Learning Center · Glossary

HVAC & Water-Heating Glossary

The terms you'll hear from technicians, see on quotes, and find on equipment spec sheets — explained in plain English, with no jargon and no upsell.

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SEER2: Seasonal Energy Efficiency Ratio 2 — the current U.S. rating for how efficiently a central air conditioner or heat pump cools over a season (it replaced SEER in 2023 using more realistic test conditions). Higher is better: a 17 SEER2 system uses meaningfully less electricity than a 14 SEER2 one. [GATHER: confirm the current California minimum SEER2 for new residential installs.]
Learn more: AC & heat-pump installation cost
HSPF2: Heating Seasonal Performance Factor 2 — the heating-mode companion to SEER2 for heat pumps. It measures how much heat is delivered per unit of electricity over a heating season; a higher HSPF2 means lower winter running costs.
Learn more: How a heat pump works
AFUE: Annual Fuel Utilization Efficiency — how much of a furnace's fuel becomes usable heat over a year. An 80% AFUE furnace turns 80¢ of every fuel dollar into heat; a 96% condensing furnace turns 96¢. Higher AFUE lowers gas bills but costs more upfront.
Learn more: Heat pump vs. furnace
COP (Coefficient of Performance): A measure of a heat pump's efficiency: the ratio of heat delivered to energy consumed. A COP of 3 means the system moves three units of heat for every unit of electricity — far above the 1.0 ceiling of electric-resistance heat. It's the underlying reason heat pumps are so efficient.
Learn more: How a heat pump works
EER (Energy Efficiency Ratio): A measure of cooling efficiency at a single fixed operating condition, unlike SEER2 which averages across a season. EER is useful for comparing performance on the hottest days; higher is better. For overall seasonal efficiency, SEER2 is the more common spec.
Learn more: What SEER2 do I need?
BTU: British Thermal Unit — the basic unit of heating and cooling energy (the heat needed to raise one pound of water 1°F). Equipment capacity is rated in BTU per hour; 12,000 BTU/hr equals one 'ton' of cooling.
Learn more: AC & heat-pump installation cost
Ton / Tonnage: A measure of air-conditioning or heat-pump capacity: one ton equals 12,000 BTU/hour of cooling. Most Bay Area homes need roughly 2–4 tons, but the right size should come from a Manual J load calculation — not square footage alone.
Learn more: Manual J load calculation
Manual J: The industry-standard load calculation that sizes a system to your specific home — accounting for square footage, insulation, windows, orientation, and air leakage. Correct Manual J sizing prevents the short-cycling and humidity problems an oversized unit causes.
Learn more: How to compare HVAC quotes
Heat Pump: A single system that both heats and cools by moving heat rather than burning fuel — it works like an air conditioner in summer and runs in reverse to warm your home in winter. In the Bay Area's mild climate, a heat pump can efficiently replace both a furnace and an AC.
Learn more: How a heat pump works
Air Handler: The indoor component that circulates conditioned air through your ductwork, housing the blower and the indoor coil. Paired with an outdoor condenser, it's the indoor half of a split system — and its blower capacity sets the highest filter MERV your system can safely handle.
Learn more: MERV ratings explained
Condenser (Outdoor Unit): The outdoor unit of an AC or heat pump, containing the compressor, outdoor coil, and fan. It releases heat outside when cooling (and absorbs it when a heat pump is heating). Because it lives outdoors, it's the part most exposed to coastal salt-air corrosion.
Learn more: Salt-air coastal HVAC protection
Evaporator Coil: The indoor coil (in or above the air handler or furnace) where refrigerant absorbs heat from your home's air in cooling mode. A dirty or frozen evaporator coil is a common cause of weak cooling — often traced back to a clogged filter restricting airflow.
Learn more: Why is my AC not cooling?
Mini-Split / Ductless: A ductless heat pump: one outdoor compressor linked to one or more wall- or ceiling-mounted indoor heads. Ideal for additions, ADUs, garages, sunrooms, and homes without ductwork — and each head can be zoned for room-by-room control.
Learn more: High-velocity vs. ductless
High-Velocity (Unico): A small-duct, high-velocity system (Unico is the leading brand) that delivers conditioned air through flexible 2-inch tubing fished through existing walls and ceilings. It brings central heating and cooling to older and architecturally sensitive homes without large, visible ductwork.
Learn more: High-velocity vs. ductless
MERV: Minimum Efficiency Reporting Value — a 1–16 scale for how well an air filter captures particles. MERV 8 catches dust and pollen; MERV 11–13 adds finer smoke and bacteria particles. Too high a MERV for your system can restrict airflow, so match the filter to the equipment.
Learn more: Improve your indoor air quality
HEPA: High-Efficiency Particulate Air — a filtration standard that captures 99.97% of 0.3-micron particles, beyond the top of the MERV scale. Most central HVAC blowers can't push air through true HEPA without a dedicated bypass, so HEPA is most practical in portable room air cleaners; whole-home filtration usually relies on high-MERV media.
Learn more: Do I need a whole-home air purifier?
ERV / HRV: Energy- and Heat-Recovery Ventilators — devices that bring filtered fresh air into a tightly sealed home while recovering heat (HRV) or heat and humidity (ERV) from the outgoing stale air. They improve indoor air quality without wasting the energy you paid to condition.
Learn more: Improve your indoor air quality
Tankless Water Heater: A water heater that heats water on demand as it flows through the unit instead of keeping a tank hot 24/7. It never runs out of hot water and saves standby energy, but needs adequate gas or electrical capacity and proper sizing for simultaneous demand.
