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  1. #1
    Join Date
    Nov 2008
    Posts
    6

    Cold Air Returns Not Physically Connected to Furnace

    I bought my house (ranch with basement) last year and the previous owner had mostly removed the plaster/drywall ceiling in the basement to access electrical and plumbing when renovating. I noticed the other day that this "disconnected" the cold air returns from the duct running along the one side of the basement back to the furnace area.

    The cold air returns open up to the joist bays, which used to be sealed by the drywall ceiling. In each bay where a cold air return vent was, there was an opening in the cold air return duct in the basement. This duct runs across the house and then opened to another joist bay that ran over to the top of the furnace.

    My question is: when the ceiling was removed and the joist bays were no longer sealed by the drywall and the cold air returns essentially just open into the basement, how does this effect the system? I was assume its probably less efficient since more cold basement air is mixing with the warmer air from upstairs. Is it hurting anything for now? I'd rather not have to spend the money right now to run ductwork to reconnect everything. Also the furnace seems to run fine and my gas bills arent excessive (especially considering my leaky single pane windows)

  2. #2
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,741
    should not be pulling return from area where heater is, could cause backdraft on chimney.

  3. #3
    Join Date
    Nov 2008
    Posts
    6
    i dont understand how pulling air from the basement instead of upstairs would cause it to backdraft and pull air back down the chimney.

    the system is a lenox whisperheat if that helps. i can look harder for an exact model number if that matters.

  4. #4
    Join Date
    Oct 2007
    Location
    BC, Canada
    Posts
    55
    a
    Last edited by jibroni69; 11-10-2008 at 10:38 PM. Reason: none

  5. #5
    Join Date
    Aug 2007
    Location
    South central WI
    Posts
    257
    Quote Originally Posted by DaveWW00 View Post
    i dont understand how pulling air from the basement instead of upstairs would cause it to backdraft and pull air back down the chimney.

    the system is a lenox whisperheat if that helps. i can look harder for an exact model number if that matters.
    My dad almost killed himself in this very same situation with very same furnace be very very careful. Negative pressure (your furnace pulling air) in the basement sucks the flue gas out of the furnace and chimney into the living space. CO KILLS silently and no smell.

  6. #6
    Join Date
    Nov 2008
    Posts
    6
    I'm kind of annoyed that the home inspector and the heating guy who replaced my a/c last year and serviced the furnace didnt mention anything about this.

    i guess i will have to re-seal those joist bays with some sheet metal panning or drywall.

  7. #7
    Join Date
    Sep 2002
    Location
    Northeast Ohio
    Posts
    4,842
    Quote Originally Posted by DaveWW00 View Post
    I'm kind of annoyed that the home inspector and the heating guy who replaced my a/c last year and serviced the furnace didnt mention anything about this.

    i guess i will have to re-seal those joist bays with some sheet metal panning or drywall.
    We have a winner! Tell him what he's won Johnny! A much longer life expectancy. For what ever reason they "missed" it, shame on them. There are plenty of "older" installs still out there that draw return air into the basement. They become dangerous and even deadly when the configuration of air infiltration openings are changed or eliminated creating a negative pressure zone in the basement when the system blower runs. Remember, "you can't live in a vacuum" at least for very long . You already have figured out the cure for what ails your system.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  8. #8
    Join Date
    Aug 2008
    Posts
    416

    Definitely Can Cause Backdraft

    This will be even worse if:
    If there are no supply ducts in basement and there is door between first floor and basement. Now your return is getting all air from "sealed" basement and will create negative pressure. It will backdraft down chimneys, etc. Joist cavities should have been panned with sheet metal and sealed to horizonal duct runs.

    Its easy enough to test, start furnace and open door to basement so there is a 1 inch gap (or so). If door gets pulled shut or moves to close you got a problem. Or, if door has gap at bottom and you can feel air being pulled into basement from first floor, then be concerned.....

    It doesn't take much, my house was built in 93 with panned joists and unfinished basement, BUT the joints where the floor joist panning met the horizontal return ducts wasn't sealed (we're talking 1/4" gap). This was enough to backdraft my water heater. Sealed return ducts and no more back drafting and the above basement door test holds open even if gap is next to nothing.

    Oh and did I mention air infiltration. With an older home thats leaks you are probably pulling a lot of air from outside and running up your heating bill. The short of it is: Pull return air ONLY from areas where you SUPPLY air.

  9. #9
    Join Date
    Nov 2008
    Posts
    6
    can someone explain exactly what a backdraft is. i sort of understand but not totally.

    it wont be too hard to pan the bottom of the joist where the returns are. also the duct that connects them together and runs over near the furnace is still there to connect into. i did just notice two returns drywalled over upstairs so i will cut those open and reinstall them tonight to give it more air.

    i do have air being pulled under the door slightly. not anymore than what can be felt being pulled throught the return openings. I dont quite understand why it pulling air from under the door is a bad thing. doesnt that mean its just pulling it from upstairs like it should?

    when i reinstall all the returns and pan the joists and seal it all up wont that be somewhat restricting on the supply to the furnace? i understand thats how its suppose to be, but intuitively to me it seems like giving the furnace all the air it wants would be better for it.

    while i'm asking all these questions: where does the furnace pull the combustion air from? i see an exhaust pipe heading up and out of the house and i see the return duct going in but no other pipework going in/out (except gas/electric).

    i'm probably just overthinking this whole thing. I am an engineer, thinking is what we do best.

  10. #10
    Join Date
    Aug 2007
    Location
    South central WI
    Posts
    257
    Back draft is when you create a negative pressure around the furnace and are sucking the flue gas out of the chimney and furnace, think vacuum effect. If you pull intake air from the upstairs the air has to flow from the upstairs all the way down to the furnace though the duct.

  11. #11
    Join Date
    Mar 2008
    Location
    Shelby Twp MI
    Posts
    820
    Your not going to restrict the airflow if you button the return up, your going to redirect it and pull it from the living space which is where you want it to come from. Now in theory if your chimney is sized correctly you won't get a backdraft situation under normal circumstances, and you want your home to be in a positive pressure and not a negative. Depending on the age of your home and how it's built depends on the pressure +/- . You can create a backdraft condition as well by mechanical means when you run exhaust fans. Usually engineers don't really overthink things, they try and RE engineer things. After all that's what you actually do for a living.
    Jim

  12. #12
    Join Date
    Nov 2008
    Posts
    6
    just got back from home depot. i looked at the panning they sell and it seemed pretty expensive; $5.50 for a 36" by 18" sheet. is this reasonable? any suggestions where else to look? also i searched online and saw this thermopan, sheet metal alternative, stuff. looks easier to use and from what i can find online it is cheaper than the metal. any thoughts on this?

    thanks for all your help everyone.

  13. #13
    Join Date
    Mar 2008
    Location
    Shelby Twp MI
    Posts
    820
    BTW, you really don't want to pan the joists with drywall, you might want to use a material that if it gets wet, it will dry and not create mold.
    Jim

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