Reference
HVAC & Plumbing Glossary
Plain-English definitions of the terms you'll hear from technicians, on invoices, and in equipment specs — so you can make informed decisions about your home.
- AFUE
- Annual Fuel Utilization Efficiency. Measures how efficiently a furnace converts fuel to heat over a heating season. An 80 AFUE furnace converts 80¢ of every dollar of gas into heat; a 96 AFUE condensing furnace converts 96¢. Higher AFUE = lower heating bills but higher equipment cost.
- BTU
- British Thermal Unit. The amount of energy needed to raise one pound of water by one degree Fahrenheit. Used to measure heating and cooling capacity — a 36,000 BTU system is a 3-ton AC. Rule of thumb: 20 BTU per square foot of living space, adjusted for insulation, ceiling height, and climate.
- Condensate Line
- The drain line that removes moisture extracted from indoor air by the air conditioner's evaporator coil. Algae buildup can clog the line, causing water to back up and trip the system's safety shutoff — one of the most common summer AC failures. Flushing with diluted bleach during a tune-up prevents it.
- Heat Pump
- A year-round system that moves heat rather than generating it — it cools like an AC in summer and extracts heat from outdoor air to warm your home in winter. Up to 3× more efficient than a gas furnace for heating. California's Title 24 (2026) makes heat pumps the default for new residential construction.
Learn more: California Heat Pump Rules 2026
- HEEHRA Rebate
- Home Energy and Efficiency Home Rebate Act. Federal rebate program (funded through the Inflation Reduction Act) offering up to $8,000 toward a heat pump installation for qualifying California households. Income-based eligibility applies. Your contractor files on your behalf.
Learn more: California Heat Pump Rules 2026
- HSPF2
- Heating Seasonal Performance Factor 2. Efficiency rating for heat pumps in heating mode — the updated version of HSPF. Higher numbers mean less electricity used per unit of heat produced. Minimum federal standard for split-system heat pumps is 8.8 HSPF2 as of 2023.
- MERV
- Minimum Efficiency Reporting Value. Rates how well an air filter captures particles on a 1–16 scale. MERV 8 catches most dust, pollen, and mold spores. MERV 11–13 adds bacteria and smoke particles. MERV 16 hospital-grade. Higher MERV can restrict airflow if your system isn't designed for it — ask your technician before upgrading.
- Mini-Split / Ductless System
- A ductless heat pump consisting of one or more indoor air handlers connected to an outdoor compressor. Ideal for home additions, garages, ADUs, and rooms that are hard to condition with central ductwork. Requires no duct installation and can be zoned per room.
- R-Value
- A measure of insulation's resistance to heat flow. Higher R-value = better insulation. California Title 24 requires R-38 in attics for new construction. Most older Ventura County homes have R-11 to R-19; upgrading to R-38 or R-49 meaningfully lowers cooling and heating costs.
- Refrigerant (R-410A / R-454B)
- The chemical refrigerant that circulates through an AC or heat pump to transfer heat. R-410A was the standard until EPA regulations phased it out in 2025; new equipment uses R-454B or similar low-GWP refrigerants. Refrigerant leaks require a licensed technician with EPA 608 certification to handle.
- SEER2
- Seasonal Energy Efficiency Ratio 2. The current federal rating for central air conditioner and heat pump efficiency (replaced SEER in 2023). Higher is more efficient — a 16 SEER2 unit uses roughly 20% less electricity than a 13 SEER2 unit. California's 2026 minimum for new residential AC installations is 15 SEER2.
Learn more: California Heat Pump Rules 2026
- Short Cycling
- When an AC or heat pump runs for a few minutes, shuts off before the house reaches the set temperature, and immediately restarts. Causes include an oversized system, low refrigerant, dirty filters, or a failing compressor. Wastes energy and accelerates wear on the compressor.
- Slab Leak
- A water or sewer line leak beneath the concrete foundation of a home. Signs include warm or wet spots on the floor, rising water bills, and reduced water pressure. Repairs range from spot-fixing a single pipe to full repipe. Requires leak detection equipment to locate without breaking up the entire slab.
- Tankless Water Heater
- A water heater that heats water on demand rather than storing a tank of hot water 24/7. Uses 10–15% less energy than a tank heater and never runs out of hot water. Higher upfront cost; SoCal Gas offers rebates for qualifying mobile home conversions.
Learn more: Is a Tankless Water Heater Worth It in California?
- Title 24
- California's energy code, updated every few years by the California Energy Commission. The 2026 version makes heat pumps the prescriptive standard for new construction and shapes equipment choices during major system replacements. Does not require removing a working gas furnace.
Learn more: What Title 24 Means for Your Home
- Ton (Cooling)
- A unit of air conditioner capacity. One ton = 12,000 BTU/hour. A typical 1,800 sq ft Ventura County home usually needs a 3–4 ton system. Oversizing causes short-cycling and poor dehumidification; undersizing causes the system to run constantly and never cool adequately.
- TXV (Thermostatic Expansion Valve)
- A precision metering valve that controls refrigerant flow into the evaporator coil based on suction temperature. Improves efficiency and protects the compressor compared to a fixed orifice. Most modern AC and heat pump systems include a TXV as standard.
- Whole-House Fan
- A large fan mounted in the ceiling that pulls cool outdoor air in through open windows and exhausts hot indoor air through the attic and vents. Most effective in Ventura County's mild coastal climate when evenings cool below 70°F. Can significantly reduce AC runtime and energy bills.
Still have questions about a term or your system?
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