8.2 Combustion Appliance Zone (CAZ) Testing and Worst-Case Depressurization
Key Takeaways
- The Combustion Appliance Zone (CAZ) houses indoor combustion appliances; worst-case depressurization testing under ANSI/BPI-1200 systematically measures the maximum negative pressure the CAZ experiences under operating exhaust fans and air handlers.
- Air handler return duct leakage in mechanical rooms acts as a powerful exhaust mechanism, while closing bedroom doors without return grilles starves central returns and induces severe CAZ depressurization.
- The CAZ Depressurization Limit (CDL) screening table used across weatherization field guides comes from BPI Building Analyst practice, not from ANSI/BPI-1200-S-2017: roughly -2 Pa to -5 Pa for open-flue Category I appliances with draft hoods, -15 Pa for fan-assisted units, and -50 Pa for sealed combustion direct-vent systems.
- ANSI/BPI-1200-S-2017 judges the appliance on measured spillage and CO rather than on a depressurization limit: spillage is assessed at 2 minutes of main burner operation for warm vents and domestic water heaters, at 5 minutes for cold vents, and undiluted flue gas CO is measured at 5 minutes.
- Flue draft pressure must be verified with a static pressure probe connected to a digital manometer, achieving minimum outdoor-temperature-dependent negative draft thresholds between -1.0 Pa and -3.0 Pa.
8.2 Combustion Appliance Zone (CAZ) Testing and Worst-Case Depressurization
Quick Answer: The Combustion Appliance Zone (CAZ) is any enclosed space or room housing combustion appliances that draw combustion air from inside the building. Under ANSI/BPI-1200, building analysts perform worst-case depressurization testing to identify the maximum negative pressure the CAZ can experience when mechanical exhaust devices, clothes dryers, and the central air handler operate simultaneously with interior doors configured to create maximum suction. Most weatherization field guides compare that reading against the CAZ Depressurization Limit (CDL) table from BPI's Building Analyst combustion-safety practice—roughly -2 to -5 Pascals for open-flue appliances with draft hoods, -15 Pascals for fan-assisted units, and -50 Pascals for sealed combustion direct-vent systems. ANSI/BPI-1200-S-2017 itself does not publish a CDL table: it holds the CAZ in its worst-case depressurized state and judges the appliance directly on spillage and carbon monoxide, assessing spillage at 2 minutes of main burner operation for domestic water heaters and warm vents, at 5 minutes for cold vents, and measuring undiluted flue gas CO at 5 minutes.
The Physics of Building Depressurization and Flue Reversal
A chimney or vertical vent pipe acts as a thermal engine. Hot combustion flue gases inside the chimney have a lower density than the cooler ambient air surrounding the house. This density differential generates an upward buoyant pressure differential known as thermal draft or stack pressure:
Where $h$ is chimney height, $P_{\text{atm}}$ is atmospheric pressure, and $T$ represents absolute temperatures in Rankine or Kelvin. In residential atmospheric Category I appliances, this thermal buoyancy is exceptionally weak—typically generating only 2 to 5 Pascals (0.008 to 0.020 inches of water column) of upward draft suction.
Simultaneously, modern homes contain powerful mechanical devices that actively exhaust indoor air out of the building envelope:
- Kitchen Range Hoods: Standard residential hoods exhaust 100 to 400 CFM; commercial-style downdraft or overhead pro-style range hoods can exhaust 600 to 1,200 CFM.
- Clothes Dryers: Standard residential electric or gas dryers exhaust 100 to 150 CFM of air to the outdoors.
- Bathroom Exhaust Fans: Intermittent spot fans exhaust 50 to 110 CFM each. In a typical two-story home, three operating bath fans extract 200 to 300 CFM.
- Central Vacuum Systems: Exhaust 100 to 200 CFM to the outdoors or garage.
- Whole-House Mechanical Ventilation: Continuous exhaust ventilation fans extract 40 to 120 CFM.
