Combustion-air-fullHere in Michigan, builders have been required to install an outdoor combustion air supply into homes to supply power vent and natural draft appliances with the air necessary for proper operation without backdrafting or spilling chimney fumes.

Typically we find a 6″ piece of duct hanging in the floor joists of a basement ceiling. One end has a screen and hood on the outside through the band joist and the other end is open and has an orange sticker that ominously warns of “bodily harm or DEATH” if it is plugged.

The code actually says you may NOT need it!

The purpose of the supply is to ensure that the water heater or furnace chimney is drafting properly. If a natural draft appliance does not have enough combustion air, or if there is a weak drafting chimney it can spill chimney gasses or backdraft causing all kinds of trouble. Don’t go there.

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Poorly drafting atmospheric water heater

The Michigan Residential Code describes this all in section 2407.5.2

Standard Method allows the use of interior air for combustion supply where the volume of indoor air (in the combustion appliance zone) must be 50 cubic feet per 1000 BTUs of appliance input.

Where the infiltration rate of the home is known to be less than .4 ACH, the calculation below must be followed to determine the required volume of indoor air in the combustion appliance zone (CAZ).




Before we get into the mathematics of it all, you can download the worksheet or the stickers (Avery 5168 labels) to use on new or existing homes. but further explanation is below. It’s actually very easy math.

To claim the exception from using an outside air intake

You’ll need two know a couple pieces of data:

  1. The air infiltration rate of the house (in cfm@50, ACH50, or NACH). You’ll typically get this from having a blower door test done to measure air leakage. In Michigan, you can find blower door professionals at
  2. The BTU input of the water heater, typically listed on the label, most are about 40,000 BTU
  3. The calculated volume in cubic feet of the whole house and the room where the water heater is located. The code is not specific on this, but it’s typically the space that can be closed off by a door

Now you need to do some math. We need to calculate the required volume and compare it to the available volume:

NOTE: The ACH in the formula above references NACH or ACHn which is the “natural” air infiltration, not the energy code ACH50 which is the air changes “at test pressure” of -50 Pascals. To convert ACH50 to ACH, it is approximately ACH50 divided by 17.

FIRST:   Convert your CFM@50 blower door number into ACH50:

Multiply CFM@50 x 60

Divide that result by the volume of the entire house to get ACH50

SECOND: Convert ACH50 to ACH (Air changes per hour at the natural state of the house):

Divide ACH50 by 17 to get ACH

THIRD: Calculate the volume required:

21 ÷ ACH x (Appliance BTU input ÷ 1000) = volume required

NOTE: for natural draft appliances, use 15 instead of 21 which applies to fan assisted draft appliances like a power vent water heater

DONE: Compare the volume required against the volume of the room where the water heater is installed and as long as the available volume is greater than the required volume, YOU DO NOT NEED EXTERIOR COMBUSTION AIR!

Last Words of Advice

This information has been reviewed and approved by several code officials with the State of Michigan as being accurate and enforceable parts of the current code. Your HERS Rater can do these calculations for you at any time. You may need to point them to this post to download the worksheet or the stickers (Avery 5168 labels) for use in your homes.

Until more builders and code officials become aware of this procedure, you should expect some push-back. Take the time to educate your officials and show them the sections of the code referenced in this post.