HERS Combustion Safety 2 — 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.
HERS Combustion Safety 2
What is the primary purpose of a combustion appliance zone (CAZ) depressurization test?