Hydronic Static Fill Pressure

A closed loop must be pressurised enough to keep water — and air — where they belong at the top of the building. The static requirement is pure hydrostatics, ρgH, and North American technicians carry it as H ÷ 2.31 = psi, since a 2.31 ft column of water weighs 1 psi (62.3 lb/ft³ ÷ 144 in²/ft²). To that you add a margin, conventionally 4 psi (28 kPa), so the highest point stays positively pressurised and the automatic air vent up there can actually vent instead of sucking air in.

Worked example: a top emitter 30 ft above the boiler-room gauge needs 30 ÷ 2.31 + 4 ≈ 17 psi, which is why residential PRVs ship set at 12 psi and every three-storey job needs that setting raised — a fact discovered most often by the tenant on the top floor whose radiator never gets hot. In metric, 15.3 m of lift needs about 150 kPa plus margin. Two traps: measure H from the gauge, not from grade, and remember the relief valve. A standard 30 psi relief with an 18 psi fill leaves only 12 psi of headroom for thermal expansion, so the tank sizing and the fill pressure have to be chosen together or the valve will tell you about it every evening.