HI HVAC flashcards

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

Sensible heat equation

Btu/h = 1.08 × CFM × ΔT for standard air at sea level. The constant combines air density, specific heat, and 60 minutes per hour to convert airflow and temperature change into heat rate.

AFUE

Annual Fuel Utilization Efficiency is the seasonal efficiency rating for heating equipment, accounting for steady-state combustion efficiency plus cycling, standby, and jacket losses over a typical heating season.

Condensing furnace

A furnace that extracts latent heat by cooling flue products below the water vapor dew point, reaching 90%+ AFUE. The acidic condensate requires PVC venting and a drain with neutralizer.

Hydronic Btu formula

Btu/h = 500 × GPM × ΔT for water. The 500 constant reflects 8.33 lb per gallon, specific heat of 1.0, and 60 minutes per hour, so 10 GPM at 20°F delivers 100,000 Btu/h.

Heat pump balance point

The outdoor temperature where heat pump capacity equals the building heat loss. Below this point, supplemental heat must make up the shortfall to maintain indoor setpoint.

Flame rectification sensor

A rod that proves burner flame by sensing the tiny DC current that flows when flame conducts ions between the rod and grounded burner. A dirty rod causes lockout or short cycling.

Breathing-zone ventilation rate

ASHRAE 62.1 formula Vbz = Rp × Pz + Ra × Az, where people and area components are summed to give the minimum outdoor airflow required for a zone.

MERV rating

Minimum Efficiency Reporting Value rates filter capture of particles from 0.3 to 10 microns on a 1–16 scale. Higher MERV means finer capture but greater pressure drop and fan energy.

Demand-controlled ventilation

A control strategy that modulates outdoor air based on measured occupancy, often via CO2 sensors. It saves energy in variable-occupancy spaces while maintaining acceptable indoor air quality.

Building pressure relationship

A space is negative when exhaust exceeds supply and positive when supply exceeds exhaust. Contaminant sources such as bathrooms and kitchens are held negative to prevent migration.

Makeup air requirement

Outdoor air supplied to replace air exhausted by hoods or fans. Unbalanced exhaust depressurizes the building, causing backdraft of combustion appliances and infiltration of unconditioned air.

Radon action level

EPA recommends mitigation when indoor radon exceeds 4 picocuries per liter. Sub-slab depressurization is the standard mitigation method for existing homes.

Superheat definition

The difference between actual suction line temperature and saturated suction temperature at the same pressure. It confirms that refrigerant is fully vapor before entering the compressor.

Subcooling definition

The difference between saturated condensing temperature and liquid line temperature. It confirms that liquid refrigerant is fully condensed and indicates charge level on a TXV system.

Sensible heat ratio

Sensible capacity divided by total capacity. A typical residential value near 0.75 means about three-quarters of the cooling effect lowers temperature and the rest removes moisture.

Refrigeration cycle order

Compressor raises pressure and temperature, condenser rejects heat and condenses, metering device drops pressure, and evaporator absorbs heat before returning vapor to the compressor.

Evaporator freezing threshold

Coil temperature must remain above 32°F to prevent condensate freezing. Low airflow, low charge, or a restricted metering device can drop coil temperature below freezing and ice the coil.

Compression ratio

Absolute discharge pressure divided by absolute suction pressure. Ratios above roughly 10:1 on single-stage equipment produce high discharge temperatures that degrade oil and compressor life.

Zeotropic refrigerant blend

A mixture whose components boil at different temperatures, causing a temperature glide. R-410A must be charged as liquid to avoid fractionation that changes blend composition.

Equal friction duct sizing

A method that selects duct sizes so friction loss per unit length stays constant along the run. It uses available static pressure and equivalent length to set the friction rate.

Velocity pressure formula

VP = (V ÷ 4,005)² in inches of water column, where V is velocity in feet per minute. Total pressure equals static pressure plus velocity pressure.

Equivalent length of fittings

The straight-duct length that produces the same pressure loss as a fitting. A radius elbow is about 30 ft; a hard tee can equal 60 to 100 ft of duct.

Return grille sizing

Return free area is commonly sized near 2 square feet per ton of cooling. Undersized returns raise system static pressure and reduce delivered airflow.

Suction line oil return

Suction line velocity must stay above roughly 500 feet per minute to carry oil back to the compressor, while staying below about 4,000 feet per minute to avoid noise.

TXV function

A thermostatic expansion valve modulates refrigerant flow to maintain a set superheat at the evaporator outlet, responding to bulb temperature and evaporator pressure.

Ohm's law and power

V = I × R and P = V × I = I²R. These relationships let technicians compute current, resistance, and wattage for heaters, motors, and control circuits.

Thermostat terminal letters

R is 24 V hot, W calls heat, Y calls cooling, G calls fan, O or B reverses the heat pump valve, and C provides common power for smart thermostats.

Run capacitor failure

A failed run capacitor prevents the motor from starting under load, causing hum and overload trips. A bulged case or capacitance below 10% of rating confirms replacement.

Temperature rise check

Measure supply minus return air temperature and compare with the data plate value, typically within plus or minus 10%. High rise suggests low airflow; low rise suggests excess airflow.

Manual J load calculation

The ACCA method that sums transmission losses (U × A × ΔT) and infiltration losses (1.08 × CFM × ΔT) for heating, and radiant plus internal gains for cooling.

Manual S equipment selection

The ACCA procedure for choosing equipment whose capacity matches the Manual J load without exceeding it. Oversizing causes short cycling, poor dehumidification, and higher energy use.

A2L refrigerant classification

ASHRAE 34 class for mildly flammable refrigerants such as R-32 and R-454B. Installations require ignition-source controls, leak detection, and charge limits in occupied spaces.