Combustion Appliance Zone Depressurization Limit (Worst-Case Backdraft Test)

ScopeThe physics is that an atmospherically vented appliance draws its combustion air from the room and relies on a weak buoyant draft to push flue gases up the chimney -- a draft worth only a few pascals. Any exhaust that pulls the room more negative than that draft can overcome reverses the flow, and the flue becomes an inlet: combustion products, including carbon monoxide, enter the house instead of leaving it. That is why the limits are small numbers and why they differ by venting type, from a couple of pascals for a natural draft water heater to well over ten for a sealed-combustion unit. The zone is judged by the WEAKEST appliance in it, and that is the part that gets misread. A basement with a natural draft water heater and an induced draft furnace is governed by the water heater, so a reading that the furnace passes comfortably can still fail the zone -- and it is reported that way here rather than as a single pass or fail, because the fix depends on which appliance is the problem. Worst case is a test condition, not an observation. It means every exhaust appliance running, interior doors positioned to maximise the depressurization, and the air handler tested both on and off, because a duct leak on the return side can depressurize the zone by itself. A CAZ test done without establishing worst case has measured something, but not the thing the limit refers to. The connection to air sealing is why this belongs in a weatherization workflow rather than only a service call. Every hole sealed makes the house tighter and the same exhaust more effective at depressurizing it, so a zone that passes today can fail after the crew leaves -- and a house that fails should not be sealed further until the appliance or the exhaust is resolved. The limits are ENTERED because they vary by protocol and by jurisdiction. This does not perform the test, establish worst case, measure spillage or draft, test for carbon monoxide (which is a separate and non-optional measurement), or evaluate the venting system, its sizing or its condition. The BPI or equivalent protocol in force, the appliance manufacturer, and the technician's own judgment govern.

Formula or tablethe zone passes when the measured worst-case depressurization is at or below the limit for the WEAKEST vent type present; limits are entered by appliance class, and the margin is limit minus measured.

Source and editionThe combustion appliance zone depressurization test as BPI and CAZ field protocol states it, with the limits ENTERED by appliance class because they differ between protocols and jurisdictions. It is a pressure screen: it does not measure spillage, draft, or carbon monoxide, all of which the full protocol requires.

Free accessOne comparison against an entered limit.

What governsEstimate. AHJ and licensed professional govern.

API field namesmeasured_depressurization_pa, has_natural_draft_water_heater, natural_draft_wh_limit_pa, has_natural_draft_furnace, natural_draft_furnace_limit_pa, has_induced_draft, induced_draft_limit_pa, has_direct_vent, direct_vent_limit_pa, largest_exhaust_cfm, governing_limit_pa, margin_pa, appliance_count

• Net of base pressure the reading is the worst case LESS the base (house at rest) reading: base +1 Pa and worst case -3 Pa is a net -4 Pa, which fails a 3 Pa limit the raw reading passes; BPI puts direct vent and sealed combustion at -50 Pa and a mechanically assisted draft furnace or boiler vented alone at -15 PaBPI Building Analyst Standard; CT DEEP Form 5
• Limits are entered by appliance class they differ between protocols and jurisdictionsBPI / the adopted protocol
• Worst case is a test condition every exhaust on, interior doors positioned to worsen itthe CAZ procedure
• Not a spillage or CO test pressure onlythe full combustion safety protocol