Formula & Engineering Notes
Recoverable heat (sensible, vs cold make-up):
Both enthalpies are hf at their temperature. Annual energy = rate × hours; fuel avoided = energy ÷ boiler efficiency; savings = fuel avoided × fuel cost.
All steam properties are interpolated from the shared IAPWS-IF97 steam-table datasets — see the full steam tables and the Steam Property Calculator.
Every kilogram of condensate you send to drain is paid for three times: the heat to make it, the fresh water to replace it, and the chemicals to treat that fresh water. This calculator only counts the first — the sensible heat between hot condensate and cold make-up — and it's usually already enough to justify the return line.
The defaults (make-up at 15 °C, 85% boiler efficiency, $0.05/kWh) are starting points — tune them to your site. The real-world payback is almost always faster than the heat-only number here, because the water and chemical savings ride on top for free.