[ EXTERIOR ATMOSPHERE (0 Pa Reference) ]
^
| (Exhaust Fans Sucking Air Out)
|
+---------------------------------+---------------------------------+
| CONDITIONED HOUSE |
| |
| Kitchen Hood (300 CFM) + Dryer (150 CFM) + Bath Fans (100 CFM) |
| = 550 CFM Total Air Extraction |
| |
| Interior Doors Closed ===> Creates Pressure Imbalances |
+-------------------------------------------------------------------+
|
v (Air pulled through basement cracks)
+-------------------------------------------------------------------+
| COMBUSTION APPLIANCE ZONE (CAZ) |
| |
| Base Pressure drops to: -6 Pascals relative to outside |
| |
| Water Heater Natural Chimney Draft: Only +3 Pascals Upward |
| |
| RESULT: -6 Pa Room Suction > +3 Pa Chimney Draft |
| ===> FLUE FLOW REVERSES (BACKDRAFTING / SPILLAGE INTO ROOM) |
+-------------------------------------------------------------------+
When mechanical exhaust fans remove air from a tightened building envelope faster than outdoor air can infiltrate to replace it, the indoor living space depressurizes relative to the exterior atmosphere. If the negative pressure in the Combustion Appliance Zone becomes stronger than the upward buoyancy pressure inside the appliance chimney, air flow reverses: air is pulled down the chimney from the outside. When the combustion appliance fires, its toxic combustion products cannot rise against this downward suction; instead, they pour out of the draft hood or draft diverter into the mechanical room. This dangerous failure mode is called combustion backdrafting or continuous flue gas spillage.
Air Handler Return Leaks and the "Bedroom Door Effect"
While exhaust fans are obvious drivers of depressurization, forced-air HVAC systems frequently generate even larger, more dangerous pressure imbalances through duct leakage and closed interior partition doors.
1. Air Handler Return Leaks in the CAZ
Central air handlers circulate substantial airflow—typically 350 to 450 CFM per ton of heating or cooling capacity (a 3-ton system circulates 1,050 to 1,350 CFM). If the furnace or air handler is located in a basement or enclosed mechanical room and has return duct leaks, disconnected return seams, or an unsealed filter access slot:
- The high-pressure suction side of the blower pulls hundreds of CFM of air directly out of the mechanical room air volume.
- The blower then pushes that air through supply ducts into the upstairs living spaces.
- This turns the central heating blower into a massive internal exhaust fan that aggressively depressurizes the CAZ, dropping room pressure by 5 to 15 Pascals in seconds and inducing violent water heater backdrafting whenever the heating system runs.
2. Supply Leaks in Unconditioned Spaces
When forced-air supply ductwork running through an unconditioned attic or vented crawlspace contains leaks, conditioned air is blown directly outdoors. Because the blower continues drawing its full volume of return air from inside the house via central return grilles, the living space is starved of replacement air. This induces a net negative pressure (depressurization) across the entire conditioned building envelope and CAZ.
3. The Closed Bedroom Door Effect (Return Air Starvation)
In many homes, supply registers are installed in every bedroom, but return grilles are located only in a central hallway or living room. When occupants close bedroom doors at night:
- The central blower forces supply air into the closed bedrooms, creating positive pressure (+5 to +15 Pa) inside those rooms.
- This positive pressure drives conditioned air out of the bedrooms through ceiling cracks, exterior wall penetrations, and window seals.
- Meanwhile, the central return grille in the hallway continues trying to pull its full design CFM. Starved of the air trapped inside the closed bedrooms, the return side of the system pulls air from wherever it can—sucking air through basement stairwells, floor cracks, and mechanical room bypasses.
- This induces severe negative pressure (-4 to -10 Pa) in the central core and basement CAZ, immediately backdrafting atmospheric water heaters.
+-------------------------------------------------------------------------+
| THE BEDROOM DOOR EFFECT |
+-------------------------------------------------------------------------+
| CLOSED BEDROOM (Supply Only) CENTRAL HALLWAY & BASEMENT CAZ |
| * Blower pumps air IN (+10 Pa) * Blower starves for Return Air |
| * Room is PRESSURIZED * Hallway/CAZ DEPRESSURIZED (-6Pa)|
| * Air forced out through walls * Natural draft flues BACKDRAFT |
+-------------------------------------------------------------------------+
ANSI/BPI-1200 Worst-Case Depressurization Protocol
To ensure consistent, diagnostically rigorous combustion safety assessments, BPI established the ANSI/BPI-1200 Standard Practice for Basic Analysis of Buildings. Building analysts must follow a strict, standardized sequence to identify the worst-case negative pressure that can occur in the CAZ.
