HERS Combustion Safety Principles — Questions and Answers

Question 1: What is the primary purpose of a combustion air opening in a mechanical room?

To exhaust flue gases to the outside
To provide adequate oxygen for complete combustion (Correct answer)
To reduce indoor humidity levels
To supply makeup air for kitchen exhaust fans

Correct answer: To provide adequate oxygen for complete combustion

Combustion air openings supply sufficient oxygen to support complete combustion and prevent CO production from fuel-fired appliances.

Question 2: During a worst-case depressurization test, which combination of conditions is typically induced?

All windows open, all fans off
All exhaust fans on, fireplace damper open, supply fans off (Correct answer)
Only the furnace blower running
HVAC system on heating mode with all doors closed

Correct answer: All exhaust fans on, fireplace damper open, supply fans off

Worst-case depressurization maximizes negative pressure by operating all exhaust devices and opening fireplace dampers simultaneously.

Question 3: A gas water heater located in a garage must have its burner and pilot elevated at least how high above the floor?

6 inches
12 inches
18 inches (Correct answer)
24 inches

Correct answer: 18 inches

Garage-installed water heaters must have ignition sources elevated at least 18 inches above the floor to avoid igniting gasoline vapors that pool at floor level.

Question 4: Which test instrument is used to measure carbon monoxide concentration in appliance flue gases?

Manometer
Combustion analyzer (Correct answer)
Blower door
Infrared camera

Correct answer: Combustion analyzer

A combustion analyzer measures CO, CO2, O2, and flue gas temperature to assess combustion efficiency and safety.

Question 5: What does a yellow or orange burner flame on a natural gas appliance most likely indicate?

Efficient complete combustion
Insufficient combustion air causing incomplete combustion (Correct answer)
Excess combustion air diluting the flame
High gas pressure at the appliance

Correct answer: Insufficient combustion air causing incomplete combustion

Yellow or orange flames indicate incomplete combustion from insufficient air, which can produce dangerous levels of carbon monoxide.

Question 6: What is the term for the phenomenon where combustion appliance exhaust is pulled back into the living space through the flue?

Backdrafting (Correct answer)
Short-cycling
Flue recirculation
Stack effect reversal

Correct answer: Backdrafting

Backdrafting occurs when negative house pressure or adverse weather conditions reverse the intended upward flow of combustion gases through the flue.

Question 7: Which RESNET standard governs combustion safety testing procedures for HERS raters?

RESNET/ICC 301
RESNET/ACCA 310
RESNET/ICC 380 (Correct answer)
RESNET 55

Correct answer: RESNET/ICC 380

RESNET/ICC 380 establishes standard test methods for building enclosures, including combustion safety diagnostics performed by HERS raters.

Question 8: What level of ambient CO in a home (in ppm) is considered immediately dangerous to life and health (IDLH) by NIOSH?

35 ppm
200 ppm
1,200 ppm (Correct answer)
400 ppm

Correct answer: 1,200 ppm

NIOSH defines the IDLH level for CO as 1,200 ppm, at which exposure can cause death or irreversible health effects within 30 minutes.

Question 9: A Category I vented appliance relies on which mechanism to exhaust combustion gases?

Powered inducer fan forcing gases out under positive pressure
Natural draft buoyancy created by temperature difference between flue gas and outside air (Correct answer)
Sealed direct-vent system drawing combustion air from outside
Balanced pressure mechanical ventilation

Correct answer: Natural draft buoyancy created by temperature difference between flue gas and outside air

Category I appliances use natural draft, relying on the buoyancy of hot flue gases rising through a vertically vented system.

Question 10: When testing a atmospherically vented furnace, the CAZ (combustion appliance zone) pressure should ideally read:

Strongly negative (below -5 Pa) relative to outside
Slightly negative or neutral relative to outside (Correct answer)
Strongly positive (above +5 Pa) relative to outside
Equal to attic pressure

Correct answer: Slightly negative or neutral relative to outside

A CAZ that is only slightly negative or neutral supports proper natural draft without creating conditions that cause backdrafting.

Question 11: Orphaned water heaters—left alone on a large flue after a furnace is removed—are at heightened risk of:

Over-firing due to excess gas supply
Backdrafting because the oversized flue reduces draft velocity (Correct answer)
Short-cycling from excessive heat exchanger capacity
Pilot outage from high combustion air volumes

Correct answer: Backdrafting because the oversized flue reduces draft velocity

An oversized flue slows flue gas velocity and allows cold outside air to cool and reverse the flow, causing backdrafting in the remaining appliance.

Question 12: What does CO ppm stand for in combustion testing?

