Sizing and Design of Expansion Tanks

Expansion Tank Sizing

Expansion Tank Sizing Calculator


  • Total System Volume includes water content in all coils, pipes, and equipment.
  • If temperatures are not known, allow 30 °C max temperature for chilled water and 15 °C minimum temperature for heating water.
  • All the inputs must be filled accurately for a valid calculation & selection.
  • For any application other than HHW, CHW or CCW, please contact Automatic Heating for selections.

Joule expansion tank

Static expansion tanks are supplied with a pre charge pressure of 150kPa.

Careful calculation of the expansion tank size is critical to the correct functioning of the system.

Expansion tanks come in a variety of sizes. Selecting the appropriate size depends on several factors, such as water pressure, the total water capacity of the system, and the temperature of the water entering and exiting the system. Using these parameters, the recommended standard size is usually determined through charts or software.

The first step is to estimate the expansion coefficient using the table and chart below. The x-axis displays values representing the difference between the cold system water temperature (when the heating is off) and the maximum working temperature. On the other hand, the y-axis displays the corresponding expansion coefficients.

°C Coefficient
0 0.00013
10 0.00025
20 0.00174
30 0.00426
40 0.00782
50 0.01207
60 0.0145
65 0.01704
70 0.0198
75 0.02269
80 0.0258
85 0.02899
90 0.0324
95 0.0396
100 0.04343

Estimating the expansion coefficient from the difference between the cold system water temperature and the max working temperature.

expansion graph
Once the expansion coefficient is obtained, the volume of the expansion tank can be calculated using Boyle’s Law.

Heating System

The expansion tank sizing formula is as follows (based on Boyles Law):

Vf = e x C = Vu
1 – Pi/Pf 1 – Pi/Pf


Vu = Total useful volume of tank = Vi-Vf
Vi = Initial volume
Vf = Final volume
e = Expansion coefficient
Pi = Initial charge pressure (absolute) of vessel.
This pressure must not be lower than the hydrostatic pressure at the point where the tank is connected to the system.
Pf = Maximum operating pressure (absolute) of the pressure relief (safety) valve.
(taking into account any differences in level between the vessel and the safety valve.)
C = Total water capacity of the system in litres:
boiler, pipework, radiators etc
(as a general approximation, C is between 4 and 8 litres for every kW of boiler output)

Note: Calculations must be done in Absolute Pressure
e.g. 100kPa = 200kPa absolute.

In standard heating systems:

e = 0.04318 (Tmax = 99°C, Tmin = 10°C, Δt = 89ºC, C = 0.035)

Cooling System

The vessel sizing formula is as follows (based on Boyles Law):

Vf = e x C
1 – Pi/Pf

In standard cooling systems:

e = 0.011 (Tmax = 50°C, T min = 4°C)
Pi = Maximum plant pressure, corresponding to the maximum achievable temperature, equal to the ambient temperature, which is recommended to be fixed at 50°C
Pf = The final working pressure achieved at minimum temperature, using 4°C


C = 500 litres
Pi = 150kPa (250kPa Abs)
Pf = 400kPa (500kPa Abs)
V = 0.04318 x 500 = 43.2 litres
1 – (250/500)

Select the nearest size tank 50 litres

Calculating Expansion Tank Pre-charge Pressure

Please use the below calculation to correctly determine the expansion tank pre-charge pressure:

Pi = [Hm x 10] + 20kPa
Pi = Initial charge pressure (absolute) of vessel
Hm = System height (metres) above the location of the expansion tank

What if the system volume is not known?

When the system volume is unknown, the following ‘rule of thumb’ calculations may be used.

Hot Water: 6 x kW of the system = volume.

Chilled Water: 12 x kW of the system = volume.