How to Calculate Infiltration Rate HVAC
Infiltration is the uncontrolled leakage of air into a building through cracks, crevices, and other openings. For HVAC professionals and homeowners alike, calculating the infiltration rate is critical for accurately sizing heating and cooling equipment and estimating energy costs.
This calculator determines the Cubic Feet per Minute (CFM) of outside air entering the space based on the building's volume and air tightness, and estimates the resulting sensible heat load.
1. The Infiltration Formula (CFM)
The most common method for estimating infiltration for HVAC load calculations is the Air Change Method. The formula used to calculate the airflow rate is:
CFM = (Volume × ACH) / 60
- CFM: Cubic Feet per Minute of infiltrating air.
- Volume: The total volume of the room or building in cubic feet (Length × Width × Height).
- ACH: Air Changes per Hour. This represents how many times the entire volume of air in the room is replaced by outside air in one hour.
- 60: The conversion factor from hours to minutes.
2. Estimating Air Changes Per Hour (ACH)
The "ACH" variable is the hardest to pinpoint without a Blower Door Test. However, for standard load calculations, the following guidelines are often used:
- Tight Construction (0.35 – 0.50 ACH): Modern homes with vapor barriers, sealed windows, and gasketed doors.
- Average Construction (0.75 – 1.00 ACH): Standard insulation, standard windows, caulked joints. Typical of homes built in the 1990s-2000s.
- Loose Construction (1.50+ ACH): Older homes, unsealed windows, no vapor barrier, gaps around doors and molding.
3. Calculating Sensible Heat Load
Once you know how much air is leaking in (CFM), you can calculate how much energy is required to heat or cool that air to the desired indoor temperature. This is known as the Sensible Heat Load formula:
Qs = 1.08 × CFM × ΔT
- Qs: Sensible Heat Load in BTU/hr.
- 1.08: A constant derived from air density and specific heat capacity at standard conditions.
- ΔT (Delta T): The absolute difference between indoor and outdoor temperatures (°F).
Note: This calculation provides the sensible load. In humid climates, the latent load (moisture removal) is also a significant factor but requires psychrometric data to calculate accurately.
Why Infiltration Matters
High infiltration rates lead to significant energy waste. In the winter, your heating system works overtime to heat cold air rushing in. In the summer, humidity and heat infiltrate the envelope, causing discomfort and higher electric bills. Properly sealing a building envelope is often more cost-effective than upgrading the HVAC equipment itself.