Guide

Flow rate in heating and plumbing

Volumetric flow describes how much liquid passes through a pipe per unit of time. It is independent of pipe diameter — the diameter only affects velocity.

Three notations for the same quantity

Heating engineering works in cubic metres per hour, pump and fitting data often in litres per minute, some datasheets in litres per second. The relation is simple: 1 m³/h equals about 16.667 l/min or 0.2778 l/s. One litre per minute equals 0.06 m³/h. Because the numeric ranges differ widely, factor errors creep in easily when estimating.

Diameter and velocity

Volumetric flow does not change because a pipe becomes narrower — the same amount still passes through. What changes is the flow velocity, and with it the pressure loss. A constriction raises velocity and therefore friction, which is why narrow pipes need more pump power at the same volumetric flow.

US gallons per minute

Anglo-American datasheets use gpm, US gallons per minute. A US gallon measures 3.785411784 litres, so 1 gpm equals about 3.785 l/min. The imperial gallon is a different unit at 4.54609 litres and is practically never used for flow — when in doubt, check the datasheet footnote.

What volumetric flow alone does not say

Volumetric flow is not sufficient for sizing a heating system. The power transferred follows from volumetric flow, specific heat capacity and the temperature difference between flow and return. Volumetric flow alone describes how much water is moved, not how much heat it carries.

Matching conversions

Matching tables

Unit family: Flow rate

Sources