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How to Size a Cold Water Break Tank

Introduction: when to use a break tank

Break tanks are used in pressure boosting water systems to supply sufficient net positive suction head 'NPSH' to a pump in situations where mains water pressure is not sufficient to supply the requirements of the system. Break tanks typically feature an air gap between the inlet and maximum water level to prevent backflow.

Please note that some of the guidance in this article is based on BS EN 806 Part 2. Any further recommendations are for guidance only and should always be checked by a qualified mechanical, public health or building services engineer, as the requirements of different projects can vary greatly. 

There are two primary considerations in determining the size of a break tank. Firstly, the use and occupancy of the building should be considered. For example, a hotel requires more water storage per bed space than a hostel. 

1. Building Type and Use

The following table shows the recommended minimum storage levels for each type of premises:

Type of building Minimum storage (l)
Hostel 90 per bed space
Hotel 200 per bed space
Office with canteen 45 per employee
Office without canteen 40 per employee
Restaurant 7 per meal
Day school (nursery/primary) 15 per pupil
Day school (secondary/technical) 20 per pupil
Boarding school 90 per pupil
Children's home/residential nursery 135 per bed space
Nurse's home 120 per bed space
Nursing or convalescent home 135 per bed space

Source: BS EN 806 Part 2 (see full acknowledgement below).

The likely peak occupancy of the building should be taken into consideration when making this calculation. When sizing for a domestic building, a standard property should be sized as a hostel, with storage of 90 litres a person sufficient for most installations. High end properties may require additional storage.

2. Timed Storage

Dutypoint recommend that a minimum of 15 minutes' storage, according to the peak flow rate of the booster set, should be provided in a cold water break tank.

Secondly, the expected peak flow rate of the premises should be considered. This is calculated when sizing the booster set (see separate article for details). In determining the peak flow rate, we must consider how much water storage must be provided to facilitate this flow rate.

Dutypoint recommend that a minimum of 15 minutes' storage, according to the peak flow rate of the booster set, should be provided in a booster set break tank within a commercial installation. For example, a booster set with a peak design flow rate of 1.1 litres/second, or 66 litres/minute, requires a break tank with a capacity of at least 990 litres. 

Other factors such as the tank inlet flow rate and the usage patterns of the building should also be taken into account - buildings with a poor rate of mains water supply will require greater storage. This will also be influenced by the siting of the break tank - a tank on the ground floor of a building will fill at a higher rate than a tank on an upper floor or in the roof space. Siting a tank in the roof space of a building is likely to necessitate a larger storage volume.

The following graph shows the minimum storage required for a range of booster set peak flow rates:

15 minute storage according to booster set flow rate

If you have any questions about selecting the right break tank for your system, please get in touch and our knowledgeable consultants will be happy to help you.

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Acknowledgement: Permission to reproduce extracts from British Standards is granted by BSI Standards Limited (BSI). No other use of this material is permitted.


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