You already own two heat pumps
Your refrigerator and your freezer. A refrigerator doesn't "make cold" — it pumps heat out of the food compartment and dumps it into your kitchen through the warm coils on the back. A home heat pump is the same machine scaled up and pointed the other way: it pumps heat out of the outdoor air and dumps it into your living room. Flip a reversing valve and it runs backward in summer, becoming a high-efficiency central air conditioner.
The four-step refrigeration cycle
- Evaporate (outside, absorb heat). Liquid refrigerant at very low pressure boils at extremely low temperatures — modern refrigerants like R-410A and R-454B can evaporate below −40°F. Because the refrigerant in the outdoor coil is colder than even a −22°F morning, heat flows from the "cold" outdoor air into the refrigerant. Physics doesn't care that the air feels cold to you; it only cares which substance is colder.
- Compress (concentrate the heat). The compressor squeezes the now-gaseous refrigerant to high pressure, which drives its temperature up to 120–150°F. This is the step that consumes electricity — and it's the only meaningful energy input in the whole cycle.
- Condense (inside, release heat). The hot, high-pressure gas flows to the indoor coil, where a quiet fan moves room air across it. The refrigerant condenses back to liquid, releasing all the heat it collected outside plus the heat added by compression.
- Expand (reset). An expansion valve drops the pressure, the refrigerant gets very cold again, and the loop repeats — continuously, dozens of times per minute.
A 95%-efficient propane furnace turns 1 unit of fuel energy into 0.95 units of heat — that's the ceiling for anything that burns. A heat pump uses 1 unit of electricity to move 2–4 units of free heat from outdoors. Delivered heat divided by electricity consumed is called the Coefficient of Performance (COP). A COP of 3.0 means 300% effective efficiency. Cold-climate units maintain a COP around 1.5–2.0 even near their −22°F limit, and 3–4+ in mild weather — averaging out to roughly 2.5–3.2 over a full Maine heating season.
What changed: the cold-climate breakthrough
Heat pumps earned a bad reputation in the 1980s and 90s because early single-speed units genuinely did quit below about 25°F. Three engineering advances created the modern cold-climate machine:
- Inverter-driven variable-speed compressors. Instead of a compressor that's either off or at 100%, an inverter compressor runs anywhere from ~15% to ~120% of nominal speed. When it's brutally cold, it spins faster and harder to maintain output; when it's mild, it idles along sipping electricity. This is also why the heat feels steady instead of blasting on and off.
- Enhanced vapor injection (EVI) / two-stage compression. A portion of refrigerant is flash-evaporated and injected mid-compression, cooling the compressor and boosting both capacity and reliability at extreme low temperatures. EVI is the core technology behind −22°F operation and behind "hyper-heating" designs that hold 100% of rated capacity all the way down to +5°F.
- Smarter defrost and coil design. Large outdoor coils, base-pan heaters, and demand-based defrost cycles manage the frost that forms when extracting heat from humid winter air — briefly and automatically, without freezing you out.
The main system types
| Type | What it looks like | Best fit |
|---|---|---|
| Ductless single-zone (mini-split) | One outdoor unit feeding one indoor wall, floor, or ceiling unit | Open floor plans, additions, the classic Maine "heat the main living space, keep the boiler as backup" strategy. Also the configuration Efficiency Maine's per-unit rebates are built around. |
| Ductless multi-zone | One larger outdoor unit feeding 2–5+ indoor heads | Whole-home coverage without ducts. Note: multi-zone outdoor units are currently not eligible for Efficiency Maine's per-unit rebate — two single-zone systems often out-rebate one multi-zone. |
| Ducted (central) | Outdoor unit paired with an air handler on conventional ductwork | Homes that already have ducts from a furnace or central AC; qualifies for Efficiency Maine's larger whole-home lump-sum rebates when sized for 100% of heating load. |
| Compact-ducted / hybrid | Small concealed air handlers serving a few rooms each | Bedroom wings and finished capes where wall units aren't wanted. |
Ground-source (geothermal) heat pumps use buried loops instead of outdoor air and reach even higher efficiencies, but at several times the installed cost. This site focuses on air-source systems, which represent the overwhelming majority of Maine installations.
Next: what actually happens to output and efficiency when the thermometer heads for the basement — Cold-climate performance.