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  1. #1

    Ventilating sealed crawlspace

    Hi,

    My first post here.

    I'm an owner builder now spec'ing the mechanicals on a 2300 s.f. wood frame house with EIFS in a mixed humid heating climate. Heat calc calls for 52,000 BTU of cooling and 66,000 of heating, and the shell of the house should end up extremely tight.

    Crawlspace will be sealed and insulated with foam around perimeter and poly under the slab.

    Projected HVAC setup to include a single high velocity air handler for the AC and, in a separate system, a single ERV with hydronic heat coil for the ventilation, supplemented by a gas fireplace. These systems are merely hypothetical, have not been designed and integrated yet.

    Question: I understand that I need another separate mechanical ventilation system for the crawlspace, which should draw some of the dirty air out of the living space while mopping up soil gases that seep into the crawlspace.

    Option A is a single pipe with inline fan venting to the rooftop (Building Science's recommendation)

    Option B is a squirrel cage blower with several inlet hoses stationed at the corners of the crawlspace (my local GC does this)

    Option C is to put air return(s) for the ERV ventilator down in there and use the crawlspace as an exhaust plenum for the ventilation system (I read about this somewhere)

    Any opinions about these options or other suggestions about how to integrate crawlspace ventilation into the overall system requirements? The basic mission is cost effectiveness and efficiency with high emphasis on IAQ.

    I realize I should be talking to an engineer, but I haven't hired one yet.

  2. #2
    Join Date
    Apr 2007
    Location
    Rossland, B.C.
    Posts
    95
    It sounds like your crawl will be within the building envelope which will need heat and ventillation. Check with the local building inspector to see what they want.

  3. #3
    We have a pretty primitive set of regulations here. Basically, all I have to do is affirm that I've installed a 24/7 ventilation system of some sort and that's it, no HVAC inspections, nothing.

    Moreover, there seem to be a variety of different opinions about what to do with the crawlspace. Most builders leave the sealed crawlspace with no exhaust fan, as there's not much concern about soil gases.

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,436
    Is the home located in a green grass climate or arrid climate? Your discription of the crawlspace is similar to a small basement. With your ventilation strategy, outdoor dew points and cooling load is the issue. The crawlspace has earth contact with a cool slab. What's you highest outdoor dew point? TB

  5. #5
    Climate similar to Richmond, VA. August temperatures of 95% and 95% percent humidity, at times. Things can get pretty moldy down in the crawlspace without the right strategy.

  6. #6
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,436
    You have many hours of +70^F dew point. Clothes drier, kitchen hood, bathfans, and in your case an ERV contibute to the moisture load humidifying your home by sucking in fresh damp air. The peak moisture load approaches 90 lbs. of moisture per day to maintain <50%RH inside. This is the level required for comfort, control of mold and dust mites. A week of high indoor humidity starts mold growth. You are betting the a/c will remove the 90 lbs. of moisture while a/c is cooling the home. There are many days without cooling loads and with +70^F dew points. A one ton a/c removes 2 lbs. of moisture per hour @ 75^F, 50%RH. Turn off the ventilation, do not operate the clothes drier, bath fans, and kitchen hood. The home will still slowly get damp with the crawlspace being worst.
    Thousands are opting for a high efficiency/capacity whole house dehumidifier. They are capable of removing 4-6 lbs. moisture per KW verses <2 lbs./kw with a/c. Brands like Santa Fe, Ultra-Aire and Honeywell plus new brands everyday. Because of your mild climate, suggest investigating the ventilating dehumidifier for your climate. About the same cost. Dehu TB

  7. #7
    The only thing I am legally required to have running is the ERV. Is it a realistic idea to put an aggressive dehumidifier inline with the ERV system?

  8. #8
    Join Date
    Mar 2005
    Posts
    997
    Spray foam the joist and you should not have to do anything else.

  9. #9
    The crawlspace will be properly insulated up to the joists.

    What about ventilating the crawlspace? Most people I've talked to say it's not necessary, but won't there be a mildly toxic atmosphere of mold, soil gases, radon, etc. just stagnating there under the first floor?

