Excessive pressure drop in a heating system means the pump works against too much resistance, heating circuits receive uneven flow and energy is wasted. Undersized pipes are often the cause of noise, flow rumbling and poor heat distribution throughout a building. The pipe pressure drop calculator uses the Darcy-Weisbach equation to determine pressure loss and shows whether your pipe size is suitable for the planned flow rate.
Step by Step: How to Use the Pipe Pressure Drop Calculator
- Enter pipe diameter: Enter the inner diameter of the pipe in mm. For heating pipes: DN 15 (inner approx. 15.5 mm), DN 20 (approx. 21 mm), DN 25 (approx. 26 mm).
- Enter pipe length: The total length of the pipe section being calculated in meters – e.g. 25 m for a heating loop.
- Enter flow rate: The flow rate in litres per hour or m³/h. For a 2 kW radiator circuit with a 10 K temperature spread: Q = P / (c × ρ × ΔT) = 2000 / (4186 × 1 × 10) = 0.0478 l/s = 172 l/h.
- Select pipe material: Copper (roughness 0.0015 mm), PE pipe (0.0015 mm), steel (0.046 mm). Roughness affects the friction coefficient.
- Check the result: For heating systems, a maximum of 100–150 Pa/m is recommended. If the value is higher, use a pipe with a larger diameter.
Practical Examples
Example 1 – Radiator connection: DN 15 copper pipe, 8 m length, flow rate 200 l/h. Flow velocity v = Q/A = 0.0556 l/s / (π × 0.00775²) = 0.295 m/s – good (0.3–0.5 m/s is recommended for heating). Pressure drop approx. 50 Pa/m × 8 m = 400 Pa – acceptable.
Example 2 – Underfloor heating circuit: PE pipe 17 × 2 mm (inner 13 mm), 80 m length, 60 l/h. Flow velocity: very low, approx. 0.13 m/s – within the recommended underfloor heating range (0.1–0.3 m/s). Pressure drop is manageable.
Example 3 – Main distribution pipe: DN 50 steel pipe, 30 m, 3,000 l/h. High flow rate: velocity approx. 0.43 m/s – still within the permitted range. Pressure drop approx. 80 Pa/m = 2,400 Pa total – acceptable for a main distribution line.
Pipe Pressure Drop Calculation
Formula (Darcy-Weisbach): Δp = λ × (L/D) × (ρ × v²/2). Friction factor λ: copper 0.02, PE 0.015, steel 0.025. Guideline: max. 100–150 Pa/m for heating systems.
Frequently Asked Questions (FAQ)
- What are local pressure losses and does the calculator account for them?
- Local losses arise at bends, tees, valves, elbows and transition fittings. They are expressed as equivalent pipe length (in meters) or as a resistance coefficient ζ (zeta). As a rule of thumb: local losses amount to roughly 30–50% of the straight-pipe friction losses. The calculator computes pipe friction losses only – add 30–50% manually to account for fittings.
- What is the optimal flow velocity in heating pipes?
- Too slow (below 0.1 m/s): risk of sludging and poor heat distribution. Too fast (above 0.8 m/s): flow noise and elevated pressure drop. Recommended: 0.2–0.5 m/s for main pipes, 0.1–0.3 m/s for underfloor heating loops.
- How do copper and PE pipes compare in terms of pressure drop?
- Copper and PE pipes have very similar internal roughness (both approx. 0.0015 mm – hydraulically smooth). The practical difference in pressure drop is minimal. Steel and cast-iron pipes have significantly higher roughness (from 0.046 mm) and therefore 10–20% higher friction losses at the same diameter.