+-------------------------------------------------------------------------+
| ANSI/BPI-1200 WORST-CASE DEPRESSURIZATION |
+-------------------------------------------------------------------------+
| 1. PRE-TEST SAFETY SCREEN |
| * Zero CO monitor in clean outdoor air; check ambient CAZ CO. |
| * At 36-69 ppm, ventilate and shut off sources; at >= 70 ppm, ABORT. |
+-------------------------------------------------------------------------+
|
v
+-------------------------------------------------------------------------+
| 2. INSTRUMENT SETUP (Digital Manometer) |
| * Channel A: Input = CAZ, Reference = Outdoors |
| * Channel B: Input = Flue Draft Probe, Reference = CAZ |
| * Record Baseline CAZ pressure with all systems OFF |
+-------------------------------------------------------------------------+
|
v
+-------------------------------------------------------------------------+
| 3. CONFIGURE BUILDING ENVELOPE |
| * Close all exterior doors, windows, and fireplace dampers. |
| * Close interior doors EXCEPT rooms with an exhaust fan or a return. |
| * Close CAZ doors first, then open them if the CAZ gets more negative|
+-------------------------------------------------------------------------+
|
v
+-------------------------------------------------------------------------+
| 4. ACTIVATE MECHANICAL EXHAUST APPLIANCES |
| * Turn on clothes dryer, kitchen range hood, and all bath fans. |
+-------------------------------------------------------------------------+
|
v
+-------------------------------------------------------------------------+
| 5. EVALUATE AIR HANDLER & INTERIOR DOOR POSITIONS |
| * Turn on central HVAC blower. |
| * Systematically open/close bedroom doors while watching manometer. |
| * Leave doors in position that creates MAXIMUM CAZ DEPRESSURIZATION. |
+-------------------------------------------------------------------------+
|
v
+-------------------------------------------------------------------------+
| 6. TEST CAZ DOOR POSITION (Open vs. Closed) |
| * Check CAZ pressure with CAZ door OPEN, then CLOSED. |
| * Record the most negative pressure as WORST-CASE DEPRESSURIZATION. |
+-------------------------------------------------------------------------+
Step-by-Step Diagnostic Sequence
Step 1: Pre-Test Ambient Carbon Monoxide Screening
Before setting up equipment, the auditor must power on and zero a calibrated electronic carbon monoxide detector in clean outdoor air. Measure ambient CO in the living area and CAZ. If ambient CO is 36 to 69 ppm, open windows and doors, recommend that all possible CO sources be turned off, and do not begin combustion testing until levels fall. If ambient CO is 70 ppm or greater, ANSI/BPI-1200-S-2017 Section 7.3.3.3.1 requires terminating the inspection outright: evacuate the building and notify emergency services from outside. Symptomatic occupants trigger the same evacuation response at any reading.
Step 2: Digital Manometer Configuration
Connect a precision two-channel digital differential manometer (such as an Energy Conservatory DG-700 or DG-1000):
- Channel A Setup (CAZ with reference to Outdoors): Run a length of pressure tubing to the outdoors, positioning the outdoor end away from direct wind gusts and exhaust fan discharge streams. Connect the outdoor tubing to the Reference port on Channel A. Connect the Input port on Channel A to open air inside the CAZ.
- Channel B Setup (Flue with reference to CAZ): Insert a rigid stainless steel static pressure probe into the appliance flue vent connector (1 to 2 pipe diameters upstream of the draft hood relief opening). Connect this probe to the Input port on Channel B, leaving the Reference port open to the CAZ.
Step 3: Record Natural Baseline CAZ Pressure
With all exhaust fans, clothes dryers, central air handlers, and combustion appliances turned completely OFF, record the baseline pressure on Channel A. Natural stack and wind effects typically produce a baseline reading between 0.0 Pa and -1.5 Pa.
