Loop Water Expansion Volume

Heat water and it swells. The volumetric coefficient β is small — about 2.1 × 10⁻⁴ per K near 20 °C, rising to 4.6 × 10⁻⁴ at 60 °C and 7 × 10⁻⁴ at 90 °C — but multiplied by a few thousand litres of system content it becomes tens of litres that have to go somewhere. Heating 500 L of loop water by 60 K at β = 4.6 × 10⁻⁴ produces 500 × 4.6 × 10⁻⁴ × 60 = 13.8 L of expansion, which is precisely the volume your expansion tank must accept.

The honest caveat: β for water is strongly temperature-dependent, so this linear form is an approximation over any wide range, and serious tank sizing uses the net expansion factor from specific-volume tables (v₂/v₁ − 1) rather than a single β. Use the average β over your range and you will land within a few percent. Water's other oddity earns a mention: below 4 °C it expands as it cools, which is why lakes freeze from the top and why a loop left unheated in an unprotected building splits pipes rather than merely stressing them. Glycol mixes expand roughly 10–20 % more than plain water over the same rise, another reason antifreeze systems need larger tanks.