Building Depressurization
Combustion appliance zone (CAZ) testing, worst-case depressurization procedures, spillage testing, pressure differential limits, and identifying sources of building depressurization that affect combustion safety.
- Define the combustion appliance zone (CAZ) and explain why depressurization testing is required
- Perform worst-case depressurization testing by activating all exhaust devices and closing interior doors
- Measure CAZ pressure relative to outdoors and compare to the -3 to -5 Pascal action level
- Identify common sources of building depressurization and recommend corrective actions
Lesson 1
Understanding Building Depressurization and the CAZ
What Is Building Depressurization?
Building depressurization occurs when more air is exhausted from a building than is being supplied. Exhaust fans, clothes dryers, range hoods, bathroom fans, fireplaces, and even the HVAC system's return duct leakage in unconditioned spaces can all remove air from the building. If replacement air does not enter the building at the same rate, the indoor pressure drops below outdoor pressure - the building becomes depressurized.
For most building functions, slight depressurization is harmless. But for natural draft combustion appliances, depressurization is potentially deadly. A natural draft furnace or water heater relies on a tiny pressure differential (as little as -0.01 inches w.c. or about 2.5 Pascals) to pull combustion products up the flue. If the building is depressurized by more than this amount, the draft is overcome and combustion products - including carbon monoxide - spill into the living space.
The Combustion Appliance Zone (CAZ)
The Combustion Appliance Zone (CAZ) is the room or space where fuel-burning appliances are installed. It may be a furnace closet, utility room, basement, or mechanical room. NCI testing procedures focus on the CAZ because this is where combustion product spillage first occurs.
The CAZ pressure relative to outdoors is the critical measurement. If the CAZ is at -5 Pascals relative to outdoors, the building is actively pulling air inward - including pulling it down the chimney flue, which is the path of least resistance. This reverses the flow in the vent system and brings combustion products from a running appliance (or cold chimney air from a non-operating appliance) directly into the CAZ.
Sources of Depressurization
Understanding what creates depressurization helps technicians identify the root cause when CAZ testing reveals a problem:
Exhaust fans: Kitchen range hoods are the most powerful residential exhaust devices, ranging from 100 CFM for basic models to over 1,200 CFM for commercial-style hoods. Bathroom exhaust fans typically move 50-110 CFM. Every CFM exhausted must be replaced by infiltration or makeup air.
Clothes dryers: A standard residential dryer exhausts approximately 150-200 CFM. This alone can depressurize a tight house enough to cause spillage.
Fireplaces: An open fireplace damper can draw 200-500 CFM up the chimney, even without a fire. A burning fireplace may draw 400-800 CFM. This is often the single largest depressurization source in a home.
HVAC system duct leakage: If the return duct system has leaks in unconditioned spaces (attic, crawlspace), the blower pulls outdoor air into the return side and depressurizes the building. A system with 10-15% return leak rate can depressurize a home by 3-5 Pascals.
Stack effect: In winter, warm air rising in a multi-story building creates a natural chimney effect. The upper floors become positively pressurized while the lower floors become negatively pressurized. Combustion appliances in basements are particularly vulnerable.
The Hidden Killer
A 1,200 CFM range hood in a modern tight home can depressurize the building by 10+ Pascals - enough to completely reverse the flow in a natural draft chimney. The homeowner turns on the range hood to cook dinner while the furnace is running in the basement, and CO pours out of the draft hood into the living space. This scenario has caused numerous CO poisoning incidents. Always test for worst-case depressurization.
Building depressurization occurs when exhaust devices remove more air than enters the building. The CAZ pressure relative to outdoors must be measured under worst-case conditions. Natural draft appliances operate at only 2.5 Pascals of draft force - depressurization of -3 to -5 Pascals can overcome this and cause CO spillage into the living space.