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Category I Venting Questions

Mike2 Member Posts: 31
I have a couple of questions on Chapter 13 of NFPA54.

According to 13.1.2, Vent downsizing is permitted if we follow the following rules:

1:total height at least 10'

2:flue collar 12" or smaller not reduced by more than one table size

3:flue collar larger than 12" not reduced by more than 2 table sizes

4:maximum capacity for fan assisted is reduced by 10%

5:draft hood outlet is greater than 4"

Also according to 13.1.7:

we should use the tables 13.1a and 13.1b (b-vent tables) for corrugated stainless steel liner systems, but maximum capacity is reduced by 20%

Question 1:

If I am downsizing and using a corrugated stainless liner, do I have to first reduce my Fan Max by 10% for the downsizing, then also reduce by 20% for the liner?

Question 2:

If I have a boiler such as an older HB Smith that had a 7" breach but may have only had a 90,000 firing rate, can I only reduce this down to a 6" liner according to 13.1.2 #2. even though a 5" liner would work better at increasing velocity to carry away condensate? 

Thanks for taking the time to read.


  • Tim McElwain
    Tim McElwain Member Posts: 4,524
    Here are the two sections of

    NFPA 54 you reference:

    13.1.2 Vent Downsizing.

    Where the vent size determined from the tables is smaller than the appliance draft hood outlet or flue collar, the use of the smaller size shall be permitted provided that the installation complies with all of the following requirements:

    (1) The total vent height (H) is at least 10 ft (3 m).

    (2) Vents for appliance draft hood outlets or flue collars 12 in. (300 mm) in diameter or smaller are not reduced more than one table size.

    (3) Vents for appliance draft hood outlets or flue collars larger than 12 in. (300 mm) in diameter are not reduced more than two table sizes.

    (4) The maximum capacity listed in the tables for a fan-assisted appliance is reduced by 10 percent (0.90 X maximum table capacity).

    (5) The draft hood outlet is greater than 4 in. (100 mm) in diameter. Do not connect a 3­in. (80-mm) .diameter vent to a 4-in. (lOO-mm) diameter draft hood outlet. This provision shall not apply to fan-assisted appliances.


    There is an economic incentive to using smaller vents wherever possible. If the vent is smaller that the draft hood or flue collar, venting problems can occur. The restrictions in subsection 13.1.2 recognize and avoid these venting problems. Limits are placed on "downsizing" (see Exhibit 13.2). In particular, note that a 4-in. (lOO-mm) draft hood outlet may not be reduced to 3 in. (80 mm).

    13.1.7* Corrugated Chimney Liners.

    Listed corrugated metallic chimney liner systems in masonry chimneys shall be sized by using Table 13.1 or Table 13.2 for Type B vents with the maximum capacity reduced by 20 percent (0.80 X maximum capacity) and the minimum capacity as shown in Table 13.1 or Table 13.2. Corrugated metallic liner systems installed with bends or offsets shall have their maximum capacity further reduced in accordance with 13.1.3. The 20 percent reduction for corrugated metallic chimney liner systems includes an allowance for one long radius 90­degree turn at the bottom of the liner.

    A.13.1.7 A long radius turn is a turn where the centerline radius is equal to or greater than 1.5 times the vent diameter.


    Because properly installed corrugated chimney liners have heat loss similar to a Type B vent, they are sized using Table 13.1 or Table 13.2, as stated in subsection 13.1.7. However, such liners' corrugations and their tendency to spiral in the chimney require a 20 percent maximum capacity reduction.

    Many liners begin at the breaching and then bend up vertically. This 90-degree elbow at the beginning of the liner is included in the table capacity. The 2002 edition was revised to clearly state that the capacities in Table 13.1 and Table 13.2 include one 90-degree elbow, which is formed when the liner is bent to exit the chimney at the bottom. This revision was developed with the assistance of the researchers who developed the tables. See Exhibit 13.7. 

    Yes liners have 20% less capacity than an equal size "B" Vent.

    The 20% reduction on the liner from "B" Vent tells you how much the liner will handle in BTU's so the 20% is calculated from the size "B" vent you determine to be correct then reduce the BTU amount by 20%. That figure would have to match the input of the equipment you are venting into the liner.

    You stated "Fan Max" reduction from 10%, is this a "Fan Assisted" boiler?  Or is it power vented? if power vented it is not a Category I appliance. Yes the fan assistied 10% reduction must be made first because you are reducing one size.

    You can only reduce one size and reducing size does not increase flue gas velocity.

    I would be careful reducing size at the breach when using a chimney liner.

    If you give specific equipment information it would help in determining what you can and can't do.

  • Mike2
    Mike2 Member Posts: 31

    Thanks Tim. 

    An example of a problem job that I might see would be an older HB Smith Series 8-3L.

    Listed input is 105,000 and collar size is 7".  If I had a 15' tall chimney with a 5' lateral offset, according to 13.1b and downsizing by one size only to a 6" corrugated liner, my minimum btu input would need to be 144,000.  I would understand the max to be 235,000 because of the 10% and 20% reduced capacities.

      I understand that I could use 13.1a and replace the single wall vent connector with b-vent and then be within range for the fan minimum column.  But, is there a way that this could be done using single wall pipe for the vent?
  • Tim McElwain
    Tim McElwain Member Posts: 4,524
    Mike I recxommend to

    everyone today to stay away from using any single wall vent pipe. "B" vent is going to give you more capacity and with the lower flue temps we are looking at now days we need to conserve as much Delta T as we can.
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