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Thread: Make up air

  1. #1
    Join Date
    Feb 2009
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    MA
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    Make up air

    Hello Everyone,

    I have enjoyed lurking in the forum. This is a great resource.

    I have a question about make up air in houses. I have a pretty tight house (2200 CFM@50 Pascales), I have bathroom and kitchen fans, fireplaces that I will rarely use and a hot water heater that draws air from the inside. I have geo thermal heatpumps to forced air. I have an ERV/HRV for exchanging air inside/outside that the manufacture says is not appropriate for make up air.

    My question is do I need additional part of the ventilation system to add outside air when inside air is expelled by the fans? It is as simple as adding a vent to the return?

    Thanks - John

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,638
    Quote Originally Posted by amatuana View Post
    Hello Everyone,

    I have enjoyed lurking in the forum. This is a great resource.

    I have a question about make up air in houses. I have a pretty tight house (2200 CFM@50 Pascales), I have bathroom and kitchen fans, fireplaces that I will rarely use and a hot water heater that draws air from the inside. I have geo thermal heatpumps to forced air. I have an ERV/HRV for exchanging air inside/outside that the manufacture says is not appropriate for make up air.

    My question is do I need additional part of the ventilation system to add outside air when inside air is expelled by the fans? It is as simple as adding a vent to the return?

    Thanks - John
    The operation of exhaust devices causes negative pressure. Negative pressure is not bad unless it causes back-drafting of combustion devices or sucks moisture sensitive areas full of moisture. Fireplaces and open stack heating devices back draft easily when operating intermitantly. You are right about HRV devices not providing makeup air. Clothes driers and kitchen hoods are the big exhaust devices. The old leaky homes had enough air leaks that the open combustion where difficult to get enough negative pressure to back draft combustion devices.
    Your blower door pressure indicate 100 cfm of infiltration/exfiltration on a average winter day. Keep in mind that on a warm calm day, you have 0 cfm of infiltration. Consider that operating the clothes drier and the kitchen hood on high cause the most negative. My experience says that operating the drier and the kitchen hood on high speed on a warm calm day will make most non-operating gas water heaters/fireplace open chimimies flues back-draft.
    At least test you home by operating all of you exhaust devices on high with heaters in a non-heating mode. Check your chiminies for back-flow and then cycle the heaters and check for Proper flow. If the chiminies back-draft you have a serious big problem. Rather than make-up air on your return, it would be better to provide mechanical make-up air directly to the whenever the chiminies are being used.
    To help the above problem, I am a big fan of mechanical make-up air ventilation in tight homes instead of balanced device. During the coldest windy weather, few home need alot of fresh air, during the calm warm times of the year, homes need much more fresh air. Unfortuantely the benefit of balanced heat transfer devices is not needed when their heat transfer benifit is available.
    At least do the back-draft test on your home during the different weather conditions. Monitor the CO levels in your home constantly. Test you CO monitoring device by exposing them to CO. CO meters decline over a couple years.
    A side line is maintain <50%RH while getting adequate fresh air into your all year.
    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
    Join Date
    Feb 2009
    Location
    MA
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    TB,

    Thanks for the response. Two follow up questions: 1) do you recommend a particular product for mechanical make up air or is it a fan controlled by a manometer? 2) how do you control humidity in the winter with out a humidifier? Is there a reason you

    Building Sciences is very clear that a humidifier should not be used. The link below is a "homeowner" report, but there are many on their site about humidity.

    http://www.buildingscience.com/docum...rm=humidifiers

    Thanks - John

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,638
    I am involved with the venitlating dehumidifier. For green grass climate that have significant amounts of moisture in the fresh air during the grass growing season, I favor the dehu part for maintaining <50%RH throughout the home. The ventilating dehu will also provide fresh air during the cold weather.
    For controlling humidity during winter, decrease the amount of fresh air as the home becomes too dry. Limit minimum ventilation during hours of occupancy to conserve the moisture being added by the occupants. Adding 60 cfm of fresh for 10 hours to the natural fresh air is better than over-ventilating.
    Buildingscience does not understand that 2 occupants in a mid-large home with adequate very dry fresh air flow may need some humidification to maintain 30% RH. In the past, I shared the BS veiw. After extensive testing, I feel that two occupants in +midsize home need some humidification during the driest cold weather to maintain 30%RH with 80 cfm of fresh air ventilation during occupancy.
    I am attaching a graph showing the natural fresh ventilating from .2 ach house. Regards TB
    Attached Images Attached Images  
    Attached Images Attached Images
    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
    Join Date
    Feb 2009
    Posts
    131
    Quote Originally Posted by amatuana View Post
    Hello Everyone,

    I have enjoyed lurking in the forum. This is a great resource.

    I have a question about make up air in houses. I have a pretty tight house (2200 CFM@50 Pascales), I have bathroom and kitchen fans, fireplaces that I will rarely use and a hot water heater that draws air from the inside. I have geo thermal heatpumps to forced air. I have an ERV/HRV for exchanging air inside/outside that the manufacture says is not appropriate for make up air.

    My question is do I need additional part of the ventilation system to add outside air when inside air is expelled by the fans? It is as simple as adding a vent to the return?

    Thanks - John
    is there a chart that shows how many cfms vs pascals equals a tight house and a not so tight house? basically just a reference sheet?

  6. #6
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,638
    Typically, you divide the cfm leakage @ 50 Pa by 20 to establish the cfm of air leakage of your home average winter temperature and wind. Next calculate the cubic foot volume of your home's interior space. Divide the volume of your home by the cfm of leakage for the minutes to change the air in your home on an average winter day. Divide by 60 for the hours to change the air. Any thing more than 3 hours is moderately tight. Less than 10 hours is extremely tight. Google air tight homes for more info. The graph in attached was for a home with an air change in 5 hours. 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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