Thread: Combustion Air and M1703.2.1

Yesterday I observed two 8x16 ducts supplying combustion air from outdoors for appliances totaling 402,000 BTUs. My question is that since these ducts run horizontally for about three feet before becoming vertical, should the 1/2000 for the horizontal or the 1/4000 for the vertical be used to calculate the proper sizing?

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I would go with the 1/2000 or the 1/3000 that is allowed with the one permanent opening method. (G304.6.2 One-permanent-opening method)

Originally Posted by Ed Bliss

My question is that since these ducts run horizontally for about three feet before becoming vertical, should the 1/2000 for the horizontal or the 1/4000 for the vertical be used to calculate the proper sizing?

First, if you were to use one of them, the most restrictive would apply as that is the basic premise for conflicting code requirements/allowances.

However, ... there is no section which 'allows for' combining horizontal and vertical ducts in one duct run, thus *neither* can really be applied.

Any change in direction of combustion air ducting would fall under 304.8 Engineered installations, or 304.9 mechanical combustion air supply. These are from IFGC. Engineered systems must be approved, mechanical systems require .35 cubic feet per minute per 1000 BTH/h of outdoor air.

It looks like the fuel gas code says the duct can be either vertical or horizontal with no mention of a change of direction being a problem. This is the code I use for combustion air.

G304.6.2 One-permanent-opening method.

One permanent
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

than the sum ofthe areas of all vent connectors in the space.

The opening shall directly communicate with the
outdoors or through a vertical or horizontal duct


One or the other,,not both

From GE website,

If air is supplied from outdoors, directly
or through vertical ducts, there must be
two openings located as specified above
and each must have a minimum net free
area of not less than one square inch per
4000 Btuh of the total input rating of
all the appliances in the enclosure.
If horizontal ducts are used to
communicate with the outdoors, each
opening must have a minimum net free
area of not less than one square inch per
2000 Btuh of the total input rating of all
the appliances in the enclosure. If ducts
are used, the minimum dimensions of
rectangular air ducts shall not be

less than
oth

Our local mechanical inspector, the NCHILB, and the NCDOI all got involved when I asked this question and I was told to go to this section in the Mechanical Code for the answer to my question. My problem was I was convinced that you needed two combustion air vents but I was proven wrong by the fuel gas code. Your state may be different.

701.1 Scope. The provisions of this chapter shall govern the
requirements for combustion and dilution air for fuel-burning
appliances other than gas-fired appliances. The requirements
for combustion and dilution air for gas-fired appliances shall be

in accordance with the International Fuel Gas Code.

James,

I will change the highlighting on what you posted, then explain the problem.

Originally Posted by James Duffin

It looks like the fuel gas code says the duct can be either vertical or horizontal with no mention of a change of direction being a problem. This is the code I use for combustion air.

G304.6.2 One-permanent-opening method.

One permanent

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

than the sum ofthe areas of all vent connectors in the space.

"OR" does not carry the same meaning as "AND"

Yes, you can have vertical ducts - use the applicable code section.

Yes, you can have horizontal ducts - use the applicable code section.

Yes, you can have vertical AND horizontal sections in the same duct - use the applicable code section, which would mean this would need to be engineered, that you are not allowed to apply either the vertical or the horizontal code section (been there, said that before).

Since I have been told otherwise by the NC folks I'll go with their interpretation (been there, said that before). And I agree with them. If the code said and you would have to run both vertical and horizontal ducts to meet the code but it says or you can run either or both. Thanks for the input though.

Originally Posted by James Duffin

but it says or you can run either or both. Thanks for the input though.

James,

You are correct as far as you are taking your thinking ...

"but it says or you can run either or both" - yes, you can run one duct horizontally, or, you can run one duct vertically, but you are not allowed to run one duct horizontally and vertically ... the duct must all be the same direction for the length of the duct.

You could, using what you are thinking, run one duct vertically 5', horizontally 5', then vertically 5' more, then horizontally 5' more ... right?