Hydronic Water Flow Rate Calculator (Heating and Chilled Water, kW to m³/h)
The water flow a heating or cooling load needs for a chosen temperature difference, and the pipe diameter or the velocity that flow gives — drawn against ΔT.
Results
The dashed line marks the value you entered.
What this tool does
The energy balance P = ṁ·c_p·ΔT turns a load in kW into a flow once you choose how much the water is to warm up or cool down: 20 kW at 20 K needs about 0.86 m³/h of water. The smaller the ΔT, the larger the flow and the pipe — which is why low-temperature systems such as heat pumps and chilled water, often designed for ΔT of 5–10 K, need bigger pipes and pumps than old 20 K radiator systems. Heating pipework is commonly sized for velocities of around 0.5–1.5 m/s, lower in small bore where noise matters. Use the right c_p and density for glycol mixtures.
Formula
ṁ = P ÷ (c_p × ΔT) · V̇ = ṁ ÷ ρ · d = √(4 V̇ ÷ (π v)) (kW, kJ/(kg·K), K, kg/m³, m/s)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
pl | Heat load or cooling load | kW |
dt | Temperature difference between flow and return | K |
cp | Specific heat of the fluid | kJ/(kg·K) |
ro | Density of the fluid | kg/m³ |
v | Velocity you want in the pipe | m/s |
di | Inside diameter of the pipe you have | mm |
QH | Water flow needed | m³/h |
QL | Water flow in litres per second | L/s |
QM | Mass flow | kg/s |
DR | Inside diameter needed for your velocity | mm |
VA | Velocity in the pipe you have | m/s |
Worked example
- Heat load or cooling load20 kW
- Temperature difference between flow and return20 K
- Specific heat of the fluid4.18 kJ/(kg·K)
- Density of the fluid1000 kg/m³
- Velocity you want in the pipe1 m/s
- Inside diameter of the pipe you have20 mm
- Water flow needed0.861 m³/h
- Water flow in litres per second0.239 L/s
- Mass flow0.239 kg/s
- Inside diameter needed for your velocity17.5 mm
- Velocity in the pipe you have0.76 m/s
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.