Jargon. I forget how much of it I use until I get that blank stare from a client. So here is a primer, and a reminder that if I start using too many acronyms in a conversation, you are to poke me in the chest and say “Hey! Speak English!”

  • CFM50: Cubic feet per minute at 50 pascals. The standard way of finding out if a house is leaky or airtight is to depressurize the house with a fan to minus 50 pascals compared to the outside. When the fan has created this pressure difference, measure how many cubic feet per minute are going through the fan. Then, compare that number to your BTL.
  • BTL: Building tightness limit. The highest number of three equations that determines what your ideal CFM50 should be for a house. One equation deals with the number of people, one with the volume of the home, and one with the number of bedrooms. All of the equations are based on ASHRAE standards. However, as you tighten a house to BTL, you may affect the CAZ.
  • CAZ: Combustion Appliance Zone. A space defined by walls, a floor, a ceiling, and usually a door, that contains a combustion appliance (like a natural gas furnace). Sometimes, if you turn on all the exhaust fans in a house (dryer, bath and kitchen fans, the furnace itself) you can create a vacuum in the CAZ that can pull the exhaust out of the appliance and into the living space. A CAZ test measures that vacuum in pascals, and provides pressure limits for different types of appliances.