NATE Gas Heating Service key terms

The exact amount of air needed to completely burn a given quantity of gas, supplying just enough oxygen with no excess. Real appliances always use more than this to ensure complete combustion.

Air supplied beyond the stoichiometric requirement, usually expressed as a percentage. It ensures every gas molecule finds oxygen, but too much cools the flame and lowers efficiency.

Burning in which all fuel carbon becomes carbon dioxide and all hydrogen becomes water vapor. It requires adequate oxygen, proper mixing, and a flame that is not quenched.

A poisonous, odorless gas produced by incomplete combustion. It signals too little oxygen, poor flame stability, or a cold surface quenching the flame, and must be kept out of the occupied space.

Burning that leaves some fuel unburned or partly oxidized, producing carbon monoxide and soot. Causes include insufficient air, bad gas/air mixing, and flame impingement on cool surfaces.

A flame touching a heat exchanger or other cool surface directly, which quenches combustion and forms carbon monoxide. Caused by misaligned burners, overfiring, or blocked passages.

A condition where the flame detaches from the burner ports and floats or blows out. It typically results from excessive gas pressure or too much primary air.

The flame travels back into the burner tube or mixer instead of staying at the ports. It usually means the gas/air mixture is burning too fast for the port velocity, often from too little primary air.

Air mixed with gas before the mixture reaches the burner ports. It controls flame shape and softness; too little gives a lazy yellow flame, too much can cause lift-off.

Air drawn from the surrounding space into the flame after the gas leaves the burner ports. It completes combustion and depends on proper clearances around the burner.

Air supplied to the appliance for burning fuel, separate from dilution or ventilation air. Insufficient combustion air causes carbon monoxide and is a common venting-related service call.

A fitting on a natural-draft appliance that limits stack effect and prevents backdrafts from pushing flue gases into the space. It mixes room air with flue products and must not be blocked.

Flue gases escaping from the draft hood or relief opening into the room instead of going up the vent. It indicates a blocked flue, poor draft, or depressurization and is a serious safety hazard.

Reverse flow in a vent, where room air is pulled down the flue and combustion products enter the space. Caused by depressurization, exhaust fans, or a blocked or undersized vent.

A gas appliance that operates with negative vent pressure and produces flue gas cool enough that a vertical metal vent is acceptable. Most conventional furnaces and water heaters are this type.

A high-efficiency appliance with positive vent pressure and low flue-gas temperature, producing condensate. It is vented with listed plastic pipe and slopes back to the appliance.

A device that collects acidic water formed in a condensing appliance and drains it while keeping flue gas from escaping. It must be primed and kept clear to prevent gas leaks or water backup.

The gas pressure measured at the burner manifold, set by the appliance manufacturer. It determines input rate and flame size, and must be checked with a manometer during startup.

The control that opens or closes gas flow to the burners and regulates manifold pressure. It may be a simple redundant valve or a modulating valve controlled by the board.

A valve design with two operators in series so a single failure cannot leave gas flowing to the burners. It is a required safety feature on modern automatic gas controls.

A device that reduces supply pressure to a steady value for the appliance or burner. It responds to downstream pressure and can be adjusted to set manifold pressure.

A capped pipe section installed at the bottom of a gas line before the appliance to catch dirt and scale. It protects the gas valve and orifices from debris.

A precisely sized hole that meters gas into a burner. Its size sets the input rate for a given pressure, and it must match the gas type and appliance rating.

An electrically heated element that glows to light the burners. Its resistance changes with temperature, letting the control board sense whether it is hot enough before opening the gas valve.

A rod or electrode that uses flame rectification to prove a flame is present. It produces a small DC current through the flame; a dirty or corroded sensor prevents the burner from staying lit.

An ignition method that uses a high-voltage spark at the burner to light gas directly, without a standing pilot. The same electrode often serves as the flame sensor.

A temperature-sensitive safety device that opens when flames exit the burner area abnormally. It shuts off gas and indicates a blocked heat exchanger, bad draft, or delayed ignition.

A temperature-activated safety control that opens the burner circuit when the appliance overheats. Repeated trips point to low airflow, a dirty filter, or a failing blower.

The difference between return-air and supply-air temperatures across the appliance. It must fall within the manufacturer's range; too high means low airflow, too low means excessive airflow.

The resistance to airflow measured in the duct system outside the appliance, usually in inches of water column. High values indicate restrictive ducts, filters, or coils and reduce airflow.

The volume of air moved through the appliance and duct system, typically in cubic feet per minute. Correct airflow is essential for heat transfer, temperature rise, and safe operation.

The metal component that separates combustion products from the air being heated. Cracks or holes let carbon monoxide into the supply air and require replacement, not repair.

A condition where gas builds up before the burner lights, causing a small explosion or roll-out. Causes include weak ignition, dirty burners, or low gas pressure.

Operating an appliance above its rated input, usually from excessive manifold pressure or oversized orifices. It shortens heat exchanger life and can produce carbon monoxide.

Measuring flue-gas oxygen, carbon monoxide, and temperature to calculate efficiency and confirm safe, complete combustion. It is the definitive test after installation or repair.

A method of timing the gas meter dial while the appliance runs to calculate actual input rate. Comparing it to the rating plate confirms correct gas flow and orifice sizing.