HVAC System Glossary for San Francisco Property Owners
San Francisco's HVAC service sector operates under a layered framework of state energy codes, local reach codes, seismic standards, and air quality regulations that produce terminology not always self-evident from general industry usage. This glossary defines the technical terms, regulatory categories, and equipment classifications that appear most frequently in permit applications, contractor proposals, and inspection reports for San Francisco properties. Coverage spans residential and commercial contexts, with attention to the city's specific climate conditions, building stock, and electrification mandates. Property owners, building managers, and real estate professionals navigating HVAC systems in local context will encounter these terms across permitting documents, utility rebate programs, and contractor communications.
Definition and scope
This glossary functions as a reference index for HVAC terminology as applied within San Francisco's regulatory and environmental context. Terms are drawn from four overlapping domains:
- Mechanical engineering and equipment classification — the physical components, refrigerant circuits, airflow mechanics, and thermodynamic principles that define how systems operate.
- California regulatory codes — Title 24, Part 6 (California Energy Code), administered by the California Energy Commission (CEC), and California Mechanical Code (Title 24, Part 4), which govern equipment efficiency ratings, ventilation minimums, and installation standards.
- San Francisco local amendments — the San Francisco Building Code (SFBC) and SF Environment Department guidelines, which extend or tighten state minimums in areas including electrification and equipment siting.
- Air quality compliance language — terminology from the Bay Area Air Quality Management District (BAAQMD), which regulates combustion equipment and enforces Regulation 6 governing residential and commercial appliance emissions.
Scope: This reference covers HVAC terminology as it applies within the City and County of San Francisco. It does not extend to adjacent jurisdictions such as Oakland, Daly City, or unincorporated San Mateo County. State-level codes referenced here apply California-wide, but local amendments described are specific to San Francisco. For permitting procedures that apply within city boundaries, the relevant authority is the San Francisco Department of Building Inspection (SFDBI). Terms related to federal environmental regulation — such as EPA Section 608 refrigerant handling certification requirements — are noted where directly relevant to San Francisco HVAC practice but are not administered locally.
How it works
HVAC terminology in San Francisco documents generally organizes around five operational categories. Understanding these groupings clarifies how terms are used in practice:
1. System classification terms
These identify the type of HVAC system by heat transfer method or distribution architecture:
- Forced-air system: Uses a blower to distribute conditioned air through ducts. Covered in detail at forced air systems in San Francisco properties.
- Hydronic system: Circulates heated or cooled water through pipes to terminal units (radiators, fan coils, radiant panels). See hydronic heating systems in San Francisco.
- Ductless mini-split: A refrigerant-based system connecting an outdoor compressor to one or more indoor air handlers without ductwork. Directly relevant to older building stock — detailed at ductless mini-split systems in San Francisco.
- Variable refrigerant flow (VRF): A multi-zone refrigerant-based system that modulates compressor speed to match simultaneous heating and cooling demands across zones. Common in San Francisco commercial buildings.
2. Efficiency rating terms
- SEER2 (Seasonal Energy Efficiency Ratio 2): The updated efficiency metric for cooling equipment, replacing SEER under Department of Energy regulations effective January 1, 2023. Minimum SEER2 ratings vary by equipment type and climate zone under Title 24 compliance requirements.
- HSPF2 (Heating Seasonal Performance Factor 2): Measures heat pump heating efficiency. Higher HSPF2 values indicate greater efficiency per watt of electricity consumed.
- EER (Energy Efficiency Ratio): A steady-state cooling efficiency metric used for specific equipment categories and design load calculations.
- COP (Coefficient of Performance): The ratio of heat output to electrical input for heat pumps; a COP of 3.0 means 3 units of heat energy delivered per unit of electrical energy consumed.
3. Regulatory and permitting terms
- Mechanical permit: Required from SFDBI for most HVAC equipment installations, replacements, and significant modifications. Tracked through the city's permit database.
- Reach code: A local energy ordinance that exceeds state minimums. San Francisco's reach codes, effective as of the city's adoption process, impose all-electric requirements on new construction and qualifying alterations. Covered at San Francisco reach codes and HVAC implications.
