Boiler and Steam Fitting (C-4) · Lesson 3 of 7

Combustion Air for a Boiler Room (CMC Chapter 7)

"Combustion Air for a Boiler Room (CMC Chapter 7)" is lesson 3 of 7 in the Boiler and Steam Fitting (C-4) Learning Hub. It opens with "Where Boiler Combustion Air Is Governed" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Where Boiler Combustion Air Is Governed
  2. Test the Room First: Indoor Air
  3. Outdoor Air: Two Openings or One
  4. Louvers, Ducts and Dampers
  5. Mixed Air Sources and Mechanical Supply

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Total input and room volume" through "Other methods for comparison".

What you'll be able to do

  • Explain the main idea of "Combustion Air for a Boiler Room (CMC Chapter 7)" in plain language you can use on the exam.
  • Apply this step with confidence: "Outdoor Air: Two Openings or One".
  • Apply this step with confidence: "Louvers, Ducts and Dampers".
  • Apply this step with confidence: "Mixed Air Sources and Mechanical Supply".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Boiler Room Combustion Air (CMC Chapter 7)BOILER ROOM (SECTION VIEW)B1B2900,000 Btu/htotal inputOUTDOORSwithin 12 inof ceilingOUTDOORSwithin 12 inof floorRoom 12 x 15 x 10 = 1,800 cu ftSTEP 1: INDOOR AIR TEST (701.4.1)Input 500,000 + 400,000 = 900,000 Btu/hNeed 900 x 50 = 45,000 cu ft of room volumeRoom has 1,800 cu ft: TOO SMALLso bring in outdoor airSTEP 2: OUTDOOR OPENINGS (701.6, 701.10)Direct or vertical duct: 900,000 / 4,000= 225 sq in net free, EACH of twoMetal louver 75% free: 225 / 0.75 = 300 sq inHorizontal ducts: 900,000 / 2,000 = 450 sq inOne opening: 900,000 / 3,000 = 300 sq inMIXED AIR AND MECHANICAL SUPPLYAdjoining 18,000 cu ft room: ratio (1,800 + 18,000) / 45,000 = 0.44; reduction factor 1 - 0.44 = 0.56 (701.7.3)Outdoor opening = 225 x 0.56 = 126 sq in each (minimum dimension 3 in)Mechanical supply (701.9): 0.35 cfm per 1000 Btu/h = 0.35 x 900 = 315 cfm, interlocked with the burnersTwo openings: 225 sq in net free each (300 sq in metal louver), one high and one lowSource: 2025 California Mechanical Code 701.1, 701.4.1, 701.6, 701.7.3, 701.9, 701.10, 1001.3CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Where Boiler Combustion Air Is Governed

2025 CMC §1001.3 says air for combustion and ventilation for a boiler shall be provided in accordance with Chapter 7, and §701.1 says air for combustion, ventilation and dilution of flue gases comes from one of the methods in §701.4 through §701.9.3. The sizing rules below are written for natural-draft (Category I) gas appliances; §701.1.1 sends appliances that are not natural draft, are not Category I, or have power burners to the appliance manufacturer's instructions. The input in every calculation is the total input rating, in Btu/h, of all appliances in the space.

Step 2: Test the Room First: Indoor Air

Under the standard method in §701.4.1 the room must provide at least 50 cubic feet per 1000 Btu/h of total input. Where the infiltration rate is known, §701.4.2 allows a smaller volume, but an infiltration rate above 0.60 ACH may not be used, and if the rate is below 0.40 ACH that method must be used. Rooms that open into the boiler room without doors, or through openings sized under §701.5, count as part of the volume; those indoor openings are 1 square inch per 1000 Btu/h, not less than 100 square inches, one within 12 inches of the top and one within 12 inches of the bottom, with a 3-inch minimum dimension.

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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