Tightening a home’s envelope may reduce the air supply needed for combustion, and when there’s not enough combustion air, equipment could have combustion ventilation problems. Thus, the people who sealed up homes (often referred to as the weatherization industry) needed a way to determine if sealing a home up would undermine the safe operation of combustion equipment. To meet this safety need, they embraced combustion appliance zone (CAZ) depressurization testing.