Learning Center
HVAC & Plumbing Glossary
The words you will hear from a technician, explained in plain English. Each term links to the guide that covers it and the service it relates to.
A
- A2L refrigerant
- A2L is a safety class for refrigerants that are lower in toxicity and only mildly flammable. Newer refrigerants like R-454B fall into this class. Equipment built for A2L refrigerants includes safety features such as leak sensors, and installers follow updated handling and installation practices. The U.S. EPA's limits on higher-impact HFC refrigerants drove the shift to A2L systems.
- AFUE
- AFUE, or Annual Fuel Utilization Efficiency, is the rating that shows how much of a gas furnace's fuel becomes usable heat in your home over a typical year. A furnace rated 95 percent AFUE turns about 95 cents of every fuel dollar into heat, with the rest leaving through the vent. Higher AFUE furnaces usually use sealed combustion and PVC venting.
- air handler
- An air handler is the indoor cabinet that moves air through your ductwork. It holds the blower motor, the evaporator coil and often electric heat strips, and it pairs with an outdoor heat pump or air conditioner in a split system. Homes with a gas furnace use the furnace as the air handler instead of a separate unit.
- anode rod
- An anode rod is a metal rod, usually magnesium or aluminum, that hangs inside a tank water heater. It corrodes on purpose so the steel tank lining does not. Once the rod is mostly eaten away, the tank itself starts to rust. Checking the anode rod during routine maintenance or a flush can help a tank water heater last longer.
- auxiliary heat
- Auxiliary heat, often shown as AUX on a thermostat, is the backup heat a heat pump uses when it cannot keep up alone, usually on the coldest days or during defrost. In most homes it comes from electric heat strips in the air handler. Seeing AUX now and then is normal, but if it runs constantly, the heat pump may need service.
B
- backflow preventer
- A backflow preventer is a valve assembly that lets water flow into a building but not back out into the public water supply. It protects drinking water from contamination through cross-connections such as lawn irrigation systems, commercial kitchen equipment and fire sprinkler lines. Many water utilities require certain assemblies to be tested on a schedule by a certified tester.
- BTU
- A BTU, or British thermal unit, is the amount of heat needed to raise one pound of water by one degree Fahrenheit. HVAC equipment is rated in BTUs per hour: a furnace by how much heat it delivers and an air conditioner by how much heat it removes. Twelve thousand BTUs per hour of cooling equals one ton.
C
- cleanout
- A cleanout is a capped access fitting on a drain or sewer line, often found outside near the foundation or in a crawlspace, garage or utility room. Plumbers remove the cap to run a drain snake, hydro-jetting hose or sewer camera into the line without pulling a toilet or cutting pipe. An easy-to-reach cleanout makes drain service simpler.
- compressor
- The compressor is the pump inside your outdoor air conditioner or heat pump that pressurizes refrigerant and moves it through the system. It is the heart of the cooling cycle and one of the most expensive parts to replace. Single-stage compressors run at one speed, while two-stage and variable-speed compressors can slow down for steadier comfort and humidity control.
- condenser
- The condenser is the outdoor unit of a central air conditioner. Its coil and fan release the heat that refrigerant carried out of your home, and it houses the compressor. Keeping the condenser coil clean and clear of grass, leaves and shrubs helps it shed heat, which matters during long Arkansas summers.
- cross-connection
- A cross-connection is any point where your drinking water piping links, or could link, to a source of contamination, such as a lawn irrigation system, a hose sitting in a bucket, or commercial equipment. If water pressure drops, contaminated water can be pulled backward into the supply. Backflow preventers and air gaps protect against this.
D
- descaling
- Descaling is the process of flushing a tankless water heater's heat exchanger with a cleaning solution to dissolve mineral scale. Hard water leaves calcium and magnesium deposits that can reduce efficiency, restrict flow and trigger error codes. Manufacturers such as Rinnai list descaling as routine maintenance, and how often you need it depends on your water.
- dip tube
- A dip tube is a plastic pipe inside a tank water heater that carries incoming cold water to the bottom of the tank, where it gets heated. If the dip tube cracks or breaks, cold water mixes with hot water near the top, so showers run lukewarm sooner than they should, and bits of plastic can show up in faucet screens.
- drain cleaning (snaking)
- Drain cleaning, often called snaking or cabling, uses a motorized cable with a cutting head to break up or pull out a clog in a sink, tub, toilet or main drain. It is a common first step for slow or backed-up drains. When grease, scale or roots keep coming back, plumbers may recommend a camera inspection or hydro-jetting.
