Expansion Tank Sizing Calculator

This calculator sizes a thermal expansion tank from the real physics of water density change: Expansion Volume is System Water Volume times the fractional change in water density between Cold Fill Temperature and Maximum Hot Temperature, computed from Kell's equation -- G.S. Kell's 1975 closed-form fit for liquid water density published in the Journal of Chemical & Engineering Data -- rather than a simplified polynomial that can misbehave at higher temperatures. Minimum Tank Size then inflates Expansion Volume using an acceptance factor built from Fill Pressure and Relief Valve Pressure -- the tighter that pressure margin, the larger a tank is needed to absorb the same expansion volume without tripping the relief valve. That relationship is a genuine physical near-singularity as Fill Pressure approaches Relief Valve Pressure (a tank with zero pressure margin truly cannot absorb any expansion), and it is not only a boundary case at Fill Pressure equal to or above Relief Valve Pressure -- legal input pairs well inside the declared ranges, with only a few psi of margin between them, already push the acceptance factor toward unreasonable multiples of Expansion Volume.

To keep the recommendation useful rather than spiking, Acceptance Factor is capped at 10, so Minimum Tank Size never exceeds 11 times Expansion Volume no matter how far the margin collapses. Pre-Charge Pressure is the simplest output here: it is set equal to Fill Pressure exactly, with no contribution from System Water Volume, Cold Fill Temperature, Maximum Hot Temperature, or Relief Valve Pressure at all, since the tank's air-side pre-charge is meant to match the system's static fill pressure by definition, not to be derived from the sizing math. A Fill Pressure entered close to or above Relief Valve Pressure is a configuration that would never be specified on a real job, since the relief valve must be set well above the system's static fill pressure to leave room for expansion -- the cap exists so the tool degrades gracefully rather than recommending an absurd tank size if it is.