CT HVAC flashcards
CT 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.
- Section 608 venting prohibition
- Under EPA Section 608, knowingly venting ozone-depleting refrigerants and their non-exempt substitute refrigerants to the atmosphere during service, maintenance, or disposal is prohibited.
- Recovery cylinder 80 percent rule
- Recovery and refrigerant cylinders must never be filled beyond 80 percent of capacity by weight, because liquid refrigerant expands as it warms and can rupture a completely full cylinder.
- A2L refrigerant flammability class
- A2L refrigerants are mildly flammable, with a lower flammability limit and a burning velocity no greater than 10 cm/s; charge limits and ignition-source control follow ASHRAE 15 and UL 60335-2-40.
- Nitrogen pressure testing rule
- Pressure-test systems only with dry nitrogen and a pressure regulator; never use oxygen or compressed air, because oxygen combined with compressor oil can ignite or explode.
- Micron gauge purpose
- An electronic micron gauge reads absolute pressure during evacuation; it resolves levels far below what manifold gauges indicate, so it is required to confirm the system reached 500 microns.
- Superheat calculation formula
- Superheat equals the actual suction line temperature minus the saturation temperature corresponding to the measured suction pressure, expressed in degrees Fahrenheit.
- Subcooling calculation formula
- Subcooling equals the saturation temperature at the measured liquid-line pressure minus the actual liquid-line temperature; it confirms a solid liquid seal at the condenser outlet.
- Manometer reading units
- A manometer measures duct static pressure in inches of water column; one inch of water column equals about 249 pascals, or 0.036 psi.
- One refrigeration ton
- One ton of refrigeration equals 12,000 Btu per hour, or about 3.517 kilowatts, the rate of heat removal needed to freeze one ton of ice in 24 hours.
- Latent versus sensible heat
- Sensible heat changes a substance's temperature without changing its state, while latent heat changes state at constant temperature, as when refrigerant boils or condenses in a coil.
- Compression ratio definition
- Compression ratio is absolute discharge pressure divided by absolute suction pressure, both in psia; higher ratios raise discharge temperature and reduce volumetric efficiency.
- Zeotropic blend temperature glide
- A zeotropic refrigerant blend evaporates and condenses across a temperature range called glide, and it must be charged as a liquid to prevent fractionation of the mixture.
- Ohm's law relationship
- Voltage equals current multiplied by resistance (V = I × R); rearranged, current equals voltage divided by resistance and resistance equals voltage divided by current.
- Thermostat terminals R Y W G
- R supplies 24-volt control power, Y energizes the compressor contactor for cooling, W energizes the heating circuit, and G starts the indoor blower.
- Run capacitor microfarad test
- Discharge the capacitor, then measure microfarads with a capacitance meter; a reading more than about 10 percent below the nameplate rating indicates a weak capacitor.
- Three-phase power formula
- For a balanced three-phase load, true power in watts equals the square root of three multiplied by volts, amps, and power factor.
- Airflow from velocity
- Cubic feet per minute equals air velocity in feet per minute multiplied by the duct cross-sectional area in square feet at the measurement point.
- Sensible heat airflow equation
- Sensible Btu per hour equals 1.08 multiplied by cfm multiplied by the dry-bulb temperature difference across the coil or the furnace heat exchanger.
- External static pressure limit
- Residential blowers are typically rated at 0.5 inches of water column total external static pressure; readings above the fan table value mean restricted ducts, coils, or filters.
- Duct leakage test pressure
- A duct blaster pressurizes the duct system to 25 pascals and reports leakage in cfm at that pressure, allowing comparison against program or code thresholds.
- Natural gas manifold pressure
- Most natural gas furnaces are set to about 3.5 inches of water column manifold pressure, while propane appliances typically require 10 to 11 inches.
- Furnace temperature rise
- Temperature rise is supply air temperature minus return air temperature; it must fall within the range printed on the furnace rating plate at the correct airflow.
- Flame sensor microamp reading
- A flame sensor proves ignition by flame rectification; a healthy reading is roughly 2 to 6 microamps, and lower values indicate a dirty or failing sensor.
- Condensing furnace secondary exchanger
- A condensing furnace extracts additional heat in a secondary heat exchanger, cooling flue gas below its dew point; the acidic condensate must drain through a trap.
- TXV superheat control
- A thermostatic expansion valve modulates refrigerant flow to hold a set evaporator superheat, responding to the temperature and pressure sensed by its remote bulb.
- Subcooling charging method
- Systems with a thermostatic expansion valve are charged by the subcooling method, targeting the manufacturer's specified value, commonly around 10 to 12 degrees Fahrenheit.
- Superheat charging method
- Fixed-orifice systems are charged by superheat, using a chart based on indoor wet-bulb and outdoor dry-bulb temperature to determine the target superheat value.
- Crankcase heater function
- A crankcase heater warms compressor oil so refrigerant does not migrate and condense into the oil during the off cycle, which would dilute lubricant and damage bearings.
- Cooling temperature split
- The dry-bulb temperature difference between return and supply air in cooling is typically 16 to 22 degrees Fahrenheit when airflow and refrigerant charge are correct.
- Commissioning airflow checked first
- Airflow must be verified before refrigerant charge is evaluated, because incorrect evaporator airflow skews superheat and subcooling readings and misleads the technician.
- High head high suction diagnosis
- High discharge pressure together with high suction pressure usually indicates an overcharged system, excessive heat load, or insufficient condenser airflow.
- Blower door ACH50 test
- A blower door measures envelope airtightness by pressurizing the house to 50 pascals and reporting air changes per hour at that pressure, called ACH50.