Step 4: Configure Building Envelope Boundaries
- Close all exterior doors, windows, and storm assemblies.
- Extinguish any fire in a woodstove or fireplace (no hot coals or embers), then close fireplace glass doors, ash pit cleanouts, and fireplace dampers.
- Per ANSI/BPI-1200-S-2017 Section 7.9.1.3, close the interior doors of all rooms except rooms that contain an exhaust fan and rooms that contain a central forced-air return. Outdoor combustion-air openings stay open.
- Close all CAZ doors to start. Section 7.9.1.9 then has you open the door(s) leading directly into the CAZ and keep them open only if the CAZ becomes more negative with reference to outside.
- Turn mechanical ventilation and forced-air blowers off while you record the baseline.
Step 5: Engage All Exhaust Appliances
Turn on all mechanical exhaust devices that vent air to the outdoors:
- Clean the clothes dryer lint trap, set the dryer to high heat or air fluff, and turn it on.
- Set the kitchen range hood exhaust fan to its highest operating speed.
- Turn on all bathroom exhaust fans, utility room exhaust fans, and continuous ventilation fans.
- Do NOT turn on whole-house attic fans or attic gable ventilators (which are seasonal cooling devices).
Step 6: Systematically Explore Air Handler and Bedroom Door Positions
Turn on the central air handler blower (by adjusting the thermostat to call for continuous fan or heat/cool mode). Then, while continuously observing Channel A on the digital manometer:
- Systematically open and close interior bedroom doors.
- Test different permutations (e.g., all bedroom doors closed; master suite closed with other doors open; all doors open).
- Identify the specific door configuration that drives Channel A to its most negative pressure reading.
- Leave the bedroom doors configured in this maximum depressurization state.
Step 7: Evaluate the CAZ Door Position
With the exhaust fans running and bedroom doors set in the worst configuration, observe Channel A with the CAZ door OPEN, and then with the CAZ door CLOSED:
- In some homes, closing the CAZ door isolates the appliances from exhaust fans upstairs, making the CAZ less negative.
- In other homes with return duct leaks in the basement, closing the CAZ door traps return suction inside the small mechanical room, making the CAZ severely more negative.
- Record the most negative value displayed on Channel A as the Worst-Case Depressurization ($WCD$) in Pascals.
- Calculate the net depressurization induced by mechanical systems:
CAZ Depressurization Limits (CDL)
The measured worst-case depressurization is conventionally compared against a CAZ Depressurization Limit (CDL): the maximum negative room pressure under which a given venting category can still exhaust combustion gases reliably.
[!IMPORTANT] Know where the CDL comes from. The CDL table below originates in BPI's Building Analyst combustion-safety practice and is reproduced in most state weatherization field guides and DOE Weatherization Assistance Program protocols. ANSI/BPI-1200-S-2017 does not contain a CDL table. That standard instead places the CAZ in its worst-case depressurized state and then judges each appliance on two measured outcomes — flue gas spillage and carbon monoxide — with action levels in Annex D. Treat the CDL as a practical screening tool that tells you when spillage is likely; treat spillage and CO as the pass/fail evidence.
+-------------------------------------------------------------------------+
| BPI CAZ DEPRESSURIZATION LIMITS (CDL) BY TYPE |
+-------------------------------------------------------------------------+
| APPLIANCE VENTING CATEGORY CDL THRESHOLD |
+--------------------------------------------------+----------------------+
| Open-flue Natural Draft (Draft Hood / Diverter) | |
| - Mild Outdoor Weather (> 90°F) | -2.0 Pascals |
| - Moderate Outdoor Weather (10°F to 90°F) | -3.0 Pascals |
| - Freezing Outdoor Weather (< 10°F) | -5.0 Pascals |
| - Exterior Masonry Chimney (All temps) | -2.0 Pascals |
+--------------------------------------------------+----------------------+
| Natural Draft Fireplace or Open Hearth Woodstove| -3.0 to -5.0 Pascals|
+--------------------------------------------------+----------------------+
| Category I Fan-Assisted (Induced Draft Fan) | -15.0 Pascals |
+--------------------------------------------------+----------------------+
| Category III / IV Mechanical Draft (Indoor Air) | -15.0 to -20.0 Pa |
+--------------------------------------------------+----------------------+
| Category IV Direct-Vent (Sealed Combustion) | -50.0 Pascals |
+--------------------------------------------------+----------------------+
Why Do Limits Vary by Venting Category?
