A 300 cfm exhaust fan can depressurize the leakiest 2,000-square-foot home allowed by code (blower door at 900 cfm50, as indicated by dotted line) to -10 Pa (red dot). No natural-draft gas appliance in the home could safely draft under these conditions.
Adopted from The Energy Conservatory A 300 cfm exhaust fan can depressurize the leakiest 2,000-square-foot home allowed by code (blower door at 900 cfm50, as indicated by dotted line) to -10 Pa (red dot). No natural-draft gas appliance in the home could safely draft under these conditions.

There appears to be a major conflict—one that poses a real danger to the life and safety of building occupants—in the 2012 International Energy Conservation Code (IECC) and 2012 International Residential Code (IRC). The 2012 IECC sets a mandatory blower-door-tested maximum air-infiltration rate at a tight 3 ACH50 (50 air changes per hour) in most of the country (climate zones 3 to 8) and 5 ACH50 in hot, humid regions (climate zones 1 and 2).

The problem lies in the fact that the new IECC/IRC also allows builders to install natural-draft gas appliances in these homes with no safety testing. There is a significant danger that negative pressure near combustion appliances will cause backdrafting, drawing carbon monoxide into the building. The IRC says that if your home meets the ACH50...

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