• Combustion air: Air required to complete combustion (primary air and secondary air)
  • Ambient air: This is the air that surrounds us
  • Dilution air: The ambient air admitted to the draft hood, draft diverter, or draft regulator. It’s used to cool the venting system and vent the flue gases
  • Excess air: The portion of the combustion air supplied to ensure complete combustion can happen. This is added to combustion air
  • Primary air: The portion of air added to the air/gas mixture prior to ignition
  • Secondary air: The portion of air supplied prior to ignition around the flame

Aldehydes are a product of incomplete combustion. They’re irritating to the eyes, throat, and nose and smell terrible. Tastes somewhat metallic and are likely accompanied by carbon monoxide

The ambient air temperature is the dry-bulb temperature of the surrounding air

Applied voltage
The voltage supplied or applied to the electrical circuit
(also known as supply voltage)

At rest
The appliance main circuit disconnect is in the open position not allowing the circuit to be energized. This could be the door switch in a furnace. Any heating or cooling thermostats connected to the circuit would be sitting in a non-operating position.

British Thermal Unit is the measurement used to measure heat. It takes 1 BTU to raise the temperature of 1LB of water 1°F

British Thermal Units per hour

Calorific value
The calorific value (also known as heat content) of a fuel is the amount of energy released when a volume of fuel is burned expressed in BTU per cubic foot of natural gas or propane
(also known as: heat content)

Capacitor start/capacitor run
Has a start capacitor in series with the secondary (aka start or auxiliary) winding for high starting torque. It also has a run capacitor that is in series with the secondary (aka start or auxiliary) winding after the start capacitor is taken out of the circuit

Capacitor start/induction run
Similar to the capacitor start/capacitor run motors. Uses a start capacitor for the secondary (aka start or auxiliary) winding for high starting torque. A potential relay removes the capacitor from the circuit at about 3/4 of the startup speed. No run capacitor is used.

Carbon monoxide
A byproduct of incomplete combustion. A colorless, odorless, and tasteless gas with a relative density of 0.96

Closed switch
The position a switch must be in to allow electricity to flow

CO air-free. Flue gas CO readings are inaccurate since the added excess air dilutes them. The COAF measurement compensates for the excess air and is used to express CO levels in the flue gas reading

Materials that allow electricity to flow. Copper, aluminum, silver, gold are some examples of conductors. Wiring in your house would be a conductor wrapped with an insulator

The flow of electrons through a conductor. Measured in amperes (amps)

Dew-point is the temperature that air must be cooled to in order for water vapor to condense (at which the air is 100% moisture). The closer the dew-point is to the actual temperature, the more moisture in the air.

Dirt pocket
Used in a piping system to collect dirt.

Drip pocket
Used in a piping system to collect condensate.

Dry-bulb temperature is the temperature of the air around you measuring only the sensible heat and is not affected by the moisture in the air. This would be the temperature you measure when using a normal thermometer.

Flame impingement
Flame touching metal surface inside heat exchanger or cold air blowing around the flame and cooling it below ignition temperature.

Flue gas
Appliance exhaust removed through exhaust venting or chimney.
(also known as exhaust)

Ignition control board
Common in older systems. They control the gas valve, igniter, flame sensor, and the damper (when used). This is like a step-down from a full furnace control board.

Ignition temperature
Temperature the air:gas mixture will support combustion. Between 1200°F and 1300°F for natural gas and 920°F for propane.

Inshot burner
Used in most modern furnaces. A combustion blower is used to pull the flames through the burner tubes into the heat exchanger.

An insulator does not allow an easy flow of electricity. Glass, plastic, etc. An insulator can also be called a non-conductor.
(also known as non-conductor)

Integrated Furnace Control
Integrated Furnace Control is also called the IFC or the control board. The IFC controls everything an ignition control board would and more. All safety controls, combustion blower, flame proving, gas valve, and the blower are all controlled by the IFC.
(also known as: control board, IFC)

Isolation relay
Normally a SPST (single pole single throw) relay. Used to separate two different voltages in a system.

Longest Measured Run. It’s the distance from the supply (meter or regulator) to the furthest appliance to be connected. Used in pipe sizing.

(With regard to electrical) a good load in a circuit does useful work. This could be as simple as turning on a light

(With regard to gas piping) The appliance input in BTUh. This can be found on the appliance rating plate

The amperage a motor will draw when the supply voltage is applied, and the rotor is locked (not turning). This lasts for a second or less before the motor gets up to speed.
(also known as lock rotor amperage)

Added smell to natural gas to make it easier to detect since by default natural gas is both odorless and colorless. Without this smell, gas leaks wouldn’t be detected in most cases until it’s too late

Unit of measurement based on meter, cm, kg, ect

Ohm’s Law
The current flowing in a circuit is directly proportional to the applied voltage. As current increases, resistance decreases. If resistance increases, current decreases.

Gas valve pressure adjusted higher than the data-plate requires.

Perfect combustion
Combustion that provides the exact fuel to air ratio and extracts all energy from the fuel at 100% efficiency. Perfect combustion does not exist in the real world.
(also known as stoichiometric combustion)

A popular pilot starter. They have the ability to generate a high voltage to supply a spark when a mechanical force is applied. Popular with BBQ starters.


  • Continuous pilot: AKA standing pilot. Burns the entire time without turndown regardless if main burner fires or not
  • Expanding pilot: Burns at low turndown throughout the entire time the burner is in service, except when a call for heat. Upon a call for heat, the pilot is increased to expand the flame to ignite the main burner
  • Intermittent pilot: Activated each call for heat and the pilot stays active until the main burner powers off
  • Interrupted pilot: Activated each call for heat, but the pilot does not function after the main burner powers on

Supply Plenum
Sheet metal box connected directly to the supply outlet of the HVAC appliance that directs the heated or cooled air to the ductwork.

Return Plenum
Most units have multiple, or at least one return duct(s) in the house moving conditioned room air to the HVAC unit. When air is forced through the supply ductwork, conditioned room air gets pulled throughout the structures return ductwork into the central return air plenum to be moved through the HVAC appliance to be heated or cooled.

Polarity is the direction of current flow in a circuit from the two poles, positive and negative

In an AC circuit, each pole alternates between positive and negative 60 times per second

In a DC circuit, it’s between the negative and positive with current flowing from negative to positive

Permanent Split Capacitor motors have a run capacitor permanently connected in series with the start winding. That makes the start winding the auxiliary winding once the motor reaches running speed.

Relative density
The comparison between the weight of a volume of a liquid or vapor and the weight of an equal volume of air or water.
(also known as specific density)

Relative humidity
Relative humidity is the amount of water vapor in the air, expressed as a percentage, compared to the amount of water vapor it could hold at that temperature if fully saturated.

Series circuit
A series circuit provides only one path for current to flow.


  • Step up transformer: A transformer with a higher voltage at the secondary winding terminals than at the primary winding terminals. The secondary coil has a greater amount of windings
  • Step down transformer: A transformer with a lower voltage at the secondary winding terminals than at the primary winding terminals. The secondary coil has a fewer amount of windings. Normally line voltage (120v) to control voltage (24v)

When gas passes through a passage that has a reduction in size this causes an increase in speed to make the pass-through possible to happen and draws air in with it. Paint spray guns use the venturi effect to pass paint through the gun

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