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.
Formula and source
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.
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