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  1. #1

    fresh air to hvac unit in sealed attic

    i have searched on here and internet in general and cant seem to find an answer to my specific needs. a little background on home: located in new orleans (hot and humid), natural gas water heater in main living area, natural gas furnace in attic. im getting ready to spray foam my attic so it will be 100% sealed and all contractors say i need a fresh air intake for "cobustion" for the hvac unit. we rarely use the heater down here and when we do its usually very cold outside. wouldnt bringing in fresh air from outside make the heater work harder and vice versa. if it gets plumbed into the return air track then im bringing in 95 degree humid air and trying to cool it during the summer. this just doesnt sound right.
    also does the air that the furnace is using to burn come through the return air also? i dont disagree with the contractors im just trying to understand. please help me understand how all this works. thanks

  2. #2
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    Quote Originally Posted by 94slowbra View Post
    i have searched on here and internet in general and cant seem to find an answer to my specific needs. a little background on home: located in new orleans (hot and humid), natural gas water heater in main living area, natural gas furnace in attic. im getting ready to spray foam my attic so it will be 100% sealed and all contractors say i need a fresh air intake for "cobustion" for the hvac unit. we rarely use the heater down here and when we do its usually very cold outside. wouldnt bringing in fresh air from outside make the heater work harder and vice versa. if it gets plumbed into the return air track then im bringing in 95 degree humid air and trying to cool it during the summer. this just doesnt sound right.
    also does the air that the furnace is using to burn come through the return air also? i dont disagree with the contractors im just trying to understand. please help me understand how all this works. thanks
    Fire needs oxygen, fuel source, and heat to survive. This is true with gas furnaces, they have natural gas as the fuel and the outside air is the oxygen (aka combustion air).

    The law requires a way for the furnace to get combustion air to burn or the fire won't last (http://publicecodes.citation.com/ico...?bu2=undefined).

    This air is separate from the return air, you wouldn't want the furnace to burn all the oxygen in your return air, because the return air will be reheated and pushed back into the house.

    Does this mean the code contradicts itself? You bet it does... they want you to seal up your house, but then because of CO poisoning, they make you bring in outside air... But the sealing will reduce your heating and cooling, while the outside air is to protect you... so there is logic to their madness.

    Also, don't forget you need a flue to expel the burned gases.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  3. #3
    so technically i would need two fresh air sources right? one for me and one for the hvac unit. well then i guess they are talking about putting one in for the unit so it can breathe. this makes more sense now, a dedicated o2 source for the unit so i can function properly. there is already a vent coming off the unit venting out the roof for burned gasses.
    so where would the intake for the furnace connect, or does it?
    also i guess a fresh air intake for people wouldnt be a bad idea either.

  4. #4
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,060
    Quote Originally Posted by 94slowbra View Post
    so technically i would need two fresh air sources right? one for me and one for the hvac unit. well then i guess they are talking about putting one in for the unit so it can breathe. this makes more sense now, a dedicated o2 source for the unit so i can function properly. there is already a vent coming off the unit venting out the roof for burned gasses.
    so where would the intake for the furnace connect, or does it?
    also i guess a fresh air intake for people wouldnt be a bad idea either.
    How old is the original furnace? If it has age and you are foaming the attic a category iv furnace would be your best bet, both intake and exhaust piped to the outside and not compromising the attic space.

  5. #5
    dont know for sure but im guessing 7yrs old. changing out the furnace really isnt in the plans though. i agree a more efficient model would be nice but like i said before we run it very little, maybe 20 days out of the year. and hopefully with better insulation that will be decreased

  6. #6
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    The code doesn't really say that people need any outside air in residences (in futures codes it will and maybe in your state/city) but you can still put it in if you like... just have a vent to the outside with a damper attached to your return duct before it enters the furnace (make sure there is a filter downstream).

    Commercial buildings have fresh air requirements for humans, but for houses people can open windows. This has slightly changed in some states, CA requires fresh air for new houses now.

    The combustion air could be from a permanent opening in the attic to the outside, but second opinion is right, if you are going to make the attic into a living space you might want to have a pipe/duct/vent for both combustion and exhaust.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  7. #7
    not going to be a living space just trying to save on energy cost and be more efficient. so if i already have a vent for exhaust i just need to add one for intake/combustion? thanks

  8. #8
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,060
    Quote Originally Posted by 94slowbra View Post
    not going to be a living space just trying to save on energy cost and be more efficient. so if i already have a vent for exhaust i just need to add one for intake/combustion? thanks
    Do you know the size of the furnace? I will tell you what size whole you will need for the combustion duct.

  9. #9
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,163
    technically you only need it for the furnace.

    IF the house is tight enough to require fresh air
    can be determined by a blower door test.
    the highest concentration of energy raters in the state
    are in new orleans.
    LaDept of Natural Resources has a list of raters for
    the HERO program.
    if you are truly concerned, give one of them a call.
    btw..you have to do a LOT of sealing on an existing
    home to require fresh air.

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  10. #10
    Where and what am i looking for to check size. Sorry, hvac is definitely foreign to me. Thanks

  11. #11
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,060
    Quote Originally Posted by 94slowbra View Post
    Where and what am i looking for to check size. Sorry, hvac is definitely foreign to me. Thanks
    The model # of the unit. Example Trane TUD100R----

  12. #12
    Join Date
    Aug 2001
    Location
    Pavilion, NY
    Posts
    2,123
    Another thing to consider would be a
    Fresh air intake kit. This is essentially a dryer vent sized opening in the
    Exterior of the home and it has a blower attached. It will turn on and provide combustion air to the space only when the furnace calls for heat. It has a safety interlock so the furnace will not turn
    On in the event of motor failure. This may be a win/win for your situation. "field controls" is one manufacturer that makes them. Kgod
    ...

  13. #13
    Thanks for the heads up kangeroogod, that does sound like something id be interested in.
    second opinion: ill check model # tomorrow. My attic entrance is right next to my sons room and he is sleeping

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