Seattle HVAC Systems Glossary of Terms

The terminology used in heating, ventilation, and air conditioning work spans engineering standards, regulatory codes, equipment classifications, and trade-specific language that varies by application and jurisdiction. This page defines the core terms encountered across residential and commercial HVAC contexts in Seattle, organized by functional category. Familiarity with this vocabulary is prerequisite to evaluating contractor proposals, interpreting permit documentation, or assessing system specifications against Washington State Energy Code requirements.

Definition and scope

An HVAC glossary in the Seattle context is not simply a general industry dictionary. The definitions that matter most to property owners, permitting staff, and licensed contractors in this market are shaped by the 2021 Washington State Energy Code (WSEC), the Seattle Mechanical Code (Seattle Municipal Code Title 22), and standards published by ASHRAE — the American Society of Heating, Refrigerating and Air-Conditioning Engineers. When a term like "minimum efficiency" appears in a contractor bid or permit application, its threshold value is set by these specific instruments, not by generic industry convention.

Key foundational terms include:

  1. HVAC — Heating, Ventilation, and Air Conditioning; the integrated system category covering thermal comfort, air movement, and indoor air quality.
  2. BTU (British Thermal Unit) — The standard unit for measuring heating or cooling capacity; 1 BTU equals the energy required to raise 1 pound of water by 1°F.
  3. Ton (refrigeration ton) — A cooling capacity unit equal to 12,000 BTU per hour, derived from the heat absorption rate of melting ice; commonly used to size central air conditioning systems.
  4. Load calculation — An engineering procedure, typically performed per ACCA Manual J, that determines the heating and cooling demand of a specific building based on insulation, window area, occupancy, and climate data.
  5. Conditioned space — Any building area that is heated, cooled, or both, and is enclosed within the building thermal envelope.
  6. Thermal envelope — The physical boundary separating conditioned interior space from the unconditioned exterior, including walls, roof, floors, windows, and doors.

For Seattle's climate and HVAC system requirements, ASHRAE Climate Zone 4C is the applicable classification — a marine climate with mild, wet winters and dry summers — which directly affects equipment sizing standards and minimum insulation requirements under WSEC.

How it works

HVAC terminology functions as a structured classification system with three primary domains: equipment ratings, system architecture, and regulatory compliance language.

Equipment efficiency ratings are assigned by standardized test protocols and carry acronyms that appear on equipment labels, contractor quotes, and WSEC compliance documentation:

System architecture terms define how components are assembled:

Common scenarios

HVAC terminology appears in four primary operational contexts within Seattle's service sector:

Permit applications filed with the Seattle Department of Construction and Inspections (SDCI) require accurate use of terms such as "mechanical permit," "equipment replacement," "new installation," and "alteration." An equipment replacement that involves no ductwork changes is classified differently from a system alteration and carries distinct inspection requirements. The Seattle building permits for HVAC systems page covers these distinctions in detail.

Contractor proposals routinely include efficiency ratings, equipment model specifications, and load calculation references. A proposal citing SEER2 18 for a heat pump installation is referencing a specific performance threshold that can be cross-checked against manufacturer data and utility rebate eligibility criteria from Seattle City Light or Puget Sound Energy.

Energy code compliance documentation uses terms such as "prescriptive path," "performance path," and "R-value" to describe how a proposed installation meets WSEC minimums. R-value measures thermal resistance of insulation materials; higher R-values indicate greater resistance to heat flow.

Indoor air quality and ventilation contexts introduce terms like ERV (Energy Recovery Ventilator), HRV (Heat Recovery Ventilator), MERV rating (Minimum Efficiency Reporting Value for air filters), and ACH (Air Changes per Hour). MERV ratings run from 1 to 16 for standard commercial filters, with MERV 13 being the threshold recommended by ASHRAE Standard 62.1-2022 for many commercial occupancies. The indoor air quality Seattle HVAC systems page addresses these terms in application context.

Decision boundaries

Understanding where one classification ends and another begins is operationally consequential in Seattle's permitting and contracting environment.

Heat pump vs. furnace terminology: A "heat pump" moves heat rather than generating it through combustion; a "furnace" generates heat through combustion of natural gas or propane. A hybrid heat pump system combines both: a heat pump handles mild-temperature heating loads while a gas furnace handles extreme cold conditions. The switchover point — expressed in degrees Fahrenheit and called the "balance point" — is a programmable threshold, not a fixed equipment characteristic.

Replacement vs. alteration vs. new installation: SDCI distinguishes these three categories because each triggers different permit and inspection requirements. Replacing a furnace with an identical-capacity unit in the same location is treated differently from replacing it with a heat pump that requires new electrical service, a new outdoor unit location, and refrigerant line routing.

Zoning terminology: A "zone" in HVAC refers to a separately controlled thermal region within a building, not a geographic or regulatory zone. A zoned HVAC system uses dampers, multiple thermostats, or multiple equipment units to manage different zones independently.

Scope, coverage, and limitations: The terminology defined on this page reflects Seattle-specific regulatory instruments — the Seattle Mechanical Code, the 2021 WSEC as adopted by Washington State Building Code Council, and SDCI permitting classifications. Definitions may differ under the Uniform Mechanical Code as adopted in other Washington jurisdictions. This page does not address HVAC terminology as it applies to unincorporated King County, Bellevue, or other municipalities, each of which may operate under different local amendments. Federal equipment standards (DOE efficiency minimums, EPA refrigerant regulations) apply uniformly regardless of municipality and are noted where relevant. For the full scope of this reference resource, see the Seattle HVAC Systems directory purpose and scope page.

References

📜 4 regulatory citations referenced  ·  🔍 Monitored by ANA Regulatory Watch  ·  View update log

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