thanks

    [Edited by rkpatt on 10-12-2004 at 10:53 AM]
    We are ATMOX
    Too bad you consider us too "complicated". We did not invent the nasty crawlspaces which can be complex in their nature. Bad we have a cure for it that works and can be mastertailored.

    If you talk to us maybe we can explain the system! It's not that complicated and the fans are the most quiet in the industry! Just call 704-867-3772

  10. #10
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
    Posts
    2,919
    Try this link for an explanation of crawlspace moisture control.

    Solve Crawlspace Moisture Problems:

    http://www.contractingbusiness.com/2...le/False/7253/


    Also check out www.rlcengineering.com.

    This guy did his doctoral thesis on crawlspaces.

    AdvancedEnergy has a good site as well. So does BuildingScience.
    Remember, Air Conditioning begins with AIR.

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