MS HVAC flashcards
MS HVAC · 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.
- Sensible heat formula
- Qs = 1.08 × CFM × ΔT. It calculates the sensible heat load in BTU/h for standard air, where 1.08 combines air density and specific heat at sea level.
- Latent heat formula
- QL = 0.68 × CFM × ΔW, where ΔW is the moisture difference in grains per pound. It estimates the BTU/h needed to remove or add moisture from the air.
- One ton of refrigeration
- Equal to 12,000 BTU/h, 200 BTU/min, or 3.517 kW. It represents the heat absorption rate of melting one ton of ice in 24 hours.
- Superheat definition
- The difference between the actual suction line temperature and the saturation temperature at the suction pressure. It confirms the refrigerant is fully vapor before the compressor.
- Subcooling definition
- The difference between the saturation temperature at the liquid line pressure and the actual liquid line temperature. It confirms the refrigerant is fully liquid before the metering device.
- Pressure unit conversion
- 1 psi equals 2.31 feet of water column or 27.7 inches of water column. 1 inch of water column is about 0.036 psi or 249 pascals.
- Sensible heat factor
- SHF equals sensible load divided by total load. A value near 1.0 means the space load is mostly sensible with little latent moisture removal.
- Refrigeration cycle order
- Compressor, condenser, metering device, evaporator, then back to the compressor. Heat is rejected in the condenser and absorbed in the evaporator.
- TXV function
- The thermostatic expansion valve meters refrigerant into the evaporator and controls superheat, typically holding 8 to 12 degrees Fahrenheit at the evaporator outlet.
- Low superheat diagnosis
- Low superheat with a cold, sweating suction line suggests overcharge or a stuck-open TXV. Liquid refrigerant may reach the compressor and cause damage.
- High discharge pressure causes
- Dirty condenser, non-condensable air, overcharge, low condenser airflow, or high ambient temperature. Each raises condensing pressure and compressor load.
- Filter-drier purpose
- Liquid line filter-driers remove moisture and acid from the refrigerant. Suction line filter-driers are temporary and removed after a burnout cleanup.
- Evacuation standard
- Evacuate to 500 microns and perform a standing vacuum test. This confirms moisture and non-condensables are removed before charging the system.
- Scroll compressor caution
- Scroll compressors are unidirectional. Running one backward from reversed three-phase power damages the scroll set, so verify phase rotation before startup.
- Ohm's law and power
- E = I × R and P = E × I. For single-phase AC, include power factor: P = E × I × PF when the load is not purely resistive.
- Run capacitor test
- A good run capacitor reads within its rated microfarad tolerance. A bulging, leaking, or open capacitor prevents compressor or fan motor start.
- Thermostat wiring letters
- R is 24V power, W is heat, Y is cooling compressor, G is indoor fan, O/B is reversing valve, and C is common. These follow standard color conventions.
- LRA versus RLA
- LRA is locked rotor amps, the inrush current at startup. RLA is running load amps, the normal operating current. Drawing near LRA while running indicates a fault.
- Blown low-voltage fuse
- A blown 24V control fuse usually means a shorted control wire or failed component. Locate the short before replacing the fuse to avoid repeat failure.
- Contactor coil versus contacts
- The coil is in the low-voltage control circuit and the contacts switch the high-voltage load. Pitted contacts cause voltage drop and compressor overheating.
- Equal friction duct method
- Duct sizing that holds a constant pressure drop per 100 feet, typically 0.1 inch water column. It simplifies design for supply and return trunk lines.
- CFM from velocity
- CFM equals velocity in feet per minute times free area in square feet. A duct traverse with a pitot tube or anemometer gives average velocity.
- High static pressure meaning
- Static pressure above about 0.5 inch water column at the blower outlet suggests restricted ductwork, a dirty filter, or an undersized return.
- MERV filter ratings
- MERV 8 to 13 filters capture particulate matter for general IAQ. Higher MERV means finer capture but more airflow resistance and fan energy.
- Coil icing cause
- A dirty evaporator coil or filter reduces airflow, lowers suction pressure, and can cause coil icing. Restored airflow usually clears the ice.
- Furnace ignition sequence
- Inducer starts, pressure switch proves draft, igniter heats, gas valve opens, flame sensor proves flame, then the blower starts after a delay.
- AFUE meaning
- Annual fuel utilization efficiency rates furnace efficiency. Units at 90 percent or higher condense flue gas and require a condensate drain.
- Heat pump balance point
- The outdoor temperature where heat pump capacity equals building load. Below it, auxiliary or emergency heat supplements the heat pump.
- Defrost cycle purpose
- The heat pump reverses refrigerant flow to melt frost on the outdoor coil. Auxiliary heat runs during defrost to temper the supply air.
- Furnace temperature rise
- The difference between supply and return air temperatures across the furnace, typically 35 to 65 degrees Fahrenheit. It confirms proper airflow and firing rate.
- Combustion CO safety
- A flue gas CO reading above 100 ppm or any measurable ambient CO indicates a problem. Cracks in the heat exchanger can leak CO into the space.
- SEER2 and HSPF2
- Efficiency ratings for cooling and heating. Higher values mean lower operating cost, and SEER2 and HSPF2 replace the older SEER and HSPF metrics.