Size an expansion tank and the system volume

Expansion & Volume totals the water a circuit holds and sizes the expansion tank that takes up its expansion.

• –System volume is the pipework plus every item entered.
• –Expansion tank gives the fluid's expansion between the fill and maximum temperatures, from its density at each; the net expansion the tank must accept; the tank's acceptance and total volumes; and, for a diaphragm tank, its Pre-charge, which is the fill pressure.
• –Each kind of tank has its own equation in the ASHRAE Handbook — HVAC Systems and Equipment, and the result names the one used. They aren't interchangeable: a plain closed tank comes out considerably larger than a diaphragm tank for the same system, and an open tank is sized at twice the net expansion.

• –For a diaphragm tank the fill cell reads Fill (pre-charge). Both pressures are gauge, at the tank, and the maximum must be above the fill.
• –Set the maximum at the relief setting less the margin you hold below it. A tank sized to the relief setting itself has no margin at all.
• –An open tank is at atmospheric pressure, so its pressure cells aren't used.
• –On a chilled-water circuit, the expansion that matters runs from the temperature the system is filled at to the hottest it reaches standing.
• –Absolute pressures add atmospheric pressure from the elevation on an HVAC project's Design Conditions, or standard sea-level pressure otherwise, which the result says.
• –If the maximum temperature isn't above the fill temperature, there's no expansion to accept and no tank is sized.

With the height from the tank connection to the system's highest point entered, Kelvin shows the least fill pressure that holds the fluid up there: the height times the fluid's density at the fill temperature times g, plus your margin. It's a check beside the fill pressure, never a value it sets, and the result warns when the fill entered is below it. Kelvin holds no cited figure for the margin, so it's nil unless you enter one. The tank connects at the pump suction, so set the fill pressure there.

Under Pipe expansion, a Coefficient α for the pipe material, with its Source of α, takes the pipework's own expansion off the fluid's, for a slightly smaller tank. Without one, the tank is sized on the fluid's expansion alone, the larger tank. Kelvin holds no table of material properties, and won't save a coefficient without its source.