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Old 01-27-2011, 03:37 PM   #1
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combustion air supply to gas units?


Hey all - I'm thinking out my basement refinishing project. The gas air handler/furnace and gas hot water heater are just sitting in one of the big rooms I'm going to finish. It was suggested if I box them in, I'll need combustion air supply high and low. Others suggested just vented doors on my framed and finished "box".

I'm thinking of an alternate approach to the "box". I don't want doors. I'd have removable panels to service or replace the HVAC unit or HWH. One of these access panels might even be built-in (but solid and removable) bookshelves. I'd like it to actually be "sealed", or so no one really knows what's back their. So no doors and no wall vent plates, etc.

So can I just run a galvanzied metal pipe (like 4-6 in diameter into that room that runs to the outside of my house with an open/screened dryer-vent type cap on it?

Somebody else noted "hot and cold" air supply, or something like that. Like the combustion air ducts I'm talking about - one would need to be high, and one low.

So with that - can I run just one vent "pipe" from the outside into this "box" - then split it off so there is one opening at the top, and one at the bottom?

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Old 01-27-2011, 05:12 PM   #2
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combustion air supply to gas units?


Section 703 "Outdoor Air" - Where all combustion and dilution air is to be provided by outdoor air, the required combustion and dilution air shall be obtained by opening the room to the outdoors. Openings conncting the room to the outdoor air shall comply with Section 703.1.1 through 703.1.4.

703.1.1 Number and location of openings, Two openings shall be provided, one within 1 foot of the ceiling of the room and one within 1 foot of the floor.

703.1.2 Size of direct openings. The net free area of each direct opening to the outdoors, calculated in accordance with Section 709, shall be minimum of 1 square inch per 4.000 Btu/h of combined input rating of the fuel-burning appliances drawing combustion and dilution air from the room

703.1.3 Size of horizontal openings. The net free area of each opening, calculated in accordance with Section 709 and connected to the outdoors through a horizontal duct, shall be a minium of 1 square inch per 2,000 btu/h of combined input rating of the fuel-burning appliances drawing combustion and dilution air from the room. The cross-sectional area of the duct shall be equal to or greater than the required size of the opening.

703.1.4 Size of vertical openings. The net free area of each opening, calculated in accordance with Section 709 and connected to the outdors through a verticl dcut, shall be a minium of 1 square inch per 4,000 btu/h of combined input rating of the fuel-burning appliances drawing combustion and dilution air from the room. The cross-sectional area of the duct shall be equal to or greater than the required size fo the opening.

709.1 General THe required size of openings for combustion and dilution air shall be based on the net free area of each opening. the net free area of an opening shall be that specified by the manufacturer of the opening covering. In the absence of such infomation, openings covered with metal louvers shall be deemed to have a net free area of 75 percent of the area of the opening, and openings covered with wood louvers shall be deemed to have a net free area of 25 percent of the area of the opening. Louvers and grells shall be fixed in the open position.

Other considerations would be for allowable clearances, I believe a miniumum of 3"

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Old 01-27-2011, 05:26 PM   #3
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combustion air supply to gas units?


Quote:
One of these access panels might even be built-in (but solid and removable) bookshelves. I'd like it to actually be "sealed", or so no one really knows what's back their. So no doors and no wall vent plates, etc.
Ayuh,.... Codes aside,...

That'll be alotta Fun at 2:am when the heating system Quits for some reason...
It'll take ya til 3:am just to get To the unit...
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Old 01-27-2011, 05:50 PM   #4
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combustion air supply to gas units?


Thanks for the replies - yeah, I didn't mean I was boarding it up in the walls. Both HVAC and HWH will have complete accessability. I was simply hoping to do it without doors. I just wanted panels or something that looked more like part of the wall. In actuality, they could be 7 feet high by 4 feet wide - even larger than a door. Or - even a studded drywalled wall section that swings on a hinge.

That aside - I appreciate the code examples provided. A tad over my head.

Am I to generally read this as that X amount of air volume must be availale to the units based on their power/gas usage? It sounds like my idea of a 4 inch sheet metal pipe might from outside wouldn't be enough. It sounds like louvered vents would additionally be needed.

I'll nail down specifics later - but can you folks confirm that what I'd likely need would be:

1. Two combustion air sources from the outside into my "box" - one a foot from the ceiling, and one a foot from the ground. (NOTE: would I need two separate pipes to the outside - or just one that splits?)
2. AND - I'd need louvered vents in the faces of my "box"

AND the sizes of the combustible air pipes and the louvered vents would be determined by the consuption/power of my units?
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Old 01-27-2011, 06:19 PM   #5
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combustion air supply to gas units?


One pipe from the outside then Tee it to get a high and low. Add up the furn and WH BTU and divide by 4000 to get the square inches of the pipe size needed.
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Old 01-27-2011, 08:57 PM   #6
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combustion air supply to gas units?


Thanks - I feel like I'm getting warmer - no pun intended.

So it sounds like through calulation of BTUs etc. - I can determine the size of the pipe needed. It would be interesting if those calculations told me the pipe needed to be 14 inches - if so, I'm dead in the water with a pipe-only solution.

So at that point - my guess is I'd have to go with a combo of whatever size pipe I could fit in my ceiling (to be finished) - then add louvered panels on sides of my box.

Anyone care to venture a guess for if I can accomplish this with air supply pipes alone? I'd say my furnace and WH are "average" in size.
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Old 01-27-2011, 09:28 PM   #7
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combustion air supply to gas units?


Say your furnace is 85,000 and your hot water is 15,000 that would be 25 square inches, a 6" pipe has 28 square inches. But you will probably will need an 8" given 75% of an 8" louver has to be greater than 25 square inches.

Now thats all just guessing of course

85000+15000=100000
100000/4000=25
((6/2)^2)*PI = 28.3

((8/2)^2)*PI=50.26
50.26*.75=37.7

Looks like 8" in this example.

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Old 01-29-2011, 04:22 PM   #8
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combustion air supply to gas units?


Quote:
Originally Posted by denemante View Post
So can I just run a galvanzied metal pipe (like 4-6 in diameter into that room that runs to the outside of my house with an open/screened dryer-vent type cap on it?

Somebody else noted "hot and cold" air supply, or something like that. Like the combustion air ducts I'm talking about - one would need to be high, and one low.

So with that - can I run just one vent "pipe" from the outside into this "box" - then split it off so there is one opening at the top, and one at the bottom?
I did the box thing with bifolds front and rear. Ran 4" pipe to a 12x12x20 insulated box sitting on the floor. I then put a return grill high in one of the walls.

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