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  1. #1
    Join Date
    May 2005
    Posts
    6

    Cold Basement/Combustion Air

    My basement is finished except for the mechanical room. I have 2 nat gas fired appliances a furnace (75,000 btus) and water heater at (40,000 btus).

    Question: am I reading it right, each unit states something along the lines of "Hourly Input Btu's" with the above numbers, I have a combined 115,000 btu's, this is what I have to base my combustion air requirments on correct?

    The basement seems too cold, it has 5 registers with no returns and is about 1300 square feet. The mechanical room leaks cold air out from under the door when the furnace is not running, the gap is about an inch or so. I have two six inch flex pipes supplying the room with combustion air, one high one low. That gives me a total of 56.5 square inches of atmospheric combustion air deliverd from the attic for combustion air. According to this calculator (which I have no idea if it is correct) http://www.houseofcraig.net/combustion_air_calc.html

    I need 58 square inches of outside combustion air for 115000 btus, like I stated earlier with my calculations (3 x 3) 3.1415 times two pipes equals 56.5 inches squared. If I seal off the bottom of this door with some sort of rubber flap to help keep out the cold, given the requirement of 58" and only having 56.5" will this cause any problems or decrease efficency?

    Also, is there an inexpensive mechanical flap that would open and close my combustion air pipes when the appliances are in service or in other words when the room is under a vacuum.

    Thanks Matt

  2. #2
    Join Date
    Sep 2006
    Location
    Leavenworth KS
    Posts
    305

    Air requirement

    That air requirement is for each amd not both.

    Do not seal the door and have your service provider install a return duct into
    the basement space near the floor

  3. #3
    Join Date
    May 2005
    Posts
    6
    Well, according to the calculator the 75000 btu furnace requires 38" and the water heater at 40000 btus requires 20" (bottom number on the calculator). Adding the two together which would be the requirement if each were running simulateously would be 58"sq. Like I said I have 56.5", would the 1.5" be that big of a deal?

    Thanks Matt

  4. #4
    Join Date
    Jan 2007
    Location
    Minneapolis, MN
    Posts
    415
    Technically you are short of free area but the calculation is for both units

    running at the same time. Both units are usually not running at the same time

    and continuously. You can add two two openings (one high and one low) that

    communicate directly with the basement room to make up the differnence.

    Dampers cannot be used on the outside air ducts unless the furnace and water

    heater are interlocked through an end switch on the damper to prove it is

    open.

    You should not feel cold air leaking into the basement from the outside air

    unless there in a pressure difference. Adding a return air duct will most

    likely make it worse. You could be having a stack effect problem in the house

    that could draw in out side air while the furnace is off.

    Another approach would be to increase the outside air to 8 inches each

    seal the room and add the return air in the basement to improve air

    circulation.

  5. #5
    Join Date
    Oct 2005
    Posts
    922

    Wink who ?

    Did you run the flex from the attic ? Or did the tech do that for you?

  6. #6
    Join Date
    Jul 2005
    Posts
    717
    Your basement needs some RETURN air (ducted back to the unit).,especially with it having 5 supply air outlets.A 1300sq ft basement with 5 supply outlets could sure warrant a couple of well place low return inlets.

    IF you have adequate high and low combustion air vents in this Mechanical room, the door gap could be a lot tighter so that the cold outside air does NOT enter into your heated areas, and is confined to the Mechanical room.

  7. #7
    Join Date
    Oct 2005
    Posts
    922

    combustion air

    1) What is the size of the room?
    2) Is it a confined space?
    3) Is it an unconfined space?
    Do you have 50 Cubic Ft per 1000 BTU or more of all aggregate input of all fuel burning appliances installed? Not some of, part of , or if they run some times.

  8. #8
    Join Date
    May 2005
    Posts
    6
    Barson, Not sure that you can not feel cold air in a basement unless there was a pressure difference. After all cold air sinks, it is the same effect as opening a window, it would let cold air in.

    Propmanage, No it was here when I purchased the home, it was built in 2000, and it is a very common way of bringing combustion air into homes in my area.

    Deejoe, my thoughts exactly I totally agree with you, the size of the mechanical room is irrelevant as long as it has A) proper clearances to combustibles and B) it has adequate supplied combustion air. The gap in the door seems to be overkill and may even be unnecessary. Like I stated earlier and this is still my original question, I require 58sq", I am supplied with only the flex ducts even if the room was otherwise air tight with 56.5 sq". So, is the lack of 1.5" sq that big of a deal?

    Propmanage,

    Like I said earlier I need 38 sq" for the furnace at 75000 btus, and I need 20" sq for the 40,000 btu water heater. NOW if they run simultaneously I need 58sq". I do not have to have 50 cubic feet per 1000 btus, that calculation is most likely for a room that has no make-up air to it, but this is not the case I have almost entirely ample amounts of combustion air just with the flex pipes, still the question remains, I require 58sq" but if I seal the room I have 56.5"sq. Is this a problem?

    Thanks Matt

  9. #9
    Join Date
    Oct 2005
    Posts
    922

    Why ask

    (Deejoe, my thoughts exactly I totally agree with you, the size of the mechanical room is irrelevant)

    (I do not have to have 50 cubic feet per 1000 btus, that calculation is most likely for a room that has no make-up air)

    Ok then you must all ready have all the answers. You had a question about what you could do to prevent cold air from coming in your mechanical room. This air is used for combustion. This air is required under code and a formula with a premise, confined space or unconfined space is part of the formula. BTU rating and Gallon per hour are also used. Put a trap on it and have a nice day.

  10. #10
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    I didn't see anyone ask you this question.

    Is your furnace a two pipe 90% or single pipe 80%?

  11. #11
    Join Date
    Jan 2007
    Location
    Minneapolis, MN
    Posts
    415
    Your question is "is this a problem?" My answer is yes.
    Call your local HVAC contractor to make the required corrections so
    you will have the correct amount of combustion air. They can
    also correct the return air in the basement.

  12. #12
    Join Date
    Apr 2005
    Location
    new jersey
    Posts
    752
    I personally think you are ok. dont seal off the underside of the door. Add Return air. To main living space.no basement is comfortable without a low return on the floor.

  13. #13
    Join Date
    Jan 2007
    Location
    Minneapolis, MN
    Posts
    415
    And when the new return air causes back drafting because there was a lack of
    combustion air and CO enters the house because someone who did not
    understand what they were doing (home owner) fixed it.
    I would not take that liability.

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