Every cold snap brings a wave of worried heat pump calls, and a good share of them end with us explaining that nothing is broken — heat pumps just behave differently from the furnaces people grew up with. Here's the field guide to which winter behaviors are physics and which are faults.
Normal things that feel wrong
Supply air that feels merely warm. A furnace blasts 120°F+ air; a heat pump delivers a steady 90–105°F — warmer than your skin, so the house heats fine, but it can feel "cool" from a vent. That's design, not decline. Longer, gentler run times. Heat pumps are marathoners, not sprinters; near-continuous running in cold weather is how they're meant to work. Steam and gurgles during defrost. Every so often the outdoor unit pauses heating, runs briefly in reverse, hisses, and vents a dramatic cloud of steam — that's the defrost cycle clearing frost off the coil. Startling, normal.
Actually-broken things
An outdoor unit turned into an ice sculpture. Light frost is normal; a coil encased in thick ice means defrost has failed (sensor, board, or reversing valve) and the unit is suffocating. That's a service call — see our icing guide. Cold air, actually cold. Supply air at room temperature or below means the reversing valve, refrigerant charge, or compressor deserves instruments. Electric bills that suddenly doubled. Usually the auxiliary electric heat strips running far more than they should — because the heat pump side is underperforming or a thermostat setting is calling for aux constantly. Aux heat is the space heater of last resort; when it becomes the main act, something upstream needs diagnosis.
The thermostat trap
Big setbacks — dropping to 60°F at night and demanding 70°F first thing in the morning — force a heat pump to summon expensive aux heat every morning. These systems run cheapest held steady or nudged 2–3 degrees. If your bill hurts, this one habit is often half the story.
The local reality check
Our marine climate is heat pump paradise — winter design temperatures around Puget Sound sit comfortably within a modern unit's happy range, which is why they've spread across Shoreline and Edmonds so fast. A heat pump truly struggling at 35°F here isn't facing hard weather; it's facing a fault, low charge, or airflow problem, and instruments will name which. A written quote after the diagnosis, and the honest word on whether it's the machine or the settings.