How to balance radiators

Water follows the path of least resistance. Radiators close to the boiler on short pipe runs get more flow than they need; those at the end get too little. It happens when a system is never adjusted after installation and again after replacing a radiator, adding one or changing the boiler.

1. Bleed every radiator and bring the system pressure to the value your boiler specifies (often 1.2–1.8 bar cold).
2. Open all thermostatic valves (flow) and all lockshield valves (return) fully. Note how many turns open each lockshield was before you touch it.
3. Switch the heating on and let the system settle for 30–60 minutes.
4. Measure flow and return temperatures at each radiator with an infrared or clip-on thermometer. A typical target for a gas boiler is a 10–20°C difference; with a heat pump, smaller (about 5–8°C).
5. Start with the radiators closest to the boiler: close the lockshield until the flow–return difference rises towards the target. Move outwards in order of distance.
6. Check again after an hour and fine-tune. The furthest radiators usually stay almost fully open.

Many modern thermostatic valves have a pre-setting ring (values 1–6 or N) that limits the maximum flow on the flow side. Correct values come from a hydraulic calculation based on each radiator's output; without one, the same logic applies — lower settings near the boiler, higher at the end.

• even temperatures without turning cold rooms to maximum;
• fewer boiler starts and stops;
• a cooler return, which helps a condensing boiler actually condense;
• less flow noise in radiators near the boiler.