NCCER HVACR flashcards
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32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Ton of refrigeration
- The heat absorption rate of melting one ton of ice in 24 hours, equal to 12,000 Btu/h, 200 Btu/min, or 3.517 kW. It is the standard capacity unit for air conditioning equipment.
- Sensible vs latent heat
- Sensible heat changes a substance's temperature without a phase change. Latent heat changes phase at constant temperature, as when refrigerant boils in the evaporator or condenses in the condenser.
- Btu definition
- The quantity of heat required to raise one pound of water one degree Fahrenheit. One Btu equals 1.055 kilojoules, and one watt of power equals 3.412 Btu/h.
- Heat flow direction
- Heat always moves from a warmer body to a cooler body. Air conditioning does not create cold; it removes heat from the conditioned space and rejects it outdoors.
- Temperature conversion formulas
- Fahrenheit from Celsius: F = (C × 1.8) + 32. Celsius from Fahrenheit: C = (F − 32) ÷ 1.8. Absolute Rankine equals Fahrenheit plus 460; Kelvin equals Celsius plus 273.
- Relative humidity
- The percentage of water vapor actually in the air compared with the maximum the air could hold at that temperature. Warmer air holds more moisture, so cooling raises relative humidity unless moisture is removed.
- Lockout/tagout procedure
- Isolate all energy sources, lock the disconnect open, attach a warning tag, and verify zero energy with a meter. Never trust a switch position or another worker's assurance.
- Refrigerant cylinder storage
- Store cylinders upright and secured, below 125°F, away from heat and flame. Never fill above 80% liquid full so thermal expansion has room and hydrostatic rupture is prevented.
- Manifold gauge hose colors
- Blue connects to the low side suction service port, red to the high side discharge port, and yellow to the vacuum pump, recovery unit, or charging cylinder.
- Micron vacuum level
- A micron is 1/25,400 of a millimeter of mercury. A standing vacuum below 500 microns after isolation indicates acceptable dehydration and no leaks in the system.
- Nitrogen purge when brazing
- Flowing dry nitrogen through copper tubing during brazing displaces oxygen and prevents internal oxidation, which forms scale that can clog metering devices and damage the compressor.
- Recovery vs reclamation
- Recovery removes refrigerant from a system into a container. Reclamation reprocesses it at a facility to ARI-700 purity. Recycling cleans refrigerant on site but does not restore virgin specifications.
- Vapor-compression cycle order
- Refrigerant flows from compressor to condenser to metering device to evaporator and back. The compressor raises pressure and temperature; the metering device drops pressure and temperature.
- Superheat calculation
- Superheat equals measured suction line temperature minus the evaporating saturation temperature from a pressure-temperature chart. It confirms the evaporator is fully fed with liquid refrigerant.
- Subcooling calculation
- Subcooling equals condensing saturation temperature minus measured liquid line temperature. It confirms the condenser is fully condensing and providing a solid liquid seal to the metering device.
- TXV operating principle
- A thermostatic expansion valve modulates refrigerant flow to maintain a set evaporator superheat. It responds to bulb temperature and pressure, keeping the coil fed without flooding the compressor.
- R-410A characteristics
- A near-azeotropic HFC blend operating at roughly 50 to 60 percent higher pressure than R-22. It requires POE oil and cannot be used to retrofit R-22 systems.
- Pressure-temperature chart use
- A PT chart converts saturation pressure to temperature for a specific refrigerant. Each refrigerant has a unique curve, and zeotropic blends have glide between bubble and dew points.
- Evaporator airflow rule
- Airflow across the evaporator is typically 350 to 450 cubic feet per minute per ton. Low airflow lowers suction pressure and can freeze the coil even with a correct charge.
- Gas furnace ignition sequence
- Thermostat calls, inducer starts, pressure switch proves draft, igniter heats, gas valve opens, flame sensor proves flame, blower delays then starts. Any failed proof stops the sequence.
- Combustion air requirement
- Confined spaces need roughly 50 cubic feet of combustion air per 1,000 Btu/h input when using outdoor air. Inadequate air causes incomplete combustion, carbon monoxide, and flame rollout.
- Natural gas manifold pressure
- Most natural gas furnaces operate at about 3.5 inches water column manifold pressure. Liquefied petroleum gas typically requires 10 to 11 inches. Always verify against the unit data plate.
- AFUE rating
- Annual fuel utilization efficiency measures the percentage of fuel energy delivered as heat over a heating season. Ratings of 90 percent or higher indicate condensing furnaces with secondary heat exchangers.
- Heat pump balance point
- The outdoor temperature at which heat pump capacity exactly matches the building heat load. Below this point, auxiliary or emergency resistance heat supplements the heat pump.
- Heat pump defrost cycle
- A timed or demand-initiated cycle that melts frost from the outdoor coil by reversing refrigerant flow or stopping the outdoor fan. It terminates on coil temperature or time.
- Electric heat conversion
- One kilowatt of resistance heat equals 3,412 Btu/h. Heat strips are verified by comparing measured amp draw and resistance readings with the nameplate rating.
- Triple evacuation method
- Evacuate to a target micron level, break the vacuum with dry nitrogen, and repeat three times. Finish below 500 microns and confirm the vacuum holds when isolated.
- Leak detection order
- Use an electronic leak detector first for trace leaks, then soap bubbles at suspected joints, and confirm with a nitrogen pressure test. Never use oxygen or compressed air.
- Weigh-in charging
- Charging by weight uses the nameplate charge plus additional charge for line length. It is required for critically charged systems and new installations where superheat or subcooling cannot be measured reliably.
- Superheat charging method
- Used on fixed-orifice systems: measure suction pressure, convert to saturation temperature, measure suction line temperature, and adjust charge until superheat matches the manufacturer's target.
- Subcooling charging method
- Used on TXV systems: measure liquid line pressure, convert to condensing temperature, measure liquid line temperature, and adjust charge until subcooling matches the manufacturer's target.
- High head pressure diagnosis
- High head pressure with high subcooling indicates overcharge or restricted condenser airflow. Check for dirty coils, failed fan motors, recirculated air, or noncondensables in the system.