NATE HVAC key terms

The number of degrees a refrigerant vapor is above its saturation temperature at a given pressure. Measured at the evaporator outlet, it shows whether the metering device is feeding the coil correctly.

The number of degrees a liquid refrigerant is below its saturation temperature at a given pressure. Measured at the condenser outlet, it indicates how much liquid is stacked in the condenser.

A fluid that absorbs heat by boiling at low pressure in the evaporator and releases it by condensing at high pressure in the condenser, moving heat from one place to another inside a sealed circuit.

A handheld test instrument that measures voltage, resistance and often current. Technicians use it to verify power supply, check switch continuity and confirm a component's electrical condition before replacing it.

A safety procedure in which a disconnect or valve is locked in the off position and tagged so equipment cannot be energized or pressurized while someone is servicing it.

A digital instrument that reads very deep vacuum in microns. It is used during evacuation to confirm that air and moisture have been removed from a refrigeration circuit before charging.

A component that stores and releases electrical energy to start or run a motor. A weak run capacitor causes high amp draw, slow starting and overheating of compressor or fan motors.

A DOT-approved container used to hold refrigerant removed from a system. It must never be filled above 80 percent liquid full and must be labeled with the refrigerant it contains.

Heat that changes the temperature of a substance without changing its state. In cooling, removing sensible heat lowers the dry-bulb temperature of the room air.

Heat that changes the state of a substance without changing its temperature, such as the energy absorbed when water evaporates or released when refrigerant vapor condenses.

The temperature difference between air entering and leaving a coil, or between supply and return air. It is a quick field check of airflow and equipment performance.

A U-shaped trap in the drain line that holds water to block air from being pulled through the drain, while still allowing condensate to flow away from the coil.

The outward pressure a blower creates against the duct system, measured in inches of water column. Excessively high external static pressure signals restricted airflow.

Cubic feet per minute of air moving through a system. Correct airflow is required for proper heat transfer and is estimated from blower charts or measured with a flow hood.

A box or chamber attached to the supply or return side of air-handling equipment that feeds several branch ducts. Supply plenums are pressurized; return plenums run under negative pressure.

The gas pressure measured at the burner manifold with a manometer. It must match the data plate value so the burners fire at the intended input rate.

A safety device made of two dissimilar metals that generates a small millivolt signal when heated, holding the gas valve open only while the pilot flame is proven.

The movement of combustion gases up a vent or chimney, caused by the natural rise of hot gas or by an induced fan. Proper draft carries flue products outdoors and prevents spillage into the space.

A four-way solenoid valve in a heat pump that switches refrigerant flow direction, swapping the jobs of the indoor and outdoor coils between heating and cooling.

A timed or demand-initiated mode in which a heat pump temporarily reverses to melt frost from the outdoor coil, usually stopping the outdoor fan and bringing on indoor supplemental heat.

The outdoor temperature at which a heat pump's heating capacity exactly equals the building's heat loss. Below that point the system needs supplementary heat to keep the space warm.

Supplemental electric resistance or fossil-fuel heat that energizes when the heat pump alone cannot meet the load, during defrost, or when the thermostat is set several degrees higher.

A precision orifice that meters and atomizes fuel oil into the combustion chamber. Its size and spray pattern must match the burner firing rate and the shape of the chamber.

A light-sensitive resistor in an oil burner that drops sharply in resistance when it sees flame, proving combustion to the primary control and shutting off oil if flame is lost.

A pump that moves hot water from the boiler through the piping to the terminal units and back. Circulators are selected by required flow rate and pump head.

An air-cushioned tank connected to a hydronic system that absorbs the volume increase as water heats, keeping system pressure stable and preventing relief valve discharge.

A temperature-sensing control on a boiler that cycles the burner or circulator to hold a set water temperature, and often includes a high-limit safety function.

A rooftop unit feature that uses outdoor air for free cooling when conditions are suitable, modulating dampers open and closing them again when outdoor air is too warm or humid.

A terminal unit with a modulating damper that varies the volume of cool air delivered to a zone in response to thermostat demand, letting one air handler serve many zones.

An evaporator pressure regulator that holds a minimum evaporator pressure, and therefore a minimum coil temperature, protecting product in one circuit while other circuits run colder.

A control that keeps condenser pressure high enough for the metering device to work in cold weather, often by cycling the condenser fan or holding refrigerant back in the receiver.

A control that ends the defrost cycle once the evaporator reaches a set temperature, or after a maximum time limit, so heating does not continue longer than needed.

A thermostatic expansion valve that modulates refrigerant flow into the evaporator in response to the temperature and pressure at the coil outlet, maintaining a set superheat.

A large refrigerated room for storing perishable product, typically built from insulated panels and served by an outboard condensing unit and a matched evaporator with defrost control.