Fundamentals: Sizing Furnace Vents

February 23, 2002
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The problem: Replacing an existing gas furnace with a new, more efficient furnace.
The apprentice: Allen Charles Edwards (ACE) — First-year mechanic.
The technician: N. Arthur Thomas Egan (NATE) — 15-year veteran.

NATE and ACE have been working to replace an older gas furnace with a more efficient furnace. They have determined that the new unit is a Category I appliance. They are going to use a 5-in. Type B venting system. The next step in assembling materials for the job is to determine the length of the system. (See Figure 1 for a schematic drawing of the installation.)

ACE: “Why are we going to pick up 30 ft of vent piping when, according to my calculations, we only have to go up 24.5 ft to get through the roof? With the termination cap, we’ll be around 25.5 ft. Extra material cuts into the profit for the job.”

NATE: “Ace, there are some other factors that have to be considered besides just getting the vent system through the roof. Let’s look at them.”

NATE: “When we cut the hole for the roof penetration, what are we going to use to prevent leaks?”

a) Tar
b) Flashing collar
c) Rain collar
d) Vent system top

NATE: “The flashing collar adds height to the overall vent system depending on our penetrating a flat or pitched roof.

NATE: “What do you think is the minimum height above the roof?”

a) No minimum required
b) 1 ft
c) 2 ft
d) 3 ft

NATE: “The National Fuel Gas Code — 1999, Article 7.5.2 — ‘Termination,’ and the International Fuel Gas Code Article 503.5.4 — ‘Chimney Termination’ state that ‘a chimney for residential-type or low-heat gas utilization equipment shall extend at least 3 ft (0.9 m) above the highest point where it passed through a roof of a building…’ but that is not the complete article. There are other factors to consider.”

NATE: “What is the minimum distance that the vent system must extend above any other portion of the building?”

a) No minimum distance
b) 1 ft
c) 2 ft
d) 3 ft

NATE: “There is another component to the distance above another portion of the building. What is the minimum horizontal distance from the roof penetration that must be used to measure the venting system extension above the roof?”

a) No minimum required
b) 5 ft
c) 10 ft
d) 15 ft

NATE: “You know Ace, trying to explain this information using the question and answer method is difficult. Let’s look at the typical termination examples from the Codes.” (See Figure 2.)

NATE: “Those examples are worth a thousand words, Ace. And knowing that we have to maintain the clearances, from the sketch we have, what is the distance we should extend above the roof penetration?”

a) 2 ft
b) 3 ft
c) 4 ft
d) 5 ft

NATE: “Then how many feet of vent system pipe will we need?”

a) 25 ft
b) 28.5 ft
c) 30 ft
d) 32.5 ft

Answers: 1) b; 2) d; 3) c; 4) c; 5) c; 6) c.

Patrick Murphy is director of technical development, North American Technician Excellence (NATE). If you have any further questions or comments on this Fundamentals quiz, contact Murphy at (e-mail).

Publication date: 02/25/2002

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