Fundamentals: Determining Furnace Vent Size

January 26, 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 are working together to replace an old gas furnace with a new gas furnace. The new equipment is more efficient than the system being replaced. So far, they have determined that the new unit is a Category I appliance. The next step in assembling materials for the job is to determine the venting system size.

NATE: “Ace, we have to make sure we get the right size Type B venting system materials for the job. How do you think we’ll go about it? What is the first thing we need to know?”

a) Furnace output
b) Furnace input
c) Furnace AFUE rating
d) Furnace efficiency

NATE: “What do you think is the next thing we need to know?”

a) Supply duct size
b) Return duct size
c) Gas pipe connection size
d) Flue collar outlet size

NATE: “Good, Ace. Before we leave the shop, let’s check the furnace to get the rating, flue collar size, and the installation instructions. Well, the furnace nametag tells us that the furnace has a rating of 130,000 Btu input, the collar size is 4 in., and the furnace is a natural-draft type. Let’s look up the size of the vent system we will need in the code book.”

ACE: “Nate, I have a problem. There are several venting tables. Which one do we use? They have heights, laterals, fan min/max, natural max — and each table seems to have a different criteria or combination of venting materials listed on the top.”

NATE: “It’s OK, Ace. Now you see the complexity of venting a furnace and understand why we all have to keep up with changes in the codes and read the manufacturer’s installation instructions. Have you noticed the similar information between the codebook information and the installation instructions?”

ACE: “The installation instructions reference the code in a number of places. But which information do you follow?”

NATE: “Generally, the term is ‘the authority having jurisdiction.’ In the town we are going to work in, that authority is the code enforcement official. I have worked with him on a number of jobs and he usually refers to the manufacturer’s installation instructions as the main information for reference. If the code supersedes the installation instructions, then he will let us know when we pick up the permit for the replacement. Ace, we’re going to have to swing by the jobsite and the building department to check on the other information before we go for materials. Let’s load the furnace, and we can drop it off, determine the rest of our materials, and head to the supply house.”

NATE: “Well Ace, we’re in luck. The code official says the installation instructions and the code match up for this job. We’ll use the code to determine our sizing and then check it with the installation instructions to make sure we’re correct. Also, the furnace is going into a location that has the venting system going straight up without any laterals. Nothing else is being connected to the venting. We’ll connect the vent system Type B venting material directly to the furnace. I measured the height to be no more than 30 ft.”

NATE: “Which Table do we use?”

a) Table 10.1
b) Table 10.3
c) Table 10.5
d) Table 10.7

NATE: “What is the size of the vent system we will be installing?”

a) 6 in.
b) 5 in.
c) 4 in.
d) 3 in.

NATE: “What is the capacity of the vent system?”

a) 128,000 Btu
b) 213,000 Btu
c) 220,000 Btu
d) 374,000 Btu

Answers: 1. b) Furnace input; 2. d) Flue collar outlet size; 3. a) Table 10.1; 4. b) 5 in.; 5. c) 220,000 Btu.

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 pmurphy@natex.org (e-mail).

Publication date: 01/28/2002

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July 13, 2008
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supply vent sizes

Eaglenest61
September 20, 2009
I have 1400 cfm being supplied. How do I figure out what size vents to put in each room?

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