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Thread: House Makeup Air

  1. #1
    Join Date
    Apr 2017
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    Michigan
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    House Makeup Air

    This one has me baffled. In theory a house needs makeup air to replace the air removed by appliances. Also needed is fresh air to keep us from asphyxiation. Few houses have any specific source but we survive on leakage.

    I had my house built pretty tight and there is a full set of exhausting devices.

    Gas furnace
    Gas water heater
    Electric dryer vent
    Cat latrine vent powered 24/7
    Kitchen exhaust
    1200 sf ranch plus full basement

    Why are we still alive?

  2. #2
    Probably not as tight as you think. There are companies that test and seal houses. Any one or two of those device you listed that are running will steal air from the ones that aren't and for short periods when they are all running it would take a while to remove all the breathable oxygen from 18,000 cubic feet of space inside your home. It just so happens that a person needs about 20 cubic feet of oxygen to survive 24 hours. Since the oxygen is 20% of air, 100 cubic feet will have about 1 days worth. After 12 hours you would probably get pretty sleepy in that 100 cubic feet as oxygen level start to drop below 18%.
    Last edited by Lee Schierer; 05-31-2020 at 8:04 AM.
    Lee Schierer
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  3. #3
    Join Date
    Dec 2005
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    West Lafayette, IN
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    Do you have a chimney? And opening exterior doors is another great way to get makeup air.

    I built a few BSL4 labs that were negative pressure, meaning that the air was always being removed from the lab so toxins wouldn’t escape. There is a lot of work that goes into that, and fart fans and range hoods aren’t going to do the same.

  4. #4
    Join Date
    Feb 2016
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    Quote Originally Posted by Lee Schierer View Post
    Probably not as tight as you think. There are companies that test and seal houses. Any one or two of those device you listed that are running will steal air from the ones that aren't and for short periods when they are all running it would take a while to remove all the breathable oxygen from 18,000 cubic feet of space inside your home. It just so happens that a person needs about 20 cubic feet of oxygen to survive 24 hours. Since the oxygen is 20% of air, 100 cubic feet will have about 1 days worth. After 12 hours you would probably get pretty sleepy in that 100 cubic feet as oxygen level start to drop below 18%.
    Definitely not as tight as one thinks.

    One way to get a feel for how leaky buildings really are is to look at results of door blower tests on real houses. Unless a house is meticulously built to an "airtight" standard, about the best people get is 3 ACH50. That is a shorthand way of saying that to keep the air pressure .01 psi lower than the outside, you have to pump the entire air volume out of the house out 3 times per hour. That's a very good, tight, traditionally constructed house. Even the most "airtight" modern construction rarely gets that below 1.

    It takes about 10 cubic feet of air per 1,000 BTUs to run a gas appliance. So your furnace and water heater might be, on a cold day, consuming 500 cubic feet per hour. All your fans, if running simultaneously might add up to 200 cfm (although I doubt it). At their actual duty cycle, maybe 50. So there's another 3000 cubic feet/hour.

    So, your house - say 18,000 cubic feet of enclosed space - let's in 3 times its volume or 54,000 cubic feet per hour at a trivial pressure difference with the outside, but exhausts maybe 3,500.

    And as Lee says, you only need 100 cubic feet of fresh air per day to keep your oxygen up.

    You'll be fine.

  5. #5
    Join Date
    Mar 2003
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    SE PA - Central Bucks County
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    Formal attention to makeup air in residential structures is a more recent thing as energy efficiency requirements grew and products like closed cell spray foam and extensive infiltration sealing became more prevalent. But appliances improved, too. Typical gas fired high efficiency furnaces and water heating appliances have both outside air inlet and exhaust so they "steal" minimal air from the interior during their combustion process. Range hoods and other exhaust fans generally do not have any native air replacement provisions. In addition, "fresh" air is still necessary to create and maintain a healthy interior environment. There are air exchangers that can deal with temperature differences that can be utilized for a "pretty tight structure" to provide both make-up air and general refreshment. I've never looked into them, however, because, well...our current home has enough "natural" air exchange already.

    -----

    Hmmm...a vented cat latrine. What a concept!
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  6. #6
    Join Date
    May 2018
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    Lancaster, Ohio
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    My house, 1650 sq ft ranch built in 1999, has a 4: round duct from outside to return air duct on furnace for make up air. House stays fairly draft free except for one window that I had replaced with a door to the hot tub outside and now is back to a window again. Missed something when i reinstalled the window.
    Buildings leak more than most people think and a blower door test is the best way to find out how much and where the major leaks are.
    Commercial buildings that have provisions for exhaust air relief dampers to keep building pressurization under control have problems if the building envelope is not properly detailed. They will have air coming in the exhaust air relief dampers if air is being sucked out of the building elsewhere that it is not supposed to be.
    Ron

  7. #7
    While I agree with Lee that your house is likely not as air tight as you think, unless your gas fired appliances are exhausting into the house (which I doubt) they do not reduce the oxygen content % in the house air. They would reduce the barometric pressure inside the house. This reduction in barometric pressure inside the house would depend on how much (if any) the CFM of air exhausted was greater than the CFM of air infiltration caused by the reduced barometric. If I remember my physics correctly (actually I looked it up) the relationship between pressure and volume is linear (PV=nRT) So, if your house was airtight which as Lee points out it is likely not, and the temperature is held constant, the reduction in pressure would equal the reduction in volume. So using Steve's numbers, exhausting 3500 cu ft from a 18,000 cu ft house would drop the barometric pressure by 20%, I'm sure your eardrums would pop long before it go that bad.

