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Sizing a heat pump for King County starts with understanding what the system is actually being asked to do. Homes in King County, WA run heat about ten months a year, from September through June. A heat pump's output drops as outdoor temperatures fall, so the coldest weeks of that heating season are the weeks the equipment is most likely to fall short if the contractor undersized the unit. Backup resistance heat fills the gap, but it runs at a much higher operating cost than the heat pump itself. A contractor quoting a new system here establishes the full heating load before specifying equipment, because the install that looks adequate in October gets tested hard by February.
Ductwork sized for a furnace and central air setup does not automatically carry a heat pump's airflow demands, and in a county where much of the housing predates the equipment now running in it, that gap between what was built and what is being installed is where a contractor's assessment spends most of its time. Half the homes in King County, Washington were built in 1983 or earlier (2020-2024 ACS 5-year). That means the electrical service and the existing duct runs get opened and measured before any equipment gets priced, because what a heat pump draws and what the existing system can move are the two numbers that decide whether the installation is straightforward or not.
What Gets Installed With a New Heat Pump
The outdoor unit is the piece a homeowner sees first, set on a concrete pad or wall brackets beside the house, housing the compressor and the coil that pulls heat from the outside air. Inside, the air handler mounts in a utility closet, attic, or mechanical room and pushes conditioned air through the home.
A refrigerant line set runs between the two units, carrying the working fluid that connects them. The thermostat goes on an interior wall and gives the homeowner control over the whole system. Some installations also include a supplemental electric strip heater inside the air handler for backup on the coldest nights.
The drawing shows a typical layout, and a licensed contractor determines what applies to this particular house.
Get matched in King CountyKing County, by the numbers
1 severe weather event was recorded in King County, Washington between 2015 and 2024.
NOAA National Centers for Environmental Information, Storm Events Database, bulk CSV export, events 2015-01-01 through 2024-12-31.Half the homes in King County, Washington were built in 1983 or earlier (2020-2024 ACS 5-year).
U.S. Census Bureau, 2020-2024 American Community Survey 5-Year EstimatesKing County, Washington holds 1,003,284 housing units (2020-2024 ACS 5-year).
U.S. Census Bureau, 2020-2024 American Community Survey 5-Year EstimatesOwners occupy 55.4% of the occupied homes in King County, Washington (2020-2024 ACS 5-year).
U.S. Census Bureau, 2020-2024 American Community Survey 5-Year EstimatesRules & Standards Governing Heat Pump Installation in King County, Washington1 sourced rules and standards
Contractor Rules in King County, Washington
Washington requires contractor registration rather than an HVAC licence: every contractor must register with the Department of Labor and Industries, and advertising, bidding or working as a contractor without that registration is a gross misdemeanour.