Plain-English definitions for every HVAC term you'll hit.
37 terms, organized by category — search below or browse a section. Looking for a deeper guide instead? Visit the Learning Center.
Air Conditioning
Condenser
The condenser is the outdoor unit of a central air conditioning or heat pump system. It releases heat collected from inside the home into the outdoor air, using a fan, a coil, and a compressor working together to complete the refrigerant cycle.
Evaporator Coil
The evaporator coil is the indoor coil, usually mounted above or near the furnace, where refrigerant absorbs heat from indoor air as it passes over the coil. That heat absorption is what cools the air before it's circulated back through the home's ductwork.
R-454B / A2L Refrigerant
R-454B is one of the newer, lower-global-warming-potential refrigerants that the HVAC industry is transitioning to under the EPA's AIM Act phasedown of R-410A. It's classified A2L (mildly flammable), which is why new equipment using it has different handling and installation requirements than older R-410A systems.
Deep diveRefrigerant
Refrigerant is the chemical compound that circulates between the indoor and outdoor units of an AC or heat pump, absorbing heat indoors and releasing it outdoors as it changes between liquid and gas states. It's the working fluid that makes the whole cooling cycle possible.
SEER2
SEER2 is the current U.S. Department of Energy testing standard for how efficiently a central air conditioner or heat pump cools a home over a season. It replaced the older SEER standard in 2023 using more realistic duct-pressure test conditions. The higher the SEER2 number, the more cooling a system delivers per unit of electricity consumed.
Deep diveTonnage (Cooling Capacity)
Tonnage describes an air conditioner or heat pump's cooling capacity — one ton equals 12,000 BTUs of heat removal per hour. Residential systems commonly range from 1.5 to 5 tons depending on home size, insulation, and layout.
Heating
AFUE
AFUE stands for Annual Fuel Utilization Efficiency — the standard U.S. measurement of how much of the fuel a furnace burns actually turns into usable heat, expressed as a percentage. A 90% AFUE furnace converts 90% of its fuel into heat delivered to your home, with the rest lost mainly through exhaust venting.
Deep diveCondensate Line
The condensate line is the tubing that drains moisture produced by a high-efficiency furnace, or by an air conditioner's evaporator coil, safely out of the home. On high-efficiency furnaces, it carries away the water byproduct of extracting extra heat from exhaust gases.
Flame Sensor
A flame sensor is a safety component that confirms a furnace's burner has actually ignited. If it doesn't detect a flame within a set time, it shuts the gas valve to prevent unburned gas from accumulating — a common cause of a furnace that tries to start and then shuts back off.
Heat Exchanger
The heat exchanger is the component inside a furnace that transfers heat from combustion gases to the air circulating through your home's ductwork, without letting the two air streams mix. It's a core safety component, not just an efficiency one.
Short-Cycling
Short-cycling is when a heating or cooling system turns on and off more frequently than it should, rather than running a full, normal cycle. It's typically caused by incorrect sizing, a dirty filter, or a thermostat or airflow problem, and it reduces both comfort and equipment lifespan.
Deep diveBoilers
Bleeding a Radiator
Bleeding a radiator means releasing trapped air from inside it, using a small valve, so hot water can fill the radiator completely. Trapped air causes cold spots — typically at the top of the radiator — even though the boiler and circulator pump are working normally.
Circulator Pump
The circulator pump moves hot water from the boiler through the piping system to radiators or baseboard heaters throughout the home, then back to the boiler to be reheated. It's the component that actually distributes the heat the boiler generates.
Kettling
Kettling is a rumbling, gurgling, or banging noise from a boiler, typically caused by mineral scale or sediment buildup restricting water flow inside the heat exchanger and creating trapped steam pockets. It's named for the sound resembling a kettle coming to a boil.
Deep diveRadiant Heat
Radiant heat is a heating method that warms objects and people directly through radiators or in-floor piping, rather than heating and circulating air the way a furnace does. Boiler-based systems in older homes typically deliver heat this way through cast-iron radiators.
Ductless & Mini-Splits
Ductless Mini-Split
A ductless mini-split is a heating and cooling system that conditions individual rooms or zones without central ductwork, using an outdoor compressor connected to one or more indoor air-handling units. Each indoor unit can be controlled independently for zone-by-zone temperature control.
Deep diveIndoor Air-Handling Unit (Head Unit)
The indoor air-handling unit, sometimes called a head unit, is the wall-, ceiling-, or floor-mounted component of a ductless mini-split system installed inside a room. It contains a coil and fan that condition and circulate air for that specific zone.
Line Set
A line set is the pair of small refrigerant lines that connect a ductless mini-split's outdoor compressor to its indoor air-handling unit(s). It's what allows the two components to be installed apart from each other without central ductwork.
Thermostat Zoning
Thermostat zoning divides a home into separate temperature zones, each controlled independently, rather than relying on a single thermostat for the whole house. Ductless mini-split systems achieve this naturally since each indoor unit has its own control.
