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Thread: Need HRV ??

  1. #1
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    Need HRV ??

    Hi there, customer in the middle of a renovation to an old house. Complete gut of 2nd and 3rd floor for a residence. Going to be spray foaming everything, walls and roof, also new windows. Putting in new 2 stage HP, AH and ductwork. Thinking this place should probably be awfully tight when they are done and we should be considering and HRV. What do you all think.
    Could install later on I guess after they move in but the AH is up on the 3rd fl. and pretty tight for space, would rather do now if needed.
    Thanks Ron

  2. #2
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    Jun 2003
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    Madison, WI/Cape Coral, FL
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    Where in God's green grass earth is this home? Tell us about the kitchen hood cfm exhaust.

    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"

  3. #3
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    In Maine. Not sure of kitchen CFM, not yet installed, would be standard though, nothing over the top. Also there are two bathrooms in building. Thanks

  4. #4
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    Homes respond to all the pressures we put the surfaces of the home under. On a calm summer day, there is little pressure on the home and very little air enters or leaves unless we operate a fan.
    On typical winter day, the wind blows 10 mph and the hot air balloon effect (stack) makes home breathe too much, like and air change in 3-4 hours. Plus we have clothes drier, bath fans, and a kitchen hood removing air from the home.
    Try as you might, it is difficult to tighten a home up enough to not get excess fresh air moving through the home on the extreme cold windy days. Plus other mechanicals change air, making the benefit of the HRV marginal.
    But the rest of the year, good home need a small amount of fresh air when occupied to purge indoor pollutants and renew oxygen.
    On more complication, moisture from the occupants and adequate fresh air during high outdoor dew points raises the indoor moisture levels to the point of growing mold and dust mites.
    After many years of selling many HRVs and living with mold and dust mites, many of us have opted for mechanical make-up fresh air ventilation to provide air change when occupied and calm weather. This supports clothes driers, bath and kitchen exhaust. Also wood burners, fireplaces and water heaters have a source of air to function.
    Include a good dehumidifier in the cool spaces like the lower levels and basement to maintain <55%RH during the wet times of year.
    Some use the whole house ventilating dehumidifier like Ultra-Aire as solution in one device. This is a rather new solution but very effective.
    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"

  5. #5
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    So are you saying that an HRV is overkill most of the time. What would you recommend? Can't imagine there will be much air entering the building after it's all sealed with 4-6" of foam. Simply pulling in some fresh air into the return as they used to do, doesn't seem very efficient or adequate for today. Thanks TB

  6. #6
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    It is complicated. You need make-up air for the exhaust devices. An HRV exhaust as much air as it brings in and does nothing for kitchen hood or the clothes drier. If the home is occupied 24/7, maybe a small HRV is justified. All the testing I have done indicates that this type of home will get enough fresh during the coldest winter weather.
    The for sure part is that this home will need fresh air during the other 3 seasons of the year when occupied and the windows are closed.
    Do your thing. You have to live in it. If there are any open combustion devices in the home, an HRV will not help supply make-up air.
    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"

  7. #7
    Join Date
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    Location
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    Let me give you a little insight into what you need.
    You need a fresh air change when the home is occupied. This is about 80 cfm with a 2,000 sqft. home. You need a home air tight enough to not leak fresh air excessively. You need heat, cool, dry (<55%RH) yet moisture enough to maintain +35%RH.
    You need enough fresh air to operate the clothes drier, kitchen hood, and bath fans.
    In Maine, the cost to heat 80 cfm of fresh cold air through the winter 24/7 is about $250 for the year. The exhaust devices need make-up air only not balanced flow like a HRV.
    A HRV may be 50% effective at heat recovery and needed 50% of the time. This could be 50% of $250 if the home is occupied 100% and the wind never blows. When the wind blows +10 mph, a foam home will breath enough naturally that any additional mechanical ventilation will be excessive, making the home dry. I am testing a foam home in WI that confirms this.
    If the home is occupied 50% of the time, the benefit of the HRV is less than $75 for the year. Do you need the HRV for a benefit of saving $100 per year?
    You do need supplemental dehumidification when the outdoor dew points are +60^F to maintain 55%RH in the home during the summer much more than you need a HRV?
    You will need a humidifier during the winter if you overventilate your home when the outdoor dew points are <30^F.
    Go with your gut reaction.
    Keep us posted on how it works.
    My brother built a foam house in WI, installed an ERV, and started without a dehumidifier two years ago. First year, he installed a whole house dehumidifier to avoid a damp home during low/no cooling loads and +60^F outdoor dewpoints, put a CO2 controller on the ERV to limit the ventilation to time occupied without wind to avoid overventilation. Last winter the ERV operated 18% of total winter. They are in the home +50% of the time. This year, he is installing a humidifier to increase the %RH from about 30% RH to 40%RH for comfort during the winter.
    In his case the ERV has a small benefit and was not needed. If the home was occupied 100% of time, the benefit doubles. Better but not great.
    Hope this helps.

    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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