Double wall vent clearance

[Anthony]

I think I am correct that a double wall vent is class b, if it has a locking mechanism.

What is the minimum clearance to combustibles? I think I read 1inch in the mechanical code book.
Someone told me it was 2 inches.

I am preparing to take the Ky. Journeyman exam if you can help further are these statements true?

A gas furnace vent should rise 1/4" every 12"?

2-45° bends are allowed OR one 60°? or are they saying you can have 2 45's and a 60° bend?

Condensate drain fall 1/8" for every 12" or 1%?

Thanks for any help you can give me. I have been out ofthe field for awhile and am preparing for the fact that I may need it to fall back on.

[Jerry Peck - Codeman]

Hi Anthony,

The following answers are based on the 2006 IRC - Fuel Gas Chapter 24, which would be the same in the 2006 IFGC.

I think I am correct that a double wall vent is class b, if it has a locking mechanism.

You have described a Type B Gas Vent, however, only those which are also listed and marked/labeled as Type B Gas Vent are Type B Gas Vents, i.e., one could have an unlisted/unmarked/unlabeled vent as you described and it would not be a Type B Gas Vent.

What is the minimum clearance to combustibles? I think I read 1inch in the mechanical code book.
Someone told me it was 2 inches.

The minimum clearance will be as listed by the manufacturer, but for a Type B Gas Vent the standard clearance to combustible material AND INSULATION (most people forget that part) is 1 inch.

A gas furnace vent should rise 1/4" every 12"?

True - for the horizontal portion of the vent, see below for a definition of horizontal.

2-45° bends are allowed OR one 60°? or are they saying you can have 2 45's and a 60° bend?

They are saying that you can install 45-degree offsets (plural, meaning more than one) except that you could install one (limited to one offset) if you installed a 60-degree offset. The vent is required to extend "in a generally vertical direction". If an offset if more than 45-degrees that part is considered to be "a horizontal section" and not a vertical section. Additionally, the maximum total offset of a vent and vent connector serving an appliance with a draft hood is 75 percent of the total vertical height as measured from the draft hood to the discharge opening within the Type B Gas Vent Cap.

Let's say the vertical distance from the draft hood to the discharge within the Type B Gas Cent Cap is 8 feet, which would be quite possible for a single story house (the minimum total height required is 5 feet), that means the greatest total offset allowed would be 75 percent of 8 feet, or 6 feet maximum allowed offset.

Condensate drain fall 1/8" for every 12" or 1%?

True.

[Anthony]

Thanks! That was a great help! So 2-45's in addition to a 60 would be ok?

[Jerry Peck - Codeman]

So 2-45's in addition to a 60 would be ok?

Depends.

From the 2006 IRC (underlining and bold are mine):

- G2427.6.9.1 (503.6.9.1) Category I appliances. The sizing of natural draft venting systems serving one or more listed appliances equipped with a draft hood or appliances listed for use with Type B gas vent, installed in a single story of a building, shall be in accordance with one of the following methods:
- - 1. The provisions of Section G2428.
- - 2. For sizing an individual gas vent for a single, draft-hood-equipped appliance, the effective area of the vent connector and the gas vent shall be not less than the area of the appliance draft hood outlet, nor greater than seven times the draft hood outlet area.
- - 3. For sizing a gas vent connected to two appliances with draft hoods, the effective area of the vent shall be not less than the area of the larger draft hood outlet plus 50 percent of the area of the smaller draft hood outlet, nor greater than seven times the smaller draft hood outlet area.
- - 4. Approved engineering practices.
- G2427.6.9.2 (503.6.9.2) Vent offsets. Type B and L vents sized in accordance with Item 2 or 3 of Section G2427.6.9.1 shall extend in a generally vertical direction with offsets not exceeding 45 degrees (0.79 rad), except that a vent system having not more than one 60-degree (1.04 rad) offset shall be permitted. Any angle greater than 45 degrees (0.79 rad) from the vertical is considered horizontal. The total horizontal distance of a vent plus the horizontal vent connector serving draft hood-equipped appliances shall be not greater than 75 percent of the vertical height of the vent.

Being as most vent installation will be designed according to either 1. Section G2428; or 4. approved engineering practices, Section G2427.6.9.2 will seldom come into play.

It a question was "Is it allowed to design a vent system using a 60-degree offset and a 45-degree offset in accordance with G2428 or approved engineering practices, the answer would be 'No.'

However, if the question was simply "Is it allowed to install a 60-degree offset and a 45-degree offset in a vent, then the answer would be 'Yes.' ... because it is allowed, just would likely never be done.

[Anthony]

Why does the code call out gas vent and gas connector?
i.e. "effective area of the vent connector and the gas vent shall be not less than the area of the appliance draft hood outlet," are "connectors" simply elbows etc.?
I apologize for the stupid questions but some of the wording of codes are confusing.
Thanks for your help you've cleared up a lot of my doubting my own knowledge.

[Jerry Peck - Codeman]

Why does the code call out gas vent and gas connector?

The connector which connects an appliance to its vent is treated differently than the vent itself, and all appliances are required to have a connector ... unless the vent connects directly to the draft hood.

Don't you love the way the code says 'all must have' and then says 'except'?

The "effective area of the vent connector and the gas vent shall be not less than the area of the appliance draft hood outlet," is basically just saying the obvious - that you are not allowed to install a 3" vent connector on a 4" draft hood, etc.