Natural Draft vent termination

[Marc M]

Whats the clearance of a natural draft B vent to operable window, door, deck?
Vert wall?

[Jerry Peck - Codeman]

From the 2006 IRC:
- M1804.2 Termination. Vent termination shall comply with Sections M1804.2.1 through M1804.2.6.
- - M1804.2.1 Through the roof. Vents passing through a roof shall extend through flashing and terminate in accordance with the manufacturer’s installation requirements.
- - M1804.2.2 Decorative shrouds. Decorative shrouds shall not be installed at the termination of vents except where the shrouds are listed and labeled for use with the specific venting system and are installed in accordance with the manufacturer’s installation instructions.
- - M1804.2.3 Natural draft appliances. Vents for natural draft appliances shall terminate at least 5 feet (1524 mm) above the highest connected appliance outlet, and natural draft gas vents serving wall furnaces shall terminate at an elevation at least 12 feet (3658 mm) above the bottom of the furnace.
- - M1804.2.4 Type L vent. Type L venting systems shall conform to UL 641 and shall terminate with a listed and labeled cap in accordance with the vent manufacturer’s installation instructions not less than 2 feet (610 mm) above the roof and not less than 2 feet (610 mm) above any portion of the building within 10 feet (3048 mm).
- - M1804.2.5 Direct vent terminations. Vent terminals for direct-vent appliances shall be installed in accordance with the manufacturer’s installation instructions.
- - M1804.2.6 Mechanical draft systems. Mechanical draft systems shall be installed in accordance with their listing, the manufacturer’s installation instructions and, except for direct vent appliances, the following requirements:
- - - 1. The vent terminal shall be located not less than 3 feet (914 mm) above a forced air inlet located within 10 feet (3048 mm).
- - - 2. The vent terminal shall be located not less than 4 feet (1219 mm) below, 4 feet (1219 mm) horizontally from, or 1 foot (305 mm) above any door, window or gravity air inlet into a dwelling.
- - - 3. The vent termination point shall not be located closer than 3 feet (914 mm) to an interior corner formed by two walls perpendicular to each other.
- - - 4. The bottom of the vent terminal shall be located at least 12 inches (305 mm)above finished ground level.
- - - 5. The vent termination shall not be mounted directly above or within 3 feet (914 mm) horizontally of an oil tank vent or gas meter.
- - - 6. Power exhauster terminations shall be located not less than 10 feet (3048 mm) from lot lines and adjacent buildings.
- - - 7. The discharge shall be directed away from the building.

From the above, there are not specific requirements addressing this part of your question:
- Whats the clearance of a natural draft B vent to operable window, door, deck?

There is a requirement for this part of your question:
- Vert wall?

- G2427.6.4 (503.6.4) Gas vent termination. A gas vent shall terminate in accordance with one of the following:
- - 1. Gas vents that are 12 inches (305 mm) or less in size and located not less than 8 feet (2438 mm) from a vertical wall or similar obstruction shall terminate above the roof in accordance with Figure G2427.6.4.
- - 2. Gas vents that are over 12 inches (305 mm) in size or are located less than 8 feet (2438 mm) from a vertical wall or similar obstruction shall terminate not less than 2 feet (610 mm) above the highest point where they pass through the roof and not less than 2 feet (610 mm) above any portion of a building within 10 feet (3048 mm) horizontally.
- - 3. As provided for direct-vent systems in Section G2427.2.1.
- - 4. As provided for appliances with integral vents in Section G2427.2.2.
- - 5. As provided for mechanical draft systems in Section G2427.3.3.

If the gas vent is *not* less than 8 feet from a vertical wall or similar obstruction, then the gas vent shall terminate in accordance with Figure G2427.6.4. That Figure also includes a table of minimum vent heights above sloped roofs as follows:
ROOF SLOPE --> H (minimum) ft
Flat to 6/12 slope --> 1.0 ft
Over 6/12 slope to 7/12 slope --> 1.25 ft high minimum
Over 7/12 slope to 8/12 slope --> 1.5 ft high minimum
Over 8/12 slope to 9/12 slope --> 2.0 ft high minimum
Over 9/12 slope to 10/12 slope --> 2.5 ft high minimum
Over 10/12 slope to 11/12 slope --> 3.25 ft high minimum
Over 11/12 slope to 12/12 slope --> 4.0 ft high minimum
Over 12/12 slope to 14/12 slope --> 5.0 ft high minimum
Over 14/12 slope to 16/12 slope --> 6.0 ft high minimum
Over 16/12 slope to 18/12 slope --> 7.0 ft high minimum
Over 18/12 slope to 20/12 slope --> 7.5 ft high minimum
Over 20/12 slope to 21/12 slope --> 8.0 ft high minimum

However ... if the vent *is* within 8 feet of a vertical wall or similar obstruction, then the vent ... shall terminate not less than 2 feet above the highest point where they pass through the roof and not less than 2 feet above any portion of a building within 10 feet horizontally.

