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Water Flow Rate Calculator
Calculate the water flow rate needed for hydronic heating and cooling systems. Enter the heat load and flow/return temperature difference to get the flow rate in L/s and L/min.
How to Calculate Water Flow Rate
The water flow rate for a heating or cooling system is calculated as: Flow (kg/s) = Heat Load (W) ÷ (Specific Heat × ΔT), where specific heat of water is 4.18 kJ/kgK.
For a typical UK radiator system with ΔT = 11°C (75°C flow / 65°C return), a 15 kW heat load requires approximately 0.36 L/s flow rate.
Frequently Asked Questions
What ΔT should I use for radiator systems?
Traditional UK radiator systems use ΔT = 11°C (75/65°C). Low-temperature heat pump systems may use ΔT = 5–8°C (40/35°C).
What ΔT should I use for chilled water?
Chilled water systems typically use ΔT = 5–6°C (12°C flow / 6°C return). Wider ΔT reduces flow rate and pipe sizes but may reduce coil performance.
Water Flow Rate Calculator
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