Why keep water velocity under 4 ft/s?

Q: How many GPM do I need for a given BTU heating load?

Flow is found with the standard hydronic relation GPM = BTU/hr ÷ (500 × ΔT), where ΔT is the supply-to-return temperature drop. At a 20 F design ΔT, every 10,000 BTU/hr needs about 1 GPM. This calculator uses that formula and lets you set ΔT to 10 for radiant or 20 for baseboard.

Q: What pipe size should a hydronic loop be?

The tool selects the smallest nominal pipe size whose water velocity stays at or below 4 ft/s, the common residential noise-and-erosion ceiling, for the computed flow. Friction loss is then calculated with the Hazen-Williams equation using the C-factor for your chosen material. For radiant loops, a 2–3 ft/s target is preferred to limit air entrainment.

Question

Why keep water velocity under 4 ft/s?

Accepted Answer

Sustained velocity above roughly 4 ft/s causes flow noise, accelerates erosion-corrosion of pipe walls, and raises pumping head unnecessarily. ASHRAE pipe-sizing guidance and standard residential practice cap velocity at about 4 ft/s, which is the threshold this tool enforces when recommending a size.

Why keep water velocity under 4 ft/s?

Answers