Filling and expansion in a closed heating system

Measure the static height: the vertical distance in metres from the expansion vessel up to the highest point of the installation. The height determines the minimum pressure needed to keep water at the top of the installation.

As a guide, the pre-charge on the air side is set to the static height in metres divided by 10 (i.e. 10 m ≈ 1 bar), but at least 1 bar — often 0.2–0.3 bar is added to ensure pressure at the system's highest point. The pre-charge is measured and adjusted while the vessel is drained of water.

For maximum operating stability, the expansion vessel is connected on the suction side of the circulation pump. This prevents the pump from pulling the pressure below the pre-charge at the vessel's connection point.

Fill the system until the pressure is approx. 0.2 bar above the pre-charge — that overpressure is needed for any liquid to enter the vessel against the air cushion at all. Fill slowly so air can escape.

Vent via air separators and radiator bleed valves until no more air comes out. Trapped air causes poor heat distribution, noise and can give false pressure readings. Top up to the correct operating pressure after venting.

The safety valve is the last protection against overpressure and blows off at its set opening pressure. It must not operate during normal operation, and it does not replace the expansion vessel. The discharge from the valve must be routed safely and visibly in accordance with the applicable regulations.

The size of the vessel itself (and the expansion volume ΔV = V · e/100) is sized according to DS 469 and the manufacturer's instructions — use the «Expansion volume» calculator as a starting point and add the pipe and component contents together. Check the pre-charge pressure regularly with the vessel drained.

Pre-charge is always measured on the EMPTIED vessel — otherwise you get the wrong figure.