NC HVAC flashcards
NC 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 equation
- Q = 1.08 × CFM × ΔT, where Q is BTU/h and ΔT is the dry-bulb temperature difference between entering and leaving air.
- Total heat equation
- Q = 4.5 × CFM × Δh, where Δh is the enthalpy difference in BTU per pound of dry air between entering and leaving conditions.
- AFUE definition
- Annual Fuel Utilization Efficiency, the ratio of annual heat output to annual fuel energy input, expressed as a percentage for furnaces and boilers.
- Heat pump balance point
- The outdoor temperature at which heat pump heating capacity equals the building heat loss; below this point supplemental heat is required.
- Vapor-compression cycle order
- Refrigerant flows from compressor to condenser to metering device to evaporator and back to the compressor in a closed loop.
- Superheat definition
- The difference between the actual suction line temperature and the evaporator saturation temperature, used to verify proper refrigerant charge in fixed-orifice systems.
- Subcooling definition
- The difference between the condenser saturation temperature and the liquid line temperature, indicating how much the refrigerant has cooled below condensing temperature.
- TXV function
- A thermostatic expansion valve modulates refrigerant flow to maintain a constant superheat at the evaporator outlet regardless of load changes.
- Non-condensable effect
- Air or nitrogen trapped in a system raises discharge pressure and subcooling and increases head temperature, reducing capacity and efficiency.
- Manual J purpose
- ACCA residential load calculation method that determines heating and cooling loads from envelope, infiltration, internal gains, and duct losses.
- Sensible heat ratio
- The ratio of sensible load to total load; equipment SHR must not exceed the calculated SHR to avoid excess latent capacity and humidity problems.
- One ton of cooling
- Equal to 12,000 BTU per hour of heat removal, the standard nominal capacity unit for residential and light commercial air conditioning.
- Oversizing consequences
- Excess capacity causes short cycling, poor latent removal, elevated indoor humidity, and reduced equipment life; avoid oversizing beyond about 15–20%.
- Manual D purpose
- ACCA residential duct design standard that sizes ducts using friction rate, equivalent length, and available static pressure.
- Equivalent length concept
- Fitting losses converted to straight-duct length; a 90-degree elbow typically adds 20–30 feet to the effective duct run.
- External static pressure
- The sum of supply and return static pressures measured at the blower; typical residential blower ratings are about 0.5 inches water column total.
- Flex duct support rule
- Flexible duct must be supported at intervals no greater than 4 feet and cannot be compressed more than 20% of its diameter.
- Ohm's law
- V = I × R; voltage equals current times resistance, the basis for troubleshooting electrical circuits in HVAC equipment.
- Locked-rotor amps
- The current drawn by a motor at startup with the rotor stationary, typically five to six times the full-load running amps.
- Run capacitor function
- A capacitor that shifts phase in a permanent-split capacitor motor to improve running efficiency and torque; failure causes overheating and high current.
- Thermostat terminal R
- The terminal that supplies 24 VAC control power from the transformer to the thermostat and switching circuits.
- Reversing valve terminal O
- On most brands, the O terminal energizes the reversing valve in cooling mode; B terminal configurations energize it in heating.
- Combustion air requirement
- Complete combustion of natural gas needs about 10 cubic feet of air per cubic foot of gas; 15 cubic feet provides a design safety margin.
- Sediment trap purpose
- A drip leg installed upstream of gas controls that captures debris and condensate to protect the gas valve from contamination.
- Carbon monoxide source
- CO is produced by incomplete combustion from insufficient air, dirty burners, or blocked flues, and is a lethal, odorless gas.
- Category IV venting
- Condensing appliances with positive flue pressure use PVC venting and produce acidic condensate that must be drained and neutralized.
- NC HVAC licensing board
- The North Carolina State Board of Examiners of Plumbing, Heating, and Fire Sprinkler Contractors licenses heating contractors under classifications H-1, H-2, and H-3.
- NC permit requirement
- North Carolina requires a permit before installing or replacing HVAC equipment, with rough-in and final inspections by the local code official.
- EPA Section 608
- Federal rule requiring certification for technicians who service, repair, or dispose of equipment containing ozone-depleting or regulated refrigerants.
- Gross profit margin
- Calculated as (price minus cost) divided by price; a 30% margin corresponds to a 42.9% markup on cost.
- Change order purpose
- A written, signed document authorizing out-of-scope work and its price before performance, protecting the contractor's right to payment.
- Refrigerant recovery limit
- Recovery cylinders may be filled to no more than 80% of capacity by weight and must be DOT-approved and hydrostatically tested.