Blower Doors ACH50 vs. Natural Infiltration

Two of the great air and infiltration expert minds of our time, Gary Nelson and Steve Rogers from TEC (The Energy Conservatory), come on the podcast to talk about blower doors. They also discuss blower door testing and how it compares to real-life infiltration.

A blower door has a fan to measure the air flowing through it, and you generally install it in a doorframe. Blower doors hook up to manometers to measure the pressure differential between the inside of a building and the outside. Generally, you need to adjust the fan speed to bring the pressure differential down 50 Pascals. Then, you measure the airflow through the blower. That is how you determine how leaky a building is; all the leakage comes out through the blower door due to the pressure differential. We use blower door data and mathematical models to estimate the natural infiltration rate.

ACH50 is a means of expressing the leakage (air changes per hour at 50 Pascals). You take the CFM50 and multiply that by 60 to get the cubic feet per hour. Then, you divide that product by the cubic feet of the building to get your ACH50. You can apply a similar process to the surface area of a building instead of volume (though that’s more common in commercial buildings).

During natural infiltration, the leaks can move inside or outside the envelope, so it’s difficult to use the blower door test to measure infiltration accurately. Wind and extreme temperatures also affect natural infiltration, and testing can’t account for those.

Gary, Steve, and Bryan also discuss:

• Testing pressure variations
• Analogies for measuring infiltration at 50 Pascals
• PSIG vs. PSIA
• Stack effect
• CO₂ and infiltration
• Predicting infiltration rates with models
• Infiltration in cold climates
• How infiltration affects the latent load
• Duct leakage and building pressure
• Transfer grilles
• Balancing with precision manometers

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