Quick answer
A heat pump doesn't make heat — it moves it. In winter, refrigerant in the outdoor coil gets colder than the outside air, soaks up the heat that even a 48° Puget Sound morning holds, and a compressor concentrates that warmth and releases it inside. In summer a reversing valve flips the same loop and your home's heat gets moved outdoors. Because moving heat takes far less energy than making it, a heat pump delivers roughly 2–4 units of heat for every unit of electricity it uses.
- Four parts do all the work: outdoor coil, compressor, indoor coil, and expansion device — connected in a refrigerant loop.
- The reversing valve flips the loop's direction, so one system heats in winter and cools in summer.
- Cold air still holds heat: modern cold-climate models keep extracting it well below freezing, and mild Puget Sound winters are close to ideal.
- Moving heat beats making it — that's the 2–4× efficiency edge over electric-resistance and combustion heating.
Why are heat pumps such a good fit for Washington?
Because our winters are mild by heat pump standards and our summers increasingly need cooling. A heat pump's efficiency depends on how much heat the outside air holds — and a typical Puget Sound winter day in the 40s is easy pickings compared to the climates these systems are engineered for. Then July arrives, and the same machine becomes an air conditioner for the hot stretches many Washington homes were never built to handle. One system covers the whole year, and heat pumps are among the most heavily incentivized upgrades in the state — HEAR and PSE programs may apply, and we confirm eligibility and handle the paperwork.
Who should read this guide?
Anyone weighing a furnace or AC replacement, anyone heating with electric baseboards or wall heaters, and anyone who's heard 'heat pumps don't work in the cold' and wants the real answer before making a five-figure decision. Understanding the loop below is the fastest way to see through both the hype and the myths.
How it works
Stage 1 — the outdoor coil collects heat (yes, from cold air)
Heat always flows from warmer to colder. The refrigerant arriving at the outdoor coil is very cold — colder than a winter morning — so heat flows out of the outside air and into the refrigerant, no flame required. Air holds usable heat energy far below freezing, which is why the same physics runs your refrigerator and freezer every day.
Stage 2 — the compressor concentrates it
Low-grade warmth spread through refrigerant isn't hot enough to heat a house — so the compressor squeezes it. Compressing the refrigerant raises its temperature sharply, turning 'slightly warmed' into 'hot enough to warm your living room.' The compressor is the heart of the system and the part doing most of the electrical work.
Stage 3 — the indoor coil releases the heat
The hot refrigerant flows through the indoor coil while the air handler blows your home's air across it. The air picks up the heat and the supply registers deliver it through the house. The refrigerant, having given up its heat, moves on cooler than it arrived.
Stage 4 — the expansion device resets the cycle
The expansion device drops the refrigerant's pressure, which drops its temperature back below the outdoor air — ready to collect heat all over again. The loop runs continuously, moving heat steadily instead of blasting on and off like a furnace.
The reversing valve — how one machine heats AND cools
Flip the thermostat to cooling and the reversing valve swaps which coil does which job: the indoor coil becomes the collector (absorbing heat from your home's air) and the outdoor coil becomes the releaser (dumping that heat outside). Same four parts, opposite direction — that's the whole trick, and it's why a heat pump replaces both a furnace and an air conditioner.
Key terms and context
This guide is written for heating & air decisions in the Puget Sound. It uses the same terminology you'll hear from inspectors, technicians, and permit offices.
"Heat pumps don't work when it's cold" — the honest version
Older single-speed units did lose ground in deep cold, and the reputation stuck. Modern cold-climate heat pumps use variable-speed compressors engineered to keep extracting heat well below freezing — and Puget Sound winters rarely go there anyway. Systems here are set up with backup heat for the rare deep-cold snap, so you're never without warmth. What you might notice: a defrost cycle that briefly steams the outdoor unit on frosty mornings. That's normal operation, not a malfunction.
The diagram can't save a bad installation
The physics works every time; installations don't. An oversized or undersized system short-cycles or falls behind, badly designed ductwork chokes airflow, and a sloppy refrigerant charge quietly taxes efficiency for the life of the unit. That's why a real load calculation — not a rule of thumb off the old furnace's label — is the non-negotiable first step of a good heat pump install.
How we build this guidance
- Written by the team that installs and services heat pumps across the Puget Sound — the seasonal diagram reflects how we explain the cycle in real living rooms.
- Cold-weather guidance matches what we see on service calls: defrost cycles are normal, and sizing/installation quality decide real-world performance.
- Efficiency framing (2–4 units of heat per unit of electricity) is the same conservative range we use in estimates — no cherry-picked lab numbers.
Methodology: Explanation follows the standard vapor-compression refrigerant cycle; cold-climate and defrost behavior reflect the heat pump installations and service calls Eco performs across the Puget Sound. Rebate availability changes — we verify current HEAR and PSE eligibility per project.
Last updated: 2026-07-03
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Common questions
Does a heat pump really work on a freezing Washington morning?
Yes. Air holds usable heat energy well below freezing, and modern cold-climate heat pumps are built to extract it — Puget Sound winters are mild by heat pump standards. Systems are installed with backup heat for the rare deep-cold snap, so the house stays warm no matter what.
How can it be more than 100% efficient?
Because it moves heat rather than making it. Electric-resistance heat converts one unit of electricity into one unit of heat, full stop. A heat pump uses one unit of electricity to move 2–4 units of already-existing heat from outside to inside — the heat itself is free; you only pay to relocate it.
Is the steam coming off my outdoor unit in winter a problem?
Usually the opposite — that's the defrost cycle working. Frost naturally builds on the outdoor coil on cold, damp mornings, and the system briefly reverses to melt it, which releases steam. A unit encased in solid ice for hours is a different story and worth a service call.
Do I still need a furnace or an air conditioner?
No — one heat pump does both jobs, which is the point. The reversing valve turns the heater into an air conditioner for the increasingly hot Washington summers. Some homes pair a heat pump with an existing furnace as backup (a 'dual fuel' setup); we'll tell you honestly whether that fits your home.
Is a ductless mini-split the same thing?
Same technology, no ducts. A ductless mini-split runs exactly the loop in the diagram — outdoor unit, refrigerant lines, indoor coil — but delivers the air from wall-mounted heads instead of ductwork. It's often the right call for older Puget Sound homes without ducts, additions, and ADUs.