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HVAC glossary
The terms on quotes, equipment labels and in our guides, defined in plain language.
- A2L refrigerant
- A safety classification for refrigerants with lower toxicity and lower flammability. Newer lower-impact refrigerants such as R-454B are A2L, and equipment using them follows specific installation requirements. Repair or Replace? A Practical Framework for an Aging HVAC System
- AFUE
- Annual fuel utilization efficiency. It describes how much of a furnace's or boiler's fuel becomes usable heat over a season, expressed as a share of the fuel burned. The rest leaves through the flue. Heat Pump vs. Furnace in the Sierra Foothills: What Elevation Changes Day & Night systems
- Air handler
- An indoor unit containing a blower and a coil, used with a heat pump or air conditioner when there is no furnace. It often includes electric auxiliary heat. Day & Night Systems Explained: Furnaces, Heat Pumps and Air Handlers
- Auxiliary heat
- Backup heat, usually electric resistance strips in the air handler, that supplements a heat pump in very cold weather or during defrost. It is effective but costly to run, so it should support the heat pump, not replace it. Cold-Climate Heat Pumps at 6,000 Feet: What They Can and Can't Do
- Boiler
- An appliance that heats water for a hydronic system. Boilers can run on natural gas, propane, oil or electricity, and high-efficiency condensing boilers pair well with low-temperature radiant floors. Radiant & hydronic heating Radiant heating repair
- BTU
- British thermal unit, the standard measure of heat energy. Heating and cooling equipment capacity is usually stated in BTUs per hour. HVAC replacement
- CFM
- Cubic feet per minute, the measure of how much air a blower or duct moves. Correct airflow in CFM is essential for both comfort and equipment performance. Wildfire Smoke and Indoor Air: Filtration and Fresh-Air Options
- Cold-climate heat pump
- A heat pump engineered to keep producing useful heat at low outdoor temperatures, usually with a variable-speed inverter compressor. It still loses some capacity as it gets colder, so it must be sized for the local design temperature. Cold-Climate Heat Pumps at 6,000 Feet: What They Can and Can't Do Heat pumps
- Combustion analysis
- A test using an electronic analyzer to measure exhaust gases from a fuel-burning appliance. It shows whether the burner is operating safely and efficiently, which matters at elevation where air is thinner. HVAC repair Heat Pump vs. Furnace in the Sierra Foothills: What Elevation Changes
- Condensing furnace
- A high-efficiency furnace with a secondary heat exchanger that captures heat from exhaust gases. That process produces water, so it needs a condensate drain and typically plastic venting. Day & Night Systems Explained: Furnaces, Heat Pumps and Air Handlers
- Defrost cycle
- A normal heat pump function that melts frost from the outdoor coil in cold, humid weather by briefly reversing the refrigerant flow. A coil that stays solidly iced over points to a fault. Cold-Climate Heat Pumps at 6,000 Feet: What They Can and Can't Do Heat pump repair
- Design temperature
- The cold winter and hot summer outdoor temperatures a system is sized to handle at a specific location. It is much colder in Truckee than in Grass Valley or Rocklin. Heat Pump vs. Furnace in the Sierra Foothills: What Elevation Changes HVAC in Truckee
- Dual fuel
- A system that pairs a heat pump with a gas or propane furnace. The heat pump heats in milder weather, and the furnace takes over below a set outdoor temperature. Heat Pump vs. Furnace in the Sierra Foothills: What Elevation Changes Heat pumps
- Ducted mini-split
- A mini-split with a compact, concealed air handler that serves a few rooms through short duct runs, offering zoned inverter comfort without visible wall units. Ducted vs. Ductless Mini-Split Systems for Foothill and Mountain Homes Mitsubishi systems
- EER2
- Energy efficiency ratio under the updated federal test procedure. Unlike SEER2, it rates cooling efficiency at a single hot outdoor condition, which makes it useful for comparing performance on the hottest days. HVAC replacement
- ERV and HRV
- Energy recovery and heat recovery ventilators bring in fresh outdoor air while exhausting stale air, transferring heat between the two streams. An ERV also transfers some moisture. Air quality installation Wildfire Smoke and Indoor Air: Filtration and Fresh-Air Options
- Evaporator coil
- The indoor coil, usually on top of a furnace or inside an air handler, where refrigerant absorbs heat from the air passing over it during cooling. In a heat pump it works in reverse in heating mode. Day & Night Systems Explained: Furnaces, Heat Pumps and Air Handlers
- Flue and venting
- The pipe system that carries combustion gases from a furnace, boiler or water heater safely outdoors. Venting must be the correct type, size and slope for the appliance. HVAC repair
- Furnace
- An appliance that heats air by burning natural gas or propane, or with electric resistance, and uses a blower to move that air through ducts. Day & Night Systems Explained: Furnaces, Heat Pumps and Air Handlers HVAC repair
- Heat exchanger
- The sealed metal chamber in a furnace or boiler that transfers heat from combustion to air or water while keeping combustion gases separate. A cracked heat exchanger is a safety concern. Repair or Replace? A Practical Framework for an Aging HVAC System HVAC repair
- Heat pump
- A system that uses refrigerant to move heat rather than make it. It pulls heat from outdoor air into the house in winter and reverses to cool the house in summer. Heat pumps Heat Pump vs. Furnace in the Sierra Foothills: What Elevation Changes
