Choosing a pipe for heating and don't know which diameter to take? A simple way to find the required diameter and check that the flow velocity and pressure drop on the section are within range.
What you'll learn
You're laying out the heating and holding pipes of different diameters — which one to take? Too small a pipe "hums" and makes life hard for the pump; too large is an overpayment for material. In this article, with no formulas, we show how to choose the right diameter for one section of the loop and check that everything will run quietly.
What the diameter depends on
To deliver heat to the radiators, hot heat-carrier (water) flows through the pipe. The required diameter depends on three things:
- how much heat this section carries (the loop power, kW);
- the temperature difference between the supply and the return (ΔT, usually 10–20 °C);
- the section length over which we calculate the pressure drop.
From this data the calculator works out the heat-carrier flow rate and picks the nearest standard diameter of pipe.
Why flow velocity matters
Once a diameter is chosen, a certain water velocity is established in the pipe. If it's too high, noise and extra resistance appear; if it's too low, heat arrives sluggishly. For heating, the comfortable velocity range is roughly 0.3–0.7 m/s. The calculator shows the actual velocity so you can see whether it's within range.
The section power is conveniently taken from the heat-loss calculator: it gives how many kW a house or room needs — that's the heat your loop carries.
What else the calculator shows
Besides the diameter and velocity, you'll see:
- the flow regime — calm (laminar), transitional or turbulent;
- the pressure drop — per metre, per 100 m and over the whole section.
The pressure drop matters for understanding whether the pump will "pull" this section.
How to do it
- Open the hydraulic calculator.
- Enter the section power (kW), the temperature difference (ΔT) and the length.
- Get the recommended diameter, velocity, flow regime and pressure drop.
- If the velocity is too high or too low, try the neighbouring diameter.
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Example
A section carries 10 kW, ΔT = 15 °C, length 20 m. The calculator picks a standard inner diameter, shows a velocity of about 0.5 m/s (within range), a turbulent regime and a moderate pressure drop — a section like this the pump will move without trouble.
The calculator handles one section of the loop. Balancing a complex network with many loops is a separate task that isn't done here.
Frequently asked questions
What is ΔT and what value should I take? It's the difference in temperature between the supply and the return. For domestic heating, 15–20 °C is often used.
Why is the diameter "standard"? Pipes are produced in fixed sizes, so the calculator rounds to the nearest available one.
And if there are several sections? Calculate each one separately. The calculator doesn't do overall network balancing.
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