NATE Heat Pump Service flashcards
NATE Heat Pump Service · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Reversing valve function
- A four-way solenoid valve that swaps the roles of the indoor and outdoor coils, allowing the same refrigerant circuit to provide both heating and cooling.
- Balance point
- The outdoor temperature at which heat pump heating capacity equals the building heat loss; below this point, supplementary heat is required to maintain indoor temperature.
- Coefficient of performance
- The ratio of heat delivered to electrical energy consumed, expressed as a dimensionless number; higher COP means more efficient heating at a given outdoor temperature.
- Accumulator purpose
- A vessel on the suction line that traps liquid refrigerant and oil, releasing them gradually to the compressor to prevent liquid slugging during heating operation.
- Crankcase heater function
- An electrical resistance heater wrapped around the compressor sump that keeps oil warm and drives off migrated refrigerant while the compressor is off, preventing startup slugging.
- Zeotropic blend glide
- The temperature difference between bubble and dew points of a refrigerant blend during phase change; affects charging accuracy and evaporator performance.
- Evacuation target
- A vacuum of 500 microns or lower held for 10 minutes with the pump isolated; a rising reading indicates moisture or a leak in the system.
- Nitrogen purge brazing
- Flowing dry nitrogen through the piping while brazing to displace oxygen and prevent internal oxidation scale that can later clog metering devices.
- Line set sizing rule
- Suction and liquid line diameters must follow manufacturer tables; undersized lines raise pressure drop, oversized lines reduce velocity and oil return.
- Condensate in heating
- In heating mode the outdoor coil runs below ambient and produces condensate; the drain path must slope away and not refreeze under the unit.
- Commissioning baseline data
- Recorded static pressure, delta-T, superheat, subcooling, and amp draw at startup; these values become the reference for future diagnostics.
- Defrost initiation logic
- Demand defrost starts when the coil temperature sensor and accumulated compressor run time both reach set thresholds, signaling frost buildup on the outdoor coil.
- Defrost cycle sequence
- The reversing valve shifts to cooling, the outdoor fan stops, and auxiliary heat energizes indoors; termination occurs when the coil temperature rises above the set point.
- Anti-short-cycle delay
- A time-delay relay or control board lockout, typically five minutes, that prevents the compressor from restarting too quickly after shutdown.
- Low-voltage fuse location
- A 3 A or 5 A fuse on the control board protects the 24 V circuit; a blown fuse usually indicates a shorted thermostat wire or contactor coil.
- High-pressure switch role
- A safety control that opens on excessive discharge pressure; tripping indicates airflow restriction, overcharge, or noncondensables, not a defective switch.
- Superheat formula
- Superheat equals suction line temperature minus the saturation temperature at suction pressure; high superheat with low suction pressure indicates undercharge or restriction.
- Subcooling formula
- Subcooling equals the saturation temperature at liquid pressure minus the liquid line temperature; low subcooling with bubbles suggests undercharge.
- Cooling delta-T range
- The temperature drop across the indoor coil in cooling is typically 16 to 22 F; a low value with normal airflow points to low charge or a dirty coil.
- Heating delta-T range
- The temperature rise across the indoor coil in heating is typically 25 to 40 F; compare measured value to the manufacturer chart for the outdoor temperature.
- Restricted drier symptom
- A temperature drop across the liquid line filter drier plus low subcooling downstream indicates a restriction; the drier should be replaced.
- Airflow before charging
- Always correct airflow before charging; low airflow depresses suction pressure and raises superheat, mimicking an undercharged system.
- TXV charging method
- Thermostatic expansion valve systems are charged by subcooling in cooling mode; target subcooling is typically 8 to 12 F at design conditions.
- Fixed-orifice charging method
- Piston systems are charged by superheat, adjusted for indoor wet-bulb and outdoor dry-bulb temperatures using the manufacturer charging chart.
- Weigh-in charging
- The most accurate charging method when line set length and indoor coil volume are known; add refrigerant per the manufacturer chart for line length.
- Total external static pressure
- Measured across the blower; residential target is typically 0.5 in. w.c. or less; higher readings indicate duct restriction or dirty filter.
- 400 CFM per ton rule
- A common residential airflow target of 400 cubic feet per minute per ton of cooling capacity; heating airflow may be set higher per manufacturer.
- Sensible capacity estimate
- Sensible Btu/h is estimated by multiplying delta-T by 1.08 times the airflow in CFM; useful for verifying delivered capacity.
- AHRI certificate check
- The AHRI certificate lists rated SEER2 and HSPF2 for the matched system; compare installed performance to these values during efficiency verification.
- Outdoor unit clearance
- Manufacturer-specified distance from walls and obstructions that prevents air recirculation; insufficient clearance reduces capacity and raises head pressure.
- Defrost sensor failure
- A faulty coil temperature sensor prevents defrost termination or initiation; symptoms include heavy frost buildup or repeated short defrost cycles.
- Noncondensable symptom
- Air or moisture in the system raises head pressure and subcooling while suction pressure stays normal; indicates poor evacuation during installation.