HVAC Excellence key terms

Refrigerant vapor temperature above its saturation temperature at the same pressure. Measured as suction line temperature minus saturated evaporating temperature, it confirms the evaporator is fully fed with liquid and protects the compressor from liquid return.

Liquid refrigerant temperature below its saturation temperature at the same pressure. Measured at the condenser outlet, it shows how much liquid is stacked in the condenser and is used to verify charge on fixed-orifice and TXV systems.

The temperature at which a refrigerant boils or condenses at a given pressure. On a pressure-temperature chart each pressure corresponds to a saturation temperature, which technicians compare against measured line temperatures.

The temperature difference between the bubble point and dew point of a zeotropic blend at a given pressure. Because the blend boils and condenses over a temperature range, charging and superheat readings must account for it, and blends are charged as liquid.

A vacuum instrument that reads absolute pressure in microns of mercury to confirm a system has been evacuated before charging. It is far more accurate at deep vacuum than a manifold gauge set.

Air or nitrogen trapped inside a refrigeration circuit. These gases collect in the condenser, raise discharge pressure and temperature, and cannot be corrected by adding refrigerant; the charge must be recovered and the system re-evacuated.

A metering device that controls liquid refrigerant flow into the evaporator by sensing superheat at the coil outlet. It opens as superheat rises and closes as superheat falls, holding a set superheat under varying loads.

Liquid refrigerant or oil entering the compressor and being compressed as a liquid, which can bend valves, break connecting rods, or wash oil from bearings. It usually results from refrigerant flooding during off cycles or an overfed evaporator.

Removing refrigerant from a system into a recovery machine and cylinder instead of venting it to the atmosphere, as required by federal rules. Recovery may be liquid or vapor, and cylinders must never be overfilled.

A component that stores and releases electrical energy to shift phase and improve running torque on single-phase motors. A weak, bulged, or open run capacitor causes the compressor or fan to draw high current or fail to start.

An electrically controlled switch that uses a low-voltage coil to close high-current line-voltage contacts. It switches compressors and condenser fans, and pitted or welded contacts are a common cause of no-cool calls.

The current a motor draws when its rotor cannot turn, several times higher than running current. The nameplate value is used to size overload protection and to interpret readings taken when a compressor is seized or stalled.

A voltage-sensing relay wired in parallel with a motor start winding that removes the start capacitor once the motor reaches speed. Rising back-EMF through the coil opens the relay contacts.

A small heater or resistor in a thermostat that warms the sensing element slightly ahead of the setpoint so the cycle ends before room temperature overshoots. Incorrect adjustment causes wide temperature swings or rapid cycling.

A step-down transformer that reduces line voltage to 24 volts to power thermostats, relays, gas valves, and circuit boards. A shorted low-voltage circuit often opens its fuse or burns out the secondary winding.

The relationship V = I x R, in which voltage equals current multiplied by resistance. Technicians use it to predict expected current or resistance and to interpret meter readings on motors, heaters, and controls.

The control that ends a heat pump defrost cycle once the outdoor coil is clear, typically sensed by coil temperature, time, or pressure. A failed sensor can leave the unit stuck in defrost or terminate it too early.

Air supplied to a burner beyond the amount theoretically required for complete combustion. Some excess air is needed for clean, safe burning, but too much carries heat up the flue and lowers steady-state efficiency.

The ideal fuel-to-oxygen ratio in which all fuel burns with no leftover oxygen or unburned fuel. Real burners always operate above this point so that some excess air guarantees complete combustion.

The negative pressure in a vent or flue that carries combustion products out of the appliance and up the chimney. Natural-draft units rely on warm flue gases, while induced-draft units use a fan; a blocked flue causes spillage or rollout.

The gas pressure delivered to the burner orifices, measured with a manometer at the valve outlet tap. It must match the appliance rating plate because overfiring or underfiring causes sooting, delayed ignition, and unsafe operation.

Air supplied to the burner for burning fuel, separate from the air being heated or cooled. Insufficient combustion air can produce carbon monoxide, so vents and piping must be sized to the appliance input rating.

The metal passages that keep flue products separate from the airstream while transferring heat into supply air. Cracks or holes allow carbon monoxide into the occupied space and require the appliance to be shut down.

A pair of dissimilar metal wires that produces a small millivolt signal when heated, used to prove a standing pilot flame. A weak or dirty thermocouple drops the gas valve out, extinguishing the pilot and main burner.

The total pressure the blower must overcome from supply and return ductwork plus accessories, measured across the blower with a manometer. Comparing it to the fan performance table shows whether airflow problems come from restrictive ducts.

The pressure lost per unit length of duct, commonly expressed as inches of water column per 100 feet. Duct runs are sized to this value so the blower can deliver design airflow without excessive static pressure.

Air drawn back from conditioned spaces to the blower for reconditioning. Undersized return ducts raise static pressure and reduce supply airflow, so return capacity must be balanced against the supply side.

A manual or motorized plate installed in a duct branch that adds resistance to shift airflow between rooms or zones. Technicians adjust dampers so each outlet delivers its design airflow.

A diagnostic test that mounts a calibrated fan and gauge in an exterior doorway to pressurize or depressurize the house. It measures total envelope air leakage in cubic feet per minute for conversion to air changes per hour.

Air changes per hour measured at 50 pascals of test pressure during a blower door test. It normalizes envelope leakage to house volume so small tight homes and large loose homes can be compared on one scale.

A calibrated fan and digital gauge assembly used to pressurize a duct system and measure leakage to outside or to the conditioned space, typically with the air handler off and all registers temporarily sealed.

A cover placed over a register during a blower door test that measures the pressure difference behind it. A high reading indicates that the ducts serving that outlet leak significantly to the outdoors.

An instrument that measures pressure difference, usually in inches of water column or pascals. Digital manometers are used for duct static pressure, gas pressure, draft readings, and building pressure diagnostics.

The temperature difference between return air and supply air across an indoor coil. A low split points to low charge, low airflow, or a dirty coil, while an unusually high split usually signals restricted airflow.

The difference between return air and supply air temperature across a furnace, which must fall within the range printed on the rating plate. Measured rise combined with blower tables verifies delivered airflow.

The difference between leaving condenser refrigerant temperature and entering air or water temperature. It gives a fast indication of condenser performance and charge without calculating full superheat and subcooling values.

A condition in which equipment starts and stops more often than intended, often from oversized capacity, a faulty thermostat, low charge, or a tripping safety. It increases wear and reduces efficiency.

The seasonal efficiency rating for central air conditioners and heat pumps produced under the newer, higher-static test procedure. It estimates seasonal cooling output divided by seasonal electrical input, with higher numbers meaning greater efficiency.

Annual Fuel Utilization Efficiency, the percentage of fuel energy a furnace or boiler converts into usable heat over a heating season. An 80% AFUE furnace sends roughly 20% of its input up the vent.