IA HVAC flashcards

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

Sensible heat ratio

Ratio of sensible heat to total heat (sensible plus latent). A coil SHR at or below space SHR is needed to control humidity; higher SHR means drier supply air relative to load.

Wet-bulb temperature

The lowest temperature air can reach by evaporative cooling at constant pressure. It reflects both temperature and moisture content and is used with dry-bulb to locate a state on the psychrometric chart.

Humidity ratio

Mass of water vapor per mass of dry air, usually lb water per lb dry air. It changes only with moisture addition or removal, not with sensible heating or cooling.

Sensible heat equation

Qs equals 1.08 times CFM times dry-bulb temperature difference for standard air at sea level. At altitude, replace 1.08 with a density-corrected coefficient.

Ton of refrigeration

A cooling rate equal to 12,000 Btu per hour, 200 Btu per minute, or 3.517 kilowatts. Equipment capacity is commonly stated in tons.

Manual J

ACCA residential load calculation procedure. It estimates heating and cooling loads from envelope, infiltration, internal gains, and design conditions; Manual S then selects equipment.

Design conditions

Outdoor and indoor temperature and humidity values used for load calculation, typically ASHRAE 1% cooling and 99% heating design values adopted by local code.

Conduction load formula

Q equals U times area times temperature difference, where U equals 1 divided by R. Used for walls, roofs, windows, and doors in heating and cooling load estimates.

Equipment oversizing effects

Excess capacity causes short cycling, poor latent removal, higher indoor humidity, and greater energy use. Select equipment close to the calculated load rather than by rule of thumb.

AFUE

Annual Fuel Utilization Efficiency, the seasonal efficiency rating for furnaces and boilers. Values of 90 percent or more generally indicate condensing equipment with PVC venting.

Hydronic heat output

Boiler output in Btu per hour equals GPM times 500 times water temperature drop. One GPM at a 20 F drop delivers about 10,000 Btu per hour.

Heat pump balance point

Outdoor temperature where heat pump capacity equals building heating load. Below this point supplemental heat operates to maintain indoor temperature.

Condensing furnace

A furnace that recovers latent heat from flue gas, reaching 90 percent or higher AFUE. It produces acidic condensate, uses PVC venting, and requires a drain.

Vapor-compression cycle

Sequence of compressor, condenser, expansion device, and evaporator. Refrigerant absorbs heat in the evaporator and rejects it in the condenser while the compressor raises pressure.

Superheat

Evaporator outlet temperature minus evaporating saturation temperature. It confirms the evaporator is fully flooded with vapor and is the charging method for fixed-orifice systems.

Subcooling

Condensing saturation temperature minus liquid line temperature. It indicates liquid refrigerant is present at the expansion device and is the charging method for TXV systems.

TXV operation

A thermostatic expansion valve modulates refrigerant flow to hold constant superheat at the evaporator outlet, responding to load changes and maintaining evaporator efficiency.

Compression ratio

Absolute discharge pressure divided by absolute suction pressure. High ratios reduce capacity and stress the compressor; ratios above about 10 to 1 are usually excessive.

Equal friction duct sizing

A duct design method that selects sizes for a constant pressure drop per 100 feet, commonly 0.1 inch water column, producing a self-balancing low-noise layout.

External static pressure

Total resistance the blower works against, including supply and return ducts plus coils and filters. It is compared to the manufacturer fan curve to verify airflow.

ASHRAE 62.2

Residential ventilation standard specifying whole-house mechanical ventilation rates based on floor area and occupants, plus local exhaust rates for bathrooms and kitchens.

Duct leakage class

SMACNA classification of allowable duct leakage as a percentage of design airflow. Sealing joints and minimizing leakage to outside improves efficiency and comfort.

24V thermostat terminals

Standard control terminals: R is 24V hot, C is common, W calls heat, Y calls cooling, G runs the fan, and O or B energizes the reversing valve.

Lockout tagout

Safety procedure that isolates energy sources, verifies de-energization with a meter, and prevents accidental startup during service on HVAC electrical equipment.

RLA and LRA

Rated load amps and locked rotor amps from the compressor nameplate. RLA guides running amp checks; LRA indicates inrush current at startup.

CFM per ton

Nominal cooling airflow target of about 400 CFM per ton, typically 350 to 450. Low airflow causes frozen coils, low suction pressure, and poor humidity control.

Charge verification method

Use superheat for fixed-orifice systems and subcooling for TXV systems. Verify airflow and static pressure first, since airflow errors distort charge readings.

Frozen evaporator causes

Low airflow from dirty filters or blower faults, low refrigerant charge, or restricted metering devices. Ice forms because coil temperature falls below freezing.

High head pressure causes

Dirty condenser coil, overcharge, non-condensable gases, or high outdoor ambient temperature. Head pressure rises and capacity and efficiency fall.

SEER2 and HSPF2

Current efficiency ratings for cooling and heat pump heating under updated test procedures. Values are generally lower than legacy SEER and HSPF for the same equipment.

A2L refrigerant

Lower-flammability refrigerant classification such as R-32 and R-454B. Handling requires updated leak detection, ventilation, and charging procedures per ASHRAE 15 and 34.

ASHRAE 90.1

Energy standard for commercial buildings covering envelope, lighting, HVAC efficiency, economizers, and controls. Local codes adopt or amend it as the minimum requirement.