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  1. #1
    Join Date
    Aug 2011
    Posts
    6

    Fresh air intake on cold air return

    House is in Michigan, about 20 years old, 2 story 2200 sqaure feet with walkout basement. When I moved in 10 years ago it was recommeded that I put in a 6" fresh air intake on the cold air return and a 6" combustion supply that just hung open ended in the furnace room. I Had both put in, but last year I had my water heater and furnace replaced, both are direct power vents. They capped the combustion air duct, but left the fresh air intake. A new service guy came out on the annual maintenance visit and noticed the fresh air intake. He said that it wasn't necessary and that I was wasting $$ of energy having it open. It has a manual damper that typically is half open.

    What's the concensious these days on the fresh air intake?

  2. #2
    Join Date
    Oct 2008
    Posts
    126
    You do need some fresh air intake in the home. Although it does waste a bit of energy, it is usually regulated that for every sq ft you would need a % of fresh air intake, it varies for certain municipalities or states. You can consult ASHRAE to see if you are over or under ventilating.

    If you are concerned with the money aspect, you could have a Heat recovery ventilator installed that recovers around 80% of the heat loss over a standard exhaust fan and air intake setup.

    I would not remove it, the service guy may not be informed as to minimum fresh air intake requirements.

  3. #3
    Join Date
    Oct 2008
    Posts
    126
    https://www.hvacquick.com/sysbuilder/hrvbuild.php

    This is a site which will calculate the fresh air required and recommends an HRV for you.
    If you do not wish to install and HRV, you could have a contractor verify that the required CFM is being pulled through the 6 inch duct you have installed already, if it is over ventilating you would close the manual damper, under ventilating would open the damper. I'd guess you'll be somewhere in the 150-200cfm range with the heat running to meet air change code.

  4. #4
    Join Date
    Aug 2011
    Posts
    6
    Thanks. When they originally installed the intake they marked a line on the duct and said the damper should be opened a little more than half way. Is there an another type of damper I could use to prevent air from coming in when the furnace is not on.

  5. #5
    Join Date
    Jun 2008
    Location
    Madison, WI
    Posts
    33
    Quote Originally Posted by jp102 View Post
    Thanks. When they originally installed the intake they marked a line on the duct and said the damper should be opened a little more than half way. Is there an another type of damper I could use to prevent air from coming in when the furnace is not on.
    You could have a gravity damper that keeps the damper almost closed, but when thee is negative pressure (i.e. bathroom fan or dryer extracting air) it opens. My 2005 ENERGY STAR sealed house has that. i assume your house is less tight and furnace/waterheater have sealed combustion it may not be necessary... but if you seal your home you'll need it for sure. I think they are called backdraft dampers. Some have springs. Mine has a weight on the lever.

  6. #6
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,198

    With a 20 year old home

    Are you sure you need OA damper? Don't you think you have enough infiltration to take care of your needs?
    Do you need to humidify during the winter? If so you have enough Nat Infiltration
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  7. #7
    Join Date
    Jun 2008
    Location
    Madison, WI
    Posts
    33
    Quote Originally Posted by genduct View Post
    Are you sure you need OA damper? Don't you think you have enough infiltration to take care of your needs?
    Do you need to humidify during the winter? If so you have enough Nat Infiltration
    it likely is required by code.

  8. #8
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,642
    Most well built homes get enough fresh air on the colder, windy days from the stack effect and wind pressure. But what about the calm winds and moderate temperatures. Most experts suggest that an air change in 4-5 hours is adequate fresh air which is common during colder windy days. During moderate weather, an air change 10-24 hours is common. The focus should be on getting adequate fresh air when the home is occupied regardless what the weather is. This is more than a damper on a make-up air duct.
    Also keep the home comfortable, dry, and the air clean. A whole house ventilating dehumidifier is a good device for green grass climates. Limit fresh air ventilation to the time of occupancy and during the mild times of the year.
    This makes sense for indoor air quality, operating cost, and comfort.
    This is a no brainer.
    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"

  9. #9
    Join Date
    Jun 2008
    Location
    Madison, WI
    Posts
    33
    Quote Originally Posted by teddy bear View Post
    Limit fresh air ventilation to the time of occupancy and during the mild times of the year.
    This makes sense for indoor air quality, operating cost, and comfort.
    This is a no brainer.
    Regards TB
    good point. but during milder seasons i have the windows open every once a while anyway. So in residential application we wouldn't need extra ventilation besides makeup air for all the fans.

    Edit: since residential homes have large envelope surface per occupant and most residential codes don't require active ventilation. Obviously this doesn't work in commercial

  10. #10
    Join Date
    Dec 2011
    Posts
    33
    fresh air is good!

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