Carbon monoxide, or “CO” is an odorless colorless gas that can kill.

The purpose of the following information is not to tell someone how to fix a space that does provide enough air for combustion and the dilution of combusted gases, the purpose is to help the inspector identify if a room or space is too small and needs outside air.

Let’s start with a few definitions.
Combustion Air: Air necessary for complete combustion of a fuel, including theoretical air and excess air.
Theoretical air: The amount of air theoretically required for complete combustion.
Excess air: The percentage of excess air is an amount of air above required air for complete combustion.
The inspector should be concerned with the amount or volume of air necessary to complete combustion of fuel in gas appliances located inside a building not the definitions.
Oxygen depletion, a subject that is not often discussed, is generally associated with water and is one of the main contributors of water pollution, however in the context of combustion air requirements if air in a room or space is not of an adequate volume to complete combustion oxygen depletion can occur and under the right conditions can do great harm to the occupants, especially in very tight house. The reason for including this is to identify that only small gas fired appliances call for the use of oxygen depletion safety shutoff systems, all other gas fired systems rely on a volume of air as determined by a formula discussed below.

The International Fuel Gas Code (IFCG) defines prohibited locations for the installation of fuel gas burning appliances. Appliances shall not be located in or obtain combustion air from any of the following rooms or spaces:

  1. Sleeping rooms.
  2. Bathrooms.
  3. Toilet rooms.
  4. Storage closet.
  5. Surgical Rooms.

When finishing a basement or attic space that contains gas fired appliances a homeowner and/or a contractor overlook the need to provide adequate amounts of combustion air.
The IFGC uses the term “confined space”. This is any space that contains gas appliances that is not large enough to provide combustion air without bringing air in from outside the space.
Determining the amount of space required for the safe operation of gas appliances is based on the total or combined Btu/hr rating of gas appliances located in one space. For example, if one space there is a water heater with a 40,000 Btu/hr rating and a furnace with a 90,000 Btu/hr rating how do we determine how much space is required.
50 cubic feet of space provides enough air for 1,000 Btu/hr. We have 130,000 Btu/hr so we need 6500 cubic feet of space. If the basement ceilings are 9 feet divide 6500 by 9 for 722 square feet. Now we need to determine how big the room should be to not require additional air. You can play around with the numbers, for example use 12 feet for the width and divide divide 722 by 12 for a room size 12X60. If the ceiling height is 10 feet divide 6500 by 10 for 650 sq. ft. A 12-foot-wide room would need to be 54 feet long.
The purpose of doing the math is not to tell someone how to fix the problem. Fixing the problem is up to a qualified and licensed HVAC contractor. The purpose of doing the math is to validate the inspector’s findings.
In a finished basement, to be in compliance, one can install metal grills between the unfinished and finished space as long as the grills do not open into a sleeping room or bathroom or a louvered door between the unfinished and finished space.
Then we have the prohibited locations part to contend with. Many of the codes have exceptions and this issue is no different.

Exceptions to the ventilation portion are:

1. Direct-vent appliances that obtain all combustion air from the outside.
2. Certain room heaters, wall furnaces, vented decorative appliances, and decorative appliances for installation in vented solid fuel burning fireplaces, provided the room is not a confined space and the building is not of unusually tight construction.
3. A single wall mounted unvented room heater equipped with an oxygen depletion safety shutoff system and installed in a bathroom provided the input rating does not exceed 6,000 Btu/hr and the bathroom is not a confined space.
4. Same as the above for a bedroom as long as the rating does not exceed 10,000 Btu/hr.
5. The space is acceptable if it is a dedicated enclosure and all combustion air is taken directly from outdoors and access into the space is protected by a solid weather stripped door in accordance with air leakage requirements of the International Energy Conservation Code and the door is equipped with an approved self-closing device.

An appliance that is not part of IFGC, but should be mentioned is the whole house fan and the impact they can have on water heater flame rollout and gas appliance venting in general. I identify water heaters because a furnace is not apt to be operating during the season when a whole house fan is used.