- Open-Flue Atmospheric Appliances (-2 Pa to -5 Pa): Atmospheric water heaters and boilers have no mechanical draft assistance. In warm or mild weather (>90°F), the temperature difference between flue gas and outdoor air is minimal, yielding almost zero thermal buoyancy; thus, even a tiny room depressurization of -2.0 Pa causes backdrafting. Cold weather (<10°F) boosts chimney draft, raising the threshold to -5.0 Pa. Exterior chimneys remain at -2.0 Pa regardless of outdoor temperature because cold outdoor bricks chill the flue gas.
- Fan-Assisted Category I Appliances (-15 Pa): Modern 80% AFUE furnaces feature an induced-draft fan that generates mechanical suction across the heat exchanger. This mechanical pull resists room depressurization up to -15.0 Pa before burner rollout or backdrafting occurs.
- Category IV Direct-Vent Sealed Combustion (-50 Pa): Because sealed combustion units draw 100% of combustion air from outdoors through a sealed pipe and discharge through a sealed exhaust pipe, indoor room pressure cannot affect burner operation. The -50.0 Pa figure functions as a practical upper boundary protecting appliance cabinet seams from secondary air infiltration — which is why replacing an atmospheric appliance with a direct-vent or power-vent unit (or a heat pump) is the durable fix for a CAZ that cannot pass.
Flue Draft Pressure Diagnostics
Once the building is established in its worst-case depressurization configuration, the auditor must directly verify that the appliance establishes adequate static negative draft inside its vent connector pipe.
+-------------------------------------------------------------------------+
| FLUE DRAFT PRESSURE TESTING |
+-------------------------------------------------------------------------+
| |
| TO OUTSIDE CHIMNEY |
| ^ |
| | Flue Gases |
| +------+------+ |
| | Vent Pipe | |
| | | |
| Static Probe =====> * | <-- 1-2 Pipe Diameters Above |
| (To Manometer) | | Draft Hood or Breech |
| +------+------+ |
| | Draft Hood | <=== Dilution Air |
| +------+------+ |
| | |
| +------+------+ |
| | Water Heater| |
| | Burner Box | |
| +-------------+ |
+-------------------------------------------------------------------------+
Draft Testing Protocol
- Insert the static pressure probe (connected to Manometer Channel B) into the test hole drilled in the vent connector pipe:
- For appliances with a draft hood, the probe must be located 1 to 2 pipe diameters upstream (below) the draft hood relief rim.
- For appliances with a barometric damper or fan-assisted draft, drill 12 to 18 inches downstream of the appliance flue collar.
- Fire the appliance under worst-case depressurization. Always fire the appliance with the smallest firing rate first (typically the domestic water heater, 30,000 to 40,000 BTU/hr), because its weak thermal draft is most easily defeated by building depressurization.
- Monitor Channel B on the digital manometer. Within 1 to 2 minutes of burner ignition, the chimney must establish a continuous negative draft.
Minimum Allowable Flue Draft Pressure Standards
The minimum required negative draft is governed by outdoor temperature. In colder weather, stronger draft is required to confirm proper chimney performance:
| Outdoor Ambient Temperature Range | Minimum Allowable Negative Draft Pressure |
|---|---|
| Freezing Cold (< 10°F) | -2.5 Pa (or more negative) |
| Moderate (10°F to 90°F) | ($T_{outside}$ / 40) - 2.75 Pa — for example -2.5 Pa at 10°F, -1.75 Pa at 40°F, -1.25 Pa at 60°F, -0.5 Pa at 90°F |
| Mild / Warm (> 90°F) | -0.5 Pa (or more negative) |
Work the formula rather than memorizing a band: at 45°F the requirement is (45 / 40) - 2.75 = -1.625 Pa, so a measured -1.4 Pa fails while -2.0 Pa passes. Colder outdoor air produces a stronger buoyant stack, so the standard legitimately expects a stronger measured draft in winter.
