How to use the chilled and hot water flow calculator
Water in the piping is what actually carries the chiller or boiler output to each space. For the same load, the delta T you choose between supply and return sets the flow rate, and that flow rate then sets the pipe size and the pump. Enter the load and the two water temperatures to get the circulation flow, and add a pipe inside diameter to check the velocity as well.
A wider delta T means less flow. Flow is inversely proportional to delta T, so opening a chilled water system from 10°F to 16°F cuts the flow to about five eighths and drops pump power sharply. The trade-offs are larger coil surface and less margin for terminal control.
Velocity usually belongs between 2 and 10 ft/s. Too slow and trapped air never sweeps out while sludge settles on the bottom of the pipe. Too fast and you get flow noise, water hammer and erosion at the inside of elbows. If the check falls outside the band, step the pipe size up or down and run it again.
All of this assumes plain water. Glycol mixtures change specific heat and density enough to need a correction factor, and pump head has to be built up separately from pipe friction, fittings, valves and coils, so have a mechanical engineer review the final selection.
Frequently asked questions
Chilled water is commonly 44°F supply and 54°F return, a 10°F delta T, while hot water typically runs a 20°F delta T. Widening the delta T cuts the flow for the same load so pipes and pumps shrink, at the cost of larger coils and tighter terminal control.
Below about 2 ft/s trapped air will not sweep out of the piping and sludge settles, both of which hurt heat transfer. Above roughly 10 ft/s flow noise and erosion at elbows become the concern, so adjust the pipe size until the velocity lands between them.
No. Pump selection needs head as well as flow, and head comes from pipe friction loss plus valve and coil losses plus static lift. This calculator stops at flow and velocity, so run the head calculation separately.