HVAC Glossary: Terms Every Chicago Homeowner Should Know

Efficiency Ratings

AFUE (Annual Fuel Utilization Efficiency): Measures furnace or boiler efficiency as a percentage of fuel energy converted to heat. An 80% AFUE furnace converts 80 cents of every dollar of gas into heat; 20 cents goes up the flue. Higher is better. Minimum for new furnaces in the Chicago climate zone is 80% AFUE; high-efficiency is 90%+.
SEER2 (Seasonal Energy Efficiency Ratio 2): Measures air conditioner or heat pump cooling efficiency over a full season. Higher is better. Minimum for new central AC in Illinois is 14.3 SEER2; high-efficiency is 18+ SEER2.
HSPF2 (Heating Seasonal Performance Factor 2): Measures heat pump heating efficiency. Higher is better. Minimum for new heat pumps is 7.5 HSPF2; high-efficiency is 9+ HSPF2.
EER2 (Energy Efficiency Ratio 2): Measures AC efficiency at a single operating condition (95°F outdoor, 80°F indoor). Used alongside SEER2 for some rating programs.
COP (Coefficient of Performance): Measures heat pump efficiency as a ratio of heat output to electrical input. A COP of 3.0 means the heat pump delivers 3 units of heat for every 1 unit of electricity consumed.

BTU (British Thermal Unit): The basic unit of heat energy. One BTU is the energy needed to raise one pound of water by one degree Fahrenheit. HVAC equipment capacity is measured in BTU/hour (BTUh). A 60,000 BTUh furnace produces 60,000 BTU of heat per hour.
Ton: A unit of cooling capacity. One ton = 12,000 BTUh of cooling. A 3-ton air conditioner removes 36,000 BTU of heat per hour. Most Chicago homes need 2–4 tons of cooling capacity.
Manual J: The industry-standard heat loss/heat gain calculation method (ACCA Manual J). Used to properly size HVAC equipment for a specific home. A Manual J calculation accounts for square footage, insulation, windows, ceiling height, and local climate. Proper sizing prevents oversizing and short-cycling.

Heat exchanger: The component in a furnace or boiler that transfers heat from combustion gases to the air or water without mixing them. A cracked heat exchanger is a safety hazard — combustion gases including carbon monoxide can enter the living space.
Evaporator coil: The indoor coil in a central AC or heat pump system. Refrigerant evaporates in the coil, absorbing heat from the indoor air. Located in the air handler or on top of the furnace.
Condenser coil: The outdoor coil in a central AC or heat pump system. Refrigerant condenses in the coil, releasing heat to the outdoor air.
Compressor: The heart of a refrigeration system. Compresses refrigerant vapor, raising its temperature and pressure. Located in the outdoor unit.
Air handler: The indoor unit in a split system that contains the blower, evaporator coil, and filter. May be a standalone unit or combined with a furnace.
Plenum: The air distribution box connected to the furnace or air handler. The supply plenum distributes conditioned air to the supply ducts; the return plenum collects return air.
TXV (Thermostatic Expansion Valve): A refrigerant metering device that controls refrigerant flow into the evaporator coil based on superheat. More precise than a fixed orifice; improves efficiency and capacity across a range of conditions.

Aquastat: The primary temperature controller for a hot water boiler. Monitors water temperature and controls the burner and circulator pump.
Pressurestat: The pressure controller for a steam boiler. Monitors steam pressure and shuts off the burner when pressure reaches the set point.
Variable speed (ECM motor): A blower motor that can operate at multiple speeds, typically 40–100% of full speed. More efficient than single-speed motors; provides better humidity control and more even temperatures.
Two-stage: Equipment that can operate at two capacity levels — typically 65% and 100%. Runs at low stage most of the time for better efficiency and comfort; switches to high stage only when needed.
Modulating: Equipment that can vary capacity continuously across a range (typically 40–100%). The most efficient and comfortable option; matches output precisely to the load.
Short-cycling: When HVAC equipment turns on and off too frequently. Usually caused by oversized equipment. Reduces efficiency, increases wear, and causes poor humidity control.

R-410A: The refrigerant used in most residential HVAC equipment manufactured between 2010 and 2024. Being phased out due to its high global warming potential (GWP).
R-32 and R-454B: Lower-GWP refrigerants replacing R-410A in new equipment. Required in new residential equipment manufactured after January 1, 2025.
R-22 (Freon): The refrigerant used in older equipment manufactured before 2010. No longer manufactured in the US. If your system uses R-22, replacement is the most cost-effective option when it needs refrigerant.