Page 1 of 2 12 LastLast
Results 1 to 13 of 16
  1. #1
    Join Date
    Nov 2009
    Location
    Northern Alberta
    Posts
    8

    Furnace combustion air

    I'm installing a 70000 BTU furnace into a confined room below a house. The room is approximately 8 feet square by about 7 feet high, so I'll need to bring in combustion air from outside. I did the calculation, and should require 2 vents, one for intake within 12" of the floor and one for exhaust within 12" of the ceiling. According to my calculation, they should be a minimum of 35 sq in surface area each. The closest round pipe size is 8" which is about 50 sq in. I mentioned this to another pro and he figures its way too much and if it's -40C outside or colder, the furnace won't be able to heat the home efficiently. He figures a 5" pipe for each is good enough, but this of course would not satisfy the code requirements. We live in northern Alberta where it can get very very cold! Any thoughts on how to alleviate this?

  2. #2
    Join Date
    Nov 2008
    Posts
    1,673
    How much combustion CFM you need, at how many inches of water?

  3. #3
    Join Date
    Feb 2006
    Location
    Tn.
    Posts
    1,344
    yea get somebody that knows about it to install it.
    not guessing at things

  4. #4
    Join Date
    Nov 2009
    Location
    Northern Alberta
    Posts
    8
    How much combustion CFM you need, at how many inches of water?
    Furnace is a Lennox (mid efficient G40UH(X)). I don't see the CFM's listed for it, but they specify in the manual to have 2 air openings free area of at least one sq inch per 2000 BTU per hour of the total input rating (of all equipment in the enclosure). It has an input rating of 70000 BTU thus the calculation of 35 sq inches per vent.

    The home is at about 2000 feet elevation.

  5. #5
    Join Date
    Aug 2004
    Location
    S.E. Pa
    Posts
    6,142

    Cool top end heated power MUA

    http://www.sheltersupply.com/corpora...t.asp?cwpID=16

    You could use a Hoyme damper made right in Alberta but it does not have the duct heater.

    HTH,
    Hearthman

  6. #6
    Join Date
    Nov 2009
    Location
    Northern Alberta
    Posts
    8
    Does anyone know how significant the impact is of bringing in extremely cold combustion air to the efficiency of the system? Would perhaps installing a longer intake pipe inside the crawl space help?

  7. #7
    Join Date
    Nov 2009
    Location
    Northern Alberta
    Posts
    8
    http://www.sheltersupply.com/corpora...t.asp?cwpID=16

    You could use a Hoyme damper made right in Alberta but it does not have the duct heater.
    Thanks Hearthman, I will look in to that.

  8. #8
    Join Date
    Nov 2009
    Location
    Northern Alberta
    Posts
    8
    I found out that I can get 7" insulated flex duct and all I need for residential (here in Alberta) is one vent for the supply (from the outside) and must terminate within 2 feet of the burners and within 12" of the floor. The cold air shouldn't impact the efficiency too much.

  9. #9
    Join Date
    Aug 2009
    Location
    Beatrice, NE
    Posts
    1,989
    If a proper load had been done on the house the combustion air would have been figured into the furnace size so having the right size pipe should not be a problem for the furnace to over come the extra cold air needed.

  10. #10
    Join Date
    Nov 2009
    Location
    Northern Alberta
    Posts
    8
    If a proper load had been done on the house the combustion air would have been figured into the furnace size so having the right size pipe should not be a problem for the furnace to over come the extra cold air needed.
    In this application, none of the combustion air is coming from inside the house. The floor is sealed with vapour barrier and is insulated. The crawl space currently is unheated, but eventually an insulated foundation will be installed and can then be heated if desired. There currently will be some leakage of air from the crawl space into the furnace room. The crawl space is partially insulated but not airtight to the outside as well. This will change as this mobile home will be put on a full foundation and function more like a house.

  11. #11
    Join Date
    Dec 2007
    Posts
    4,670
    just a question/suggestion

    why not use a 2 pipe system, intake and exhaust from outside in PVC/ABS/DWV (code compliant)?

  12. #12
    Join Date
    Jul 2005
    Posts
    717
    Stokes; One way to solve the problem is to bring in a single pipe of adequate ( 5", 6", or 7") diameter and terminate it at the furnace room FLOOR level using 2-90 degree elbows so that they form the letter "U".
    This will make an "air trap" and will provide air for combustion ONLY IF NEEDED.
    Without this "U" formation unwanted (cold) air would come into the area even when it's not required. (it's known as the "anti-spill" method) make sure you install a SCREEN onto the outside intake to avoid any entry of insects, etc.
    try it , I think you will like it.

  13. #13
    Join Date
    Nov 2009
    Location
    Northern Alberta
    Posts
    8
    Stokes; One way to solve the problem is to bring in a single pipe of adequate ( 5", 6", or 7") diameter and terminate it at the furnace room FLOOR level using 2-90 degree elbows so that they form the letter "U".
    This will make an "air trap" and will provide air for combustion ONLY IF NEEDED.
    Without this "U" formation unwanted (cold) air would come into the area even when it's not required. (it's known as the "anti-spill" method) make sure you install a SCREEN onto the outside intake to avoid any entry of insects, etc.
    try it , I think you will like it.
    That sounds like a great idea. Thanks for that deejoe!

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event