Indoor Air Quality
ERV (Energy Recovery Ventilator)
An ERV (Energy Recovery Ventilator) works like an HRV — exchanging stale indoor air for fresh outdoor air — but also transfers some humidity between the two air streams, not just heat. That makes it useful in climates with both hot-humid and cold-dry seasons, like Chicago.
HEPA Filter
A HEPA (High-Efficiency Particulate Air) filter is designed to capture a very high percentage of small airborne particles. True whole-home HEPA filtration for ducted HVAC systems typically requires a dedicated bypass unit, since standard duct filter slots aren't sized for full HEPA-grade media.
HRV (Heat Recovery Ventilator)
An HRV (Heat Recovery Ventilator) brings fresh outdoor air into a home while exhausting stale indoor air, recovering some of the heat from the outgoing air to precondition the incoming air. It improves ventilation without simply losing all that conditioned energy outside.
MERV Rating
MERV (Minimum Efficiency Reporting Value) rates how effectively an air filter captures particles, on a scale generally from 1 to 16 for residential use. Higher MERV filters trap smaller particles but also restrict airflow more, which matters for whole-system performance.
Deep diveUV Air Purifier
A UV air purifier installs ultraviolet lamps inside the ductwork or near the evaporator coil to help neutralize mold, bacteria, and other microorganisms as air passes through the system. It targets biological growth rather than filtering particles the way a standard filter does.
Whole-House Dehumidifier
A whole-house dehumidifier integrates with a home's HVAC ductwork to remove excess moisture from the air throughout the entire home, rather than treating one room at a time the way a portable unit does. It's controlled independently of the thermostat's heating/cooling calls.
How HVAC Works
BTU
A BTU (British Thermal Unit) is the standard unit used to measure heating and cooling capacity — specifically, how much heat a furnace can produce or an air conditioner can remove per hour. Residential equipment is commonly rated in BTUs per hour, or grouped into "tons" for cooling capacity.
Freeze-Thaw Cycle
A freeze-thaw cycle is the repeated swing across the freezing point that's typical of the Western Suburbs of Chicago's humid-continental climate, rather than a steady, consistently cold winter. That repeated expansion and contraction places extra mechanical stress on outdoor equipment and condensate drainage.
Heat Pump
A heat pump is a system that can both heat and cool a home by moving heat rather than generating it directly, reversing its refrigerant cycle depending on the season. Ductless mini-split systems are commonly heat pumps, providing both functions from one piece of equipment.
Deep diveManual J Load Calculation
A Manual J load calculation is the industry-standard method for determining exactly how much heating and cooling capacity a specific home needs, based on square footage, insulation, window area, orientation, and other factors. It's the proper way to size equipment, rather than guessing based on the old system's rating.
Static Pressure
Static pressure is the resistance to airflow inside ductwork, influenced by duct size, length, and restrictions like closed vents or dirty filters. It's a factor in both overall system efficiency and how modern testing standards, like SEER2, are measured.
Thermostat
A thermostat is the control device that reads a home's indoor temperature and signals the HVAC system to heat or cool as needed to reach a setpoint. Modern smart thermostats add scheduling, remote control, and usage data on top of that basic function.
Money, Code & Permits
AHJ (Authority Having Jurisdiction)
AHJ (Authority Having Jurisdiction) is the local building department or agency responsible for enforcing code and issuing permits in a given municipality or county. Each of WeaveAire's 20 service-area towns has its own AHJ, or falls under DuPage, Cook, or Will County jurisdiction.
Amana Authorized Dealer
An Amana authorized dealer is a contractor trained and certified by Amana to install and service its equipment lines. WeaveAire INC holds this designation, meaning its technicians are trained specifically on the Amana equipment they install and service.
Federal Tax Credit (25C)
The 25C tax credit, formally the Energy Efficient Home Improvement Credit, is a federal tax provision that has offered credits for qualifying high-efficiency home HVAC and heat pump equipment. Current dollar caps and percentages change, so confirm specifics with a tax professional or IRS.gov rather than relying on a fixed figure here.
HVAC Financing
HVAC financing lets homeowners spread the cost of a new heating or cooling system over monthly payments rather than paying the full amount upfront. WeaveAire offers financing through Optimus; specific rates, terms, and approval criteria come from the financing provider, not WeaveAire directly.
Deep diveMechanical Permit
A mechanical permit is a permit issued by a local building department that's typically required for furnace, air conditioner, or boiler installation or replacement work. Requirements, fees, and processes vary by municipality and change over time — this is general information, not legal advice.
Deep diveUtility Rebate
A utility rebate is money back offered by an electric or gas utility — like ComEd or Nicor Gas in Illinois — for installing qualifying high-efficiency HVAC equipment. Program availability, amounts, and eligibility rules change and vary, so this page doesn't cite specific current figures.
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