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Sizing a heat pump for this house is a local question, not a catalog exercise. Homes in Marin County, CA run heat about nine months a year, from October through June. The refrigerant circuit cycles through the full heating season here, and a unit that is undersized or matched to a milder load will run closer to its limits, wearing the reversing valve and compressor harder than a correctly sized system would. A contractor working a quote here runs a load calculation against this house's envelope and the local temperature range before settling on capacity, because the backup heat threshold and the balance point are both specific to where this house sits.
Ductwork laid in for an earlier generation of forced-air equipment was sized to move a different volume of air at different pressures, and a heat pump asks different things of it. A contractor quoting a new installation has to assess what is already in the walls before the equipment selection means anything. Half the homes in Marin County, California were built in 1967 or earlier (2020-2024 ACS 5-year). That spread across the housing stock means a significant share of homes here have ductwork that predates the kind of system now going into them. Where the existing runs are undersized, poorly sealed, or routed for a layout that has since changed, the duct work becomes the larger part of the job.
What Gets Installed With a New Heat Pump
The outdoor unit is the piece a homeowner sees first, mounted on a concrete pad or brackets outside the house, and it holds the compressor and the main coil that pulls heat from the air. Inside, the air handler sits in a closet, attic, or utility room and pushes conditioned air through the home. Together these two units are the core of what a contractor sets in place.
A refrigerant line set runs between the outdoor unit and the air handler, connecting the two through an exterior wall. A thermostat goes on an interior wall where the homeowner controls the system, and a condensate drain line runs from the air handler to carry away moisture the coil collects.
The drawing shows a representative layout, and a licensed contractor determines what the configuration looks like in your specific home.
Get matched in Marin CountyMarin County, by the numbers
134 severe weather events were recorded in Marin County, California 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 Marin County, California were built in 1967 or earlier (2020-2024 ACS 5-year).
U.S. Census Bureau, 2020-2024 American Community Survey 5-Year EstimatesMarin County, California holds 112,116 housing units (2020-2024 ACS 5-year).
U.S. Census Bureau, 2020-2024 American Community Survey 5-Year EstimatesOwners occupy 64.7% of the occupied homes in Marin County, California (2020-2024 ACS 5-year).
U.S. Census Bureau, 2020-2024 American Community Survey 5-Year EstimatesRules and Standards for Heat Pump Installation in Marin County, California1 codes, laws, and standards
Licensing and Legal Requirements in Marin County, California
California requires a state contractor license: it is a misdemeanour to engage in the business of, or act in the capacity of, a contractor in California without one, unless the work is exempt.
Severe Weather Events in Marin County
Weather swings drive the workload a heat pump carries year after year. Sizing and installing equipment for Marin County means accounting for local conditions, and this chart shows the pattern of recorded severe weather events that shapes that load.
134 severe weather events, 2015–2024. Source: NOAA National Centers for Environmental Information, Storm Events Database.