- duct sealing
- Duct sealing closes leaks at duct joints, seams and connections using mastic or approved foil tape. Leaky ducts in an attic or crawlspace can lose conditioned air and pull in hot, humid or dusty air. Sealing them can help rooms heat and cool more evenly and lets your HVAC system deliver more of the air it treats.
- ductless mini-split
- A ductless mini-split is a heat pump system with an outdoor unit connected by a small line set to one or more wall, ceiling or floor units inside. Each indoor head heats and cools its own zone without ductwork. Mini-splits are common for additions, garages, sunrooms, bonus rooms and older homes without ducts.
E
- EPA Section 608
- EPA Section 608 is the part of the federal Clean Air Act that governs refrigerant in stationary air conditioning and refrigeration equipment. It prohibits intentionally venting refrigerant during service, repair or disposal, and it requires technicians who maintain, service, repair or dispose of that equipment to hold EPA Section 608 certification.
- evaporator coil
- The evaporator coil is the indoor coil, usually sitting on top of the furnace or inside the air handler, where cold refrigerant absorbs heat and moisture from the air passing over it. The moisture drips into a condensate pan and drains away. A dirty or frozen evaporator coil cuts airflow and cooling.
- expansion tank
- An expansion tank is a small tank, usually mounted on the cold water line near a water heater, that gives heated water room to expand. In homes with a closed plumbing system, such as one with a check valve or pressure-reducing valve, it helps keep pressure spikes from stressing the water heater, the T&P valve and fixtures. Local code may require one.
F
- flame sensor
- A flame sensor is a thin metal rod in a gas furnace that confirms the burners actually lit. If it does not sense a flame, the furnace shuts off the gas for safety. A dirty flame sensor is a common reason a furnace lights, runs for a few seconds and then shuts down, often repeating the cycle.
- flow rate (GPM)
- Flow rate is how much hot water a tankless water heater can deliver at once, measured in gallons per minute, or GPM. The usable rate depends on how cold the incoming water is and how hot you want it. Sizing a tankless unit means adding up the fixtures likely to run at the same time, like two showers and a dishwasher.
- fresh-air ventilation (ERV)
- Fresh-air ventilation brings outdoor air into a home in a controlled way and sends stale indoor air out. An energy recovery ventilator, or ERV, transfers some heat and moisture between the two airstreams, so incoming air needs less heating or cooling. Ventilation can help tight, newer homes that trap cooking odors and stale air.
G
- grease trap / interceptor
- A grease trap or grease interceptor is a tank that catches fats, oils and grease from commercial kitchen drains before they reach the sewer. Grease cools, hardens and clogs lines if it gets through. Traps sit near sinks, while larger interceptors are often buried outside. Restaurants need them pumped and maintained on a regular schedule, and many sewer utilities set rules for them.
H
- heat exchanger
- A heat exchanger is the metal chamber in a gas furnace where burner flames heat the metal and the blower moves house air across the outside of it, keeping combustion gases separate from the air you breathe. Cracks or holes in a heat exchanger are a safety concern because they can let combustion gases escape, which is why technicians inspect it during service.
- heat pump
- A heat pump is an all-electric system that cools your home in summer and heats it in winter by moving heat instead of burning fuel. A reversing valve switches the direction refrigerant flows. In mild Central Arkansas winters, heat pumps handle most heating on their own, with auxiliary heat strips or a gas furnace as backup.
- HSPF2
- HSPF2, or Heating Seasonal Performance Factor 2, rates how efficiently a heat pump heats over a typical season. It replaced the older HSPF rating in 2023 with a test that better reflects real duct systems, so HSPF2 numbers look lower than old HSPF numbers for similar equipment. A higher HSPF2 means less electricity for the same heat.
- hydro-jetting
- Hydro-jetting cleans drain and sewer lines with a high-pressure water hose and a specialized nozzle that scours grease, scale, sludge and soft roots off the pipe walls. Plumbers usually run a camera first to check that the pipe is sound. It is common for restaurant drains and lines that keep clogging after snaking.
I
- indoor head
- An indoor head is the indoor unit of a ductless mini-split system, usually mounted high on a wall, though ceiling and floor styles exist. Each head has its own fan, coil and remote or thermostat, so it heats and cools one room or zone. One outdoor unit can serve one head or several, depending on the model.
L
- leak detection
- Leak detection is the process of finding a hidden water leak without tearing out walls or floors at random. Plumbers use tools like acoustic listening equipment, pressure testing, thermal imaging and moisture meters to narrow down where a supply line, drain or slab pipe is leaking. Signs include a rising water bill, warm floor spots or the sound of running water.
