Calculate boiler flow rate in L/min or gpm from heat output in kW and the supply-return temperature difference (20 °C typical), or solve for kW or delta T.

Typical boiler design value is 20 °C (11 °C on older systems). For a combi hot-water flow, enter the temperature rise, e.g. 35 °C.

Glycol percentages are by volume; DOWFROST properties at 25°C. Presets do not specify freeze protection or corrosion treatment.

Fluid properties and pipe planning
Optional. Enter both custom properties to replace the preset.

Optional. Use properties at the mean liquid temperature.

Optional. Blank omits diameter planning. Enter a project-specific limit.

Related Calculators

Boiler Flow Rate Formula

The following formula is used to calculate the boiler flow rate.

FR = (P × 3600) / (rho × cp × ΔT)

Variables:

  • FR is the flow rate (liters per hour)
  • P is the boiler power (kilowatts)
  • ΔT is the temperature rise (degrees Celsius)

Here rho is fluid density in kg/L and cp is specific heat in kJ/(kg·K). Divide heat-transfer power times 3600 by rho × cp × the temperature difference to obtain L/h. The approximate water preset uses rho = 0.997 kg/L and cp = 4.186 kJ/(kg·K). Glycol presets use published DOWFROST properties at 25°C; enter custom properties at the actual mean fluid temperature when needed.

What is a Boiler Flow Rate?

The boiler flow rate is the volume of water that can be heated by the boiler to a certain temperature in a given period of time. It is an important parameter for ensuring that the heating system can meet the heating demands of a building. The flow rate is influenced by the boiler’s power and the temperature rise required for the heating system.

How to Calculate Boiler Flow Rate?

The following steps outline how to calculate the Boiler Flow Rate.

  1. First, determine the boiler power (P) in kilowatts.
  2. Next, determine the desired temperature rise (ΔT) in degrees Celsius.
  3. Next, gather the formula from above = FR = (P * 3600) / (rho * cp * ΔT).
  4. Finally, calculate the Boiler Flow Rate (FR) in liters per hour.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

boiler power (P) = 24 kW

desired temperature rise (ΔT) = 20 °C

Using the water preset: FR = 24 × 3600 / (0.997 × 4.186 × 20) ≈ 1,035.12 L/h, or 17.2519 L/min.