HERS Combustion Safety — Questions and Answers

Question 1: What is the primary purpose of a combustion appliance zone (CAZ) depressurization test?

To measure the BTU output of the appliance
To determine if the zone becomes sufficiently depressurized to cause backdrafting (Correct answer)
To check the flue gas temperature
To verify the gas pressure at the manifold

Correct answer: To determine if the zone becomes sufficiently depressurized to cause backdrafting

The CAZ depressurization test identifies whether the appliance zone can be depressurized enough under worst-case conditions to cause combustion gases to backdraft into the living space.

Question 2: Which of the following conditions represents a 'worst-case' scenario during a CAZ depressurization test?

All windows and doors open, all exhaust fans off
All windows and doors closed, all exhaust fans on (Correct answer)
Only the furnace running with windows open
Only the water heater running with fans off

Correct answer: All windows and doors closed, all exhaust fans on

Worst-case depressurization occurs when all windows and doors are closed and all exhaust fans, including range hoods and bath fans, are running simultaneously.

Question 3: A atmospherically vented water heater fails a steady-state spillage test. What does this indicate?

The water heater is over-firing
Combustion gases are spilling from the draft hood into the living space (Correct answer)
The anode rod needs replacement
The thermostat is set too high

Correct answer: Combustion gases are spilling from the draft hood into the living space

Steady-state spillage means the appliance is continuously venting combustion products into the conditioned space rather than up the flue, creating a CO hazard.

Question 4: What CO concentration threshold (ppm) in undiluted flue gases typically triggers a 'fail' rating during a HERS combustion safety inspection?

50 ppm
200 ppm
400 ppm (Correct answer)
800 ppm

Correct answer: 400 ppm

RESNET/BPI protocols generally flag undiluted flue gas CO readings above 400 ppm as a failure requiring investigation and remediation.

Question 5: Which diagnostic tool is most appropriate for detecting combustion gas spillage at a draft hood?

Blower door manometer
Combustion analyzer or smoke pencil (Correct answer)
Infrared camera
Duct blaster

Correct answer: Combustion analyzer or smoke pencil

A combustion analyzer measures CO and other gases directly, while a smoke pencil visually confirms airflow direction at the draft hood to detect spillage.

Question 6: A sealed-combustion furnace draws combustion air directly from outside. How does this affect CAZ depressurization concerns?

It increases CAZ depressurization risk because more air is consumed
It eliminates CAZ depressurization risk for that appliance since it does not use zone air (Correct answer)
It has no effect on CAZ depressurization
It only reduces risk if the flue is also sealed

Correct answer: It eliminates CAZ depressurization risk for that appliance since it does not use zone air

Sealed-combustion (direct-vent) appliances draw combustion air from outside and exhaust directly outside, so they are unaffected by CAZ pressure and do not contribute to backdrafting concerns.

Question 7: What is the purpose of a draft pressure test on a natural-draft appliance flue?

To measure the heating efficiency of the appliance
To verify the flue is creating adequate negative pressure to remove combustion gases (Correct answer)
To check for gas leaks in the heat exchanger
To calibrate the thermostat

Correct answer: To verify the flue is creating adequate negative pressure to remove combustion gases

Draft pressure testing verifies that the flue system is generating enough negative pressure (draft) to reliably pull combustion gases upward and out of the building.

Question 8: When testing for ambient CO in a home, what is the BPI/RESNET action level (ppm) that requires immediate evacuation?

9 ppm
35 ppm
70 ppm
35+ ppm sustained or any reading above 70 ppm (Correct answer)

Correct answer: 35+ ppm sustained or any reading above 70 ppm

BPI protocols call for immediate evacuation when ambient CO exceeds 35 ppm sustained or any single reading exceeds 70 ppm, and HVAC systems should be shut down.

Question 9: Which type of venting system uses a fan to push or pull combustion gases through the flue, making it less susceptible to backdrafting?

Type B natural-draft venting
Power-vented or induced-draft venting (Correct answer)
Gravity-fed masonry chimney
Atmospheric draft venting

Correct answer: Power-vented or induced-draft venting

Power-vented and induced-draft appliances use a mechanical fan to move flue gases, providing positive pressure in the flue and reducing the risk of backdrafting from house depressurization.

Question 10: A natural-draft gas furnace shares a masonry chimney with a wood-burning fireplace. What combustion safety concern does this create?

The gas furnace will overheat the chimney
Simultaneous use can cause the furnace flue gases to backdraft into the home (Correct answer)
The wood smoke will contaminate the gas supply
The chimney will produce too much draft for the furnace

Correct answer: Simultaneous use can cause the furnace flue gases to backdraft into the home

When a fireplace draws combustion air from the chimney, it can depressurize the shared flue and reverse the flow of furnace exhaust gases back into the living space.

Question 11: What does the term 'orphaned water heater' mean in the context of combustion safety?

A water heater that has never been serviced
A water heater left alone on an oversized flue after a furnace is replaced with a high-efficiency model (Correct answer)
A water heater installed without a permit
A water heater with a failed pilot light

Correct answer: A water heater left alone on an oversized flue after a furnace is replaced with a high-efficiency model

When a high-efficiency furnace no longer vents through a masonry chimney, the remaining atmospheric water heater may be 'orphaned' on an oversized flue, leading to condensation and draft problems.