- line set
- A line set is the pair of insulated copper refrigerant tubes that connect an outdoor unit to an indoor coil or ductless indoor head. One line carries refrigerant vapor and the other carries liquid. Line set length, size and condition matter during a replacement, and new refrigerants may require checking or replacing older lines.
M
- Manual J
- Manual J is the Air Conditioning Contractors of America (ACCA) method for calculating how much heating and cooling a home needs, room by room. It accounts for square footage, insulation, windows, orientation, air leakage and local climate. A Manual J load calculation helps size new equipment correctly instead of guessing from the old unit's size.
- MERV rating
- MERV, or Minimum Efficiency Reporting Value, rates how well an HVAC air filter catches particles, on a scale from 1 to 16 for most residential filters. Higher numbers trap smaller particles like fine dust and pollen. Very high MERV filters can restrict airflow in systems not designed for them, so filter choice should match your equipment.
P
- permit / AHJ
- A permit is permission from your local government to do regulated work, such as replacing HVAC equipment, a water heater, a gas line or a sewer line, followed by an inspection. The AHJ, or authority having jurisdiction, is the office that sets and enforces those requirements, usually your city building department. Requirements vary by city and change over time.
- PRV (pressure-reducing valve)
- A pressure-reducing valve, or PRV, sits where the water line enters a home and lowers high street pressure to a safer level for pipes, fixtures and appliances. When a PRV wears out, you may notice banging pipes, running toilets, dripping faucets or a leaking T&P valve on the water heater. Adding a PRV can make a plumbing system closed, which often calls for an expansion tank.
R
- R-410A
- R-410A is the HFC refrigerant used in most home air conditioners and heat pumps installed from the mid-2000s until the recent transition. It replaced R-22. Under EPA limits on higher-impact HFCs, new equipment is moving to lower-impact refrigerants such as R-454B. Existing R-410A systems can still be serviced and repaired.
- R-454B
- R-454B is a newer refrigerant used in many current air conditioners and heat pumps, including systems from manufacturers such as Rheem. It has a much lower global warming potential than R-410A and is classed as A2L, meaning mildly flammable, so equipment includes added safety features. R-454B systems are not designed to be charged with R-410A, or the reverse.
- refrigerant
- Refrigerant is the fluid that carries heat in air conditioners, heat pumps and ductless systems. It absorbs heat indoors at the evaporator coil and releases it outdoors at the condenser, changing between liquid and vapor as it goes. A system that needs refrigerant added has a leak, and EPA rules govern how refrigerant is handled and recovered.
- relative humidity
- Relative humidity is how much moisture the air holds compared with the most it could hold at that temperature, shown as a percentage. Many homes feel comfortable around 40 to 50 percent indoors. In humid Arkansas summers, high indoor humidity can make a home feel clammy even when the thermostat is reached, and it can encourage mold and musty odors.
- return air
- Return air is the air your HVAC system pulls back from the house through return grilles and ducts to be filtered, heated or cooled, and sent out again. Too few or undersized returns can starve the system of air, raise static pressure and leave some rooms, often upstairs bedrooms, hotter or colder than others.
- reversing valve
- A reversing valve is the part in a heat pump that switches the direction refrigerant flows, so the system can move heat out of the house in summer and into the house in winter. It also reverses during defrost. A stuck reversing valve can leave a heat pump blowing cool air when it should be heating.
- rooftop unit (RTU)
- A rooftop unit, or RTU, is a packaged heating and cooling system that sits on a commercial building's roof and holds the compressor, coils, blower and often gas heat in one cabinet. RTUs are common on offices, retail stores and restaurants. Regular maintenance covers filters, belts, coils, drains and economizers.
- RPZ valve
- An RPZ, or reduced pressure zone assembly, is a type of backflow preventer used where the contamination risk is high, such as commercial kitchens, medical offices and irrigation systems with chemical injection. It has two check valves and a relief valve that discharges water if pressure drops. RPZ assemblies are commonly tested on a schedule by a certified tester.
S
- sediment
- Sediment is the sand, scale and mineral buildup that settles at the bottom of a tank water heater over time. It can cause rumbling or popping noises, slower recovery, less hot water and extra stress on the tank. Flushing the tank on a regular schedule removes much of the sediment before it hardens.
- SEER2
- SEER2, or Seasonal Energy Efficiency Ratio 2, rates how efficiently an air conditioner or heat pump cools over a typical season. It replaced SEER in 2023 with a test that better reflects real ductwork, so SEER2 numbers run a little lower than old SEER numbers for similar equipment. Higher SEER2 means less electricity for the same cooling.