- HEPA
- High-efficiency particulate air, a very high filtration standard. HEPA filters are common in portable air cleaners but are usually too restrictive for a standard residential HVAC blower. Wildfire Smoke and Indoor Air: Filtration and Fresh-Air Options
- HSPF2
- Heating seasonal performance factor under the updated federal test procedure. It rates a heat pump's heating efficiency over a season; a higher number means more heat for the same electricity. Heat pumps Heat Pump vs. Furnace in the Sierra Foothills: What Elevation Changes
- Humidifier and dehumidifier
- A whole-home humidifier adds moisture to dry winter air; a dehumidifier removes excess moisture. Both are controlled to keep indoor relative humidity in a comfortable range. Indoor air quality
- Hydronic heating
- A heating system that circulates heated water through pipes to radiant floors, radiators, baseboard units or a water-to-air coil. Radiant & hydronic heating Radiant and Hydronic Heat vs. Forced Air for Mountain Homes
- Line set
- The copper refrigerant lines, typically with the larger line insulated, that connect an outdoor unit to an indoor coil or mini-split head. Ducted vs. Ductless Mini-Split Systems for Foothill and Mountain Homes
- Manifold
- The hydronic component that divides heated water into separate loops and collects it on the return, often with valves and flow meters for balancing each loop. Radiant heating maintenance
- Manual D
- The ACCA method for designing residential duct systems. It sizes supply and return ducts so each room gets the airflow the load calculation calls for. Ducted vs. Ductless Mini-Split Systems for Foothill and Mountain Homes
- Manual J load calculation
- The industry-standard method, published by ACCA, for calculating how much heating and cooling a home needs. It accounts for insulation, windows, orientation, air leakage and the local design temperature. HVAC replacement Heat Pump vs. Furnace in the Sierra Foothills: What Elevation Changes
- MERV rating
- Minimum efficiency reporting value, a scale describing how well an air filter captures particles of different sizes. Higher MERV filters capture more fine particles but can restrict airflow. Wildfire Smoke and Indoor Air: Filtration and Fresh-Air Options Indoor air quality
- Mini-split
- A heat pump system with an outdoor unit connected by refrigerant lines to one or more indoor units, usually without ducts. Each indoor unit can be controlled as its own zone. Ducted vs. Ductless Mini-Split Systems for Foothill and Mountain Homes Mitsubishi systems
- Mixing valve
- A valve that blends hot supply water with cooler return water so a radiant floor receives water at a safe, lower temperature than the boiler produces. Radiant & hydronic heating Radiant and Hydronic Heat vs. Forced Air for Mountain Homes
- R-22
- An older refrigerant used in air conditioners and heat pumps. It is no longer produced or imported in the US, so servicing an R-22 system depends on limited reclaimed or stockpiled supply. Repair or Replace? A Practical Framework for an Aging HVAC System
- R-410A
- A refrigerant widely used in air conditioners and heat pumps since R-22 was phased out. It is now being phased down for new equipment in favor of lower-impact refrigerants. Repair or Replace? A Practical Framework for an Aging HVAC System
- R-454B
- A lower global-warming-potential A2L refrigerant used in many newer air conditioners and heat pumps as a replacement for R-410A. Repair or Replace? A Practical Framework for an Aging HVAC System
- Radiant floor
- A heating method using tubing in or under the floor to warm the floor surface, which then warms people and objects in the room. It is quiet and even, and suits tall ceilings. Radiant heating installation Radiant and Hydronic Heat vs. Forced Air for Mountain Homes
- Refrigerant charge
- The amount of refrigerant in a system. Too much or too little reduces capacity and efficiency, and a low charge usually means a leak that should be found and repaired. Heat pump maintenance
- SEER2
- Seasonal energy efficiency ratio, measured under the updated federal test procedure. It rates an air conditioner's or heat pump's cooling efficiency over a typical season; a higher number means less electricity for the same cooling. Heat pumps HVAC replacement
- Static pressure
- The pressure a blower works against as air moves through the ducts, coil and filter, measured in inches of water column. High static pressure, often from undersized returns or restrictive filters, reduces airflow and strains the blower. Wildfire Smoke and Indoor Air: Filtration and Fresh-Air Options Air quality testing
- Thermostat
- The control that tells a heating and cooling system when to run. Heat pumps and dual-fuel systems need thermostats configured for their staging and changeover settings. HVAC repair
- Ton (cooling capacity)
- A unit of cooling capacity equal to 12,000 BTUs per hour. It describes how much heat a system can remove, not the weight of the equipment. HVAC replacement
- Variable-speed compressor
- A compressor, often called inverter-driven, that adjusts its speed to match the load instead of cycling fully on and off. It gives steadier temperatures and better part-load efficiency. Cold-Climate Heat Pumps at 6,000 Feet: What They Can and Can't Do Mitsubishi systems
- Zoning
- Dividing a home into areas that are heated and cooled independently, each with its own thermostat. Zoning can use duct dampers, separate hydronic loops or individual mini-split heads. Ducted vs. Ductless Mini-Split Systems for Foothill and Mountain Homes Mitsubishi systems
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