Learn more: Tank vs. tankless vs. heat-pump
Heat-Pump Water Heater (HPWH): A water heater that uses a heat pump to move heat from the surrounding air into the tank, using roughly one-third the electricity of a standard electric-resistance tank. It's the most efficient way to make hot water, and works best in a warm, ventilated space such as a garage.
Learn more: How heat-pump water heaters work
Anode Rod: A 'sacrificial' metal rod inside a tank water heater that corrodes instead of the tank itself, protecting it from rust. Once the anode is used up, the tank starts to corrode — so checking and replacing it before it's gone is the single most effective way to extend a tank's life, and the most commonly skipped.
Learn more: How long does a water heater last?
Expansion Tank: A small tank that absorbs the pressure increase created when water heats and expands in a closed plumbing system, protecting the water heater and plumbing from pressure stress. It's a code-required item on many water-heater installs — one of the things a permit inspection checks.
Learn more: HVAC & water-heater permits
Recirculation Pump: A pump that keeps hot water circulating in the plumbing loop so it's available almost instantly at the tap, instead of running the faucet while cold water clears the line. It improves convenience and reduces water waste, and pairs with tank or tankless systems — though sizing and controls matter for efficiency.
Learn more: Tankless water heater sizing
Harvest Thermal: A system that pairs a heat pump with a hot-water thermal-storage tank and a smart controller, so one heat pump provides space heating, cooling, and domestic hot water — and shifts energy use to cheaper, cleaner hours. Air Synergy is an authorized Harvest Thermal installer.
Learn more: What is Harvest Thermal?
Refrigerant R-454B: A low-global-warming-potential refrigerant now used in new air conditioners and heat pumps as the industry transitions away from older blends. New 2025-and-later equipment is built for R-454B (or similar), which affects parts compatibility on both installs and repairs.
Learn more: Repair or replace your AC?
R-410A / R-22 Phase-Down: Older refrigerants being phased down by EPA rules: R-22 (used in systems built before ~2010) is no longer produced, and R-410A is being scaled back in favor of low-GWP refrigerants like R-454B. If your system still uses R-22, refrigerant repairs get expensive — a real factor in repair-vs-replace math.
Learn more: Repair or replace your AC?
Inverter / Variable-Speed: Technology that lets a compressor or blower run at variable speeds instead of simply on/off. Inverter-driven heat pumps and AC units ramp up and down to match demand, delivering steadier comfort, quieter operation, and lower energy use than single-stage equipment.
Learn more: How a heat pump works
Electrification: Replacing gas-burning home appliances — furnaces, water heaters, stoves — with efficient electric equivalents like heat pumps and heat-pump water heaters. It's the focus of most current California rebate programs and often involves an electrical panel upgrade to support the new loads.
Learn more: Bay Area HVAC rebates & incentives
Electrical Panel Upgrade / Amperage: Increasing a home's electrical service capacity (measured in amps) so it can safely power added loads such as a heat pump or heat-pump water heater. Older homes may have 100-amp panels that need upgrading (often to 200 amps) before electrifying — a real cost factor in some heat-pump projects.
Learn more: Heat-pump installation cost
Demand Response: Shifting a home's energy use to times when electricity is cheaper or cleaner — for example, heating water or pre-conditioning the home during off-peak hours. Smart heat-pump systems and thermal-storage setups like Harvest Thermal use demand response to cut bills and ease grid strain.
Learn more: What is Harvest Thermal?
Zoning: Dividing a home into independently controlled comfort areas — using multiple ductless heads, or motorized dampers with multiple thermostats — so you heat and cool only the rooms in use. Especially valuable in multi-level and larger homes.
Learn more: High-velocity vs. ductless
Dual-Fuel / Hybrid: A system that pairs a heat pump with a gas furnace; the efficient heat pump handles most of the heating and the furnace takes over only in the coldest conditions. It's rarely necessary in the Bay Area's mild winters, but it is an option for some homes.
Learn more: Heat pump vs. furnace
In-Slab Radiant Heating: Hydronic (hot-water) radiant heating with tubing embedded in the concrete floor slab. Common in mid-century Bay Area homes — including many Eichlers — because it heats evenly and quietly. Aging slab systems often need careful repair or a modern replacement strategy (frequently ductless or high-velocity) that respects the home's open, glass-walled design.
Learn more: Why mid-century slab homes need specialty HVAC
Title 24: California's building energy-efficiency code, updated by the California Energy Commission every few years. It shapes equipment efficiency and ventilation requirements for new construction and major changeouts. [GATHER: confirm the current Title 24 cycle and the specific provisions that apply to HVAC and water-heater changeouts in your jurisdiction.]
Learn more: HVAC & water-heater permits
Duct Cleaning: Mechanically cleaning the supply and return ducts to remove accumulated dust and debris. It genuinely helps after construction, rodent intrusion, or visible mold — but for routine dust it's often less cost-effective than sealing duct leaks and upgrading filtration.
Learn more: Is duct cleaning worth it?
Free In-Home Estimate: A no-cost, no-obligation visit where a licensed technician evaluates your actual home — system, ductwork, electrical, and layout — and gives you a written, itemized quote. Because real sizing and pricing depend on your home, it's the honest alternative to a phone-quoted price.
Learn more: How to compare HVAC quotes

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