This HVAC glossary defines the terms you'll see on estimates, permits and equipment labels in North County, from efficiency ratings like SEER2, EER2, HSPF2 and AFUE to refrigerants like R-410A and R-454B, California's Title 24 Energy Code and SDG&E time-of-use rates. Each term links to a deeper guide where we have one.
Fallbrook Heating & Air has explained these terms to homeowners in Fallbrook and across North County since 1972. If a word on your estimate isn't here, just ask.
A
A2L refrigerant
A class of refrigerant with lower global warming potential and mild flammability. New residential split air conditioners and heat pumps manufactured from January 1, 2025 use A2L refrigerants such as R-454B or R-32. See our R-410A phase-out guide.
AFUE (Annual Fuel Utilization Efficiency)
The share of a furnace's fuel that becomes heat in your home over a year. An 80% AFUE furnace turns 80% of its gas into heat; a 96% AFUE furnace turns 96% into heat and vents through PVC pipe. We install both; see furnace installation.
Air handler
The indoor cabinet that holds the blower and, usually, the evaporator coil. It's paired with a heat pump or AC and moves air through your ducts. In a furnace system, the furnace does this job.
Auxiliary heat
Backup electric heat strips in a heat pump system that switch on when the heat pump can't keep up or is recovering from a big setback. It works, but it uses much more electricity than the heat pump itself.
B
Blower
The fan inside your furnace or air handler that pushes air across the coil or heat exchanger and through the ducts. A dirty filter makes the blower work harder.
BTU (British thermal unit)
A unit of heat: the energy needed to raise one pound of water by 1°F. Furnace and AC capacity is rated in BTUs per hour.
C
Capacitor
An electrical part that gives the compressor and fan motors the boost they need to start and run. Capacitors wear out with heat and are a common cause of summer AC failures. They hold a charge even with the power off, so leave them to a technician.
Compressor
The pump in the outdoor unit that pressurizes refrigerant and moves it through the system. It's the heart of an AC or heat pump.
Condensate drain
The pipe and pan that carry water away from the indoor coil, where moisture condenses as the system cools. A clogged drain can cause leaks; see AC leaking water.
Condenser
The outdoor unit of a central AC, with the compressor, fan and a coil that releases your home's heat outdoors. Keep it clear of plants and debris so it can breathe.
Contactor
An electrical switch in the outdoor unit that turns the compressor and fan on when the thermostat calls for cooling. Worn or pitted contactors can stop the unit from starting.
D
Damper
A movable plate inside a duct that opens or closes to control airflow. Motorized dampers are what make zoning work.
Defrost cycle
When a heat pump's outdoor coil frosts up in cold, damp weather, the system briefly reverses to melt it. A little steam from the outdoor unit during defrost is normal.
Dual fuel
A system that pairs a heat pump with a gas furnace. The heat pump handles most heating, and the furnace takes over in colder weather. See heat pump vs. furnace.
Ductwork
The supply and return ducts that carry air between your HVAC equipment and your rooms. In California, ducts are tested and sealed when equipment is replaced; see Title 24 duct testing.
E
ECC (Energy Code Compliance)
California's program for third-party field verification and diagnostic testing under the Energy Code. The California Energy Commission says the ECC Program "replaces the HERS Program." See our Title 24 permits guide.
EER2
A cooling efficiency rating measured at a single hot test condition. In the Southwest, including California, new split ACs under 45,000 BTU need at least 11.7 EER2 and larger ones 11.2 EER2, or 9.8 EER2 for either size if rated at 15.2 SEER2 or higher.
Electrostatic filter
A filter that uses an electrostatic charge to attract and hold dust and fine particles. We install non-restrictive electrostatic filtration designed to clean air without choking airflow; see indoor air quality.
Evaporator coil
The indoor coil where refrigerant absorbs heat from your home's air. Low airflow or low refrigerant can make it freeze; see AC frozen with ice on the lines.
F
Flame sensor
A small rod in a gas furnace that confirms the burners have lit. When it gets dirty, the furnace may light and then shut off a few seconds later. See furnace not working.
H
Heat exchanger
The metal chamber in a furnace that transfers heat from the burner flames to your home's air while keeping combustion gases separate. A cracked heat exchanger is a safety problem.
Heat pump
An electric system that moves heat instead of making it: indoors to outdoors in summer, outdoors to indoors in winter. One system heats and cools. See heat pumps.
HERS
Short for Home Energy Rating System, the program formerly used for third-party verification of HVAC work in California. Under the 2025 Energy Code, the ECC Program replaces it.
HSPF2
A heat pump's heating efficiency over a season. The national minimum for a split heat pump is 7.5 HSPF2, alongside 14.3 SEER2 for cooling. See our SEER2 requirements guide.
