Glossary · Heating, cooling & ventilation
Single-stage, two-stage and variable-speed
Also called: single-stage · two-stage · multi-stage · single-speed · two-speed · modulating
How many output levels heating or cooling equipment has: single-stage runs fully on or off, two-stage adds a low setting, and variable-speed adjusts smoothly.
What it means
Single-stage heat pumps and furnaces always run at full output, which is simple but leads to more on-off cycling. Two-stage and multi-stage models run on low most of the time and switch to high in extreme weather, while variable-speed (inverter) and fully modulating models adjust continuously. Heat pumps at the minimum efficiency ratings are typically single-speed.
Why it matters in an audit
More stages usually mean steadier temperatures and less short cycling, which matters most once air sealing has shrunk the home's heating and cooling needs.