Hot Water Recirculation Heat Loss & Cost Calculator

How it works

Loss (BTU/h) = coefficient (BTU/h·ft·°F, by pipe size and insulation) × ΔT × length; annual cost = loss × hours × 365 ÷ efficiency × fuel price; pump kWh = W × hours × 365 ÷ 1000

A recirculation loop keeps hot water at every tap by continuously pumping it around the house, which means the entire pipe run behaves like a long, thin radiator all day. Bare 3/4-inch copper sheds around 0.4 BTU per hour per foot per degree of temperature difference; half an inch of foam cuts that by about 70%, an inch by 80%. Over a year even an insulated 150-foot loop leaks several million BTU — often more energy than the hot water actually used — so the economics hinge on insulation and on not running the pump when nobody needs it: a timer or, better, a demand-activated pump that runs only for a minute when a button or sensor calls for it captures nearly all the water and waiting-time benefit at a tenth of the energy cost.

Worked example

A 150 ft loop of 3/4″ copper with 1/2″ foam at 125 °F in a 65 °F basement loses 0.12 × 60 × 150 = 1,080 BTU/h — 9.5 MMBTU a year, about $166 of gas at $1.40/therm plus $33 for a 25 W pump. Insulating to 1″ would trim a further 30%; an 8-hour timer saves two-thirds; the loop still saves about 8,200 gallons a year of water otherwise run down the drain.

Frequently asked questions