    Here's an image of one system that I found (Japanese)




    Here's another view of the ventilation system, the gray boxes are HRVs--looks like there are two of 'em. Green is exhaust, blue is intake, yellow and violet are returns, and red is supply. Does this arrangement make sense?


  10. #10
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,436
    Quote Originally Posted by kamenr View Post
    The only thing I am legally required to have running is the ERV. Is it a realistic idea to put an aggressive dehumidifier inline with the ERV system?
    Canada? Green grass climates have wet cool weather for several weeks/months. The infiltrating/ventilation air contains enough moisture to raise the indoor humidity to the level mold and dust mite may grow. People generate moisture. The most important issue is keep it dry. Fresh air is also important. Generally, supplemental dehumidification is required for health and comfort. Dehu TB

  11. #11
    No, this is Kyoto, Japan. Lemme tell ya we need plenty of dehumidification, especially right around now. Mold is starting to grow on the leather sofa...

  12. #12
    Join Date
    Apr 2007
    Location
    Rossland, B.C.
    Posts
    95

    Free energy?

    Here's my suggestions to you. Others can comment. These will take into account some code requirements in B.C. These are more than mimimum requirements and would work for you. You have a good grasp on what you need to achieve and one way to do it. You mentioned ERV. This system is better suited to an enviornment where you want to recover energy losses in stale air to outside .These recover humidity as well as heat. Forget that. Rather go with hrv. It will give fresh air and lower humidity better (depending on the time of year.) I dont think you need either however. You can achieve what you need simpler than the system you have a picture of. I would use hivelocity for heat and cooling. This is one system and heres how it will work. For exhaust use hi quality bath fans in bath rooms. 50 to 100 cfm in each depending on size. 50 for say a powder room 100 for a deluxe bathroom. Thats the exhaust side. The hi V should be sized accordingly to heat loss / gain and cfm per room. Some suppliers supply design instructions etc. for their systems. So hi V with supplies where needed one in each small room 2 or more for larger. Also Hi V for crawl space. You need supplies and returns according to design. Rturn also in crawl space. Hi V is good for cooling and will dehumidify well. You put a hot water coil in the hi V as well as a cooling coil. Heat the hot water with electric or gas. Or hi V can use electric resistance heat coils. A cooling coil goes in the hi V. You will need a control that can turn on the ac for dehumidifying. Perhaps a two speed condensor will work. You might need 1 stage cool and heat at the same time. You wont have to have two air systems with this just use a continuous low speed fan setting. Add a good quality air filter. Use a timer system to ventilate the house. A 4 inch damper opens drawing outside air into the return by the system fan. When this occurs a main bath fan comes on . This is a simple control that is available at supplers around here. The ventillation timer can be set for 15 minute intervals. Here we run them 4 hours twice daily. good fresh air. I know what your thinking , too much humidity. So you get a heat pump appliance that is available to work in conjunction with an electric domestic hot water tank.. It takes out humidity while it is heating the hot water. You could put it in the crawl space. Cheap heat for the hot water, extra dehumidifying for free and during summer additional free air conditioning.. This willgive you aproximately 8000 btus of free cooling.. If you make the crawl part of the envelope the floors wont be cold in the winter.

  13. #13
    Gee, thanks. There's so much information in that post I will have to outline it to grasp it all. In short, you're right, I don't want to be installing two separate systems of ductwork.

    I think I mentioned in the OP that I was thinking of getting HV, and specifically Unico with Unichiller, which is pretty much spot on what you are describing there. I like the fact that with water based coolant I could rig up some sort of TES solution for the heating via my off-peak heat pump and solar, and maybe, gasp, for off-peak cooling down the road.

    Now, Unico claims that their air handler running in continuous low speed mode draws only 120 watts--so I guess this is about the same as an HRV, if true. I will have to research to see if this amount of intake and bath fan exhaust would meet or local requirements. I think the ventilation system is supposed to be 0.35 ACH or thereabouts.

    The usual solution here is to cut a four inch hole in the wall in each living space and attach a little exhaust fan. Very cheap, but people end up disconnecting them since the room gets cold.

    But I want some sort of energy recovery device because our electricity is over 20 cents per kilowatt, and it seems insane to pour that out the exhaust.

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