Expansion tank sizing — why the tank's height in the building matters

Building section

Fill temperature to max average temperature, with the tank you sized.

Building, system, and tank

Where things are

Highest piping above the lowest piping 120 ft

Boiler room elevation above the lowest piping 0 ft

Tank elevation above the lowest piping 0 ft

Minimum pressure at the highest point 4 psig

Relief valve at the boiler

Piping

Water

System water volume 1,500 gal

Fill temperature 50 °F

Maximum average water temperature 180 °F

What the tank has to do

Water the system grows by	–gal
Pressure at the tank, cold (precharge)	–psig
Pressure at the tank, hot (limit)	–psig
Acceptance factor	–
Required tank volume	–gal
Water in the tank when hot	–of the tank

–

Tank size vs. where you put it

Same building, same water, same relief valve. Only the tank moves. The pressures at the top and bottom of the system do not change, so the tank's height only changes the pressures its own air works between.

How it is sized. ASHRAE's closed-tank equations: the expanded volume is V_s[(v₂/v₁ − 1) − 3αΔt], where v is the specific volume of water at the fill and maximum temperatures and 3αΔt is the piping growing with it (α = 6.5×10⁻⁶/°F steel, 9.5×10⁻⁶/°F copper). A diaphragm tank divides that by 1 − P₁/P₂; a plain steel tank, whose air starts at atmospheric pressure, divides it by P_a/P₁ − P_a/P₂. All pressures in those ratios are absolute.

Assumptions in this figure. Elevations are measured from the lowest piping. The boiler and its relief valve sit in the boiler room at the elevation you choose. P₁ at the tank is the minimum pressure at the highest point plus the static head between the tank and that point. P₂ at the tank is 90% of the relief setting, so the valve does not weep, corrected for the static head between the relief valve and the tank. Static head is 0.433 psi/ft (cold water). Water properties are from IAPWS-IF97. Water only: glycol expands more and needs its own density data. Precharge the diaphragm tank to P₁ at its own elevation before filling.

A teaching model, not a selection. Manufacturers list both total and acceptance volume; the acceptance volume has to cover the expansion. Check the tank, relief valve, and equipment ratings against the actual design.