Combustion Air Calculator

Combustion Air Requirement Calculator

Compliant with National Fuel Gas Code (NFPA 54) standards

Sum of all gas appliances (Furnace + Water Heater + Boiler)
Two Vertical Ducts Two Horizontal Ducts Single Opening

Results Summary

Actual Room Volume
0 ft³
Required Volume
0 ft³

Required Outdoor Air Openings

Understanding Combustion Air Requirements

In mechanical engineering and HVAC design, ensuring adequate combustion air is critical for the safety and efficiency of gas-fired appliances. When a furnace, boiler, or water heater burns natural gas or propane, it consumes oxygen from the surrounding environment. If the room volume is insufficient, negative pressure can occur, leading to dangerous backdrafting of Carbon Monoxide (CO) into the living space.

The "50 Cubic Feet" Rule

According to the National Fuel Gas Code (NFPA 54), a space is considered "Unconfined" if it provides at least 50 cubic feet of volume per 1,000 BTU/hr of the aggregate input rating of all appliances in that space. If the room is smaller than this calculated requirement, additional combustion air must be provided from the outdoors or from adjacent rooms.

Outdoor Air Opening Calculations

If your mechanical room is deemed "Confined," you must provide permanent openings to the outdoors. The sizing of these openings depends on the orientation of the ducting:

  • Two Vertical Ducts: Each opening must have a free area of 1 sq. inch per 4,000 BTU/hr.
  • Two Horizontal Ducts: Each opening must have a free area of 1 sq. inch per 2,000 BTU/hr.
  • Single Opening: Must have a free area of 1 sq. inch per 3,000 BTU/hr and be located within the top 12 inches of the enclosure.

Example Calculation

Suppose you have a 100,000 BTU/hr furnace and a 40,000 BTU/hr water heater in a room that is 10ft x 10ft with an 8ft ceiling.

  1. Total BTU: 140,000 BTU/hr
  2. Actual Volume: 10 * 10 * 8 = 800 cubic feet.
  3. Required Volume: (140,000 / 1,000) * 50 = 7,000 cubic feet.
  4. Result: Since 800 < 7,000, the room is "Confined" and requires outdoor air openings.
Warning: These calculations are for general guidance based on standard codes. Always consult local building codes, as some jurisdictions have more stringent requirements regarding Louver Free Area (e.g., metal louvers are usually 75% free area, wood louvers only 25%).
function calculateCombustionAir() { var btu = parseFloat(document.getElementById('btuInput').value); var length = parseFloat(document.getElementById('roomLength').value); var width = parseFloat(document.getElementById('roomWidth').value); var height = parseFloat(document.getElementById('roomHeight').value); var ductType = document.getElementById('ductType').value; if (isNaN(btu) || isNaN(length) || isNaN(width) || isNaN(height) || btu = requiredVolume) { volumeStatus.innerHTML = '✔ UNCONFINED SPACE: The room volume is sufficient for combustion air without dedicated outdoor openings.'; louverSection.style.display = 'none'; } else { volumeStatus.innerHTML = '✘ CONFINED SPACE: Room volume is insufficient. Additional outdoor air openings are REQUIRED.'; louverSection.style.display = 'block'; var sqIn = 0; var label = ""; if (ductType === 'vertical') { sqIn = btu / 4000; label = "Two Openings (Upper & Lower): " + sqIn.toFixed(2) + " sq. inches EACH"; louverNote.innerText = "Calculated at 1 sq. inch per 4,000 BTU/hr for vertical ducts."; } else if (ductType === 'horizontal') { sqIn = btu / 2000; label = "Two Openings (Upper & Lower): " + sqIn.toFixed(2) + " sq. inches EACH"; louverNote.innerText = "Calculated at 1 sq. inch per 2,000 BTU/hr for horizontal ducts."; } else { sqIn = btu / 3000; label = "Single Opening: " + sqIn.toFixed(2) + " sq. inches"; louverNote.innerText = "Calculated at 1 sq. inch per 3,000 BTU/hr. Must be in the top 12\" of the room."; } louverSize.innerText = label; } document.getElementById('resultsArea').style.display = 'block'; }

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