Adding combustion air

Discussion in 'HVAC Heating & Cooling' started by Lakee911, Oct 19, 2009.

  1. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

    Messages:
    1,328
    Location:
    Columbus, OH
    Hello all,

    I've made an attempt to seal up my old house pretty well, but it's nowhere near the point that my furnace requires a combustion air source. (To be honest, though, I've actually never tried turning on the bathroom fan, the dryer, the water heater and the furnace to see if I have any back draft).

    I've been thinking that maybe it wouldn't hurt, though, to provide one anyways. My furnace is only 80% efficient, although it is relatively new. My thinking is that furnace will pull air directly from outside instead of pulling the cold air in through the living space.

    I figure that the outside colder air would have more oxygen (thus more efficient) and the savings from not heating cold air infiltrating through the cracks would also be beneficial.

    I would like to put in maybe a 2 or three in PVC pipe to the outdoors, but I'm concerned that this would be able permanent source for drafts when the furnace isn't on. How could I shut it off when not needed?

    Thanks,
    Jason
  2. Cass

    Cass Plumber

    Messages:
    5,980
    Location:
    Ohio
    If the dryer and furnace are gas most combustion air is done with 6"...all the homes I go in to have a 6" run from the outside terminating right at the furnace...normaly run in thin wall flue pipe
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    A gravity type damper would help but could bang in the wind. There are motoroized dampers that could be set up to open when the burner is on. You may want to build a room around the furnace, if there isn't one, insulate it, then run air in there from outside. then, you'd be relatively assured you are getting outside air, and if the room is cooler, no big deal.
  4. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

    Messages:
    1,328
    Location:
    Columbus, OH
    In lieu of a room, could I bring that outside air right in to the combustion area of the furnace? There seems to be a 4" knock out (only held together by four tabs) there. Is that what it's for?

    Thx,
    Jason
  5. Cass

    Cass Plumber

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    Location:
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    I don't think so...
  6. nhmaster

    nhmaster Master Plumber

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    Location:
    S. Maine
    Unless there is some provision made to pre-heat the incoming combustion air, it is a bad idea. Varying temperatures of the incoming air will dramatically change how the fire burns. This was all ther rage 20 years ago and then we started running into sooted up units left and right.
  7. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

    Messages:
    1,328
    Location:
    Columbus, OH
    Hmm ... that's a bummer. I'll look in my Owner's manual.

    Thanks.
  8. Cass

    Cass Plumber

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    Location:
    Ohio
    It is still done here on a regular basis...In fact years back I did it to a rental I was living in and it helped save energy by not using air that had been heated with the furnace...it did not cause any soot problems...I don't even know how that could happen...then again I am ont in the HVAC industry...
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
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    Location:
    New England
    I'd have to calculate it, but I think you'd find there's as much density change in air between typical hot and cold limits and that you'd get in a high or low pressure system passing over...IOW, unless there were gusts of air coming in from outside to disrupt the flame, I'd be surprised if it caused a problem in the real world. Now, allowing gusts of outside air to hit the burner, rather than being difused in could easily mess with the flame.

    It's been awhile since I looked, but I think the recommendation for a burner is 1 sq in per 1K BTU of burner of vent for intake combustion air. Don't quote me on that! This is for when the appliance is in it's own room and you need to be able to bring in unrestricted air for combustion. If drawing it from the dwelling and relying on leaks, the requirement is much bigger and is usually given in cubic feet of volume in the readily available part of the structure. When I was looking at the possibility of a new tankless, it quickly got discounted because the area was too small to support it without outside air (which is hard to do in a condo).
  10. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

    Messages:
    1,328
    Location:
    Columbus, OH
    The literature that I found states, "Air for combustion and supply air ventilation must conform to the methods outlined in American National Standard (ANSI-Z223.1) National Fuel Gas Code or National Standard of Canada CAN/CGA-149.1, & CAN/CGA-149.2 Installation Code for Gas Burning Appliances"

    I need to look at the BTU rating of the furnance when I get home and see what it is anyways.
  11. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    lakee911;
    A way to bring in combustion air that will not have any effect on the temperature within the area is to terminate the combustion pipe within an inch or so of the floor (near the furnace) and in a pail. The incoming air will remain within the pail until it is called on for combustion purposes.
    This is called the "anti spill") method.This pail method works ok, but another streamlined way is to bring the combustion air pipe in from outside and run it down to the floor and use 2 - 90 degree elbows to form a "U" shape. Make sure you put an screen/mesh on the outside starting point to stop any insects.etc from getting in. The diameter of the pipe depends on the btu rating of the furnace burners, but usually a 5 " or 6" is adequate for most homes.
  12. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

    Messages:
    1,328
    Location:
    Columbus, OH
    Thanks for the info.

    The holes on the sides are actually to route the flue through the side of the unit by rotating the "combustion air inducer." I thought it was a draft inducer, but that is not what the book calls it.

    My unit is 66,000BTU and the manual requires 1 sq. in. (min. 3 sq. in.) per 4,000 BTU. So, if my math is right, that is a 5" duct. That's pretty big. I might have to figure out where to bring that in from. What kind of vent can be used outside?
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