HVAC Excellence Heat Pump flashcards

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

Heating COP

Coefficient of performance in heating equals heat delivered to the space divided by electrical energy input. Higher COP means more efficient heating; it varies with outdoor temperature.

Reversing valve function

A four-way valve that swaps evaporator and condenser roles between heating and cooling by redirecting refrigerant flow. It is solenoid-piloted and typically energized in heating mode on residential units.

Superheat definition

The difference between actual suction vapor temperature and the evaporating saturation temperature. It indicates how much the refrigerant is heated above its boiling point and is used to charge fixed-orifice systems.

Subcooling definition

The difference between condensing saturation temperature and actual liquid line temperature. It shows how much liquid refrigerant is cooled below its condensing point and is the primary charging indicator for TXV systems.

Balance point

The outdoor temperature at which heat pump heating capacity equals the building heat loss. Below this point, supplemental or auxiliary heat must operate to maintain indoor temperature.

Defrost cycle purpose

A timed or demand-initiated cycle that melts frost from the outdoor coil in heating mode by reversing the refrigerant flow and often stopping the outdoor fan. Proper termination prevents coil icing and cold blow.

Crankcase heater

A small electrical heater on the compressor sump that keeps oil warm during the off cycle, preventing refrigerant migration and oil dilution. It must be energized before startup after long shutdowns.

Auxiliary heat

Supplemental electric resistance or other heat that operates when heat pump capacity is insufficient, such as during defrost or below the balance point. It is staged by the thermostat or defrost board.

Contactor function

An electrically controlled switch that connects line voltage to the compressor and outdoor fan motor when the 24-volt thermostat circuit calls. Pitted contacts cause high current and intermittent operation.

Low-voltage transformer

A component that steps down line voltage to 24 VAC to operate the thermostat, contactor coil, and control board. A blown fuse or open secondary disables all low-voltage controls.

Line set sizing

Selection of suction and liquid line diameters per manufacturer tables. Undersized suction lines increase pressure drop and reduce capacity; oversized lines can impair oil return.

Nitrogen purge brazing

Flowing dry nitrogen through refrigerant lines during brazing to displace oxygen and prevent internal oxidation scale. Scale can clog metering devices and damage the compressor.

Evacuation to 500 microns

Pulling a vacuum of 500 microns or lower and holding it to confirm the system is dry and leak-free. A standing vacuum rise of less than 500 microns in 10 minutes is acceptable.

Filter-drier location

A device installed in the liquid line, in the direction of flow, to remove moisture and debris. It must be replaced after a compressor burnout or major system repair.

Airflow CFM per ton

Typical cooling airflow target of 350–450 CFM per ton of capacity. In heating, airflow is often set higher to avoid high discharge temperatures and nuisance high-limit trips.

External static pressure

The total resistance to airflow from filter, coil, ductwork, and grilles, measured in inches of water column. High ESP reduces airflow and can cause coil freezing or high-limit trips.

Weigh-in charging

The most accurate charging method for new installations or after recovery, using the nameplate charge plus adjustments for line-set length and indoor coil volume. It does not rely on system temperatures.

Subcooling charging target

For TXV systems, charge to a subcooling of typically 8–12°F at 95°F outdoor and 80°F indoor dry bulb. Subcooling is the primary indicator because the TXV controls superheat.

Superheat charging target

For fixed-orifice systems, charge to a superheat of typically 8–12°F at the evaporator. Superheat indicates refrigerant amount when the metering device is a fixed orifice.

Temperature rise check

In heating mode, measure the temperature difference between return and supply air across the indoor coil. Compare to manufacturer charts; typical rise is 20–35°F depending on outdoor temperature and airflow.

Forced defrost test

Manually initiating a defrost cycle to verify initiation, termination, fan operation, and reversing valve action. It confirms the defrost control and sensors are functioning correctly.

Thermostat configuration

Setting the thermostat to match equipment type, stage counts, auxiliary lockout, and balance point. Incorrect configuration can cause simultaneous compressor and aux heat or short cycling.

Lockout/tagout

A safety procedure that isolates electrical energy before service. Verify power is off at the contactor and capacitor terminals with a meter before touching components.

Capacitor discharge

Using a resistor to safely discharge stored energy from a capacitor before handling. Check capacitance with a meter against the rated microfarad value, typically within ±6–10%.

Compressor winding check

Measuring resistance across compressor terminals to verify balanced windings and no shorts to ground. Unbalanced or infinite resistance indicates a failed compressor.

Cold blow diagnosis

Heating mode delivering cool air. Check reversing valve solenoid, defrost termination, auxiliary heat operation, and refrigerant charge; a stuck reversing valve often shows equal high and low pressures.

Persistent outdoor coil ice

Ice that remains or builds heavily on the outdoor coil during heating indicates defrost control, sensor, or reversing valve failure. Normal frost should melt during each defrost cycle.

High superheat low suction

A condition suggesting undercharge or a restricted metering device. Low suction pressure with high superheat means insufficient refrigerant is entering the evaporator.

Low superheat high suction

A condition suggesting overcharge or an inefficient compressor. Excess refrigerant or poor compression raises suction pressure and reduces superheat.

EPA Section 608 recovery

Federal rules requiring refrigerant recovery before opening a system and prohibiting venting. Recover to the required vacuum level and document the recovery amount.

Voltage drop under load

Measuring voltage at the load while operating to detect loose connections or undersized wiring. Excessive drop causes overheating and erratic control operation.

Sensible heat ratio

Sensible capacity divided by total capacity in cooling. Typical residential SHR is about 0.70–0.80; low airflow lowers SHR and coil temperature, affecting comfort and humidity control.