HVAC terms, in plain English

A

A2L refrigerant

A2L is a safety class for refrigerants that are mildly flammable and low in toxicity. R-454B, the refrigerant in most new residential systems sold today, is an A2L. Equipment built for A2L refrigerants includes leak sensors and specific installation rules, which is one reason a new system should be installed by a licensed contractor rather than retrofitted. Heat Pumps →

AFUE

Annual Fuel Utilization Efficiency is the percentage of a gas furnace's fuel that becomes heat for the house over a season. An 80 AFUE furnace turns 80 percent of the gas into heat and sends the rest up the flue; a 95 AFUE condensing furnace keeps 95 percent. In San Diego County's short heating season the difference in gas bills is real but smaller than it would be in a cold climate. Heating →

Air handler

The indoor unit that moves air through the ductwork. It holds the blower, the indoor coil, and the filter slot, and in a heat pump system it may also hold electric backup heat strips. When a home has no furnace, the air handler is the indoor half of the system. Heat Pumps →

Airflow balancing

Adjusting the amount of air each room receives so the whole house reaches the thermostat setting at roughly the same time. It is done at the dampers and registers after measuring what each duct actually delivers. Large single-story homes with long duct runs, common in Rancho Santa Fe and Escondido, are the usual candidates. Ductwork & Airflow →

B

BTU

A British Thermal Unit is the amount of heat needed to raise one pound of water one degree Fahrenheit. Cooling and heating equipment is rated in BTUs per hour, and one ton of cooling equals 12,000 BTU per hour. The right number for a home comes from a load calculation, not from the square footage alone. AC Installation & Replacement →

C

Capacitor

A small cylindrical part that stores an electrical charge and gives the compressor and fan motors the push they need to start and keep running. A weak or failed capacitor is one of the most common reasons an air conditioner hums but will not start, and it is usually a same-visit repair. Air Conditioning →

Compressor

The pump in the outdoor unit that circulates refrigerant and does most of the work of cooling. It is the most expensive single component in a system, so its age and condition drive a lot of repair-or-replace decisions. Salt air near the coast is harder on the outdoor unit that houses it than inland air is. Air Conditioning →

Condenser

The outdoor unit of an air conditioner or heat pump. It contains the compressor, the outdoor coil, and a fan, and its job is to release the heat pulled out of the house. Keeping it clear of leaves, dirt, and shrubs is the simplest maintenance a homeowner can do. HVAC Maintenance →

D

Ductless mini split

A heat pump system with an outdoor unit connected by refrigerant lines to one or more indoor heads mounted in individual rooms, with no ductwork. Each head is controlled separately. It is the usual answer for a garage conversion, an addition, a home office, or the one bedroom the central system never reaches. Ductless Mini Splits →

E

Evaporator coil

The indoor coil where refrigerant absorbs heat from the air passing over it, which is what actually cools the house. A dirty or frozen evaporator coil shows up as weak airflow, warm air, or water around the indoor unit. Air Conditioning →

H

Heat exchanger

The metal chamber inside a gas furnace where combustion happens on one side and household air is warmed on the other. A crack in it can let carbon monoxide into the air stream, which is why a cracked heat exchanger is a safety finding rather than an efficiency one and usually means replacing the furnace. Heating →

Heat pump

An air conditioner that can run in reverse, moving heat into the house in winter and out of it in summer with one outdoor unit. Because San Diego County winters are mild, a heat pump rarely needs backup heat here, which makes it a strong replacement for a furnace-plus-AC pair. Heat Pumps →

HERS verification

Third-party testing required by California's energy code on many HVAC changeouts, performed by an independent rater rather than the installing contractor. It typically checks duct leakage, refrigerant charge, and airflow on the installed system. Cool Guys arranges it when a project requires it. AC Installation & Replacement →

HSPF2

Heating Seasonal Performance Factor is the heating-season efficiency rating for a heat pump, the winter counterpart to SEER2. Higher is more efficient. The 2 marks the current federal test procedure, so an HSPF2 number is not directly comparable to an older HSPF number. Heat Pumps →

L

Load calculation

The engineering estimate of how much heating and cooling a specific house needs, based on its size, insulation, windows, orientation, and climate zone. The industry method is called Manual J. It is how equipment gets sized correctly; a system sized by rule of thumb is the usual reason a new unit short-cycles or never quite keeps up. AC Installation & Replacement →

M

MERV

Minimum Efficiency Reporting Value rates how well an air filter captures particles, on a scale where higher catches smaller particles. Most homes do well between MERV 8 and 13. A filter rated higher than the system's blower can handle restricts airflow, so the right MERV depends on the equipment, not just the allergy. Indoor Air Quality →

P

Permit (mechanical)

Authorization from a city or county building department to install or replace HVAC equipment, followed by an inspection that confirms the work meets the adopted code. In California, replacing a furnace, air conditioner, heat pump, or mini split generally requires one; a like-for-like repair generally does not. AC Installation & Replacement →

R

R-454B

The A2L refrigerant used in most new residential air conditioners and heat pumps sold in the United States, replacing R-410A as production of new R-410A equipment was phased down. It is not a drop-in for older systems; equipment, line sets, and service tools are specific to it. Heat Pumps →

Refrigerant line set

The pair of insulated copper lines that carry refrigerant between the outdoor unit and the indoor coil or mini split head. Line-set covers protect them where they run up an exterior wall. A leak in the line set is a repair; a line set that is too long, too short, or the wrong diameter for new equipment is an installation problem. Ductless Mini Splits →

Return air

The air pulled back out of the rooms and into the system to be filtered and conditioned again, through the larger grilles that do not blow. Too little return air starves the system, raises noise, and is a frequent hidden cause of a house that never cools evenly. Adding returns is a ductwork job, not an equipment one. Ductwork & Airflow →

S

SEER2

Seasonal Energy Efficiency Ratio is the cooling-season efficiency rating for an air conditioner or heat pump; higher uses less electricity for the same cooling. The 2 marks the current federal test procedure. California's Southwest region has its own minimum, so equipment sold here must meet a higher floor than in northern states. AC Installation & Replacement →

Short cycling

When a system turns on and off in quick, repeated bursts instead of running full cycles. Causes range from a dirty filter or low refrigerant to a system that is oversized for the house. It wears parts out early and rarely gets the home comfortable. Air Conditioning →

T

Title 24

California's building standards code. Part 6 is the energy code that sets efficiency and installation requirements for HVAC work, including when HERS verification is required and what compliance paperwork the installer files. Each city and county enforces it through its building department. AC Installation & Replacement →

Tune-up

A scheduled maintenance visit that cleans, tests, and adjusts the system before the season it will be working hardest. It is where small faults get caught while they are still small. Cool Guys' one-time tune-up is $88; the Cool Club is $225 a year and comes with a 10 percent repair discount and priority scheduling. HVAC Maintenance →

U

UV light (germicidal)

An ultraviolet lamp mounted near the indoor coil or in the ductwork to keep mold and bacteria from growing on the wet coil surface. It treats the coil, not the whole house, and works alongside filtration rather than replacing it. Indoor Air Quality →

Z

Zoning

Dividing a house into areas that are heated and cooled independently, using motorized dampers in the ductwork and a thermostat per zone. It is the fix for a two-story or large single-story home where one thermostat cannot satisfy every room, and a common design choice in larger Rancho Santa Fe homes. Ductwork & Airflow →

Reviewed by the Cool Guys Home Services service team · Licensed, bonded & insured (CSLB #1156890)