  8. #8
    Join Date
    Sep 2013
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    Wayland, MA
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    Our house is pretty tight, I haven't paid to see how tight. But we paid a lot of attention to air sealing details and our heating bills run about 3-5x lower than neighbors in similar size houses built to the current "stretch" code. All of our appliances are sealed combustion, they draw combustion air from outside, except the range where I routinely generate huge clouds of toxic gas handled by a big 1200 cfm range hood. When I run the hood I open a window for MUA or it doesn't work. I'm glad I was able to implement such a sensible solution for the few minutes a week the hood runs at full blast. We also run an energy recovery ventilator to maintain fresh air in the house; it's hot big enough to offset the hood.

    Overall the house is very comfortable, my only complaint is that at this time of year the people and cats generate enough heat to keep it too warm inside. It can be 65 outside and, with the windows wide open (but doors closed to keep the cats out), the interior temperature goes up overnight into the high 70's in our bedroom. DW can't stand a fan so there we are.

  9. #9
    Join Date
    Sep 2006
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    Quote Originally Posted by Tom Bender View Post
    Gas furnace
    In my location, rooms with gas furnaces must have a inlet vent for outside air. Air from the attic counts as outside air.

  10. #10
    Quote Originally Posted by Stephen Tashiro View Post
    In my location, rooms with gas furnaces must have a inlet vent for outside air. Air from the attic counts as outside air.

    Gas furnace (now replaced with heat pump,) and gas water heater are in room on second floor, which has a 12" square duct to the attic to provide make up air for combustion. Duct terminates approx. 12" off floor. Dryer (also gas) is located in laundry alcove next to back door. When running dryer, we crack back door, and shim open storm door by means of a a flag of 1/4" plywood located on top corner of door frame. Same goes for when using gas oven and exhaust hood. At right at 3000 sq ft, we use less energy than most homes 1/2 of our size, most of which don't have a detached shop, nor pump water for three houses, plus a church.

  11. #11
    Join Date
    Sep 2014
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    Florida
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    Quote Originally Posted by Tom Bender View Post
    ...t we survive on leakage....
    You answered your own question. If you're not ready to do the blower test, you can see leakage for yourself with a match or lighter. Crack open a window or door, hold a flame near the crack, and have someone turn on one of those devices. Your flame will probably show an impressive air flow. Since those devices don't make your house collapse by producing a vacuum when you do not have a window open, the makeup air is seeping up from your crawl space, down from the attic and in through the walls and the closed windows and doors.

  12. #12
    Join Date
    Jan 2019
    Location
    Fairbanks AK
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    Only a few observations: blower door test, check.

    If you have cold weather, that is the best time to look for leaks around openings in the exterior walls, doors and windows obviously, but also receptacles and light switches. I hit on blue carpenter's chalk once I got rolling on our current 1980 build. Find the worst leaks, mark them with blue chalk, deal with them when the temperatures are moderate enough for spray sealant products to work correctly.

    I guess if you live in Miami you could walk around the exterior in August to feel for AC leaks.

    My observation is that once you have a house built "tight" by Alaska standards +/- AD 2000 you are going to need something like a HRV, a heat recovery ventilation system, to maintain indoor air quality. For 2400 sqft conditioned space, if the air door test comes back saying you house is 100% airtight except for one theoretical opening cut in one wall with an area of about 50 square inches you can either (up here) look into an HRV now or plan to start dealing with mold issues down the road.

  13. #13
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    Dec 2010
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    Evanston, IL
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    Our local code requires a make-up air source if you install a range hood that exhausts more than 400 or 500 cfm. I had purchased a 600 cfm hood to be installed in a renovation. We have an older home and I'm sure we have enough leakage that it would not have been a problem, but my architect and contractor said the inspector was sure to flag it, so I had to return the bigger hood and buy a 400 cfm model. Glad I learned about it prior to installing it!

  14. #14
    Join Date
    Nov 2011
    Location
    Tacoma, WA
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    IRC does require makeup air for exhaust systems larger than 400 cfm but I think only if there are one or more gas, liquid, or solid fuel burning appliances that is neither direct vent or mechanical draft venting located with in the dwelling's airbarrier. The makeup air requires a baffle controlled by the exhaust system.

    The reason is that backdrafting of combustion appliances typically presents the greatest danger associated with depressurizing a space. I think most combustion appliances are required to have their own source of combustion air but it is often sized way less than that required by a large exhaust fan. An exhaust fan greater than 400 cfm must have been determined to be a reasonable size for requiring make-up air if required.

    I do not know if there are provisions for to determine need for make-up air based on negative pressure in the house caused by the exhaust fan but it seems to me that houses built in the past 30 years will generally not have enough leakage to supply a larger kitchen exhaust fan unless the house is quite large and open. I don't think bathroom exhaust fans can be used for makeup air to a kitchen because they have a door that can be closed.

    So, if there are no combustion appliances like a wood fireplace or other non mechanically or direct vented appliance, then makeup air is not required. Go electric?

    I often but not always open a window or the door to the garage when using kitchen exhaust fan. The door to garage has spring loaded hinges so when I turn off the fan the door closes by itself. I generally turn on the furnace fan when I turn on the kitchen exhaust fan. I should have connected the makeup air baffle to the furnace fan to automatically turn on the fan. I do this furnace thing because I prefer to run makeup air through the furnace filtering system and not the return air duct. Also, in the winter the furnace will probably come on and heat the incoming makeup air. My makeup air is only 8" round duct, connects to the return air plenum, and does not supply enough air for the kitchen exhaust fan at low speed. I do not know the low speed cfm.

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