Humidifier
A device that adds moisture to dry indoor air, attached to your furnace or air handler so it treats the whole house. Useful during dry Santa Ana weather.
L
Line set
The pair of copper tubes that carry refrigerant between the indoor coil and the outdoor unit. The larger one is insulated and feels cold when the AC runs.
M
Manual J
The industry-standard method for calculating how much heating and cooling a home needs, based on its size, insulation, windows and climate. It's how a system should be sized; see what size AC do I need.
MERV (Minimum Efficiency Reporting Value)
A filter rating for how well it captures particles. Higher numbers catch more, but denser filters can restrict airflow. The EPA suggests MERV 13 or as high as your system fan and filter slot can handle.
Mini-split
A ductless heat pump or AC with an outdoor unit connected to one or more wall- or ceiling-mounted indoor units. We install, service and maintain mini-splits.
P
PSPS (Public Safety Power Shutoff)
When SDG&E turns off power in high-risk areas during extreme fire weather. Your AC, heat pump and furnace blower stop until power returns; see Santa Ana winds and your HVAC.
R
R-22
An older refrigerant found in many older air conditioners and heat pumps. It has been phased out under federal rules, so servicing an R-22 system depends on limited existing supplies.
R-32
An A2L refrigerant used in some new air conditioners and heat pumps made from 2025 on.
R-410A
The refrigerant that replaced R-22 and is in most systems installed before 2025. New residential split systems made from 2025 use A2L refrigerants instead, but R-410A systems can still be serviced, and a May 2026 EPA rule removed the installation deadline for pre-2025 R-410A equipment.
R-454B
An A2L refrigerant used in many new air conditioners and heat pumps made from 2025 on, replacing R-410A.
Refrigerant
The fluid that absorbs heat indoors and releases it outdoors as it cycles between liquid and gas. A system shouldn't "use up" refrigerant; if it's low, there's a leak. See refrigerant leak repair.
Return air
The grilles and ducts that pull air from your rooms back to the system to be filtered and conditioned. Blocking returns with furniture cuts airflow.
Reversing valve
The valve in a heat pump that switches the direction refrigerant flows, so the system can heat or cool. If it sticks, the heat pump may blow cool air in heating mode; see heat pump not heating.
S
SCAQMD (South Coast Air Quality Management District)
The regional air district whose rules cover Temecula and Murrieta. Its board rejected a zero-NOx furnace amendment to Rule 1111 on June 6, 2025. San Diego County is not in SCAQMD. Check current status at aqmd.gov.
SEER2
A seasonal cooling efficiency rating for air conditioners and heat pumps. Since January 1, 2023, new split ACs installed in the Southwest, including California, need at least 14.3 SEER2 under 45,000 BTU and 13.8 SEER2 at 45,000 BTU and above. See our SEER2 requirements guide.
Short cycling
When a system turns on and off in short bursts instead of running full cycles. It wastes energy and wears parts; causes include oversizing, a dirty filter and thermostat problems. See AC short cycling.
T
Thermostat
The control that tells your system when to heat, cool or run the fan. Programmable and smart thermostats can run schedules automatically.
Title 24
California's building code; Part 6 is the Energy Code. The 2025 Energy Code took effect January 1, 2026. Replacing an air handler, outdoor unit or coil triggers duct sealing and third-party verification. See our Title 24 permits guide.
Ton
A measure of cooling capacity equal to 12,000 BTU per hour. It has nothing to do with weight. A "3-ton" AC removes 36,000 BTU of heat per hour.
TOU (time-of-use)
An electricity rate where the price depends on the time of day. On SDG&E's time-of-use plans, on-peak pricing runs from 4 to 9 p.m. every day. See our time-of-use thermostat guide.
Two-stage
Equipment with two output levels, low and high. It runs on low most of the time for steadier temperatures and switches to high on the hottest or coldest days.
V
Variable-speed
A compressor or blower motor that adjusts its speed in small steps to match demand. It tends to run longer at lower output, which means quieter operation, more even temperatures and better humidity control.
Z
Zoning
A setup that uses motorized dampers and multiple thermostats to heat or cool different areas of the house separately, such as upstairs and downstairs. See zoning.
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Frequently asked questions
What's the difference between SEER2 and EER2?
SEER2 measures cooling efficiency averaged over a typical season, while EER2 measures efficiency at one hot test condition. In the Southwest, including California, new split air conditioners must meet both a SEER2 and an EER2 minimum. EER2 matters most on the hottest inland North County afternoons, when the system is working hardest.
Is HSPF2 the same as SEER2?
No. HSPF2 rates a heat pump's heating efficiency over a season, while SEER2 rates cooling efficiency. A heat pump has both ratings because it heats and cools. The current national minimum for a split heat pump is 14.3 SEER2 and 7.5 HSPF2. Higher numbers mean less electricity for the same comfort.