Air Source Heat Pumps: Complete Buying Guide Use the Heat Loss Calculator(/tools/heat-loss-calculator) to translate your envelope, infiltration, and design temperature(/glossary/design-temperature) into a BTU/hr load and a right-sized heat pump capacity before shopping models. Air source heat pump systems move heat from outdoor air into a home and can cut winter heating bills by 30–60% compared with resistive electric heat. This guide explains how an air source heat pump works, how to size one for a DIY eco home, cold‑climate performance limits, installation options and costs, model selection, and how to run a heat pump with solar and batteries. Readers will get practical examples, spec comparisons, a printable buying checklist, and a straightforward maintenance plan. TL;DR: - Choose a cold‑climate rated air source heat pump if winter temps regularly drop below -5°C; expect seasonal COPs from ~3.5 at mild temps to ~1.8 at very low temps for standard units. - Size using a basic heat‑los...