AFUE
AFUE (Annual Fuel Utilization Efficiency) is the percentage of fuel a furnace turns into usable heat over a year. A 95% AFUE furnace converts 95% of its fuel to heat; the rest escapes as exhaust.
Explore HeatingHVAC Glossary
HVAC Terms, Defined
Plain-language definitions for the heating, cooling, and air-quality terms you'll run into — so you can decode estimates and compare options with confidence.
A homeowner's HVAC dictionary
Search by term or browse the list below. Each definition links to related services when it's useful — start in the Learning Center for full guides.
Showing 37 of 37 terms
All glossary terms
Air filter
An air filter traps dust, pollen, and particles to protect your equipment and improve indoor air. Most systems need a filter check every 1–3 months; a clogged filter restricts airflow and strains the system.
Explore Indoor Air QualityAir handler
An air handler is the indoor unit that circulates conditioned air through ductwork. It houses the blower and evaporator coil and is commonly paired with a heat pump in place of a furnace.
BTU
A BTU (British Thermal Unit) is the energy needed to raise one pound of water by one degree Fahrenheit. HVAC capacity is measured in BTU per hour for both heating output and cooling removal.
Capacitor
A capacitor stores and delivers the electrical jolt that starts and runs an AC's motors and compressor. A failed capacitor is a common, relatively inexpensive cause of a system that won't start.
Explore Air ConditioningCompressor
The compressor is the heart of the cooling cycle. Located in the outdoor condenser, it pressurizes refrigerant so it can release heat outdoors. A failed compressor is one of the costliest AC repairs.
Explore Air ConditioningCondensate drain
As an AC dehumidifies, water collects and exits through the condensate drain line. A clogged drain can cause water damage or shut the system off via a safety switch — a common, preventable summer service call.
Condenser
The condenser is the outdoor unit of a split system. It releases the heat collected from inside your home to the outside air, using a coil, fan, and compressor.
Explore Air ConditioningDuctless mini-split
A ductless mini-split pairs an outdoor unit with one or more wall- or ceiling-mounted indoor heads, delivering heating and cooling without ductwork. It's ideal for additions, garages, and rooms ducts can't reach.
Explore DuctlessDuctwork
Ductwork is the network of channels that distributes heated and cooled air throughout a building. Leaky, undersized, or poorly designed ducts waste energy and cause uneven temperatures, even with a high-efficiency system.
EER2
EER2 (Energy Efficiency Ratio 2) rates cooling efficiency at a single high-temperature operating point, rather than across a season. It helps compare how a system performs on the hottest days.
Explore Air ConditioningERV / HRV
An ERV (energy recovery ventilator) or HRV (heat recovery ventilator) brings in fresh outdoor air while recovering heat — and, for an ERV, moisture — from outgoing air. Both improve ventilation without wasting conditioned air.
Explore Indoor Air QualityEvaporator coil
The evaporator coil sits inside, usually above the furnace or in the air handler. Refrigerant inside it absorbs heat from indoor air, cooling and dehumidifying the air before it circulates through your home.
Explore Air ConditioningFreon
Freon is a brand name often used generically for refrigerant. R-22 Freon was phased out for environmental reasons; modern systems use newer refrigerants. "Adding Freon" really means recharging refrigerant after fixing a leak.
Explore Air ConditioningFurnace
A furnace heats air by burning gas or using electric heating elements, then a blower distributes the warm air through ducts. Efficiency is rated by AFUE, and gas models require safe venting of combustion gases.
Explore HeatingHeat exchanger
The heat exchanger is the metal component in a gas furnace that transfers combustion heat to your home's air while keeping exhaust gases separate. A cracked heat exchanger is a safety concern because it can leak carbon monoxide.
Explore HeatingHeat pump
A heat pump moves heat instead of generating it, so one unit both heats and cools. In winter it pulls heat from outdoor air into your home; in summer it reverses to remove heat.
Explore Heat PumpsHEPA filter
A HEPA (High-Efficiency Particulate Air) filter captures at least 99.97% of particles 0.3 microns in size. True HEPA is usually used in portable purifiers or dedicated systems, since it's too restrictive for most standard ductwork.
