NATE AC Service flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Superheat

Suction line temperature minus evaporator saturation temperature. It verifies the evaporator is fully fed with liquid refrigerant and is the primary charging method for fixed-orifice systems.

Subcooling

Condensing saturation temperature minus liquid line temperature. It confirms the condenser has enough liquid seal and is the primary charging method for TXV systems.

Target superheat

The superheat value a fixed-orifice system needs at the current indoor wet-bulb and outdoor dry-bulb conditions, read from the manufacturer's charging chart rather than a fixed number.

Evaporator saturation temperature

The boiling temperature of refrigerant inside the evaporator, found by reading suction pressure on the refrigerant's pressure-temperature chart. It is the baseline for superheat calculation.

Condensing saturation temperature

The temperature at which refrigerant condenses in the condenser, found from the high-side pressure on the pressure-temperature chart. The baseline for subcooling calculation.

400 CFM per ton

The standard design airflow for residential comfort cooling. One nominal ton equals 12,000 Btu/h, so a 2-ton system is designed for about 800 CFM of supply air.

External static pressure

The resistance to airflow measured in inches of water column across the blower assembly, combining supply and return duct, coil, and filter pressure drops. High readings indicate restriction.

Evaporator delta-T

The dry-bulb temperature difference between return air entering and supply air leaving the evaporator coil. Normal is roughly 16-22°F when charge and airflow are correct.

Wet-bulb temperature

The temperature of air measured with a wetted thermometer, reflecting moisture content and evaporative potential. Indoor wet-bulb is required input for target superheat charts.

Dry-bulb temperature

The ordinary air temperature measured by a standard thermometer, with no regard to humidity. Used for return and supply temperature split and condenser entering air checks.

Low charge symptoms

High superheat, low subcooling, low suction pressure, low head pressure, and reduced amp draw. A fixed-orifice system also shows a starved, partially frosted evaporator.

Overcharge symptoms

High subcooling, low superheat, elevated head pressure, and high compressor amp draw. Liquid may flood back to the compressor, causing frost on the suction line at the service valve.

Restricted liquid line

A blockage or kinked line between condenser and metering device. It produces high subcooling at the condenser outlet, low suction pressure, and a temperature drop across the restriction.

Restricted suction line

A blockage or kink between evaporator and compressor. It causes low suction pressure at the compressor, high superheat at the compressor, and a frosted line at the restriction point.

TXV operation

A thermostatic expansion valve modulates refrigerant flow to hold a set superheat at the evaporator outlet, responding to bulb pressure, evaporator pressure, and spring force.

TXV hunting

Repeated over-correction by the expansion valve, causing suction pressure and superheat to swing cyclically. Causes include oversized valves, poor bulb contact, or a weak bulb charge.

Fixed orifice metering

A piston with a drilled bore that meters refrigerant at a rate set by pressure difference. Charge must be set by superheat because the device cannot self-adjust.

Dirty condenser coil

Restricted heat rejection raises condensing temperature and head pressure, increases subcooling and compressor amps, and reduces system capacity. Cleaning the coil restores normal readings.

Dirty evaporator coil

Reduced heat absorption lowers suction pressure and evaporator temperature, raises superheat, and can frost or ice the coil. Airflow drops and delta-T may appear high.

Low airflow effects

Below-design CFM lowers suction pressure, raises superheat, and can ice the evaporator. Causes include restrictive filters, closed dampers, undersized ducts, and low blower speed.

High airflow effects

Above-design CFM raises evaporator temperature and suction pressure, lowers superheat, and reduces the temperature split. It also lessens latent removal and leaves the space humid.

Compressor RLA

Rated load amps stamped on the compressor nameplate. Measured running amps are compared to RLA to judge loading; readings near or above RLA suggest high head pressure or mechanical wear.

Liquid line sight glass

A viewing port in the liquid line. Bubbles indicate flash gas from a low charge or restriction, while a clear glass with proper subcooling indicates a solid liquid seal.

Flash gas

Refrigerant vaporizing before reaching the metering device because of insufficient subcooling. It reduces capacity and appears as bubbles in the sight glass.

Non-condensables

Air or nitrogen trapped in the system. They raise head pressure and condensing temperature beyond what outdoor ambient predicts and cause high subcooling with normal suction readings.

Superheat charging method

For fixed-orifice systems, adjust charge until measured superheat matches the target from the chart using indoor wet-bulb and outdoor dry-bulb. Verify airflow first.

Subcooling charging method

For TXV systems, adjust charge until liquid line subcooling matches the manufacturer's target, typically 8-12°F, after confirming the valve is operating and airflow is correct.

Weigh-in charging

Charging by weight using the nameplate factory charge plus adjustments for line-set length and added components. Required for systems with critically charged or unknown-charge conditions.

Recovery machine operation

Refrigerant is pulled from the system into a recovery cylinder, never vented. The cylinder must be rated for the refrigerant, not overfilled, and weighed during the process.

Evacuation and micron gauge

Pulling a vacuum with a pump to remove moisture and air, verified with a micron gauge isolated from the pump. A decay test confirms no leaks or trapped moisture remain.

Leak detection methods

Electronic leak detector, soap bubbles, UV dye, and pressure testing with nitrogen. Nitrogen is preferred for standing pressure tests because it adds no moisture or refrigerant.

Temperature split measurement

Return air dry-bulb minus supply air dry-bulb after 15 minutes of steady operation. It is a quick performance check, not a substitute for superheat or subcooling verification.