Thus, if the vent is within 8 feet of the vertical wall with the window or door in it, then the vent shall terminate as stated just above - "not less than 2 feet above any portion of a building within 10 feet horizontally". That means the vent must be at least 8 feet from the window or door (that would be the closest the vent could be to the wall the window or door is in), and if the vent is less than that 8 feet from the wall, then the vent need to be 2 feet higher than any portion of the building within 10 horizontally of the vent.

Any easy way to remember all the above is: 8 feet minimum horizontally from the window or door, or 2 feet higher than the highest part of the wall/roof within 10 feet of the vent.

That was a roundabout way to get to your answer, but sometimes the code does that.

[Marc M]

I actually figured that they were inferring that the 10' rule applied encompasses the windows/doors etc... installed in "the wall". I Suppose you cant have a door or window without a wall.
Heres a question; help me to understand that if a vent is >2"H within 8' of a vert wall, then how come it then goes to a 10' radius rule? (Like the 3-2-10)

[Jerry Peck - Codeman]

Heres a question; help me to understand that if a vent is >2"H within 8' of a vert wall, then how come it then goes to a 10' radius rule? (Like the 3-2-10)

That's not exactly what it says: the Type B gas vent can be 8 feet from the window, but ... if that vent is 7 feet 15/16 inches from the window ... then that vent now needs to go 2 feet higher than the roof above that window.

The only thing the 10 feet does it possibly raise the vent higher, it does not mean the vent has to be 10 feet from the window. An example is below:

Let's say the vent is 7 feet 10 inches from the wall the window is in, and there is a chimney 9 feet 11 inches away in another direction, the vent can remain at that 7 feet 10 inches from the wall the window is in, but that vent needs to be at least 2 feet higher than any part of the structure within 10 feet - if the chimney is higher than the roof above the wall with the window in it, then the vent must be at least 2 feet higher than the chimney, not just at least 2 feet higher than the wall the window is in.

Still defies logic where the vent is allowed to be 1 foot high on a 5/12 sloped roof and 8 feet from the wall the window is in (creating a sense that the clearance required is an 8 foot radius, but it is not), then requiring that vent to be 2 feet higher than the roof above the wall that window is in - let's say that wall is only 4 feet higher than the roof the vent is in, that means the vent only needs to be 6 feet high from a horizontal line from where that roof meets the wall, and the top of the window may be 3 feet high above that line, which means the vent only needs to be 3 feet above the window, Did I say that in such a way that you could follow it?

The "2 feet higher than" may result in a tall vent, or a very tall vent, but there is no '8 foot radius' rule for the clearance. It would be a lot easier for the code to just state that the clearance from a vertical wall or a building opening is 8 feet in every direction ... but the code does not state it that simply, and it is not that simple of a rule (unless the vent is at least 8 feet from the wall, then 'no problem').

[Marc M]

Yea, you explained it pretty well. I got it.
Perhaps you have an answer for this one; Why is it that mechanical draft sems to have more regulation? Is it the force of the draft?

[Jerry Peck - Codeman]

Why is it that mechanical draft sems to have more regulation? Is it the force of the draft?

You mean versus listed mechanical draft systems such as direct vented appliances?

Type B gas vents for natural draft appliances are more specifically addressed in the codes as there is not a "specific listed way for it to be installed", the installation is left up to the person in field who is installing the venting system - meaning the likelihood of 'design' and installation errors will be much greater. As such, the code tries to help reduce those 'design' errors by covering Type B gas vents for 'design', and hopefully the installation will be in accordance with those 'design' parameters.

With listed systems, such as direct vent systems, the design is tested and passed, leaving the installation up to the installer to meet the published design information.

Either way, the installation needs to be compared to the design requirements, for natural draft vents that would be to the code, for direct vent that would be to the installation manual.

Unfortunately, far too few installers have read the code and far too few installers open the installation manual - evidenced by finding the installation manuals still in their sealed envelopes when an inspection is being done.