Hydronic Fill Pressure (Static Height)

The cold-fill make-up pressure a hydronic (hot-water heating) loop needs to lift water to its highest point and keep it above atmospheric so air is not drawn in and the pump does not cavitate. fill = height / 2.31 + margin (2.31 ft of water = 1 psi), a common margin about 4 psi. A 30 ft system needs 17.0 psi (leaving 4 psi at the top); a 35 ft system needs 19.2 psi -- each story adds about 4.3 psi of static lift. Set the automatic fill valve and expansion-tank pre-charge to this; the relief valve sits well above. A design aid; the manufacturer's instructions govern.

static = height / 2.31; fill = static + margin; 2.31 ft of water column = 1 psi; margin (~4 psi) keeps the top of the loop above atmospheric.

Hydronic fill-pressure by static height, standard heating-system practice (ASHRAE Handbook, HVAC Systems and Equipment; boiler manufacturer instructions) by name.

The height / 2.31 static-head conversion is public physics; the boiler and fill-valve settings are manufacturer data.

Estimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.

Field names used by the API: height_ft, margin_psi, fill_psi, static_psi

• Static head static = height / 2.31 (2.31 ft of water = 1 psi)public physics
• Top margin add ~4 psi so the top of the system stays above atmospherichydronic practice
• Manufacturer governs the boiler's fill and relief settings and the expansion-tank pre-charge governmanufacturer instructions