Expansion Vessel Sizing Calculator for Heating and Hot Water Systems

How big the expansion vessel needs to be for a closed water system.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Enter the water volume of the whole system and the cold and maximum temperatures.

Enter the static height of the system and the highest pressure allowed at the vessel.

The calculator gives the expansion volume, the water seal, the charge pressure and the vessel size in litres.

Formulas used

  • V_e = V_s\left(\frac{\rho_c}{\rho_h} - 1\right)
  • P_0 = \rho gH + 30\,\mathrm{kPa}
  • V_n = (V_e + V_v)\frac{P_e}{P_e - P_0}

Worked example

With the default numbers in the calculator above:

  1. Density of the water cold and hot: 999.7 kg/m3 at 10 C and 971.798 kg/m3 at 80 C
  2. Expansion of the water between the two temperatures: 5.7423 litres (2.871 % of the system)
  3. Water seal that stays in the vessel (0.5 percent of the system but at least 3 litres): 3 litres
  4. Charge (pre-fill) pressure of the vessel: the static head plus 30 kPa: 128.07 kPa gauge
  5. Nominal volume of the vessel (all pressures absolute): 20.406 litres
  6. Pick the next size up from the range of vessels, and set the charge pressure with the system empty of water: At least 20.41 litres, charged to 128.07 kPa

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

Why does a heating system need an expansion vessel?

Water expands when it is heated. In a closed system there is nowhere for it to go, so the pressure would rise until the relief valve lifts.

How much does water expand?

About 3 percent from 10 C to 80 C, and about 4.3 percent to 100 C.

What should the charge pressure be?

About the static head of the system plus a margin, set with the vessel isolated and the water side empty.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Water density from the Kell formula. Method in the style of the European heating system standards. Check the vessel maker's data.

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