HVAC Excellence Master AC flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Sensible heat formula

Q = 1.08 × CFM × ΔT, where Q is Btu/h and ΔT is the dry-bulb temperature difference in °F. Used for air heating and cooling sensible load calculations at sea level.

Total heat formula

Q = 4.5 × CFM × Δh, where Δh is the enthalpy difference in Btu per pound of dry air. Combines sensible and latent loads for coil sizing.

Carnot COP

The maximum theoretical cooling COP equals evaporator absolute temperature divided by the difference between condensing and evaporating absolute temperatures. Real systems achieve roughly 40–60% of this value.

Superheat definition

The number of degrees a vapor is above its saturation temperature at a given pressure. Measured at the evaporator outlet to verify the metering device is feeding properly.

Subcooling definition

The number of degrees a liquid is below its saturation temperature at a given pressure. Measured at the condenser outlet; 8–12°F is typical for TXV systems.

One ton of refrigeration

Equal to 12,000 Btu per hour, or 200 Btu per minute. This is the heat absorbed by melting one ton of ice in 24 hours and is the standard capacity unit.

Compression ratio

Absolute discharge pressure divided by absolute suction pressure. Ratios above roughly 10:1 raise discharge temperature and shorten compressor life, so high ratios indicate a problem.

Ohm's law and power

E = I × R; single-phase power P = E × I × power factor. Three-phase power equals √3 × E × I × power factor. These are the core electrical calculations for HVAC service.

Locked-rotor amps

The current drawn when a compressor rotor cannot turn. Typically 5–6 times running-load amps; a reading near LRA indicates a stuck rotor or failed start components.

Potential relay operation

A start relay whose coil is energized by compressor back-EMF. As the motor speeds up, voltage rises and the relay opens, dropping the start capacitor out of the circuit.

Current relay operation

A start relay with a coil in series with the run winding. High starting current pulls the armature in to engage the start winding; as current falls, the relay opens.

Defrost termination

A control that ends the defrost cycle when the coil reaches a set temperature or pressure. A failed sensor causes long defrost cycles and coil icing.

Low-pressure cutout

A safety switch that opens the compressor circuit when suction pressure falls below a setpoint, typically 20–25 psig on R-410A, protecting against low charge and freezing.

Sensible heat ratio

Sensible heat divided by total heat. Typical residential design values are 0.70–0.80; it drives coil selection and airflow because it determines latent capacity.

Airflow per ton

The rule of thumb of 400 CFM per ton of cooling capacity, with a range of 350–450 CFM acceptable. Low airflow reduces capacity and can ice the coil.

External static pressure

Total resistance to airflow measured across the blower, in inches of water column. Residential systems should stay at or below 0.5 in. w.c.

Duct velocity targets

Supply trunks 600–900 fpm, branch runs 400–700 fpm, and returns 500–700 fpm. Velocities above 1,000 fpm cause noise and excessive pressure drop.

Fan laws

CFM varies with RPM, static pressure with RPM squared, and brake horsepower with RPM cubed. Doubling RPM quadruples static and octuples horsepower.

Duct leakage target

New residential duct systems typically leak 10–30% of airflow. Sealing to 5% or less is the target for high-performance homes and is verified by a duct blaster test.

Blower door ACH50

Air changes per hour at 50 pascals of pressure. A tight home is 3 ACH50 or less, average existing homes are 5–8, and leaky homes exceed 10.

Thermal bridging

Wood studs conduct heat better than cavity insulation, reducing effective wall R-value by roughly 15–25%. Continuous exterior insulation or advanced framing reduces the penalty.

Window U-factor and SHGC

U-factor measures heat loss and lower is better. SHGC measures solar gain; lower is better in cooling climates and higher is better in heating climates.

ASHRAE 34 safety groups

A1 is lower toxicity with no flame propagation. A2L is lower toxicity with lower flammability. A3 is lower toxicity with higher flammability.

A2L flammability criteria

Burning velocity of 10 cm/s or less and minimum ignition energy greater than 10 mJ. A2L refrigerants are flammable but difficult to ignite compared with A3.

A3 charge limits

Highly flammable refrigerants such as R-290 are limited to small charges, typically 150 grams or less in residential appliances under IEC 60335-2-40.

EPA Section 608 recovery

Requires recovery of refrigerant before opening a system; venting is prohibited. Recover to a dedicated cylinder and never exceed the 80% fill rule.

Target superheat charging

For fixed-orifice systems, use the manufacturer's charging chart. Typical target superheat is 10–20°F depending on indoor wet-bulb and outdoor dry-bulb conditions.

Evaporator delta-T

Supply-to-return dry-bulb difference across the evaporator. A properly charged residential system shows 18–22°F; below 15°F suggests low airflow or low charge.

Condenser approach

Condensing temperature minus outdoor ambient temperature, typically 10–20°F. A higher approach indicates a dirty coil, low airflow, or non-condensables in the system.

SEER2 and HSPF2

Current efficiency metrics for heat pumps and air conditioners, effective 2023. Minimum is 14.3 SEER2 for split systems in most U.S. regions.

ACCA Manual J

The residential load calculation procedure used to size HVAC equipment. It accounts for envelope, orientation, occupancy, and climate; sizing by rule of thumb is not acceptable.

Refrigerant GWP values

R-410A is about 2,088, R-32 about 675, R-454B about 466, and R-290 about 3. Lower GWP reduces climate impact but may change safety classification.