DHW Recirculation Loop Calculator

Required Flow Rate is Heat Loss divided by 500 times Allowable Temp Drop, and Heat Loss is itself directly proportional to Allowable Temp Drop -- so the deltaT terms cancel algebraically. The flow rate needed to offset heat loss ends up depending only on pipe geometry and insulation, not on the allowed temperature drop.

Insulation R-Value sits in the denominator of the heat-loss formula (Q = surface area x deltaT / R-Value), so more insulation directly and predictably reduces Heat Loss across its full 0.5 to 20 R-Value range. Heat Loss is rounded to the nearest whole BTU/hr, though, so it does floor and plateau at 0 once the surface-area-times-deltaT term gets small relative to R-Value -- for a small-diameter, short, or low-deltaT loop that floor can cover most of the R-Value range, not just its extreme top end.

Return Temperature is calculated as a fixed 140 F standard supply temperature minus Allowable Temp Drop, a direct one-to-one subtraction. Raising Allowable Temp Drop by one degree lowers Return Temperature by exactly one degree, with no other input involved.

Pump Size compounds a small Required Flow Rate against a modest head-loss estimate for the loop, and the resulting horsepower is genuinely tiny across most of this calculator's input range -- including its own default inputs, not only short or well-insulated loops -- so the calculator floors the displayed value rather than showing a number that rounds to zero.