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Estimating Water System Volume

The following guide to estimating system volumes has been developed, based on field experience.

Commercial systems (pressurized):

Multiply the boiler output rating in kWs by the figure given below (for the relevant type of system) to give an estimate of the total system volume. Allow an additional 10% (multiply by 1.1) for open vented systems of all types.

Useful conversion factors:
1kW : 3,412 BTU's
1m3 : 1000 liters
4.546 liters :1 gallon

Systems comprising perimeter heating, convectors, etc. : 6 liters/kW
Ventilation systems (air handling units, fan coils, etc.) : 8 liters/kW
Steel panel radiators : 11 liters/kW
Cast iron radiators : 14 liters/kW
'Distant' heating systems in large sprawling buildings : 20 liters/kW
Underfloor heating : 23 liters/kW


Domestic systems:

All small and microbore domestic installations : 6 liters/kW

• The domestic system volume can be estimated by counting the number of single panel radiators in a property and multiplying by ten.
• Remember to count double panel radiators as two single panels.
• It is also important to make consideration for thermal stores. These can add up to 100 liters to the system capacity.