Unit 4
Burnout 
Capacitance boost
Demand defrost
Direct-acting Valve
Electronically commutated motors
Field wiring
Pilot-operated valve


Unit 5
Flame rectification|A process in which exposure of a sensing rod to flame produces a tiny current that can be sensed and used to control the gas valve. 
Modulating gas valve| A gas valve that precisely controls burner capacity by adjusting the flow of gas to burners in small increments.
Net stack temperature|A value needed to perform a combustion efficiency analysis, calculated by subtracting room temperature from the temperature measured in a furnace's vent or vent connector.
Reverberator|A wire screen used to cover the ceramic grids in a gas-fired heater that operates by submerged flame.
Submerged flame|A method of gas-fired heater operation that uses burning gas within two-layer porous ceramic grids.

Unit 6
Annual Fuel Utilization Efficiency (AFUE)|An industry standard used to define furnace efficiency.
Atomize|Reduce to a fine spray
Cad cell|A light-sensitive device (photocell) in which the resistance reacts to changes in the amount of light. In an oil furnace, it acts as a flame detector.
Coping|The upper masonry course of a chimney designed to keep water out.
Downdraft|A situation in which outdoor air is forced down the chimney, preventing proper draft. Downdraft can be caused by not extending the top of the chimney above the roof line.
Flame-retention burner|A high-efficiency oil burner.
Joist|Framing member used to support floors and ceilings.
Lockout|A safety feature that prevents a furnace from automatically cycling if the flame fails.
Over-firing|A situation caused by an oversized nozzle in which excess oil is pumped into the combustion chamber.
Primary control|A combination switching and safety control that turns the oil burner on and off in response to signals from the thermostat and safety controls. 
Refractory|Materials such as ceramics designed to withstand extremely high temperatures.
Smoke spot test|A test to determine the level of smoke in the flue gases.
Soot|Fine carbon particles resulting from incomplete combustion. An accumulation of soot on heat exchangers will reduce combustion efficiency and result in increased carbon monoxide production.
Temperature differential|The difference in temperature between the flue gases and the gases above the combustion chamber.
Weatherized furnace|A furnace that has been well insulated and has weather-resistant case as specified by government standards.

Unit 7
Free cooling|A mode of economizer operation. During cooling operation, the system compressor is turned off and the indoor fan is used to bring outside air into the building to provide cooling.
Mechanical cooling|The cooling provided in the conventional manner by the compressor and mechanical refrigeration system.
Water hammer|A condition that occurs when hot steam comes into contact with cooled condensation, builds pressure, and pushes the water through the line at high speeds, slamming into valves and other devices. Water hammer also occurs in domestic water systems. If the cause is not corrected, water hammer can damage the system; a banging noise in water pipes that occurs when water supply soenoid valve on a humidifier abruptly shuts off water to the humidifier.

Unit 8
Dump zone|Area used as a point bypass unneeded supply air from a zoning system. The dump system is typically in the conditioned space, but not an area where maintaining the space temperature at a particular setpoint is required. A dump zone generally has no supply air grilles, but a return grille is often installed to relieve room pressure and return mixed air back to the system.
Polling|A network communications arrangement where a VVT system seeks information and needs from individual zones, determines what demands have priority, and then responds accordingly. Essentially, a member of a polling system are voters. 
Pressure-dependent operation|A VVT-system approach where dampers are positioned based on demand only, or operate in two positions. The results is that the design volume of air may or may not be flowing through the damper at any time. The volume of air flow through each damper is dependent on the duct pressure; it fluctuates as the pressure fluctuates. A minimum damper position is set, but air volume is not a controlling factor, and so the minimum damper position is a fixed point.
Pressure-independent operation|A VVT-system approach where dampers are positioned by demand and by the actual volume of air flowing through the damper. To achieve a specific amount of airflow, the damper positions itself as necessary. A maximum and minimum airflow setpoint is effective, and the damper will modulate as necessary to remain with these boundaries.
Refnet joints|Engineered joints used as connection points in a VRF-system refrigerant network. Refnet joints connect the main refrigerant lines to branch runs or to indoor units.

Unit 9
Boiler horsepower (Bohp)|One Bohp is equal to 9.803 kilowatts. Also, it is the heat necessary to vaporize 34.5 pounts of water per hour at 212F. This is equal to a heat output of 33,475 Btus per hours or about 140 square feet of steam radiation.
Firetube boiler|A boiler in which the flue gases are contained inside the tubes, with the heated water on the outside.
Flash economizer|An energy-conservation processed used in chillers taht accomplishes intercooling of the refrigerant between the condenser and evaporator using a flash chamber or similar device. Flash gas of coolant produced in the flash chamber flows directly to the second stage of the chiller compressor, bypassing the evaporator and the first compressor stage.
Lithium bromide|A set solution commonly used in absorption chillers.
Secondary coolant|Any liquid, such as water, that is cooled by a system refrigerant and then used to absorb heat from the conditioned space without a change in state. Also known as indirect coolant.
Superheated steam|Steam that has been heated above its saturation temperature.
Thermal economizer|An energy-conservation process used in chillers in which warm condensed refrigerant is brought into contact with the coldest (inlet) water tubes in a condenser or heat exchanger. This causes the condensed refrigerant to subcool to a temperature below the condensing temperature.
Venturi tube|A short tube with flaring ends and a constricted throat.  It is used for measuring flow velocity by measurement of the throat pressure, which decreases as the velocity increases.
Watertube boiler|A boiler where the heated water is contained inside the tubes, with the flue gases on the outside.

Unit 10
Drip leg|A drain for condensate in a steam line placed at a low point or change of direction in the line and used with a steam trap.
Dry steam|The steam that exists after all the water has been vaporized into steam at its saturation temperature.
Flash steam|Formed when hot condensate is released to a lower pressure and re-evaporated.
Saturated steam|The pure or dry steam produced at the temperature that corresponds to the boiling temperature of water at the existing pressure.
Trim|External controls and accessories attached to the boiler itself, such as sight glasses and water feeder controls.
Turndown ratio|In steady systems, the ratio of downstream pressure to upstream pressure, usually applied to steam pressure-reducing valves. More than a 50 percent turndown ratio is generally considered a large pressure reduction.
Water hammer|A condition that occurs when hot steam comes into contact with cooled condensate, builds pressure, and pushes the water through the line at high speeds, slamming into valves and other devices. Water hammer also occurs in domestic water systems. If the cause is not corrected, water hammer can damage the system; a banging noise in water pipes that occurs when the water supply solenoid valve on a humidifier abruptly shuts off water to the humidifier.

Unit 11
Phase change|The conversion of refrigerant from one state to another, such as from liquid to vapor.
Slinger ring|A ring attached to the outer edge of the fan that picks up water as the fan rotates and slings it onto the condenser coil.
Unit cooler|A packaged unit containing evaporator coil, metering device, and evaporator fan(s) that serves as the evaporator section for a refrigeration appliance.

Unit 12
Empathy|The ability to identify with another person's situation or feelings.
Rapport|Establishing a relationship of mutual trust.