Hot Water Recovery Time
Calculator

This is a simplified model: it applies a standard engineering formula to the numbers you entered. Conversions between BTU/h and kW use the exact factor 1 kW = 3412.142 BTU/h, but every sizing figure is an estimate. Air conditioner sizing uses the common 20 BTU/h per square foot rule of thumb, not a room-by-room load calculation, and it ignores insulation, glazing, orientation, ceiling height, infiltration and local climate. Heating loads use a single volumetric heat-loss factor in W/m³·K rather than a fabric-by-fabric U-value calculation. Air properties are fixed at 1.2 kg/m³ and water at 1000 kg/m³ and 4186 J/kg·K, with no correction for temperature, altitude or glycol. Duct and pipe results use the ideal continuity equation and ignore fittings, bends, roughness and system effect unless you enter those losses yourself. The Hazen-Williams equation is valid only for water in full turbulent flow at ordinary temperatures. Water hammer uses the Joukowsky surge, an upper bound for instant closure. Tank drainage assumes a prismatic tank and steady free discharge. Hot water recovery and condensate figures ignore standing losses and coil bypass. Size real systems with a proper heat-loss survey and have the work checked by a qualified HVAC or plumbing professional.