HVAC Excellence Gas Heat Certification Exam key terms

The exact volume of air needed to burn a given amount of gas completely, with no oxygen left over and no unburned fuel. Real appliances supply more than this to ensure clean combustion.

Air supplied beyond the theoretical requirement, usually expressed as a percentage. Enough excess air ensures complete burning, but too much cools the flame and lowers efficiency and heat transfer.

The proportion of combustion air to fuel gas entering the burner. It determines flame shape, temperature, and completeness of burning, and is adjusted by gas manifold pressure and burner air shutters.

Air mixed with gas before ignition, drawn in at the burner orifice or by a blower. It sets the basic flame character and, in atmospheric burners, is controlled by the air shutter opening.

Air that reaches the flame from around the burner after ignition, completing combustion of the remaining gas. Inadequate secondary air causes yellow tips, sooting, and carbon monoxide production.

The upper and lower gas concentrations in air that will support ignition and sustained burning. Natural gas burns roughly between 4 and 15 percent in air; outside that band it is too lean or too rich to ignite.

A condition where flame travels back through the burner ports or air mixture tube instead of staying at the ports. It usually results from too little gas velocity, a clogged orifice, or a hot burner.

A flame that pulls away from the burner ports and may blow out, caused by excessive gas pressure, too much primary air, or a damaged port. It risks delayed ignition and unburned gas release.

Burning that leaves unburned fuel or partially oxidized products such as carbon monoxide and soot. Causes include insufficient air, dirty burners, misalignment, and blocked flues.

A colorless, odorless, poisonous gas produced by incomplete combustion. In flue gas it signals a combustion or venting problem and is measured in parts per million during analysis.

The heat released by burning a unit volume of gas. Natural gas is typically about 1,000 BTU per cubic foot and propane about 2,500 BTU per cubic foot, which drives orifice sizing.

The density of a gas compared with air, where air equals 1.0. Natural gas is lighter than air (about 0.6) and rises; propane is heavier (about 1.5) and settles in low areas.

A capped vertical drop installed ahead of the appliance gas valve so dirt, scale, and pipe dope fall out of the gas stream. It protects the valve and orifice from debris and must be installed per code.

Selecting gas pipe diameter from the total connected BTU load, the allowable pressure drop, and the longest run length. Longer runs and larger loads require larger pipe to keep pressure at the appliance.

A device that reduces higher supply pressure to a steady lower pressure for the appliance or manifold. It responds to a diaphragm sensing downstream pressure and opens or closes a seat accordingly.

The pressure measured at the burner manifold, set by the appliance regulator. It is checked with a manometer and adjusted to the nameplate value to control firing rate and flame quality.

An instrument that measures low gas pressures, commonly in inches of water column. Technicians use it to verify supply pressure, manifold pressure, and draft without disturbing gas flow.

Finding escaping fuel using a combustible gas detector, soap bubbles, or pressure testing. Because natural gas rises and propane settles, checks cover high and low points of the piping.

A vent system for appliances that operate with negative vent pressure and low flue gas temperature, usually a vertical metal or listed plastic vent. It depends on draft and proper vent height.

The slight negative pressure in a vent or flue that carries combustion products outdoors. It is created by hot gas rising and by vent height, and is measured in inches of water column.

Flue gases escaping from the draft hood or vent connector into the room instead of going up the flue. It indicates a blocked flue, poor draft, or a backdraft from competing exhaust fans.

Air supplied to the appliance for burning fuel, distinct from dilution or ventilation air. Insufficient combustion air causes incomplete burning, carbon monoxide, and spillage.

Measuring oxygen, carbon monoxide, carbon dioxide, and temperature in the exhaust to judge combustion quality and efficiency. Results guide air adjustment and reveal heat exchanger or venting faults.

The electrically operated valve that opens and closes fuel flow to the burners. It contains an operator, a pressure regulator, and safety shutoff features, and is controlled by the furnace circuit board.

A second safety shutoff inside a gas valve that closes independently if the main operator fails to seat. It prevents gas from passing to the burners when the valve is de-energized.

The pipe assembly that distributes gas from the valve to the individual burner orifices. It is where manifold pressure is measured and where burners are mounted and aligned.

A precisely sized hole that meters gas into a burner at the rated input. Its size depends on gas heating value, specific gravity, and manifold pressure, and must match the fuel being used.

An opening in the burner head where the air-fuel mixture exits and burns. Ports must stay clean and evenly sized; clogging or damage causes flame distortion, flashback, or lifting.

A small continuously burning flame that lights the main burner when the gas valve opens. It is simple but wastes gas and can be extinguished by drafts or dirt, requiring a thermocouple safety.

A two-metal junction that generates a small voltage when heated by the pilot flame. That voltage holds the gas valve open, and loss of flame drops the valve closed for safety.

An ignition system that lights a pilot only when there is a call for heat, then proves flame before opening the main valve. It saves fuel compared with a standing pilot and uses a spark igniter.

A system that heats a silicon carbide or nitride igniter to glowing temperature to light the burners. The igniter also often serves as the flame-proving element, so its resistance matters.

A system that uses a high-voltage spark at the burner to light gas without a pilot flame. Flame must be proven by a separate sensor, or the valve closes on a lockout.

A probe that detects flame by sensing current through the ionized gas flame. A dirty or poorly positioned sensor produces weak current, causing short cycling or ignition lockout.

The electrical resistance of a hot surface igniter, checked with an ohmmeter to judge condition. Resistance rises with temperature and an open or cracked element reads infinite, requiring replacement.

The ordered steps a furnace follows on a call for heat: thermostat demand, inducer start, pressure switch proof, ignition, valve opening, flame proving, blower start, and shutdown. Technicians compare observed behavior to it.

A temperature-sensitive safety that opens the control circuit if the furnace overheats, shutting off the gas valve. It may be automatic-reset or manual-reset and often indicates an airflow problem.

A one-time or manual-reset safety mounted near the burners that opens if flame rolls out of the combustion chamber. It signals blocked flue, bad draft, or a cracked heat exchanger.

A safety that proves the inducer is moving combustion air before ignition is allowed. It senses negative pressure through a hose and blocks the sequence if the vent or inducer fails.

A blower that pulls flue gases through the heat exchanger and pushes them into the vent. It creates the draft that the pressure switch proves and must run before the gas valve opens.

The difference between return air and supply air temperatures across the furnace. It must fall within the nameplate range; too high a rise signals low airflow and risks overheating the heat exchanger.

The resistance to airflow in a duct system, measured in inches of water column. High static pressure reduces airflow, raises temperature rise, and can trip the limit switch.

The condition of the metal passages separating flue gases from supply air. Cracks or holes allow carbon monoxide into the home and are checked visually, by flame observation, and by combustion analysis.

Flame burning outside the combustion chamber instead of inside it, often from a blocked flue or failed draft. It can trip a rollout switch and indicates a serious venting or heat exchanger fault.

Timing gas meter movement to calculate actual appliance input in BTU per hour. Comparing measured input with the nameplate rating confirms correct orifice sizing and manifold pressure.

A control state that stops ignition attempts after a failed trial for ignition and requires a reset or delay. It protects against unburned gas release and points to flame proving or gas supply faults.

The timed period during which the control energizes the igniter and opens the gas valve hoping to establish flame. If proof is not received within that window, the valve closes and lockout follows.