Carbon oxygen parts per million
Carbon monoxide parts per million (Correct answer)
Carbon monoxide pounds per meter
Combustion output pressure per minute

Correct answer: Carbon monoxide parts per million

CO ppm means carbon monoxide concentration expressed as parts per million, the standard unit for measuring this toxic gas in air.

Question 13: Which type of venting system is most resistant to backdrafting caused by house depressurization?

Category I natural draft B-vent
Masonry chimney open fireplace
Category IV sealed direct-vent system (Correct answer)
Power-vented with single-wall metal pipe

Correct answer: Category IV sealed direct-vent system

Direct-vent (Category IV) systems draw combustion air from outside and exhaust directly outside through sealed concentric pipes, making them immune to indoor pressure fluctuations.

Question 14: If a combustion analyzer reads 400 ppm CO in the undiluted flue gas of a gas furnace, what action should a HERS rater take?

Document and proceed with the rating—this is within normal range
Recommend regular filter changes to improve efficiency
Advise the homeowner of a potential safety hazard and recommend HVAC service immediately (Correct answer)
Seal the flue pipe joints to reduce dilution

Correct answer: Advise the homeowner of a potential safety hazard and recommend HVAC service immediately

400 ppm CO in undiluted flue gas exceeds safe thresholds and indicates incomplete combustion requiring immediate professional HVAC inspection and repair.

Question 15: The spillage test for a naturally vented water heater is typically performed by holding a smoke pencil or mirror near the draft hood opening. A passing result shows:

Smoke billowing into the room
Smoke drawn into the draft hood and up the flue (Correct answer)
No visible smoke movement in any direction
Smoke moving horizontally across the draft hood

Correct answer: Smoke drawn into the draft hood and up the flue

Smoke drawn into the draft hood indicates proper upward draft pulling combustion gases safely out of the home.

Question 16: What is the typical worst-case depressurization threshold (in Pascals) at which natural-draft combustion appliances are considered at high risk for sustained backdrafting?

-1 Pa
-3 Pa
-5 Pa (Correct answer)
-15 Pa

Correct answer: -5 Pa

A CAZ pressure at or below -5 Pa relative to outside under worst-case conditions indicates high backdrafting risk for naturally vented appliances.

Question 17: Which fuel produces carbon monoxide when burned with insufficient oxygen?

Natural gas only
Propane only
Any carbon-containing fuel (Correct answer)
Electricity

Correct answer: Any carbon-containing fuel

Any fuel containing carbon—natural gas, propane, oil, wood, coal—can produce CO when combustion is incomplete due to insufficient oxygen.

Question 18: A combustion safety inspection reveals an improper connector material: single-wall black iron pipe passing through a bedroom wall to the chimney. The correct action is:

Document it as acceptable because single-wall pipe is standard
Flag it as a hazard—single-wall connectors must not pass through living spaces (Correct answer)
Seal the gaps around the pipe with caulk to prevent CO entry
Increase the appliance firing rate to raise flue temperatures

Correct answer: Flag it as a hazard—single-wall connectors must not pass through living spaces

Single-wall flue connectors are not permitted to pass through or be concealed in walls, floors, or ceilings due to CO leak and fire risk.

Question 19: During a combustion safety test, which appliance configuration typically creates the greatest depressurization of the combustion appliance zone?

A sealed-combustion furnace running at full capacity
A large central-return air handler drawing air from the CAZ (Correct answer)
An HRV balanced ventilation system operating normally
A power-vented water heater with dedicated combustion air supply

Correct answer: A large central-return air handler drawing air from the CAZ

An air handler with a return located in the CAZ can draw large volumes of air from that zone, dramatically depressurizing it and inducing backdrafting in co-located appliances.

Question 20: What is the purpose of a draft diverter (draft hood) on an atmospheric water heater?

To increase flue gas temperature and improve chimney draft
To dilute flue gases with room air and protect the burner from downdrafts (Correct answer)
To measure the flow rate of combustion air into the appliance
To seal the venting system against air infiltration

Correct answer: To dilute flue gases with room air and protect the burner from downdrafts

The draft hood mixes room air with flue gases to moderate temperature and prevents downdrafts from extinguishing the pilot or burner.

Question 21: Which metric best indicates the efficiency and completeness of combustion in a gas appliance flue analysis?

Flue gas temperature only
CO-to-CO2 ratio in the flue gases (Correct answer)
The color of the pilot flame
Gas pressure at the manifold

Correct answer: CO-to-CO2 ratio in the flue gases

The CO-to-CO2 ratio reveals how much carbon burned completely (CO2) versus incompletely (CO), giving a direct measure of combustion quality.

HERS Combustion Safety Principles

What is the primary purpose of a combustion air opening in a mechanical room?

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