Intake and Exhaust Equations
Important Ventilation Terminology
To understand some of the formulas below, we’ve compiled some easy-to-understand definitions for the most frequently used terminology.
Exhaust Ventilation
This form of mechanical ventilation blows polluted or unwanted air out of a specific location in a building.
Intake Ventilation
Intake ventilation brings fresh air in from the outside world. It often works in tandem with exhaust ventilation systems.
Gable Vents
A method of passive ventilation, gable vents are installed on the exterior wall of your home — most often in attics — to assist in creating better air circulation.
Ridge Vents
Another form of passive ventilation, ridge vents are located on the ridges or peaks of sloped roofs. Where two sides of the roof meet, the ridge vent allows air to naturally rise out of the attic, valuably removing moist or humid air.
Net Free Area (NFA)
The amount of open area found in each venting product through which air escapes or is released through. Most venting products are listed with a specific NFA amount.
Common Intake and Exhaust Equations
Click the drop-down menu below to see some of the most common calculations used by HVAC professionals for understanding and determining a home’s ventilation needs.
Air Flow Rate
Q = VA Q = air flow rate (ft3/min)
– A = cross-sectional area of duct or opening (ft2)
– V = average air velocity (ft/min)
Exhaust Needs
Square footage of attic floor _________ x 0.7 = ________CFM Needed (more if over 6-12 pitch)
20W Fan is maximum 890 CFM / 30W Fan is maximum 1350 CFM
Intake Needs
Attic Sq Ft / 300 = _____________ Sq ft needed x 144 = ____________ Sq inches needed
Current Intake for Under-Eave or Continuous:
Amount of vents __________ x NFA Rating _______________ = __________________ inches
Current Intake for Gable Vents
Amount of vents __________ x NFA Rating _______________ = __________________ inches
20W Fan is maximum 890 CFM / 30W Fan is maximum 1350 CFM
Current Intake For Aluminum/Vinyl
Total Sq inches _______ / 144 = _______Sq ft x NFA Rating _________ = __________ inches
TOTAL ___________ inches
Calculation Note
To convert sq ft to sq inches: Square ft ____________ x 144 = __________ Sq inches
To convert sq inches to sq ft: Square inches ___________ / 144 = ___________ Sq ft
It is recommended that 50-60% of an attic’s intake ventilation is
clear (or totally unobstructed) intake air.
The 1/300 Rule
This rule is a common benchmark among HVAC professionals. The 1/300 rule is used to calculate just how much intake ventilation an attic needs in order to create a truly healthy home environment.
The general formula is simple: Take the square footage of area your attic covers and divide that by 300. That is the square feet of total ventilation you need. Now, you need to be sure that more than half of that is intake.
However, this is just the first step in determining true intake ventilation needs. Once the square footage is determined, there are additional measurements which must be taken. Specifically, the “net free intake rating” of the type of soffit vents existing within the attic.
Calculating this is a bit more intensive than the average homeowner is willing to handle, but the initial 1/300 Rule is fairly straightforward. To get a complete understanding of an attic’s intake ventilation needs, it is recommended that homeowners consult a certified HVAC professional for full measurements.
