HVAC Excellence Gas Heat Certification key terms

The ideal air-to-fuel ratio at which all fuel and oxygen are consumed with no excess of either, producing only carbon dioxide, water vapor, and nitrogen. For natural gas it is roughly 10 cubic feet of air per cubic foot of gas.

Burning in which essentially all carbon and hydrogen in the fuel combine with oxygen, leaving negligible carbon monoxide or unburned fuel in the flue products. It requires adequate primary and secondary air and proper flame stability.

Air supplied beyond the theoretical amount needed for complete combustion, usually expressed as a percentage. A small amount ensures clean burning, but too much lowers efficiency by carrying heat up the flue.

A colorless, odorless, toxic gas formed by incomplete combustion. In flue gas it signals poor adjustment, blocked venting, or flame impingement; in room air it is a life-safety hazard requiring immediate shutdown and repair.

The upper and lower fuel concentrations in air within which a gas will ignite and sustain a flame. Natural gas ignites roughly between 4 and 5 percent lower and 15 percent upper, which guides leak and purge procedures.

Air mixed with fuel before ignition at the burner, shaping the flame and starting combustion. In naturally aspirated burners it is drawn in by gas velocity; in power burners it is supplied by a blower.

Air that reaches the flame after ignition, completing combustion around the flame envelope. Insufficient secondary air causes yellowing, carbon monoxide, and sooting on the heat exchanger.

The heat released by burning a unit volume of gas, stated as high (gross) or low (net) heating value depending on whether water vapor latent heat is counted. Rating and derating calculations depend on it.

The gas pressure measured at the burner manifold, downstream of the gas valve, while the burners operate. It is set per the rating plate and verified with a manometer during startup and service.

The electrically operated assembly that opens, closes, and regulates gas flow to the burners. It may be single- or two-stage, modulating, or redundant, and typically includes a pressure regulator and operator coils.

A valve design using two operators in series so that if one fails open, the second still blocks gas flow. It is a required safety feature on most modern furnaces and is verified during service.

A precisely sized drilled opening that meters gas into a burner and sets the firing rate. Orifice size must match the fuel type and altitude, and changing fuels requires changing orifices.

A blower that pulls flue products through the heat exchanger and pushes them outdoors. It establishes draft on fan-assisted furnaces and proves venting before ignition through a pressure switch.

A diaphragm device that senses inducer pressure or draft and closes to permit ignition. It proves the vent is functioning and the heat exchanger path is open before the gas valve is energized.

A silicon carbide or nitride element that glows white-hot to light the burners. It is fragile, draws high current, and is checked by resistance and visual glow rather than by feel.

A metal rod that uses flame rectification to prove a flame is present, allowing the control to keep the gas valve open. It must be clean, positioned in the flame, and grounded through the burner.

The process in which a flame conducts current in one direction between the sensor rod and burner, producing a small direct current. The control reads this microamp signal as proof of flame.

The electronic module that sequences the inducer, pressure switch, igniter, and gas valve, monitors flame proof, and locks out on failure. It enforces trial-for-ignition and safety timing.

The timed interval during which the control energizes the gas valve and looks for a proven flame. If no flame is sensed, the valve closes and the control retries or locks out.

A safety state in which the control stops trying to ignite and shuts the gas valve after failed attempts. Reset is usually by cycling power or a manual reset, and the cause must be found.

The ordered steps a furnace follows on a call for heat: thermostat, inducer, pressure switch proof, ignition, gas valve, flame proof, blower on, and shutdown. Technicians use it to isolate faults.

A temperature-actuated safety that opens the thermostat circuit if the furnace overheats, shutting off the gas valve. Repeated trips point to low airflow, a dirty filter, or a failing blower.

A one-time or manual-reset thermal switch near the burners that opens if flame rolls out of the combustion chamber. Its trip indicates blocked flue, bad draft, or a cracked heat exchanger.

Flame escaping the burner area instead of entering the heat exchanger, often from blocked venting, negative pressure, or dirty burners. It is dangerous and usually trips the rollout switch.

The negative pressure that moves flue gases from the combustion zone to the outdoors. It may be natural, induced by a fan, or aided by a chimney, and must be verified during startup.

An instrument, often a manometer or inclined gauge, that measures the small negative pressure in the vent connector or flue. Readings confirm whether draft is adequate or excessive.

The air supplied to the burner for burning fuel and diluting flue products. It may come from inside the structure or be piped directly from outdoors, and its adequacy is a code requirement.

A furnace that operates with non-positive vent pressure and flue gas temperature high enough to avoid condensation, vented with a listed metal vent into a chimney or vertical flue.

A high-efficiency furnace that cools flue gas enough to condense water vapor, recovering latent heat. It uses PVC or CPVC venting, a condensate drain, and positive vent pressure.

The temperature of flue products leaving the furnace, used with room temperature to estimate efficiency. A rising stack temperature can indicate a fouled heat exchanger or overfiring.

An electronic instrument that measures oxygen, carbon monoxide, carbon dioxide, and stack temperature to calculate efficiency and excess air. It is used at startup and during service verification.

A pressure-measuring instrument, often digital or U-tube, used to check inlet and manifold gas pressure and draft. Correct readings confirm the gas valve is set to the rating plate value.

A soap-bubble or electronic check of gas piping, unions, and valve bodies for leaks. It is performed before startup and after any service that breaks a gas connection.

Removing air from gas piping or combustion components by flowing gas before ignition. Proper purging prevents a delayed or explosive light-off and is part of safe startup procedure.

Adjusting the furnace firing rate for altitude or fuel heating value by changing manifold pressure or orifices. It keeps combustion clean and within the appliance's approved input rating.

Confirming the blower delivers the designed cubic feet per minute by measuring static pressure and temperature rise. It protects the heat exchanger from overheating and confirms the installation.