Explore Indoor Air QualityHSPF2
HSPF2 (Heating Seasonal Performance Factor 2) measures a heat pump's heating efficiency across a season. Higher HSPF2 means more heat delivered per unit of electricity. It is the heating counterpart to SEER2.
Explore Heat PumpsHumidifier & dehumidifier
A whole-home humidifier adds moisture in dry winter air, while a dehumidifier removes excess summer humidity. Balanced humidity improves comfort, protects woodwork, and can let you feel comfortable at more efficient temperatures.
Explore Indoor Air QualityIndoor air quality (IAQ)
Indoor air quality describes how clean and healthy the air inside a building is. It's affected by particles, humidity, ventilation, and pollutants. Filtration, air purifiers, and humidity control all help improve it.
Explore Indoor Air QualityLine set
A line set is the pair of insulated copper refrigerant lines connecting the outdoor condenser to the indoor coil. It carries refrigerant between the units and is a common spot for leaks if damaged.
Explore Air ConditioningLoad calculation (Manual J)
A Manual J load calculation determines exactly how much heating and cooling a home needs, factoring in size, insulation, windows, and climate. It's the right way to size equipment — oversizing and undersizing both cause problems.
Manual D
Manual D is the industry method for designing ductwork — sizing and laying out ducts so air reaches every room at the right volume. Proper duct design prevents weak airflow, noise, and uneven temperatures.
MERV rating
MERV (Minimum Efficiency Reporting Value) rates how well an air filter captures particles, from 1 to 16. Higher MERV traps smaller particles, but a too-restrictive filter can strain airflow if the system isn't designed for it.
Explore Indoor Air QualityPackaged unit
A packaged unit houses all heating and cooling components in a single outdoor cabinet, often on a roof or slab. It's common where indoor space is limited or in light-commercial buildings.
Refrigerant
Refrigerant is the chemical that absorbs and releases heat as it cycles between liquid and gas, enabling cooling. Common types include R-410A and the newer low-GWP R-454B. Low refrigerant usually signals a leak, not normal consumption.
Explore Air ConditioningSEER2
SEER2 (Seasonal Energy Efficiency Ratio 2) rates how efficiently an air conditioner or heat pump cools across a season. Higher SEER2 means lower cooling energy use. It replaced SEER in 2023 using updated, more realistic test conditions.
Explore Air ConditioningShort cycling
Short cycling is when a system turns on and off too frequently without completing a full cycle. Causes include an oversized unit, low refrigerant, or a dirty filter. It wastes energy and wears out parts.
Smart thermostat
A smart thermostat connects to Wi-Fi so you can control temperature from a phone, set schedules, and track energy use. Many learn your habits and adjust automatically to cut energy waste.
Split system
A split system separates equipment between an outdoor unit (condenser) and an indoor unit (furnace or air handler with the evaporator coil). It's the most common residential setup, as opposed to an all-in-one packaged unit.
Static pressure
Static pressure is the resistance air meets as it moves through ducts and filters, similar to blood pressure for an HVAC system. Too-high static pressure reduces airflow, hurts efficiency, and shortens equipment life.
Thermostat
A thermostat is the control that senses room temperature and signals your HVAC system to heat or cool. Types range from manual dials to programmable and smart, Wi-Fi-connected models.
Ton (cooling capacity)
A ton is a unit of cooling capacity equal to 12,000 BTU per hour. A 3-ton system removes 36,000 BTU of heat per hour. Sizing depends on a load calculation, not square footage alone.
Explore Air ConditioningTwo-stage (multi-stage)
Two-stage equipment has a low setting for mild conditions and a high setting for extremes. It runs on low most of the time, improving comfort and efficiency compared with single-stage on/off systems.
Variable-speed
Variable-speed equipment runs its motor or compressor at many speeds instead of just on or off. By running longer at lower output, it delivers steadier temperatures, quieter operation, better humidity control, and higher efficiency.
Zoning
Zoning uses dampers and multiple thermostats to control temperature independently in different areas of a home. It targets comfort to where it's needed — useful for multi-level homes, additions, and rooms that run hot or cold.
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