HVAC Glossary
Common HVAC terms and definitions. Click any linked calculator to put these concepts into practice.
BTU (British Thermal Unit)
A unit of heat energy. One BTU is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. In HVAC, BTUs measure the heating or cooling capacity of equipment.
SEER (Seasonal Energy Efficiency Ratio)
A measure of air conditioning efficiency. SEER is calculated by dividing the cooling output (in BTUs) by the energy input (in watt-hours) over a typical cooling season. Higher SEER = more efficient.
MERV (Minimum Efficiency Reporting Value)
A rating system for air filters that measures how effectively they capture particles. MERV ratings range from 1-20, with higher numbers indicating finer filtration. Most residential systems use MERV 8-13.
Tonnage
A measure of cooling capacity. One ton of cooling equals 12,000 BTU per hour. The term originates from the amount of heat needed to melt one ton of ice in 24 hours. Residential AC units typically range from 1.5 to 5 tons.
CFM (Cubic Feet per Minute)
A measurement of airflow volume. CFM tells you how many cubic feet of air pass a given point in one minute. Proper CFM is essential for ventilation, duct sizing, and equipment selection.
R-Value
A measure of thermal resistance in insulation. Higher R-values indicate better insulating properties. R-value requirements vary by climate zone, with colder regions needing higher values.
ASHRAE
The American Society of Heating, Refrigerating and Air-Conditioning Engineers. ASHRAE publishes standards for HVAC design, including ventilation rates (Standard 62.2) and energy efficiency requirements.
AFUE (Annual Fuel Utilization Efficiency)
A measure of furnace efficiency expressed as a percentage. An AFUE of 95% means 95 cents of every dollar of fuel becomes heat. Modern high-efficiency furnaces achieve 95-98% AFUE.
HSPF (Heating Seasonal Performance Factor)
A measure of heat pump heating efficiency. HSPF is the total heating output (BTU) divided by total electricity used (watt-hours) during a heating season. Higher HSPF = more efficient heating.
Static Pressure
The resistance to airflow in an HVAC duct system, measured in inches of water column (in. w.c.). Excessive static pressure reduces efficiency and airflow. Typical residential systems are designed for 0.5 in. w.c.
Refrigerant
The chemical compound that cycles through an AC or heat pump system, absorbing and releasing heat. Common types include R-410A (current standard) and R-22 (phased out). Handling requires EPA certification.
Heat Load
The amount of heating energy (BTU/hr) needed to maintain a desired indoor temperature. Heat load calculations consider building envelope, insulation, windows, infiltration, and outdoor design temperature.
Design Temperature
The extreme outdoor temperature used for HVAC sizing. Heating design temp is the 99% winter design condition; cooling design temp is the 1% summer condition. These are not record temperatures but statistical values.
Ductwork
The system of ducts (channels) that distribute conditioned air throughout a building. Ducts can be made of sheet metal, fiberglass, or flexible material. Proper sizing and sealing are critical for efficiency.
Mini Split
A ductless HVAC system with an outdoor compressor and one or more indoor air handlers. Mini splits are efficient, easy to install, and allow zone control. They are ideal for room additions or homes without ductwork.
Superheat
The temperature difference between refrigerant vapor and its boiling point. HVAC technicians measure superheat to verify proper refrigerant charge. Typical superheat ranges from 10-15°F at the evaporator.
Subcooling
The temperature difference between liquid refrigerant and its condensing point. Like superheat, subcooling is used to verify refrigerant charge. Typical subcooling ranges from 8-12°F at the condenser.
Air Changes per Hour (ACH)
The number of times the entire volume of air in a room is replaced in one hour. Proper ACH ensures adequate ventilation. Typical requirements: bathrooms 8+ ACH, kitchens 7+ ACH, bedrooms 4+ ACH.
EER (Energy Efficiency Ratio)
The ratio of cooling capacity (BTU/hr) to power input (watts) at a specific outdoor temperature (usually 95°F). Unlike SEER, EER measures efficiency at a single condition. Higher EER = more efficient at peak load.
Load Calculation
The process of determining how much heating and cooling capacity a building needs. Manual J is the industry standard method. Proper load calculations prevent oversizing and undersizing of HVAC equipment.