Critical Diagnostic Rule: A negative draft reading (e.g., -2.4 Pa) indicates proper upward suction toward the outdoors. A positive reading (e.g., +1.2 Pa) indicates that the flue has reversed and is backdrafting flue gases directly into the CAZ!
Flue Gas Spillage Diagnostics and Assessment Timing
While a digital manometer quantifies static pressure, spillage testing physically verifies whether combustion byproducts are escaping from the appliance into the indoor living space.
+-------------------------------------------------------------------------+
| FLUE GAS SPILLAGE TEST PROTOCOL |
+-------------------------------------------------------------------------+
| 1. Ignite appliance under worst-case depressurization. |
| 2. Position smoke pen or mirror along draft hood relief rim. |
| 3. Cold chimney may spill smoke briefly while the vent warms. |
| 4. TIMER MILESTONE (ANSI/BPI-1200-S-2017): |
| - WARM VENT / DOMESTIC WATER HEATER: assess at 2 MINUTES |
| - COLD VENT (all other appliances): assess at 5 MINUTES |
| - UNDILUTED FLUE GAS CO: measure at 5 MINUTES |
| 5. Spillage still present at the assessment point = ACTION REQUIRED. |
+-------------------------------------------------------------------------+
Spillage Testing Protocol
- With the house under worst-case depressurization, ignite the lowest-input appliance (the water heater).
- Deploy a chemical smoke pen, smoke puff bulb, or a cool dental/inspection mirror around the entire 360-degree perimeter of the draft hood relief opening or barometric damper rim.
- The Assessment Points: A cold chimney holds dense, cold air that briefly resists flow, so the standard gives the vent time to warm before you judge it. ANSI/BPI-1200-S-2017 sets the assessment point by vent condition, not by a single stopwatch number:
- Warm vent, including domestic water heaters (Section 7.9.3.1): assess spillage at 2 minutes of main burner operation.
- Cold vent, all other natural-draft appliances (Section 7.9.2.1): assess spillage at 5 minutes of main burner operation.
- Undiluted flue gas CO: measure at 5 minutes of main burner operation in both cases.
- At the assessment point, all smoke from the smoke pen must be drawn cleanly up into the flue, with no moisture vapor condensing on an inspection mirror held at the draft hood rim.
- Legacy note: older BPI Building Analyst material and many older training decks teach a 60-second spillage cutoff. If an exam item or a state field guide cites 60 seconds, it is quoting that legacy practice; the current ANSI/BPI-1200-S-2017 timings are 2 minutes (warm vent) and 5 minutes (cold vent).
- Commonly Vented Appliances (Section 7.9.4): Test appliances in order from the lowest BTUh input rating to the highest. Assess the smallest appliance first, then — without letting the chimney cool — fire the next largest while the first is still running, retest the first appliance for spillage once the second reaches 2 minutes of main burner operation, and test the second immediately after. Continue until every commonly vented appliance is firing simultaneously.
Triage and Remediation Decision Tree
- Case A: Pass Worst-Case Depressurization: The appliance establishes proper draft and shows no spillage at the assessment point (2 minutes for a warm-vent water heater, 5 minutes for a cold vent) under worst-case conditions. The venting system passes BPI safety standards; weatherization air sealing may proceed.
- Case B: Fail Worst-Case, but Pass Natural Baseline: If the appliance is still spilling smoke at the assessment point under worst-case conditions, turn off all exhaust fans, open interior doors, and allow the flue to cool. Re-test the appliance under natural baseline conditions. If it passes under baseline, the failure is driven entirely by house depressurization. Corrective measures include:
- Installing a dedicated, interlocked motorized outdoor makeup air damper for high-CFM kitchen range hoods.
- Sealing supply and return duct leaks in the mechanical room and unconditioned attics/crawlspaces.
- Installing jump ducts or door transfer grilles in bedrooms to eliminate return starvation.
- Upgrading legacy Category I appliances to direct-vent sealed combustion units.
