Understanding pressure loss in heating hydraulics
What pressure loss covers: friction, fittings, singularities
Pressure loss is the drop in pressure that occurs as water circulates. It comes from friction in the pipes (roughness, viscosity), but also from fittings. A valve, a filter, a heat exchanger or a clogged circulator all add their own resistance, just like elbows, tees and reductions.
Linear losses vs. singular losses: where the gaps hide
Linear losses mainly depend on length, diameter and flow velocity. Singular losses are tied to each "hard point" in the network, with a coefficient per fitting. In renovation work, it's often the accumulation of connections that drives pressure loss up without anyone seeing it coming.
Why your network becomes unbalanced: flow, velocity, diameter
When flow is too high or the diameter too tight, high velocity makes losses explode. The result is that the shortest branches take the flow, and the emitters at the end of the network run short of hot water. Sizing, balancing and circulator adjustment bring the network back into line.
Diagnosing your network: measurements and calculations useful on site in 2026
Field readings: ΔP, flow, temperatures, component settings
On a hot-water network, start by measuring ΔP at the key points (circulator, filter, heat exchanger), then the actual flow rate and the flow and return temperatures. With these on-site readings, you can check whether the pump is working within its curve and whether the balancing holds up. Also note the settings of balancing valves, two-way valves, thermostatic radiator valves and check valves.
Spotting hard points: filters, sludge, valves, heat exchangers, lengths
Abnormal ΔP across a component often points to a local restriction. Look for a clogged filter, sludge, a partially closed valve, a fouled heat exchanger, or runs that are too long or too "bent". A return temperature that's too high can signal insufficient useful flow or poor balancing. The goal is to restore stable circulation.
Quick calculation: a simple method for estimating pressure loss per section
For a quick check, estimate the pressure loss of a section using an order-of-magnitude figure in Pa/m based on diameter and flow, then multiply by the length. Add a "fittings" allowance (elbows, tees, valves) equivalent to a few metres of pipe. Compare the total to the circulator's available ΔP. If it exceeds it, the cause is rarely mysterious. It's usually the flow rate, the diameter, or an obstruction.
Sizing diameters and limiting pressure loss without extra cost
Choosing the right diameter: targeting the right water velocity and avoiding noise
A diameter that's too small means water velocity that's too high. The network whistles, circulators strain, and pressure loss climbs fast. In practice, aim for a steady, moderate velocity on the main sections, then fine-tune closer to the actual flow rates in distribution.
Reducing lengths and elbows: routing, headers, path planning
Every metre and every elbow adds resistance. A direct route reduces pressure loss without changing any equipment. Think about headers close to the zones they feed, straight runs, and avoid zigzags. Where possible, replace several small elbows with a simpler path.
Selecting fittings: valves, filters, check valves with low ΔP
Fittings can be costly in terms of ΔP. Choose valves with the right Kv, correctly sized filters, and check valves suited to the flow rate. The goal is low ΔP at nominal flow, to keep control stable and limit pumping power.
Adjusting and balancing the network's hydraulics to stabilise flow rates
Balancing with valves: progressive adjustment and flow control
Start by fully reopening all valves, then make a progressive adjustment circuit by circuit. The goal is to bring each loop closer to its target flow without "starving" the others. Check with a flow meter, pressure test points, or the manufacturer's charts. Work from nearest to farthest, and note down the positions so you can go back if needed.
Pump and control: curve, variable speed, differential pressure
A pump that's too "strong" creates noise, fluctuations and excess consumption. Check the curve, then adjust the speed or the automatic mode to maintain stable pressure. The right setting depends on the network's actual pressure loss and the differential pressure level to be maintained when the valves close.
Dealing with faults: sludge build-up, bleeding, flushing and protection
If some emitters stay lukewarm, suspect air and sludge. Bleed the system, check the dirt separators, then run a suitable flush if the water is black. Finish with durable protection: a magnetic filter, an inhibitor and controlled water top-up. You'll stabilise flow rates, and the controls will breathe easier.
Practical cases: optimising pressure loss by network type
Radiator networks: single-pipe, two-pipe, risers
On single-pipe systems, pressure loss climbs fast if the radiator tappings are too "throttled". Keep diameters consistent, fit adjustable valves and carry out room-by-room balancing. On two-pipe systems, aim for a simple distribution: tee adjustment, limited velocities, and a variable-speed circulator. On risers, track down unnecessary loops and stabilise pressure with a differential valve if needed.
Underfloor heating: loops, headers, maximum lengths and admissible ΔP
On underfloor heating, everything hinges on the header. Keep loops of similar length, and avoid exceeding 80 to 120 m depending on the pipe. On the ΔP side, stay within a "sensible" range (often 20 to 30 kPa per loop) to keep margin at the circulator. A measurable flow rate on flow meters saves time. For more on design, see also sizing a low-temperature underfloor heating system.
Renovation with a heat pump: securing flow and avoiding fault shutdowns
With a heat pump, excessive pressure loss quickly translates into insufficient flow and fault shutdowns. Secure the minimum flow rate. Decouple if the network is temperamental (hydraulic separator, buffer tank), and avoid valves that close "all together". A properly adjusted bypass, and the heat pump breathes easy.
Key figures
consumption ×2 if the network is poorly sized
Circulator impact
0.7 to 1.0 m/s
Max velocity
10 to 20 mmWC/m
Target linear loss
Frequently asked questions
Read the available ΔP of the circulator (from the display or the nameplate) and compare it to the sum of estimated losses: Pa/m × length plus the "equivalent metres of pipe" for fittings. If the ΔP measured across the circulator's terminals is high but end flow rates stay low, the pump is outside its useful operating zone, or a component (filter/heat exchanger) is clogged.
Louis Meneteau
CPO of Argile
