When a technician names a part or an estimate uses a word you do not know, the definition matters because it changes what you ask next. These entries connect the term to the system, the symptom you may notice, and the point where testing is more useful than guessing.
Start with the parts and problems homeowners most often hear about when comfort, airflow, controls, or equipment decisions change.
An air handler is the indoor cabinet that moves conditioned air through the ductwork. It commonly contains the blower, filter area, indoor coil, drain pan, condensate drain, controls, and sometimes electric heat strips.
Read the definitionA heat pump is an electric HVAC system that moves heat instead of creating it through combustion. It can heat and cool one home by reversing the direction of the refrigerant cycle.
Read the definitionIndoor air quality, or IAQ, describes how the air inside a home affects comfort, health, and the building. It includes pollutants, moisture, ventilation, filtration, and the way HVAC equipment moves and conditions air.
Read the definitionAn HVAC maintenance tune-up is a planned inspection and service visit for a heating or cooling system that is still operating. It checks the parts that affect safe operation, airflow, drainage, comfort, and equipment strain before heavy seasonal use.
Read the definitionRefrigerant is the fluid inside an air conditioner or heat pump that moves heat. It changes between a liquid and a gas through a sealed circuit, absorbing heat in one place and releasing it in another.
Read the definitionShort cycling is when a heating or cooling system starts, stops, and starts again before completing a normal cycle. The pattern can leave rooms uneven, humid, or uncomfortable while adding repeated startup stress to the equipment.
Read the definitionUse the category links to narrow the list, or scan by letter when you already have the term in front of you.
AFUE rating measures how much of a fuel-burning furnace or boiler's energy becomes useful heat over a heating season. A higher rating usually means less fuel energy is lost at the equipment, but it does not describe the whole HVAC system or promise a matching utility-bill reduction.
Read definitionAn air filter is the removable panel in a forced-air heating or cooling system that catches some airborne particles before air reaches the equipment. A wrong-size, overly restrictive, or dirty filter can create HVAC problems, not just dusty air.
Read definitionAn air handler is the indoor cabinet that moves conditioned air through the ductwork. It commonly contains the blower, filter area, indoor coil, drain pan, condensate drain, controls, and sometimes electric heat strips.
Read definitionAn air purifier is a device that filters particles and, depending on its design, some gases from indoor air. Portable models serve a room; whole-home models connect to the HVAC system and must match its airflow.
Read definitionAirflow is the movement and amount of air traveling through your HVAC equipment, ducts, and registers. It carries heated or cooled air to the rooms and returns room air to the system.
Read definitionAuxiliary heat is backup heat that helps a heat pump during cold weather or recovery. It is useful, but frequent use can raise electric bills.
Read definitionA blower motor is the electric motor that drives the indoor fan in your heating and cooling system. When it struggles, the equipment may heat or cool inside the cabinet but fail to move that comfort into the rooms.
Read definitionA boiler heats water or produces steam, then sends that heat through radiators, baseboards, radiant floor tubing, or a coil in an air handler. It does not heat and push air through ducts like a furnace.
Read definitionA Btu, or British thermal unit, measures heat energy. In HVAC, Btu per hour describes the rate at which a furnace, air conditioner, or heat pump can add or remove heat.
Read definitionAn HVAC capacitor stores and releases electrical energy to help a compressor or fan motor start and keep running. A symptom can point to a weak capacitor, but it does not prove that the capacitor has failed.
Read definitionAn HVAC compressor is the motor-driven pump that pressurizes and circulates refrigerant through an air conditioner or heat pump. Diagnosis matters before a repair or replacement decision is made.
Read definitionA condensate drain carries water away from the indoor coil after the air conditioner removes moisture from the air. A clog, leak, or damaged pan can send water back toward the equipment or home.
Read definitionThe condenser coil is the outdoor coil that releases heat from your home. Dirt, leaves, and poor airflow around it can make the AC run longer and cool less.
Read definitionAn HVAC contactor is an electrically controlled switch that sends power to major outdoor-unit components such as the compressor and condenser fan. It responds to the thermostat control signal, but a contactor symptom can have several other causes.
Read definitionA damper is a control device inside ductwork that regulates the flow of heated or cooled air. It can allow more air through, restrict airflow, or direct conditioned air toward a particular part of the home.
Read definitionA defrost cycle is a short, automatic heat-pump process that warms the outdoor coil to melt frost during heating. Steam, dripping water, a stopped fan, or a brief change in indoor air can be normal while the cycle runs.
Read definitionA dehumidifier removes water vapor from indoor air. It can support comfort when the air conditioner alone does not control humidity well enough.
Read definitionAn HVAC drain pan is a shallow tray that catches water produced at the indoor evaporator coil and directs it into a condensate drain. A cracked, rusted, or overflowing pan can damage the equipment or the home.
Read definitionA ductless mini-split is an HVAC system that heats or cools a room without sending air through a central network of ducts. An outdoor unit connects to one or more indoor wall, ceiling, or floor air handlers.
Read definitionDuctwork is the network of air passages that carries heated or cooled air between HVAC equipment and the rooms in a home. Leaks, restrictions, poor support, or bad routing can waste comfort even when the equipment itself is working.
Read definitionAn electric furnace is a central heating appliance that uses electric resistance elements to warm air, then a blower sends that air through the home’s ductwork. It does not burn gas, propane, or oil.
Read definitionAn evaporator coil is the indoor part of an air conditioner or heat pump that absorbs heat and moisture from the air in your home. In many systems it is shaped like an A, so you may hear it called an A coil.
