Combustion Air Calculator

2024 IFGC / NFPA 54 Updates

The 2024 International Fuel Gas Code (IFGC) and NFPA 54-2024 introduce significant updates to combustion air requirements. Key changes include:

• Section 304 completely rewritten — modernized language and requirements
• Section 304.10 (Louvers and Grilles) — metal louvers default to 75% free area when not known
• Section 304.9.2 (Appliance Interlock) — mechanical air supply systems must interlock with appliances
• Appendix E — official Residential Combustion Air Calculation Worksheet
• ISO 23552-1:2026 — new standard for electronic fuel/air ratio control systems (reference)

How to Use This Calculator

• Standard Method (304.5.1): Use when room volume is known. Includes louver free area adjustments per 304.10.
• Known Infiltration Method (304.5.2): Use when actual air changes per hour (ACH) are known. Differentiates fan-assisted appliances.
• Outdoor Air Method (304.6): Use when combustion air is drawn from outside. Includes louver free area correction.
• Combined Method (304.7): Use when both indoor and outdoor air sources are used. Includes appliance interlock option.
• Appendix E Worksheet: Complete residential calculation per IFGC 2024 Appendix E.

Common Appliance Input Ratings

• Water Heater: 30,000 – 50,000 BTU/h
• Furnace: 60,000 – 120,000 BTU/h
• Boiler: 80,000 – 200,000 BTU/h
• Gas Range: 40,000 – 65,000 BTU/h
• Gas Dryer: 20,000 – 35,000 BTU/h

Frequently Asked Questions

• What's new in the 2024 IFGC? — Section 304 completely rewritten, new louver free area defaults (75% for metal), and appliance interlock requirements.
• What is ACH? — Air Changes per Hour. Typical homes: 0.3-0.6 ACH.
• What is the 75% free area rule? — Per IFGC 304.10, metal louvers and grilles are assumed to have 75% free area when not known.
• Do I need appliance interlock? — Yes, for mechanical combustion air supply systems per 304.9.2.
• What is Appendix E? — Official residential combustion air calculation worksheet from the IFGC.