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  1. #1

    Gas furnace combustion venting

    Thanks in advance for indulging me. We just moved into a new (old) house with a gas furnace (not high-efficiency.) It is a Rheem Criterion II. It is housed in it's own little room in a hallway. There are screened holes in the floor surrounding the furnace for combustion air to be taken from the crawlspace.

    When the gas company came to inspect and turn on service, they said we needed more combustion air; also the service man who maintained the furnace concurred, and computed the number of square inches required. Both said a vent in the access door would suffice. No talk of outside air required.

    So, I cut a proper sized hole in the door and finished it with a nice vent (8"X14" or so.) Everything looks and works fine.

    The problem is that when the furnace isn't operating, we get considerable cold air coming from the crawlspace, out through the door vent into the living space. To me the logical solution would be to put some kind of hinged baffle on the inside of the vent that would only allow air to go the one direction it was intended for - toward the furnace, not toward the living space.

    Does that make sense to you folks? Any other solutions come to mind?

    Thanks,

    Steve

  2. #2
    Join Date
    Jul 2012
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    Western KY
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    Bad set up. Ideally you would have combustion air 12" down from ceiling and 12" up form floor, splitting the total required sq in required between them. I would cover the hole to the crawl and get your air from attic or through the closet door, not both. Since you already cut the door I would use that. Increase the size to required and close off the crawl. Deffinately close off crawl.

  3. #3
    Join Date
    Aug 2007
    Posts
    72
    Your house is already at negative pressure, based on your observation that outside air is coming into the house through the new vent when the furnace isn't running. Your new vent on the access door just provides a lower-resistance path into the house for outside air. When the furnace is on, it draws air the other direction through your vent, but that makes your house even more negative. This will cause even more outside air infiltration into the house from whatever leakage points exist.

    I'm guessing that the location of your furnace makes it impractical to provide an adequately sized, dedicated path for drawing outside air for combustion. In which case your door vent may be the only option. Your "hinged baffle" idea may work if there is adequate clearance for the flap to open. The flap also needs to be very lightweight so as not to require much pressure difference to open.

  4. #4
    Quote Originally Posted by PowerPlay View Post
    Your house is already at negative pressure, based on your observation that outside air is coming into the house through the new vent when the furnace isn't running. Your new vent on the access door just provides a lower-resistance path into the house for outside air. When the furnace is on, it draws air the other direction through your vent, but that makes your house even more negative. This will cause even more outside air infiltration into the house from whatever leakage points exist.

    I'm guessing that the location of your furnace makes it impractical to provide an adequately sized, dedicated path for drawing outside air for combustion. In which case your door vent may be the only option. Your "hinged baffle" idea may work if there is adequate clearance for the flap to open. The flap also needs to be very lightweight so as not to require much pressure difference to open.
    Thanks, PowerPlay and Tommy,

    I think I might be able to provide outside air. That would be the optimal solution, would it not? The gas furnace replaced an oil furnace back in the day, and I believe the crawlspace holes were originally for that. I don't think anyone has seriously considered the need for outside air. I have no attic (it's a one story with vaulted ceilings), so I could easily get a vent pipe straight up to the roof. In your opinions, would that be the option to pursue?

  5. #5
    Join Date
    Jul 2009
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    Columbia, MD
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    I would have a professional that knows how to use a combustion analyzer check out the situation. you would be surprised how little air you need for combustion. you most likely had plenty of air before you did your modification. a 4x6 register that's kept closed, cut into the supply duct would also give you plenty of combustion air.

    I've done this many of times and all of my customers are alive and have no CO threats.

  6. #6
    Join Date
    May 2008
    Location
    Central Fla.
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    311
    Quote Originally Posted by PowerPlay View Post
    Your house is already at negative pressure, based on your observation that outside air is coming into the house through the new vent when the furnace isn't running. Your new vent on the access door just provides a lower-resistance path into the house for outside air. When the furnace is on, it draws air the other direction through your vent, but that makes your house even more negative. This will cause even more outside air infiltration into the house from whatever leakage points exist.

    I'm guessing that the location of your furnace makes it impractical to provide an adequately sized, dedicated path for drawing outside air for combustion. In which case your door vent may be the only option. Your "hinged baffle" idea may work if there is adequate clearance for the flap to open. The flap also needs to be very lightweight so as not to require much pressure difference to open.
    Sorry old man but when you pull air from outside and force it into a space that space is under "positive pressure" not negative. If this is an 80% furnace to properly have make up air a set of "Hi/LO" pipes should be installed.

  7. #7
    Join Date
    Aug 2004
    Location
    S.E. Pa
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    6,192
    First of all, you have two things to deal with: codes and what is practical or what works because they are mutually exclusive.

    The code requires the Makeup Air (MUA) in one of several prescribed methods including the hi/lo and outdoor air options. There is no guarantee these will work and often work against the unit. For any MUA passive grille to work, the Combustion Appliance Zone must be under negative pressure to 'inhale' the MUA. This means drawing in cold air in your case even when not running. This will result in a call for more heat and waste energy.

    The flap valve has been tried in various iterations but all fail because you cannot get a flap to reliably open and close against a few Pascals of pressure either way.

    The supply register is the reliable answer in your case but it should be tuned using a combustion analyzer and micromanometer and seal your accessible ducts, especially the returns.

  8. #8
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    Western KY
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    Don't over complicate it. The positive/negative pressure thing is irrelevant. You have an 80% "induced draft" furnace. It will pull the air it needs as long as air is available. If you use hi/lo through roof cover hole you cut in door as well as crawl opening. If you use the door vent close off crawl opening. Do not use any of two of the options together.

  9. #9
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    Feb 2010
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    Quote Originally Posted by Tommy knocker View Post
    Don't over complicate it. The positive/negative pressure thing is irrelevant.


    Dangerous statement...

    A simple supply register in the plenum like gravity suggested would work great.

  10. #10
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    Quote Originally Posted by chuckcrj View Post


    Dangerous statement...

    A simple supply register in the plenum like gravity suggested would work great.
    Not if you read the whole post.

  11. #11
    Join Date
    Jul 2009
    Location
    Columbia, MD
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    Some very dangerous stuff being passed around here.

    If you brought another pipe from the roof to the furnace room, who is gonna stay there the entire time and tell the pipe to bring air down? and Who's gonna tell the exhaust pipe, "Hey bud, I need you to draft up?" Maybe you can draw arrows on the pipes and this will designate which way the flow is.

    The 2 pipe method rarely works. If you don't believe me, take some tissue paper and stick it up to the next combustion air pipe you see and tell me which way the paper sucks.

    The best option is to seal your returns and cut in a small register into the supply. The supply is always a positive and will provide plenty of combustion air for the appliance.

    Also, Listen to Chuck and Hearthman(Bob). They are intelligent people.

  12. #12
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    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

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    Last edited by jpsmith1cm; 11-13-2012 at 09:27 PM. Reason: non AOP member

  13. #13
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    Tommy, while you are referring to the code, the vent in the ceiling will draft up and out if that room is full of warm air--not entrain down into the room. ASHRAE did a study on passive MUA and it is unreliable. Codes do not guarantee performance and in some cases can make it hazardous.

    Leaker2, if it has a draft hood equipped gas water heater, yes, it very likely may backdraft CO2, which will fall to the floor displacing oxygen, which can snuff out a pilot in a small enclosed room but I'd be more worried about CO getting entrained into the returns and thus distributed throughout the building.

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