Read definitionAn expansion valve is the refrigerant metering device that controls how much liquid refrigerant enters an evaporator coil. It creates a pressure drop so the refrigerant can absorb heat without sending liquid back to the compressor.
Read definitionA flame sensor is a small safety component near the burner in a gas furnace. It confirms that the flame is burning so the control can keep the gas valve open; without that proof, the furnace shuts off the gas.
Read definitionA float switch is a small HVAC safety device that watches condensate water and can stop the system before a drain pan overflows. A trip is a warning about the water path, not proof that the switch itself has failed.
Read definitionA frozen coil is an indoor HVAC coil covered with frost or ice when it should be moving heat normally. Ice is a warning sign, not a diagnosis: restricted airflow, low refrigerant, a blower problem, a dirty coil, or another fault may be involved.
Read definitionA frozen coil means ice has formed on the indoor cooling coil or nearby refrigerant line. It is usually a symptom of airflow trouble, coil dirt, or a refrigerant-side problem.
Read definitionA furnace is a heating appliance that warms air and sends it through ductwork. It may use natural gas, propane, heating oil, or electricity, and most household furnaces are forced-air systems.
Read definitionA gas furnace is a fuel-burning heating system that warms air and sends it through the ductwork in your home. Most residential units use natural gas, although some use propane. Because combustion is involved, gas furnace problems carry safety concerns beyond comfort.
Read definitionA heat exchanger transfers heat between combustion gases and household air in a furnace, or between refrigerant and air in an air conditioner or heat pump. In a fuel-burning furnace, it must keep combustion byproducts out of the supply air.
Read definitionA heat pump is an electric HVAC system that moves heat instead of creating it through combustion. It can heat and cool one home by reversing the direction of the refrigerant cycle.
Read definitionAn HSPF rating describes how efficiently an air-source heat pump provides heat over a standardized heating season. HSPF2 is the updated rating commonly shown on current equipment information.
Read definitionA humidifier adds moisture to indoor air. Portable units serve one room, while whole-home humidifiers can connect to a furnace or air handler and distribute moisture through the ductwork.
Read definitionAn HVAC ignitor is the part in a fuel-burning furnace that starts the fire needed to make heat. A furnace can show an ignitor symptom while the actual fault is elsewhere in the ignition, safety, fuel, airflow, or venting sequence.
Read definitionIndoor air quality, or IAQ, describes how the air inside a home affects comfort, health, and the building. It includes pollutants, moisture, ventilation, filtration, and the way HVAC equipment moves and conditions air.
Read definitionA furnace limit switch is a temperature-sensitive safety control. It can interrupt burner operation when the furnace gets too hot and may keep the blower running to remove heat.
Read definitionAn HVAC maintenance tune-up is a planned inspection and service visit for a heating or cooling system that is still operating. It checks the parts that affect safe operation, airflow, drainage, comfort, and equipment strain before heavy seasonal use.
Read definitionMERV rating measures how well an air filter captures particles. Higher is not always better if the HVAC system cannot move enough air through the filter.
Read definitionRefrigerant is the fluid inside an air conditioner or heat pump that moves heat. It changes between a liquid and a gas through a sealed circuit, absorbing heat in one place and releasing it in another.
Read definitionRefrigerant lines are the insulated copper tubes that connect the indoor and outdoor parts of a split air conditioner or heat pump. They carry refrigerant so the system can move heat.
Read definitionReturn air is the air pulled out of the rooms in your home and sent back to the furnace, air handler, or air conditioner. The equipment can then filter, heat, or cool that air before sending it back through the supply vents.
Read definitionSEER stands for Seasonal Energy Efficiency Ratio. It compares rated cooling delivered with electricity used across a defined cooling season for air conditioners and heat pumps.
Read definitionShort cycling is when a heating or cooling system starts, stops, and starts again before completing a normal cycle. The pattern can leave rooms uneven, humid, or uncomfortable while adding repeated startup stress to the equipment.
Read definitionStatic pressure is the resistance the blower faces while moving air through an HVAC system. Filters, coils, ducts, grilles, and zoning controls all shape that resistance, so a reading is a diagnostic clue rather than a failed-part diagnosis.
Read definitionSupply air is the air an HVAC system delivers into a room or other conditioned space through a supply register or grille. It may be heated, cooled, filtered, humidified, dehumidified, or mixed with outdoor or recirculated air, depending on the system.
Read definitionA thermostat measures the air around it, compares that reading with your selected temperature, and tells the HVAC system when to heat, cool, or run the fan. It requests comfort; the furnace, air conditioner, heat pump, and ductwork do the physical work.
Read definitionA ton is a measure of cooling capacity, not equipment weight. One ton equals 12,000 Btu per hour of nominal cooling capacity.
Read definitionTXV stands for thermostatic expansion valve. It is the refrigerant metering device that controls how much liquid refrigerant enters an evaporator coil as the cooling load changes.
Read definitionA UV light is an indoor air quality accessory that uses ultraviolet energy as an air or surface treatment. In residential HVAC, it may be installed inside the equipment to expose moving air or selected surfaces to UV energy.
Read definitionVentilation is the exchange of indoor air and outdoor air. It can bring outdoor air in or exhaust indoor air so odors, pollutants, and moisture do not stay concentrated inside.
Read definitionA zoning system divides a home into separately controlled comfort areas. It usually uses multiple thermostats and motorized dampers inside the ductwork.
Read definitionTell us the word, the system it appeared on, and what changed at home. We can connect the definition to the practical repair question.