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Thread: Combustion Air For Water Heater
03-16-2011, 06:34 AM #1
Combustion Air For Water Heater
The attached photo shows two pipes going into the attic, I suppose to draw combustion air for the water heater. The furnace (at the right) is newer and is a high efficiency furnace that now draws combustion intake air from the exterior.
The water heater (flue at left) is rated a 35,500btu input. The utiltity room measures 6.5'x14'x8.5' (773 cubic ft).
Required combustion air for the water heater is 35.5x50=1775cubic ft. Based on 50ft cubic ft per 1000 btus.
Should one of the pipes extend down to within 12" of the floor?
What about the lack of fire blocking/draft stopping from where the vent pipes go into the attic? Does this not breach those requirements?
Any other comments appreciated.
03-16-2011, 06:56 AM #2
Re: Combustion Air For Water Heater
Yes, one should be within 12" of the floor.
That smaller one may be too small. Minimum cross section dimension is 3 inches.
Now that the furnace isn't connected to the vent, it is probably oversized for the water heater.
Is the smaller one ABS? Not sure if that is an approved material.
Last edited by John Arnold; 03-16-2011 at 07:02 AM. Reason: added ABS comment"There is no exception to the rule that every rule has an exception." -James Thurber, writer and cartoonist (1894-1961)
03-16-2011, 01:56 PM #3
Re: Combustion Air For Water HeaterIn NC the one opening method is allowed per the NC Fuel Gas Code. See below for the quote.
304.6.1 Two-permanent-openings method.Two permanent
openings, one commencing within 12 inches (305 mm)
of the top and one commencing within 12 inches (305 mm)
of the bottom of the enclosure, shall be provided. The openings
shall communicate directly, or by ducts, with the outdoors
or spaces that freely communicate with the outdoors.
Where directly communicating with the outdoors, or
where communicating with the outdoors through vertical
ducts, each opening shall have a minimum free area of 1
square inch per 4,000 Btu/h (550 mm2/kW) of total inputIrating of all appliances in the enclosure [see Figures
304.6.1(1) and 304.6.1(2)].
Where communicating with the outdoors through horizontal
ducts, each opening shall have a minimum free area
of not less than 1 square inch per 2,000 Btu/h (1,100
mm2/kW) of total input rating of all appliances in the enclosure
[see Figure 304.6.1(3)].
304.6.2 One-permanent-opening method.One permanentthan the sum ofthe areas of all vent connectors in the space.
opening, commencing within 12 inches (305 mm) of
the top of the enclosure, shall be provided. The appliance
shall have clearances of at least 1 inch (25 mm) from the
sides and back and 6 inches (152 mm) from the front of the
appliance. The opening shall directly communicate with the
outdoors or through a vertical or horizontal duct to the outdoors,
or spaces that freely communicate with the outdoors
(see Figure 304.6.2) and shall have a minimum free area of 1
square inch per 3,000 Btu/h (734 mm2/kW) ofthe total input
rating of all appliances located in the enclosure and not less
03-16-2011, 04:21 PM #4
Re: Combustion Air For Water Heater
I run into the high/low combustion air pipes out in the burbs in new construction, not often in the City.
I don't have a problem with the combustion air pipes coming into the basement. However, and you guys can yell if you want, but most of the time I end up writing them as a fire hazard.
Usually the pipe comes through the ceiling over the appliance and only runs into the attic, not outside. This acts as a great channel for a fire to feed on. If run outside I don't have a problem with it.
"The Code is not a ceiling to reach but a floor to work up from"