- CF1R/CF2R/CF3R: California HERS (Home Energy Rating System) compliance forms required for permitted HVAC work in residential buildings to document equipment specifications, duct testing, and refrigerant charge verification.
4. Air quality and ventilation terms
- MERV (Minimum Efficiency Reporting Value): A filter classification scale from 1 to 16, defined by ASHRAE Standard 52.2, indicating the fraction of airborne particles captured at specified size ranges. San Francisco's wildfire smoke events have elevated practical interest in MERV-13 or higher ratings. See HVAC filtration standards for San Francisco air quality.
- ACH (Air Changes per Hour): The number of times the total air volume of a space is replaced per hour by a ventilation system.
- ASHRAE 62.2: The ventilation standard for residential buildings, referenced in California Mechanical Code for minimum fresh air requirements. The current edition is ASHRAE 62.2-2022, effective January 1, 2022, which supersedes the 2019 edition.
5. Equipment and component terms
- AHU (Air Handling Unit): The indoor component of a central HVAC system containing the blower, filter rack, coil, and sometimes heating element.
- Condenser: The outdoor unit in a split system that rejects heat to the exterior environment.
- Evaporator coil: The indoor heat exchanger coil across which refrigerant absorbs heat from interior air.
- Expansion valve (TXV/EEV): A metering device controlling refrigerant flow into the evaporator. Thermostatic expansion valves (TXV) and electronic expansion valves (EEV) differ in control method; EEVs are standard in modern variable-capacity systems.
- Zoning: A configuration using multiple thermostats and dampers or separate systems to independently control temperature in distinct building areas.
Common scenarios
Three contexts account for the majority of terminology confusion encountered by San Francisco property owners:
Equipment replacement proposals: Contractor proposals for system replacement frequently cite SEER2, HSPF2, and refrigerant type without explaining how these metrics relate to California Title 24 minimum thresholds or PG&E rebate eligibility. San Francisco sits in CEC Climate Zone 3, which carries specific minimum efficiency requirements distinct from inland California zones. PG&E rebates for HVAC systems in San Francisco are tied to equipment meeting both federal minimum standards and CEC-specified thresholds.
Permit application review: SFDBI mechanical permits reference California Mechanical Code sections, HERS compliance form numbers, and equipment classifications that assume familiarity with regulatory nomenclature. Duct leakage testing requirements — expressed as a percentage of system airflow — and refrigerant charge verification requirements appear in permit conditions for new installations and qualifying replacements.
Victorian and Edwardian building retrofits: San Francisco's pre-1940 building stock, including the approximately 48,000 Victorian-era residential units, presents specific challenges around duct installation, equipment siting, and historic preservation constraints. Terms such as high-wall cassette, concealed duct mini-split, and attic air handler appear regularly in retrofit proposals for properties addressed in HVAC systems for San Francisco Victorian homes.
Decision boundaries
The following distinctions clarify where one term ends and another begins — boundaries that carry practical consequences in permitting, compliance, and contractor communication:
Heat pump vs. air conditioner: Both use refrigerant vapor-compression cycles. A heat pump incorporates a reversing valve enabling it to operate in heating mode by extracting heat from outdoor air. An air conditioner moves heat only in one direction (interior to exterior). San Francisco's reach code environment makes this distinction consequential because all-electric mandates favor heat pump installations. See heat pump systems in San Francisco homes.
Ventilation vs. air circulation: Ventilation introduces outdoor air into the conditioned space, diluting indoor pollutants and meeting ASHRAE 62.2-2022 requirements. Air circulation (or recirculation) moves existing indoor air through filtration and conditioning without adding outdoor air. The two functions are sometimes performed by the same system component but serve distinct code-compliance purposes under California Mechanical Code.
Permit-required vs. permit-exempt work: SFDBI regulations distinguish between like-for-like equipment replacements (which may qualify for streamlined permitting) and system type changes or new installations (which require full mechanical permits with plan review). The boundary is not always self-evident — replacing a gas furnace with an electric heat pump, for instance, triggers reach code compliance review even if the physical footprint is unchanged. The permitting framework is detailed at San Francisco HVAC permit and inspection requirements.
MERV rating comparison — MERV 8 vs. MERV 13: MERV 8 filters capture particles 3–10 microns in size at approximately 70% efficiency and represent the baseline for residential