This post is a revised version of the last post about Evaluating Combustion Air. The changes are a result of the many discussions I had with the experts in the previous post. You know who you are. Thank you. I continue to value your suggestions. Feel free to comment and thanks again.

Evaluating Combustion Air

Measuring CO and O2 during worst-case conditions helps to evaluate the need for supplemental combustion air.

Measuring CO and O2 during worst-case conditions helps to evaluate the need for supplemental combustion air.

This procedure helps evaluate the need for supplemental combustion air during worst-case testing in a building with a naturally drafting chimney. Don’t make openings in the building without evaluating the need for combustion air. These new openings can lead to unintended consequences like pressurizing or depressurizing the CAZ or admitting cold drafts.

Evaluating the CAZ Volume

In the average building with more than 0.40 natural air changes per hour, the combustion appliance zone or  CAZ should contain more than 50 cubic feet of volume for each 1000 BTUH of combustion-appliance input. However, a smaller volume may provide adequate combustion air and a larger volume may not depending on the airtightness of the CAZ.

Evaluating O2 and CO Readings

During worst-case testing, use a combustion analyzer to measure both CO and oxygen (O2). The O2 is an indicator of excess combustion air, and CO may be an indicator of insufficient combustion air.

  1. Sample undiluted flue gases as they leave the appliance’s heat exchanger during worst-case conditions.
  2. If the  reading from the combustion analyzer is more than 5% with an atmospheric burner or more than 3% with a power burner or well adjusted and maintained barometric draft control, combustion air is probably adequate assuming CO is minimal
  3. If the O2 reading from the combustion analyzer is less than the above O2 values, this indicates that combustion air is inadequate or that the appliance is over-fired. We would expect significant CO to accompany such low O2 readings.
  4. If O2 is too low, measure fuel input to verify that the firing rate is at or below the manufacturer’s BTUH specifications for input. An excessive firing rate could also cause low O2 and high CO.
  5. If O2 is too low at the correct firing rate, open a door or a window connected to the CAZ. If opening the CAZ door, a nearby window, an exterior door, or any combination of these increases the O2 reading and decreases CO, then install supplemental indoor or outdoor combustion air as specified below.

The best solution to a persistent combustion-air problem may be to isolate the CAZ from the building and provide outdoor combustion air.

Installing Supplemental Combustion Air

The table shown here gives sizing guidelines for combustion air openings. If testing indicates the need for supplemental combustion air, install openings to one of these spaces.

  • Another indoor space
  • A ventilated intermediate zone, such as a ventilated attic or ventilated crawl space.
  • Outdoors
  • From outdoors into an isolated CAZ
  • From outdoors to the appliance by replacing atmospheric combustion appliances with sealed-combustion (direct-vent) appliances.