ID HVAC flashcards

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

Sensible heat equation

Qs = 1.08 × CFM × ΔT, where 1.08 derives from air density, specific heat, and 60 minutes. Used to size airflow for a required temperature difference at sea level.

Latent heat equation

QL = 0.68 × CFM × ΔW, with ΔW in grains of moisture per pound of dry air. It quantifies the moisture-removal load that does not change dry-bulb temperature.

Total heat equation

Qt = 4.5 × CFM × Δh, where Δh is the enthalpy difference in Btu per pound between entering and leaving air. It combines sensible and latent capacity in one calculation.

Dew point temperature

The temperature at which air reaches saturation at constant humidity ratio; further cooling produces condensation. Coil surface temperature must be below it for dehumidification.

Relative humidity definition

The ratio of actual water vapor pressure in the air to the saturation vapor pressure at the same dry-bulb temperature, expressed as a percentage. It changes with temperature even if moisture content is constant.

Wet-bulb temperature

The temperature reached by evaporative cooling of a wetted sensor in moving air. It equals dry-bulb at 100 percent relative humidity and indicates total heat content for evaporative equipment.

Bypass factor

BF = (leaving air DB − apparatus dew point) ÷ (entering air DB − apparatus dew point). A low bypass factor means more air contacts the coil and greater dehumidification.

Sensible heat ratio

SHR = sensible heat ÷ total heat for a space or coil. A low SHR signals a large latent load requiring more moisture removal and lower coil apparatus dew point.

Apparatus dew point

The effective coil surface temperature at which air would leave fully saturated, found where the entering-to-leaving air line intersects the saturation curve on a psychrometric chart.

Duct velocity equation

Q = V × A, where Q is cubic feet per minute, V is velocity in feet per minute, and A is cross-sectional area in square feet. Rearranged, A = Q ÷ V for sizing.

Velocity pressure formula

VP = (V ÷ 4,005)² in inches of water column, with V in feet per minute at standard air density. Total pressure equals static plus velocity pressure.

Fan law for CFM

Airflow varies directly with fan speed: CFM₂ = CFM₁ × (RPM₂ ÷ RPM₁). It assumes the same fan, air density, and system resistance curve.

Fan law for horsepower

Brake horsepower varies with the cube of fan speed: BHP₂ = BHP₁ × (RPM₂ ÷ RPM₁)³. Small speed increases can overload a motor.

Superheat measurement

Suction line temperature minus the saturation temperature corresponding to suction pressure. Measured at the evaporator outlet, it confirms the refrigerant is fully vapor before the compressor.

Subcooling measurement

Saturation temperature at the liquid-line pressure minus the actual liquid-line temperature. It verifies the condenser has fully condensed refrigerant and that the charge is adequate on a TXV system.

External static pressure

The total resistance a blower works against, measured as supply static plus return static, including filter, coil, and duct pressure drops. Blower performance tables are rated at this value.

Air changes per hour

ACH = (CFM × 60) ÷ room volume in cubic feet. It expresses how many times the full room air volume is replaced each hour for ventilation or filtration checks.

Outdoor air ventilation rate

Ventilation airflow needed for acceptable indoor air quality, set by occupancy and floor area per ASHRAE 62.1. It is separate from the airflow needed for heating and cooling loads.

Manual J load calculation

ACCA method that sums transmission, infiltration, internal, and duct gains or losses room by room. It determines equipment capacity before selecting the unit and duct system.

Manual D duct design

ACCA procedure that sizes ducts using friction rate, equivalent length, and available static pressure. It matches duct resistance to the selected blower's performance curve.

Manual S equipment selection

ACCA method for matching equipment capacity to the Manual J load, checking sensible and latent capacity at design conditions. Oversizing raises cycling and reduces latent removal.

Refrigerant glide

The temperature difference between bubble and dew points of a zeotropic blend at a given pressure. Charging and superheat calculations must use the correct point for the blend.

TXV superheat control

A thermostatic expansion valve modulates refrigerant flow to hold a set evaporator superheat. It responds to bulb temperature and pressure, adapting to load changes.

Compressor short cycling

Rapid on-off operation caused by oversized equipment, low charge, or control faults. It reduces latent removal, raises humidity, and shortens compressor life.

Furnace temperature rise

The difference between supply and return air temperatures across a furnace, checked against the data plate range. Excess rise indicates low airflow; low rise indicates high airflow or overfiring.

Combustion air requirements

Category I gas appliances need dedicated or mechanical combustion air sized by code, often 50 cubic feet per 1,000 Btu input for indoor air. Inadequate air causes incomplete combustion and carbon monoxide.

Heat pump balance point

The outdoor temperature at which heat pump capacity equals building heat loss. Below it, supplemental or auxiliary heat must make up the difference.

Heat pump defrost cycle

A timed or demand-initiated reversal that melts frost from the outdoor coil. During defrost the outdoor fan stops and auxiliary heat often energizes to temper supply air.

Economizer operation

A damper system that uses cool outdoor air for free cooling when outdoor enthalpy or temperature is below the return air condition. It reduces mechanical cooling energy.

VAV terminal box

A zone device with a damper and often a reheat coil that modulates airflow from a central air handler. Minimum airflow settings maintain ventilation and prevent coil frosting.

Filter MERV rating

ASHRAE 52.2 minimum efficiency reporting value from 1 to 16, based on particle capture by size. Higher MERV captures finer particles but adds static pressure.

Duct leakage testing

Pressurizing a duct system and measuring leakage airflow against total fan airflow, often to 6 percent or less for new construction. Sealing reduces energy loss and improves comfort.