MD HVAC flashcards
MD 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.
- Tonnage definition
- One ton of refrigeration equals 12,000 Btu per hour, which is the heat absorption rate needed to melt one ton of ice in 24 hours.
- Superheat calculation
- Superheat is the suction-line temperature minus the evaporator saturation temperature at the measured suction pressure; it indicates how much vapor is leaving the evaporator.
- Compression ratio
- Compression ratio equals discharge absolute pressure divided by suction absolute pressure; use psia, not psig, because gauge pressure ignores atmospheric pressure.
- Metering device function
- The metering device reduces refrigerant pressure from the liquid line to the evaporator, controlling refrigerant flow and creating the low-temperature saturation condition needed for heat absorption.
- Zeotropic blend glide
- A zeotropic refrigerant blend changes temperature as it evaporates or condenses at constant pressure; the difference between bubble point and dew point is called temperature glide.
- Evacuation target
- A vacuum of 500 microns is a common dehydration target; 25,400 microns equals one inch of mercury, and lower absolute pressure removes moisture faster.
- Natural gas heating value
- Natural gas contains roughly 1,000 Btu per cubic foot, so a furnace input of 100,000 Btu/h requires about 100 cubic feet of gas per hour.
- Hydronic heat formula
- For water, heat output equals 500 times flow in gallons per minute times temperature drop in degrees Fahrenheit; one gallon per minute at 20°F provides about 10,000 Btu/h.
- Electric heat conversion
- One watt of electric resistance heat produces 3.413 Btu per hour; multiply heater kilowatts by 3,413 to find Btu/h output.
- Furnace temperature rise
- Temperature rise is the supply-air temperature minus return-air temperature; it should fall within the furnace nameplate range at the designed airflow.
- Manifold pressure
- Natural gas manifold pressure is typically about 3.5 inches water column, while liquefied petroleum gas is typically about 10 to 11 inches water column; nameplate values govern.
- Boiler relief valve
- A residential hot-water boiler commonly has a 30 psig pressure relief valve; it must remain uncapped and piped to a safe discharge location.
- Airflow equation
- Cubic feet per minute equals duct velocity in feet per minute multiplied by duct cross-sectional area in square feet; use this to check airflow through a known duct size.
- Velocity pressure formula
- For standard air, velocity pressure in inches of water column equals velocity in feet per minute divided by 4,005, then squared.
- External static pressure
- External static pressure is the sum of supply and return static pressures measured at the equipment; it represents the resistance the blower must overcome.
- MERV rating
- Minimum Efficiency Reporting Value rates a filter's particle capture; higher MERV captures smaller particles but usually increases pressure drop and fan energy.
- ASHRAE 62.2
- ASHRAE Standard 62.2 sets residential ventilation and acceptable indoor air quality requirements, including whole-house and local exhaust rates.
- Ohm's law
- Ohm's law states that voltage equals current times resistance; power equals voltage times current, and also current squared times resistance.
- Thermostat terminals
- In a conventional low-voltage thermostat, R supplies 24 volts, W calls for heat, Y calls for cooling, G runs the fan, and C is common.
- Run capacitor
- A run capacitor remains in the motor circuit during operation and is rated in microfarads; it helps maintain phase shift and motor torque.
- Locked-rotor current
- Locked-rotor current is the high current drawn when a motor rotor is stationary; for induction motors it is typically five to seven times full-load current.
- Contactor coil
- A contactor coil is an electromagnetic control load that closes the contactor when energized; its voltage must match the control transformer output.
- Capacitor discharge
- Before testing a capacitor, remove power and discharge it through a resistor; verify zero voltage with a meter rated for the circuit.
- EPA Section 608
- EPA Section 608 certification is required for technicians who service, repair, or dispose of appliances containing regulated refrigerants; venting refrigerant is prohibited.
- ASHRAE Standard 15
- ASHRAE Standard 15 establishes refrigerant safety requirements, including refrigerant classification, occupancy limits, and machinery-room design.
- CO alarm placement
- Carbon monoxide alarms should be installed outside sleeping areas and on each level of a home containing combustion appliances, following manufacturer instructions.
- Lockout/tagout
- Lockout/tagout isolates electrical or other energy sources with locks and tags and verifies zero energy before maintenance, preventing accidental startup.
- Recovery cylinder fill
- A refrigerant recovery cylinder must not be filled above 80 percent of its water capacity by weight; overfilling can cause hydrostatic rupture.
- Manual J
- Manual J is the ACCA method for calculating residential heating and cooling loads from building envelope, orientation, windows, occupancy, and infiltration data.
- Manual D
- Manual D is the ACCA method for sizing and designing residential duct systems based on available static pressure and room airflow requirements.
- Change order
- A change order is a written modification to a contract that documents added or deleted work, price adjustment, and schedule impact before work proceeds.
- Commissioning
- Commissioning verifies that installed equipment and controls operate according to design, including airflow, refrigerant charge, temperature rise, and safety controls.