Question 12: Which condition would most likely cause a natural-draft appliance to backdraft even without house depressurization?

A flue that is too short or has an obstruction (Correct answer)
A flue that is too long
An oversized gas orifice
A clogged condensate drain

Correct answer: A flue that is too short or has an obstruction

A blocked or undersized flue creates resistance that prevents adequate draft, causing combustion gases to spill back into the space even when the house is at neutral pressure.

Question 13: During a combustion safety test, you measure -7 Pa in the CAZ with all exhaust fans running and the largest window open. What does this result indicate?

The CAZ is dangerously depressurized and requires remediation (Correct answer)
The CAZ is within acceptable limits per BPI standards (fail threshold is typically -5 Pa or worse for atmospherically vented appliances)
The CAZ is positively pressurized
The reading is within safe limits because it is negative

Correct answer: The CAZ is dangerously depressurized and requires remediation

A CAZ depressurization of -7 Pa exceeds the BPI failure threshold (typically -5 Pa for atmospherically vented appliances), indicating a risk of backdrafting that requires correction.

Question 14: What is the function of a draft hood on an atmospherically vented gas appliance?

It measures flue gas temperature
It dilutes flue gases with room air and protects the burner from downdrafts (Correct answer)
It seals the combustion chamber from the living space
It regulates gas pressure to the burner

Correct answer: It dilutes flue gases with room air and protects the burner from downdrafts

A draft hood allows room air to dilute flue gases and protects the burner flame from being extinguished by downdrafts or fluctuating chimney draft.

Question 15: A HERS rater tests a home with a gas range and finds elevated CO in the kitchen. What is the most likely cause if only the range top burners are running normally?

Backdrafting from the water heater flue
Incomplete combustion at the range burner due to improper air-fuel mixture or dirty burner (Correct answer)
A cracked heat exchanger in the furnace
CO migrating from the attached garage

Correct answer: Incomplete combustion at the range burner due to improper air-fuel mixture or dirty burner

Gas range burners with improper air-fuel ratios or partially blocked burner ports produce elevated CO due to incomplete combustion even during normal operation.

Question 16: Which of the following best describes the hazard of a cracked heat exchanger in a forced-air gas furnace?

Reduced heating efficiency only
Combustion gases including CO can mix with and be distributed through supply air throughout the home (Correct answer)
Increased gas consumption without a safety hazard
Refrigerant leakage into the air stream

Correct answer: Combustion gases including CO can mix with and be distributed through supply air throughout the home

A cracked heat exchanger allows combustion gases to cross into the supply airstream, distributing CO and other combustion byproducts to every room served by the duct system.

Question 17: What safety device is required by most codes in sleeping areas and on each level of a home to protect against CO poisoning?

Smoke detector
CO alarm (Correct answer)
Gas leak detector
Draft pressure sensor

Correct answer: CO alarm

CO alarms are required on each level of a home and outside sleeping areas to provide early warning of dangerous CO concentrations from combustion appliances.

Question 18: During a worst-case CAZ test, which exhaust device typically contributes the most depressurization?

Bathroom exhaust fan (50 CFM)
Range hood (400+ CFM) (Correct answer)
Clothes dryer (100-150 CFM)
Attic power ventilator

Correct answer: Range hood (400+ CFM)

Large range hoods with high CFM ratings typically dominate CAZ depressurization because they exhaust far more air than bathroom fans or dryers.

Question 19: A home has been air-sealed and tightened significantly. What combustion safety issue may arise if no other changes were made?

Increased flue gas temperatures
Reduced infiltration may starve atmospheric appliances of combustion air, increasing backdrafting risk (Correct answer)
Higher CO output from sealed-combustion appliances
Lower ambient humidity causing flue liner cracking

Correct answer: Reduced infiltration may starve atmospheric appliances of combustion air, increasing backdrafting risk

Air sealing reduces the infiltration that previously supplied combustion air to atmospheric appliances, potentially increasing depressurization and backdrafting risk.

Question 20: What is the correct procedure if a HERS rater discovers steady-state CO spillage from a water heater draft hood during testing?

Document the finding and continue testing
Immediately shut down the appliance and advise the homeowner not to use it until repaired (Correct answer)
Increase the CAZ ventilation and retest in 30 minutes
Install a CO alarm near the appliance and proceed

Correct answer: Immediately shut down the appliance and advise the homeowner not to use it until repaired

Steady-state spillage is an immediate health and safety hazard; the appliance must be shut down and the homeowner advised not to use it until a qualified technician performs repairs.

Question 21: Which type of gas appliance venting is most resistant to changes in house pressure conditions?

Natural-draft Type B vent
Atmospheric open-combustion appliance
Sealed-combustion (direct-vent) appliance (Correct answer)
Power-vented appliance with an open combustion chamber

Correct answer: Sealed-combustion (direct-vent) appliance

Sealed-combustion direct-vent appliances draw combustion air from and exhaust directly to the outdoors through dedicated pipes, making them completely unaffected by indoor pressure changes.

HERS Combustion Safety

What is the primary purpose of a combustion appliance zone (CAZ) depressurization test?

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