Section 304.11 in the 2006 IFGC

[globe trekker]

I am looking for input on this particular code section.

QUESTION: When the "two permanent openings method" is used to provide [ all ] combustion air from the outsidoors, and

using the two separate vertical ducts method, ...does each vertical duct provide the ' total amount of "required" combustion

air ' for the space/appliances being served, or does each vertical duct provide one half of the "required" amount of

combustion air ( i.e. - do the two separate vertical ducts act like an HVAC system, in that one duct serves to provide the

total Supply Air and the other one serves to remove the total Return Air being drawn in ), or something else altogether ?

Thanks for your input!

 

[Builder Bob]

Re: Section 304.11 in the 2006 IFGC

4. Ducts shall not serve both upper and lower combustion air openings where both such openings are used.

The separation between ducts serving upper and lower combustion air openings shall be maintained to the source of

combustion air.

In other words, The two openings do not provide 1/2 of the required combustion air--- the openings - have to be sized for the BTU rating of the gas appliance. each one sized in accordance with the code.

for example, if 12 square inches is required, then both duct work/vents (lower and upper) need to provide 12 square inches.

 

[globe trekker]

Re: Section 304.11 in the 2006 IFGC

Builder Bob,

Thanks for your input! That's kinda the way I was leaning too! Each vertical duct shall be sized large

enough to provide the "total" required [ space ] quantities of combustion air, in case one of the ducts

gets stopped up or otherwise, " ...is unable to communicate with the outdoors."

 

[Batwood]

Re: Section 304.11 in the 2006 IFGC

This answer references the 2009 IFGC

See section 304.6.1 Two Permanent Openings Method. Two permanent openings, one commencing within 12 in. of the top and one commencing within 12 in 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 through vertical ducts, each opening shall have a minimum free area of 1 in. per 4000 Btu/hr (550 mm2/kW) of total input rating of all appliances in the enclosure.

Where communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of 1 in. per 2000 Btu/hr (1100 mm2/kW) of total input rating of all appliances in the enclosure

It pretty much says the same thing as NFPA 54 section 9.3.3.1

I would say that if each opening has the minimum calculated free area you will be in compliance.

 

[Builder Bob]

Re: Section 304.11 in the 2006 IFGC

You are welcome....

By the way, it has to do with the ability of air to move without limitations for combustion ---

When a furnace lights off, the air is rapidly pulled into the ignition source, when it ignites, air is rapidly displaced. The duct work is nothing more than a balance damper to ensure air movement is maintained- whether a rapid influx is needed or a rapid displacement is required.

When you get a chance, Google a search for high speed furnace/pilot light ignition. You might be surprised at what you see.

 

[globe trekker]

Re: Section 304.11 in the 2006 IFGC

Thanks for your input too Batwood!

.

 

[RJJ]

Re: Section 304.11 in the 2006 IFGC

Agreed to above! Good question Globe! often misunderstood by installers and inspectors!

 

[globe trekker]

Re: Section 304.11 in the 2006 IFGC

RJJ,

I was confused too! Actually, I was kinda shocked that a contractor would even call our office

to inquire. Most of them just go ahead and do whatever they want to...

We actually DO try to assist everyone that calls in, ...we just don't get that many to call in.

.