NATE AC Installation key terms

The temperature of air measured with an ordinary thermometer, unaffected by moisture content. Installers record it at the return and supply to check the temperature split and, with wet-bulb, to estimate humidity.

The lowest temperature air can reach as water evaporates into it, read with a thermometer whose bulb is wrapped in a wet wick. It indicates how much moisture the air can still absorb.

The percentage of moisture air holds compared with the most it could hold at that same temperature. High values slow evaporative cooling, add latent load, and make the coil work longer to satisfy the thermostat.

The temperature at which air becomes saturated and moisture starts condensing on surfaces. Coil and duct surfaces must stay below it so water forms and drains rather than fogging the space.

The outdoor and indoor temperature and humidity values used to size equipment, select airflow, and set expectations. They come from local weather data and agreed comfort targets, not one afternoon's reading.

The difference between return-air and supply-air temperatures across the equipment, commonly 16 to 22 degrees Fahrenheit in residential cooling. A split outside that range points to airflow, charge, or duct problems.

The temperature at which a refrigerant boils or condenses at a given pressure. Gauges convert pressure into this value so line temperatures can be compared with it to figure superheat and subcooling.

The indoor heat exchanger where low-pressure liquid refrigerant boils and absorbs heat from household air. Its surface stays below the dew point, so the water it condenses must be trapped and drained.

The outdoor heat exchanger that rejects heat gathered indoors and turns refrigerant vapor back into liquid. It needs clean fins and unobstructed airflow to keep discharge pressure within the manufacturer's range.

The pump that raises refrigerant pressure and moves it around the circuit. It draws low-pressure vapor from the evaporator and discharges hot, high-pressure vapor toward the condenser.

A modulating metering device that regulates refrigerant flow into the evaporator to hold superheat near a set value. It reacts to evaporator outlet temperature and pressure, unlike a fixed-bore piston.

A liquid-line device with a screen and desiccant that catches debris and absorbs moisture left after assembly or service. Installers braze it in with the arrow pointing toward the metering device.

A four-way valve in heat pumps that swaps which coil acts as evaporator and which as condenser, allowing one system to heat or cool. Its solenoid coil is energized in only one mode.

A water seal formed in the drain line that blocks air from being pulled through the drain and lets condensate leave the pan. Its depth must suit the fan's static pressure or water will be held or blown out.

The copper tubing pair, usually with the control wire, that joins the outdoor unit to the indoor coil. Properly insulated suction line prevents heat gain and sweating in humid spaces.

A standardized score for how well a filter captures particles of a given size range. Higher ratings trap finer particles but resist airflow more, so the choice must match blower capacity.

The resistance to airflow measured in the supply and return ducts outside the air handler, expressed in inches of water column. High readings indicate restrictive filters, tight ducts, or a fouled coil.

Air drawn back from the conditioned space to be filtered, cooled, and supplied again. Undersized or blocked return paths starve the blower and reduce delivered capacity.

Air escaping from or entering ducts through seams, joints, and holes instead of reaching rooms. Return-side leaks pull hot attic air in, while supply leaks waste cooled air.

Fresh air brought in on purpose to dilute indoor pollutants and limit stale odors. Installers must weigh it against the extra sensible and latent load it adds to the cooling equipment.

Air that slips around a filter's edges instead of passing through the media, carrying dirt directly onto the coil. Correct filter sizing and a sealed frame eliminate it.

The current a motor draws at startup when the rotor cannot turn, printed on the nameplate. Measured inrush current is compared with this figure to judge winding and capacitor condition.

The current a motor draws while operating under normal load, listed on the nameplate or in the manufacturer's tables. Readings far above it suggest high head pressure, low voltage, or a failing motor.

One capacitor serving both the compressor and the outdoor fan motor, with three terminals and two capacitance values. A bulged or out-of-tolerance unit can stop the compressor from starting.

A relay whose 24-volt coil closes high-voltage contacts that start the compressor and outdoor fan. Pitted or burned contacts cause voltage drop and intermittent operation.

A small transformer that steps line voltage down to 24 volts for the thermostat and control circuit. A shorted low-voltage wire or miswired thermostat can burn out its secondary winding.

The green or bare wire bonding metal enclosures and the compressor shell back to the panel. It carries fault current so the breaker trips instead of leaving the cabinet energized.

A service switch installed near the outdoor unit that removes all power for safe servicing. It must be within sight of the equipment and rated for the circuit's voltage and current.

How many degrees a vapor is above its saturation temperature at the measured pressure. Taken at the evaporator outlet, it shows whether the metering device is feeding the coil correctly.

How many degrees a liquid is below its saturation temperature, measured at the condenser outlet. It reveals how much liquid is stacked in the condenser and is the main charging check on expansion-valve systems.

The superheat a manufacturer specifies for the current indoor wet-bulb and outdoor dry-bulb readings. On fixed-orifice systems, charge is adjusted until measured superheat matches this number.

Adding or removing refrigerant by weight per the nameplate charge, corrected for line-set length and component volume. It is the dependable method when conditions fall outside the charging chart.

An electronic instrument that reads deep vacuum in microns during evacuation. A reading of 500 microns or lower that holds steady after valving off confirms the system is dry and tight.

Pressurizing an isolated circuit with dry nitrogen and watching the gauge to locate leaks before charging. Nitrogen adds no moisture and is never released as a refrigerant substitute.

Finding the volume of air the blower moves in cubic feet per minute, usually from fan tables using measured static pressure, or directly with a flow hood at the registers.

A set of gauges and hoses used to read suction and discharge pressures and to move refrigerant in or out. Hoses must be purged so air and moisture never enter the circuit.