Combustion Appliance Zone (CAZ) Depressurization Testing

A homeowner on a building science forum raised a question about a safety test called combustion appliance zone (CAZ) depressurization testing. This test checks whether gas-burning appliances like water heaters, furnaces, or boilers can safely vent their exhaust when a house's fans are running. The idea is that exhaust fans, range hoods, and other equipment can pull air out of a house fast enough to create negative pressure, which in turn can pull exhaust gases back down a flue instead of letting them escape outside. That backward flow is called backdrafting, and it can bring combustion byproducts, including carbon monoxide, into the living space.

The test works by measuring air pressure inside the house, turning on all the fans, then measuring again. Published guidelines set very tight limits on how much pressure change is allowed before a given appliance is considered at risk, ranging from as little as 2 pascals for an older natural-draft water heater up to 50 pascals for sealed combustion units, which draw air from outside rather than the house itself.

The poster questioned whether these numbers are realistic, since a pascal is a tiny unit and ordinary things like elevation, weather, or even where you hold the test instrument can shift readings by that much. No answers were posted in the thread. For homeowners, the takeaway is simply that this kind of test exists and matters most for older, natural-draft gas appliances, especially in tightly sealed or heavily ventilated homes.