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  1. #1
    Join Date
    Nov 2011
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    Ventilation Strategy for 3-story home

    I'm designing a system for a 3-story new home in Charleston, SC, with ducted mini-splits (Mitsubishi). Approx. s.f.: 3,000.

    There are 4 indoor fan coils: 2 serving the first floor, 1 serving the each of the second and third floors. The house is being Designed to Earn ENERGY STAR, version 3.0, and will be built to have a very low infiltration rate. (projected: between 1 and 2 ach50).

    My infiltration strategy is to provide enough ventilation to meet ASHRAE 62.2, and connect all bath exhausts (50 cfm each) for additional recovery (and fewer penetrations...bonus!). This brought the cfm capacity, including bath exhaust, to 270cfm for the whole house. So, I have specified (2) ERV's, each at 130 cfm.

    With that, I was planning to put one 130cfm ERV on the top floor system and one 130cfm ERV on the first floor. But, when I laid out the equipment and ducting for these ERV's, I found it VERY constrained in the first floor ceiling. Almost too tight to make it work with the ducted fan coil in the same area. There is more than enough room for the 3rd floor ERV, so this is not an issue.

    The theory behind this approach is that I'm essentially creating a fresh air sandwich. The second floor will get fresh air through exchange via the open stair in the center of the house.

    MY QUESTION: If there simply is not enough room in the first floor ceiling, does it make sense to connect the second ERV to the system for the second floor (where I have more room for the equipment). Will the first floor still draw an appropriate amount of fresh air from the upper floors.

    Alternatively, I could just connect a single 300cfm ERV to the middle floor system, and rely on exchange to the upper and lower floor from there.

    Any thoughts would be appreciated

  2. #2
    Join Date
    Jun 2003
    Madison, WI/Cape Coral, FL
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    Where is your clothes drier and kitchen hood getting the fresh air needed to exhaust properly without causing negative pressure?
    I think you are over doing the amount of balanced flow ventilation that you are planning on. 100 cfm of balance flow ERV fresh air will add to the natural ventilation you will have. How are you going to deal with the moisture that the ERV is going to bring into the home? ERVs only transfer a portion of the moisture in the incoming air to the exhaust air, provide the exhaust air is dry. Proper fresh air ventilation during extended cool wet weather will make the home wet when there is low/no cooling loads. I suggest 1 ERV and 1 whole house dehumidifier as a better combination to provide the best of both worlds. Some balance fresh air ventilation, 100 cfm meets ASHRAE requirements. Include a small whole house ventilating dehumidifier for fresh air when the clothes drier and kitchen exhaust hood are used. This would keep the home <50%RH during wet cool weather.
    The cost would be comparable.
    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"

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