Sizing a hydronic circulator starts by turning the load into a flow rate. The heat a loop carries is flow times the fluid’s heat capacity times the temperature drop, so for water the required flow in gpm is simply the load in BTU/h divided by 500 times the design ΔT in °F. Then the pipe size, length, and fittings set the head the circulator must produce.
Where the 500 comes from
The 500 is not magic — it is 8.33 pounds per gallon of water, times 60 minutes per hour, times water’s specific heat of 1 Btu per pound per °F: 8.33 × 60 × 1 ≈ 500. In SI the equivalent is 4.19, the specific heat of water in kJ/kg·°C, giving litres per second from kilowatts and °C. Both numbers are baked in for water — and that is exactly the assumption that breaks when you add glycol.
Worked example
A 60,000 BTU/h heating load on a 20 °F design ΔT with water: 60,000 ÷ (500 × 20) = 6 gpm. Through 100 ft of ¾-inch copper with a fittings allowance, that costs roughly 9 ft of head — a job for a small wet-rotor circulator.
Glycol changes the numbers — in both directions
Antifreeze protection comes at a hydraulic cost, and it hits twice. Glycol has a lower specific heat than water, so a given load needs more flow to carry the same heat — a 50% propylene glycol mix needs about 13% more gpm. Glycol is also much more viscous, so that higher flow meets more friction — head loss can roughly double. Multiplying the water flow by a glycol correction and leaving the head alone, as many quick calculators do, undersizes the circulator on both counts. This tool applies the specific-heat and density correction to the flow and the viscosity correction to the head, and shows the penalty against water so you can see it. Glycol property values follow the ASHRAE Handbook of Fundamentals; verify the mix and temperature for a critical job.