HVAC (C-20) · Lesson 5 of 6

Combustion Air for Furnaces in a Confined Space

"Combustion Air for Furnaces in a Confined Space" is lesson 5 of 6 in the HVAC (C-20) Learning Hub. It opens with "Gas Flames Need Air - and a Starved One Makes Carbon Monoxide" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Gas Flames Need Air - and a Starved One Makes Carbon Monoxide
  2. Step One: Is the Space Confined?
  3. Indoor Air: Two Openings
  4. Outdoor Air: Smaller Openings
  5. Work It in This Order

The worked example in the course applies this lesson to a realistic job in 4 steps, from "Total the input" through "Compare the other options".

What you'll be able to do

  • Explain the main idea of "Combustion Air for Furnaces in a Confined Space" in plain language you can use on the exam.
  • Apply this step with confidence: "Indoor Air: Two Openings".
  • Apply this step with confidence: "Outdoor Air: Smaller Openings".
  • Apply this step with confidence: "Work It in This Order".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Combustion Air: Confined-Space OpeningsCLOSET 4 x 5 x 8 ft = 160 cu ftFURNACE100,000WATERHEATER40,000Top opening35 in²Bottom opening35 in²to outdoorsConfined: 160 is less than 7,000 cu ftCombustion Air, Confined SpaceTotal input = 100,000 + 40,000= 140,000 Btu/hConfined if under 50 cu ft per 1,000 Btu/h50 x 140 = 7,000 cu ft (closet = 160)Indoor air: 2 openings, 140 in² each(1 in² per 1,000 Btu/h, 100 in² minimum)Outdoor, direct: 140,000 / 4,000= 35 in² each (top and bottom)Horizontal ducts: 140,000 / 2,000 = 70 in²One opening only: 140,000 / 3,000 = 47 in²Outdoor air, direct: two openings of 35 in² eachOne within 12 in. of the top and one within 12 in. of the bottom of the closet.2025 California Mechanical Code Chapter 7: Sections 701.4.1, 701.5, 701.6.1 and 701.6.2.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Gas Flames Need Air - and a Starved One Makes Carbon Monoxide

A gas furnace or water heater burns room air and also uses room air to dilute and vent the flue gases. CMC Section 701.1 requires combustion, ventilation and dilution air to be provided by one of the methods in Sections 701.4 through 701.9.3, and where the indoor-air requirements of Section 701.4 are not met, outdoor air must be introduced under Sections 701.6 through 701.9.3. Appliances that are not natural-draft, not Category I vented, or that have power burners follow the appliance manufacturer's instructions (701.1.1), and direct vent appliances are excluded.

Step 2: Step One: Is the Space Confined?

Add up the INPUT rating (Btu/h) of every fuel-burning appliance in the space, furnace and water heater together. CMC Chapter 2 defines a confined space as one with a volume less than 50 cubic feet per 1,000 Btu/h of that aggregate input, and Section 701.4.1 sets the same 50 cubic feet per 1,000 Btu/h as the minimum required volume. Volume = length x width x height. Rooms that communicate directly through openings without doors, sized under Section 701.5, count as part of the volume. A small utility closet almost always fails this test.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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