- Case C: Fail Natural Baseline: If the appliance is still spilling at the assessment point even under natural baseline conditions (with all fans off and doors open), the venting system suffers from a severe physical defect: a blocked chimney, a collapsed flue tile, an undersized vent pipe, or disconnected pipe joints. Immediate BPI Protocol: The auditor must immediately shut down the appliance gas supply, apply an official warning red-tag, notify the homeowner in writing, and require repair by a licensed HVAC contractor before any weatherization work can begin.
Concrete Residential Case Study: Depressurization in a Split-Level Retrofit
An energy auditor evaluates a 1972 split-level home in Ohio following extensive attic air sealing that reduced envelope leakage from 8.8 ACH50 down to 2.9 ACH50. The basement mechanical closet houses a 34,000 BTU/hr Category I atmospheric gas water heater with a draft hood.
Worst-Case Setup:
- Exterior doors and windows closed.
- Clothes dryer (120 CFM), two bath exhaust fans (70 CFM each), and a 350-CFM kitchen range hood turned ON.
- Central air handler turned ON. The auditor monitors Channel A on the digital manometer while opening and closing interior doors. Closing three bedroom doors increases negative pressure by 2.6 Pascals due to the absence of return grilles in the bedrooms.
- Channel A records -6.2 Pascals in the CAZ relative to outside.
Appliance Evaluation:
- The outdoor temperature is 42°F. For an open-flue appliance in moderate weather, the BPI CAZ Depressurization Limit is -3.0 Pa.
- The measured depressurization (-6.2 Pa) severely violates the CDL (-3.0 Pa).
- The auditor fires the water heater. A smoke pen held at the draft hood indicates heavy spillage into the basement. At the 2-minute warm-vent assessment point, spillage continues unabated and the flue draft probe on Channel B reads +1.8 Pa (positive pressure pushing combustion products out of the draft hood).
- The auditor re-tests the unit under baseline conditions (all exhaust fans off, bedroom doors open). Under baseline, the water heater establishes a draft of -2.1 Pa and shows no spillage at the 2-minute assessment point.
Diagnostic Conclusion & Remediation: The water heater is structurally functional but is defeated by house depressurization. The weatherization agency installs jump ducts in the three bedrooms (relieving 2.4 Pa of pressure drop), seals a massive return duct seam on the furnace plenum in the basement, and installs a fresh-air makeup duct. Re-testing under worst-case yields a new CAZ pressure of -1.8 Pa; the water heater drafts safely and clears all smoke within 35 seconds, successfully passing BPI test-out verification.
BPI Exam Tips & Common Traps
- Manometer Reference Port Trap: Always remember how Channel A is configured: CAZ with reference to Outdoors. The Input port is inside the CAZ; the Reference port is connected to outdoor tubing. A negative reading means the CAZ is depressurized relative to the outside.
- The Bedroom Door Exploration Rule: Never assume whether interior doors should be open or closed during worst-case depressurization. BPI mandates actively exploring door positions with the air handler running. Closing doors to rooms that have supply vents but no return grilles almost always produces the most severe negative pressure in the CAZ.
- The Spillage Assessment Points: Memorize 2 minutes (warm vent, including every domestic water heater) and 5 minutes (cold vent, everything else), with CO measured at 5 minutes in both cases. Brief spillage while a cold vent warms up is expected and is not what you are judging; spillage persisting at 61 seconds is an automatic diagnostic failure.
- Draft Direction Trap: Remember that upward chimney draft is a negative pressure on the manometer (suction). A positive reading indicates flue gas backdrafting into the living space.
When configuring a residential building for worst-case depressurization testing under ANSI/BPI-1200 standards, how must the auditor determine the final position of interior bedroom doors?
A basement mechanical room contains an atmospheric gas water heater equipped with a draft hood. The outdoor temperature during the audit is 45°F. What is the Combustion Appliance Zone Depressurization Limit (CDL) established by BPI standards for this open-flue appliance?
Under ANSI/BPI-1200-S-2017, at what point during main burner operation is flue gas spillage assessed on a domestic water heater connected to a warm vent?