How does a heat pump heat a Seattle home?
A heat pump doesn't create heat — it moves it, using a refrigerant loop to pull warmth out of outdoor air (yes, even cold air holds usable heat) and release it inside through your ducts or ductless heads. Run the loop backwards and it cools your house in summer. Because it moves heat rather than making it, a heat pump delivers roughly 2–4 times the energy it consumes — which is why Seattle's mild-but-long winters are close to ideal for this technology.
The quick answer
- One system heats in winter and cools in summer — it's an air conditioner that runs both directions
- It moves heat instead of generating it: 2–4× the energy delivered per unit consumed (U.S. DOE)
- Seattle's mild winters keep a heat pump in its most efficient operating range nearly all season
- Modern cold-climate units hold 100% heating capacity at 5°F — far below Puget Sound weather
Why 'it's too cold here for heat pumps' is out of date
That objection dates from single-stage units of decades past. Current variable-speed, cold-climate models — like the Mitsubishi H2i line we install — hold full heating output at 5°F and keep operating to -13°F. Seattle's typical winter design conditions sit far above that; a normal cold snap here is a Tuesday for this equipment.
The climate math actually runs the other way: because our winters are long but mild, a heat pump spends nearly all its hours in the gentle part of its performance curve, which is exactly where its efficiency advantage over combustion and resistance heat is biggest.
The refrigerant loop, in plain English
Refrigerant is a fluid engineered to boil at very low temperatures. Outside, it evaporates in the outdoor coil, absorbing heat from the air — cold air still holds usable heat well below freezing. The compressor squeezes that vapor, concentrating the heat; inside, the hot refrigerant condenses in the indoor coil and your blower pushes house air across it. A reversing valve flips the direction for summer cooling.
The spec numbers describe this loop's efficiency: HSPF2 rates seasonal heating efficiency (higher is better — Seattle City Light's instant discounts step up at 8.1, 8.5, and 9.5), and SEER2 rates cooling the same way.
Ducted, ductless, or dual-fuel — the three ways to run one
A ducted heat pump replaces (or pairs with) a furnace and uses your existing ductwork — the natural path for most 1980s-and-newer homes. Ductless mini-splits skip ducts entirely with wall-mounted heads, the standard retrofit for Seattle's pre-war housing stock. Dual-fuel pairs a heat pump with a gas furnace that takes over during cold snaps — a common setup when a newer furnace is too good to scrap.
What happens below freezing (defrost, explained)
In cold, damp weather — Puget Sound's specialty — frost forms on the outdoor coil. The unit periodically reverses into cooling mode for a few minutes to melt it, which is when you'll see steam rising off the outdoor unit and hear a whoosh. That's normal defrost, not a malfunction, and modern controls do it on demand rather than on a dumb timer.
Where heat pump installs go wrong
Sizing by rule of thumb. An oversized heat pump short-cycles and never settles into the efficient steady-state running it's designed for; an undersized one leans on backup heat. The right size comes from a Manual J load calculation on your actual house — insist on it in any bid.
The other install-day mistakes: skipping the duct inspection (leaky ducts waste 20–30% of what any system produces, per ENERGY STAR), and ignoring the electrical panel until change-order time. Older Seattle homes commonly carry 60–100 amp service; an honest bid checks the panel first.
In Burien and West Seattle, we also see heat pumps set on ground that doesn't drain — defrost melt refreezes into ice that looks like an equipment failure but is really placement. Raised pads and annual fall service catch both classes of problem before the first cold snap.
Questions Seattle homeowners ask
Do heat pumps really work when it's below freezing?
Yes. Cold air still contains heat, and cold-climate models are engineered to extract it — holding 100% rated capacity at 5°F and operating to -13°F on the Mitsubishi H2i units we install. Seattle weather rarely gets close to testing those limits.
Why is steam coming off my outdoor unit in winter?
That's the defrost cycle: the unit briefly reverses to melt frost off the outdoor coil, and the melt flashes to steam. It's normal in cold, damp Puget Sound weather. Ice that builds up and never clears is the thing to call about.
Will a heat pump lower my bills?
It depends on what you're switching from. Off electric baseboard or oil, the U.S. DOE and City of Seattle both put savings around 50% of heating costs. Off a newer gas furnace, honest answer: it ranges from break-even to moderate at today's rates — but you also gain air conditioning, and gas-to-heat-pump conversions carry PSE's largest rebate ($3,000–$4,000). SCL instant discounts ($300–$600 by efficiency tier) apply in City Light territory.
How long do heat pumps last?
Typically 12–15 years, sometimes longer with consistent maintenance — a bit shorter than a furnace because a heat pump works year-round, heating and cooling. Annual service and clean filters are the difference between the ends of that range.
What size heat pump does my house need?
The only defensible answer comes from a Manual J load calculation — your home's envelope, windows, orientation, and ductwork, not its square footage alone. Any bid sized 'to match the old unit' or by square-foot rule deserves a second opinion, which we provide free on replacement quotes.
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How we know this
Efficiency mechanics per U.S. DOE air-source heat pump guidance; cold-climate capacity figures from Mitsubishi H2i published specifications; savings figures from DOE, ENERGY STAR, and City of Seattle sources as cited in our energy data. Housing-stock observations from Glendale's Greater Seattle service history since 1938.
Reviewed by the Glendale Heating & Air Conditioning team · Last updated 2026-07-15
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