Calculate Combustion Air

Combustion Air Requirement Calculator

Use this tool to determine if your mechanical room provides enough volume for gas-fired appliances (furnaces, water heaters, boilers) based on the standard 50 cubic feet per 1,000 BTU/hr rule.

Analysis Results

Required Volume 0 ft³

Available Volume 0 ft³

Ventilation Opening Requirements:

function calculateCombustionAir() { var btu = parseFloat(document.getElementById('totalBtu').value); var length = parseFloat(document.getElementById('roomLength').value); var width = parseFloat(document.getElementById('roomWidth').value); var height = parseFloat(document.getElementById('roomHeight').value); var resultsArea = document.getElementById('resultsArea'); var statusBox = document.getElementById('statusBox'); if (isNaN(btu) || isNaN(length) || isNaN(width) || isNaN(height) || btu = requiredVolume) { statusBox.innerHTML = "STATUS: UNCONFINED SPACENatural infiltration is likely sufficient for combustion air."; statusBox.style.backgroundColor = "#d4edda"; statusBox.style.color = "#155724"; statusBox.style.border = "1px solid #c3e6cb"; } else { statusBox.innerHTML = "STATUS: CONFINED SPACEWarning: Additional combustion air openings are required by code."; statusBox.style.backgroundColor = "#f8d7da"; statusBox.style.color = "#721c24"; statusBox.style.border = "1px solid #f5c6cb"; } // 5. Calculate Ventilation Openings (NFPA 54 Standards) // Indoor Air: 1 sq in per 1,000 BTU var indoorOpening = btu / 1000; // Outdoor Air (Vertical Ducts): 1 sq in per 4,000 BTU var outdoorVertical = btu / 4000; // Outdoor Air (Horizontal Ducts): 1 sq in per 2,000 BTU var outdoorHorizontal = btu / 2000; document.getElementById('indoorAir').innerHTML = "From Inside Air: 2 openings of " + Math.ceil(indoorOpening) + " sq. inches each."; document.getElementById('outdoorAirVertical').innerHTML = "From Outside (Vertical): 2 openings of " + Math.ceil(outdoorVertical) + " sq. inches each."; document.getElementById('outdoorAirHorizontal').innerHTML = "From Outside (Horizontal): 2 openings of " + Math.ceil(outdoorHorizontal) + " sq. inches each."; }

Understanding Combustion Air Requirements

Combustion air is the oxygen required for gas-fired appliances to burn fuel efficiently and safely. Without adequate air, appliances like furnaces, boilers, and water heaters can produce carbon monoxide (CO), experience "sooting," or fail to draft properly, leading to life-threatening conditions.

The 50 Cubic Feet Rule

Building codes (such as NFPA 54 and the International Fuel Gas Code) generally define a space as "confined" if it provides less than 50 cubic feet of volume for every 1,000 BTU/hr of the aggregate input rating of all appliances in that space.

Calculating Total BTU Input

To use this calculator, you must sum the BTU/hr ratings found on the data tags of every appliance in the room. For example:

• Gas Furnace: 80,000 BTU/hr
• Gas Water Heater: 40,000 BTU/hr
• Total: 120,000 BTU/hr

In this scenario, you would need at least 6,000 cubic feet of room volume (120 * 50) for the room to be considered "unconfined."

What if the Space is Confined?

If your room volume is too small, you must provide permanent openings that communicate with the outdoors or with other large indoor spaces. Common methods include:

• Two Permanent Openings: One located within 12 inches of the ceiling and one within 12 inches of the floor.
• Indoor Air: Each opening must have a free area of at least 1 square inch per 1,000 BTU/hr.
• Outdoor Air: If using vertical ducts directly to the outside, you need 1 square inch per 4,000 BTU/hr. If using horizontal ducts, you need 1 square inch per 2,000 BTU/hr.

Note: Louvers and grilles restrict airflow. When calculating the physical size of the vent, you must account for the "free area." Wood louvers usually have 20-25% free area, while metal louvers have 60-75% free area.