- service call fee
- A service call fee covers a technician's trip to your home or business and the time spent diagnosing the problem. At Central Arkansas Service Company, it is $99 on weekdays and $149 on weekends and holidays, for both HVAC and plumbing. It is not waived or credited toward the repair, and there is no travel fee within the normal 50-mile service area.
- sewer camera inspection
- A sewer camera inspection runs a waterproof camera on a flexible cable through a drain or sewer line, usually from a cleanout, to show the inside of the pipe on a screen. It can reveal roots, cracks, sags, offset joints, grease buildup and blockages, and help a plumber locate the problem before recommending a repair.
- short cycling
- Short cycling is when an air conditioner, heat pump or furnace turns on and off much more often than normal, running only a few minutes at a time. Causes include oversized equipment, a dirty filter, low refrigerant, a faulty thermostat or a safety switch tripping. It wastes energy, wears parts and leaves humidity behind in summer.
- slab leak
- A slab leak is a leak in a water or drain line running under or through a home's concrete slab foundation. Signs include warm spots on the floor, the sound of running water when fixtures are off, damp flooring, cracks or a sudden jump in the water bill. Leak detection pinpoints the spot before repair or rerouting.
- split system
- A split system is a central heating and cooling setup with one part outside, the condenser or heat pump, and one part inside, the furnace or air handler with its evaporator coil. Copper refrigerant lines and wiring connect the two halves. It is the most common setup in Central Arkansas homes and in many small commercial buildings.
- static pressure
- Static pressure is the resistance to airflow in your duct system, measured in inches of water column. Like blood pressure, too high is a warning sign. Undersized ducts, too few returns, closed vents and dirty filters or coils all raise it, which can reduce airflow, make rooms uneven and strain the blower motor.
- sump pump / sewage ejector
- A sump pump moves groundwater out of a pit in a basement or crawlspace so it does not flood. A sewage ejector pump lifts wastewater from fixtures below the sewer line, like a basement bathroom, up to the main drain. Both rely on float switches and check valves, and both should be tested so they work when needed.
T
- T&P valve
- A T&P valve, or temperature and pressure relief valve, is a safety valve on every tank water heater. It opens to release water if the temperature or pressure inside the tank gets too high. It connects to a discharge pipe that should run down toward the floor or a drain. A dripping T&P valve can point to high water pressure or a worn valve.
- tankless water heater
- A tankless water heater heats water only when a hot tap opens, using a gas burner or electric element and a heat exchanger instead of a storage tank. It does not run out of hot water the way a tank can, but it has a maximum flow rate. Gas tankless units, such as Rinnai models, need proper venting, gas supply and periodic descaling.
- thermostat
- A thermostat is the wall control that tells your heating and cooling system when to run, based on the temperature you set. Programmable and smart thermostats can follow schedules, and heat pump thermostats also manage auxiliary heat. Dead batteries, wrong settings, loose wiring or a bad location can all cause comfort problems that look like equipment trouble.
- tonnage
- Tonnage is the cooling capacity of an air conditioner or heat pump. One ton equals 12,000 BTUs per hour of heat removed, so a 3-ton system removes 36,000 BTUs per hour. Most homes use systems between about 1.5 and 5 tons. The right tonnage comes from a load calculation, not square footage alone.
U
- UV light (UV-C)
- A UV light for HVAC uses ultraviolet-C (UV-C) lamps installed near the evaporator coil or in the ductwork. The light is aimed at surfaces where mold and bacteria tend to grow, especially the damp indoor coil and drain pan. UV lamps lose strength over time and need periodic replacement to keep working as intended.
V
- variable-speed
- Variable-speed describes a compressor or blower motor that can adjust its output across a wide range instead of running only fully on or off. Variable-speed systems run longer at lower speeds, which can mean steadier temperatures, quieter operation and better humidity removal. They are often found in higher-tier air conditioners and heat pumps.
W
- water line
- The water line, or water service line, is the pipe that carries drinking water from the meter to your home. Leaks can show up as soggy spots in the yard, low water pressure, a meter that spins with everything off, or a higher water bill. Water line repair or replacement often involves locating the leak and digging to the pipe.
- whole-home dehumidifier
- A whole-home dehumidifier connects to your ductwork and removes extra moisture from the air throughout the house, separate from your air conditioner. It can help when the AC keeps a home cool but it still feels sticky, or in homes with damp crawlspaces. The collected water drains away, so there is no bucket to empty.
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