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  1. #1
    Join Date
    Sep 2004
    Location
    Round Rock, TX
    Posts
    25

    "Default" Make up air in Average Homes?

    Since no home I personally know of has a make up air system installed, (granted, none in my social circle have a custom home, all tract builder here) there must be some assumption/saftey margin that some amount of make up air is drawn from doorways or "somewhere safe" besides a dedicated make up air system. What "cfm" is typically assumed on a recent (<5yrs) home without risk of back drafting from furnaces and water heaters/chimneys?

    After all, we all run clothes dryers, kitchen vents and bathroom exhaust. So take my home for example, ~2600sf, 3 baths, 1 interior utility room. The cheapo builder bath & laundry fans are 50cfm, there was the standard 150-200cfm kitchen vent fan, and the clothes dryer.

    Is it safe to assume then that in my home I don't require make up air for 4x50cfm + 1x100cfm + 1x150cfm (bathroom/laundry, dryer, kitchen) 450cfm?

    When does an HVAC professional decide a home needs make up air when designing an HVAC system for a given home?

  2. #2
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    IT IS THE LAW HERE

    Just to be clear you are talking fresh air/makeup air. It will eliminate stagmentation of airs oxygen levels and keep co2 levels down and will help to prevent negative pressures.
    Do it right the first time.

  3. #3
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
    Posts
    2,919
    It is unlikely that you run all fans at the same time. It is also unlikely that they deliver their rated CFMs. Most houses will be leaky enough for your situation. The only way to be sure is to have it tested. My house has no fossil fuel appliances, so it would not make a problem for me. If you have a gas furnace or water heater, you could back-draft the flue(s). Testing is the only way to be safe. Rules of thumb can get you sick or killed.
    Remember, Air Conditioning begins with AIR.

  4. #4
    Join Date
    Aug 2008
    Location
    Minneapolis
    Posts
    79
    Here in Minnesota homes constructed after 12/31/99 are required to have a makeup air/ vent system.

  5. #5
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    We hear a lot about Minn

    You guys have strick codes like us and I like when you see people posting stuff you would be amazed that it gets passed.
    Do it right the first time.

  6. #6
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
    Posts
    1,582
    Yep my HVAC guy told me the other day it's the law here for all new homes to have fresh air intakes as well as makeup air for the home. I would also agree your almost never going to run all your house appliance's at the sametime so to need 100 CFM's for make-up air probably isn't really needed unless you do alot of cooking in the house and have a exahsut hood that has a 360 CFM fan or larger then your going to need some make-up air. For your HWT and dryer in the basement you can build a utility closet and have a duct run in to the utility room for make-up air which will lower your need for make-up air in the rest of your home. I would have a 6" fresh air intake brought into the home for both fresh air and make-up air for the home regardless of the age of the house unless were talking about a home that has paper thin walls with little or no insulation and the windows leak then you'll have plenty of make-up air. But if your home if fairly tight I would definately consider having one put in.

  7. #7
    Join Date
    Jun 2007
    Location
    Round Rock
    Posts
    3,651
    There is a little thing called combustion air which is generally a 6" piece of snaplock has to be added 12" from the top of closet and 12" from bottom of closet going into the attic if water heater or furnace is installed in them, to draw in fresh air, it could also go sidewall to the outside. If the furnace/hot water heater is in the attic, it has soffitt vents or gable vents to provide fresh air. Backdrafting would be the last thing I would be afraid of. You are in the same town I am and if your house is fairly new and inspected, which almost everything is here. You should be good on combustion air. As far as fart fans and exhaust hood, they move such a small amount of air that opening the front door to go in or out will provide enough air, much less being a tract home, it isn't built super tight anyways. You've been there 5 years and you're still alive to type this, so I will assume that everything is fine and you're worrying about nothing.

  8. #8
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    There should be 2

    Our local codes say you must have 1 SQ" for every 1000 BTUs for Combustion air
    As for fresh air depends on the exhaust value must equal intake value and this is Fresh air

    Now don't get me wrong you guys might call it something else. I would not depend on door to balance pressures. What happen when the door is not opened for 6 hrs in winter time. (always error on safe side)
    Do it right the first time.

  9. #9
    Join Date
    Nov 2004
    Location
    South Carolina
    Posts
    1,840
    i call it a erv or hrv-- google honeywell

  10. #10
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    What?

    Quote Originally Posted by papa_jo View Post
    i call it a erv or hrv-- google honeywell
    You still have to have a with energy/ heat recovery ventilator a pipe going outside to bring in fresh air, but what about combustion air?
    Do it right the first time.

  11. #11
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,637
    Fresh air in homes is the most neglected item of the critical indoor air quality issues. Back-drafty open combustion devices can kill you but homes that do not have enough air change to purge indoor pollutants also have a long term bad health effect. There are many respitory problems that EPA claims are caused by indoor air pollution. During cold windy weather, homes get fresh air from the stack effect and wind. During mild weather, fresh air from stack effect disapears. Leaving wind to provide fresh air. Most evenings wind lessens. Testing for CO2 reveals that there in minimal fresh air in many homes. CO2 itself is not the problem, but the lack of air change to purge indoor pollutants. Once you get accustone to fresh air, you appreciate the diffence. Providing 50-100 cfm of fresh when a home is occupied makes real difference in the freshness of a the home. As a trade, we totally neglect the fresh air issue. ASHRAE, EPA, etc recommend fresh air when occupied. 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"

  12. #12
    Join Date
    Aug 2004
    Location
    S.E. Pa
    Posts
    6,290

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