TryBuildCalcWater Heater Combustion Air Calculator (Confined Space Sizing)
Size your water heater closet's combustion air openings instantly.
🕒 Last updated: September 18, 2026
Inputs
Appliances in This Space
Add every gas appliance sharing this room/closet — furnace, water heater, boiler, etc.
Appliance 1
Space Size
The room, closet, or alcove enclosing these appliances — used to check whether it's a "confined" space under IFGC.
Weatherstripped doors/windows, sealed sheathing, no fireplace/other natural draft — even an otherwise-unconfined space still needs dedicated outdoor air here.
Outdoor Opening Details
Used for the single-opening method and the louver/grille free-area adjustment.
Only valid when every appliance in the space has a draft hood (or is fan-assisted Category I) and meets its required clearances — check your specific appliance's installation manual.
ℹ️Sets the free-area percentage used to convert required free area into the actual gross opening size you need to cut.
Confined Space — Combustion Air Required
10 sq in
each opening, outdoor two-opening method (direct/vertical duct) — see all methods below
63 cu ft actual space volume vs. 2,000 cu ft required (50 cu ft per 1,000 BTU/hr of 40,000 BTU/hr combined input) — this space is CONFINED.
| Method | Free Area (Each) | Gross Area (Each) |
|---|---|---|
| Indoor, two openings (to adjacent indoor space, same story) | 100 sq in | 100 sq in |
| Outdoor, two openings — direct or vertical duct | 10 sq in | 10 sq in |
| Outdoor, two openings — horizontal duct | 20 sq in | 20 sq in |
| Outdoor, single opening | N/A — enable single-opening eligibility above | |
The 100 sq in floor above applies to the indoor method only. Every opening also needs a minimum 3 in cross-sectional dimension — this calculator computes area only, so confirm your actual width/height meets that separately.
Alternatively, a mechanical combustion air supply delivering at least 14 CFM from outdoors (interlocked so appliances can't fire without it running) satisfies all of the above.
Appliance Load
Appliances: 1
Combined input: 40,000 BTU/hr
Opening Sizing
Louver/grille type: Unlouvered Opening (No Grille/Louver) (100% free area)
Mechanical supply alternative: 14 CFM
Assumptions Used
Confined/unconfined determined per the IFGC/NFPA 54 Standard Method: a space is confined if its volume is less than 50 cu ft per 1,000 BTU/hr of combined appliance input. Indoor two-opening sizing (to an additional indoor space on the SAME STORY only — a different, larger requirement applies to a different-story connection, not modeled here) uses 1 sq in per 1,000 BTU/hr, each opening not less than 100 sq in. Outdoor two-opening sizing uses 1 sq in per 4,000 BTU/hr (direct or vertical duct) or 1 sq in per 2,000 BTU/hr (horizontal duct) — this 100 sq in floor does NOT apply to either outdoor method. Outdoor single-opening sizing uses 1 sq in per 3,000 BTU/hr, not less than the combined vent connector (draft hood/flue collar) area you entered — a required input once this method is eligible, since it can be the figure that actually governs. An unconfined space in unusually tight construction still needs outdoor air, obtained using these SAME outdoor methods (not a separate formula). Mechanical combustion air supply uses 0.35 CFM per 1,000 BTU/hr from outdoors, with the appliance(s) interlocked to the supply fan. Gross (actual cut/product) opening size divides free area by the selected louver/grille type's free area percentage — 100% unlouvered, 75% metal louver/grille, 25% wood louver, per IFGC 304.10's own default assumptions when a specific product's free area isn't yet known. Every opening size above is rounded UP to the next 0.1 sq in, never down, so the displayed figure is never below the true mathematical minimum. Every opening also needs a minimum 3 in cross-sectional dimension, separate from this area-only calculation. This is a planning estimate, not a substitute for your local jurisdiction's adopted code or a licensed mechanical contractor's review.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Combustion & Makeup Air Sizing Calculator.
Water heater combustion air sizing
A standard atmospheric-draft gas water heater almost always qualifies as a draft-hood-equipped appliance, making the single-opening method a realistic option worth checking alongside the two-opening methods.
This page defaults to a 40,000 BTU/hr water heater in a small 3x3x7 ft closet — edit the inputs above to match your actual unit and space.
Combustion Air Formula: How Is It Determined?
Every method starts from the same combined Btu/hr input, then applies a different code-specified divisor and minimum.
Confined Space Check (Standard Method)
Required Volume (cu ft) = Combined Appliance Input (BTU/hr) ÷ 1,000 × 50
If your actual room/closet volume is less than this required volume, the space is "confined" and needs dedicated combustion air. At or above it, infiltration is normally adequate — unless the building is of unusually tight construction, which needs outdoor air regardless, sized using the same outdoor methods below (not a separate formula).
Two-Opening Methods (Indoor or Outdoor)
Indoor (to adjacent space, same story): 1 sq in per 1,000 BTU/hr, each opening
Outdoor, direct or vertical duct: 1 sq in per 4,000 BTU/hr, each opening
Outdoor, horizontal duct: 1 sq in per 2,000 BTU/hr, each opening
The indoor method's each opening also has its own 100 sq in floor — a small combined appliance load can compute to less than 100 sq in by the formula alone, but the code requires at least 100 sq in per opening regardless. This floor does NOT apply to either outdoor two-opening method. One opening goes within 12 in of the top of the enclosure, the other within 12 in of the bottom. A different, larger requirement applies to an indoor opening connecting a different story — not modeled here.
Outdoor Single-Opening Method
Free Area (sq in) = max(Combined Input ÷ 3,000, Total Vent Connector Area)
Only valid when every appliance has a draft hood (or is fan-assisted Category I) and meets its required clearances. The opening can never be smaller than the combined vent connector (draft-hood/flue-collar outlet) area of the appliances, even if the formula alone would allow it.
Gross (Actual Cut) Opening Size
Gross Area (sq in) = Free Area ÷ Louver/Grille Free Area %
Free area is the portion of an opening actually open to airflow — a louver or grille blocks some of it. Per IFGC 304.10's own default assumptions (when a specific product's free area isn't yet known): 100% for a bare unlouvered opening, 75% for a metal louver/grille, 25% for a wood louver.
Mechanical Combustion Air Supply (Alternative)
Required Supply (CFM) = Combined Appliance Input (BTU/hr) ÷ 1,000 × 0.35
Supplied from outdoors, with every served appliance interlocked to the supply fan so the main burner can't fire unless it's running. This replaces passive openings entirely where wall access is impractical.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Combined appliance input | 40,000 BTU/hr (1 appliance) |
| Space volume | 63 cu ft |
| Louver/grille type | Unlouvered Opening (No Grille/Louver) (100%) |
Step-by-Step Calculation
| Step | Calculation | Result |
|---|---|---|
| Required volume (confined-space check) | 40000 ÷ 1,000 × 50 | 2,000 cu ft |
| Space status | 63 vs. 2,000 cu ft | Confined |
| Outdoor two-opening, vertical (each) | 40000 ÷ 4,000 | 10 sq in |
| Outdoor two-opening, horizontal (each) | 40000 ÷ 2,000 | 20 sq in |
| Gross area, outdoor two-opening vertical (each) | 10 ÷ 100% | 10 sq in |
| Mechanical supply alternative | 40000 ÷ 1,000 × 0.35 | 14 CFM |
Therefore, 40,000 BTU/hr of combined appliance input in a 63 cu ft space is a confined space. Using the outdoor two-opening method with a direct or vertical duct, each opening needs at least 10 sq in of gross (actual cut) area — see the full method comparison above for the other options.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.