Glossary · Testing & air leaks

Worst-case depressurization test

Also called: worst-case depressurization · worst-case test · worst-case CAZ test · CAZ depressurization test

A combustion safety check run with exhaust fans on and doors set to create the lowest pressure the furnace room is likely to see.

What it means

The auditor closes windows and exterior doors, turns on exhaust fans, the dryer and sometimes the furnace blower, and sets interior doors to make the combustion appliance zone (CAZ) as depressurized as possible. Each appliance is then fired to check its draft, spillage and carbon monoxide. If venting fails only under these conditions, fixes can include make-up air, smaller exhaust fans or replacing natural-draft appliances.

Why it matters in an audit

It reveals backdrafting risks that a normal check would miss, which is why it's done before and after major air sealing.

In the United States

Pass limits depend on how the appliance is vented and on the standard used, such as BPI-1200 or RESNET's CAZ procedure.

Sources

Learn more here

Related terms

Combustion safety test A check that furnaces, boilers, water heaters and stoves burn cleanly and vent their exhaust outside, even with fans running.
Combustion appliance zone (CAZ) The room or area holding a fuel-burning appliance such as a furnace, boiler or water heater; it's the focus of combustion safety testing.
Depressurization When more air is being pushed out of a house than is coming in, so the inside sits at slightly lower pressure than outdoors.
Backdrafting and spillage When exhaust from a furnace, boiler, water heater or fireplace comes back into the house instead of going up the chimney or vent.
Make-up air Outdoor air deliberately brought into a house to replace air removed by big exhaust fans, range hoods, dryers or fireplaces.
Natural-draft (atmospherically vented) appliance A furnace, boiler or water heater that takes combustion air from the room and relies on hot exhaust rising up a chimney, so house pressure can reverse it.
Manometer A precise pressure gauge auditors use to read small pressure differences, in pascals, during blower door, duct leakage and combustion safety tests.

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