Calculate the required water flow rate for your HVAC or hydronic heating system.
The total heat output required (kW).
Temperature of water leaving the boiler/heat pump.
Temperature of water returning to the heat source.
Default for water is 4.186.
Calculation Results
Temperature Difference (ΔT):20.00 °C
Flow Rate (Liters per Minute):7.17 l/min
Flow Rate (Liters per Hour):430.01 l/h
Flow Rate (m³/h):0.43 m³/h
Please ensure Flow Temperature is higher than Return Temperature.
Understanding Heating Flow Rate
The heating flow rate is the volume of water moving through a central heating system per unit of time. Getting this calculation right is critical for ensuring that radiators, underfloor heating, or fan coil units receive the exact amount of thermal energy required to heat a space effectively.
The Physics of Heat Transfer
In hydronic systems, water acts as the energy carrier. The amount of heat delivered depends on three primary factors:
Mass Flow Rate: How much water is moving.
Specific Heat Capacity: The ability of the fluid to hold heat (Water is approx 4.186 kJ/kg·°C).
Temperature Drop (Delta T): The difference between the flow temperature (leaving the boiler) and the return temperature (returning to the boiler).
The Flow Rate Formula
m = Q / (Cp × ΔT)
Where:
m = Mass flow rate (kg/s)
Q = Heat load (kW)
Cp = Specific heat capacity (kJ/kg·°C)
ΔT = Temperature difference (Flow – Return)
Common Delta T (ΔT) Standards
Depending on your system type, the design temperature difference will vary:
System Type
Typical ΔT
Traditional Gas Boiler (Radiators)
11°C or 20°C (Modern condensing)
Air Source Heat Pumps
5°C to 7°C
Underfloor Heating
5°C
District Heating Heat Exchangers
30°C to 40°C
Calculation Example
Imagine a house with a heat loss calculation of 12 kW. You are using a condensing boiler designed for a 20°C drop (e.g., 70°C flow and 50°C return).