Gas-fired Appliance

 

From the Room or Adjacent Rooms (no intervening door)

Combustion appliances require 50 cubic feet per 1000 BTU per hour (BTU/h) aggregate input. Aggregate- means that if two combustion appliances such as a gas-fired furnace and a gas-fired water heater are installed in the same room, each of their input ratings in BTU/h must be included when calculating adequate combustion air.

 

Example: room ≥50 sq. ft. containing one 40,000 BTU water heater   and one   110,000 BTU furnace:

 

40,000 BTU water heater,

+110,000 BTU furnace

150,000 BTU total

÷ 1,000

150

x 50 sq. ft.

7,500  cubic feet of room air needed for proper combustion

 

From the Room or Adjacent Rooms (with intervening door)

There should be two openings between the room where the combustion appliances are located and the adjacent rooms that contribute to combustion air, one within 12” of the floor, the other within 12” of the ceiling. Each opening should have one sq. inch of net free opening for every 1,000 BTUs/hr. input of all combustion appliances in the room. Using the situation described above:

 

40,000 BTU water heater,

+110,000 BTU furnace

150,000 BTU total

÷ 1,000

150

150 x 1 sq. inch= 150 sq. inches net free area per duct

 

“Net free area” means in calculating the opening dimensions, you would subtract any items that encroach upon the opening, like louvers, screens, etc.

 

From Outside, Ventilated Attic/Crawlspace

Combustion air from outside can be supplied by two openings or two ducts, one within 12” of the floor, the other within 12” of the ceiling. The openings or ducts must open directly to the outside, or to a ventilated attic or crawlspace.

The net free area requirements are as follows:

  1. For each wall opening or vertical duct: one sq. inch of net free opening for every 4,000 BTUs/hr. input of all combustion appliances in the room.
  2. For each horizontal duct: one sq. inch of net free opening for every 2,000 BTUs/hr. input of all combustion appliances in the room.

 

Single Opening to Outside

Combustion air from outside can be supplied by a single opening located within 12” of the ceiling. The openings must be directly to the outside.

The required net free area is one sq. inch of net free opening for every 3,000 BTUs/hr. input of all combustion appliances in the room, and not less than the sum of all vent connectors of the combustion appliances being served.

 

Oil-fired Appliance

 

From the Room or Adjacent Rooms (no intervening door)

Combustion appliances require 50 cubic feet per 1000 BTU per hour (BTU/h) aggregate input.

 

From Outside, Ventilated Attic/Crawlspace

Combustion air from outside can be supplied by two openings or two ducts, one within 12” of the floor, the other within 12” of the ceiling. The openings or ducts must open directly to the outside, or to a ventilated attic or crawlspace.

The net free area requirements are as follows:

  1. For each wall opening or vertical duct: one sq. inch of net free opening for every 4,000 BTUs/hr. input of all combustion appliances in the room.
  2. For each horizontal duct: one sq. inch of net free opening for every 2,000 BTUs/hr. input of all combustion appliances in the room.

 

Note: For both gas and oil, ducts placed low and high in a room serve as intake and exhaust.