Sizing cold water storage from demand

What storage has to cover

Cold water storage gives a reserve when the mains is interrupted and smooths out peak demand. The size is a balance: enough to cover the chosen reserve period, but not so much that water sits for days and warms into the Legionella growth band.

Working out the volume

• Estimate the daily demand from the number of users and a design allowance per person, or from the fixtures served.
• Decide the reserve period to cover, for example part of a day, or a full day for some buildings.
• Multiply demand by that period to get the required usable reserve.
• Check the product’s normal stored volume and its usable draw-off capacity. Usable reserve is the water between the normal shut-off level and the lowest permitted draw-off level; the outlets cannot draw all of the nominal shell capacity, or all the water held at shut-off.

Worked example

For this exercise, assume a dwelling for 4 people and a design allowance of 150 litres per person per day. That allowance is an assumption for this exercise, not a figure for every home. Daily demand is about 4 x 150 = 600 litres. If only a quarter of a day is needed as reserve, that is about 150 litres of storage. Select a cistern that provides at least that 150-litre usable reserve between its operating levels. Check the actual stored volume at shut-off as well, because water left below the usable draw-off level still counts towards stored-water age.

Avoid simply fitting the largest cistern: oversized storage lengthens the time water is held and lets it warm up. Confirm allowances and reserve periods against BS 8558, BS EN 806 and any water undertaker guidance.

How much to store, and where the pressure comes from

• HSE's legionella guidance says stored cold water should be kept to a minimum and should not normally exceed one day's use. For healthcare premises it recommends a nominal 12 hours of total storage. This concerns all water held, including any volume below the lowest usable draw-off level. Check the drawable reserve separately; the one-day guide does not require every building to store a whole day's demand.
• Oversized storage, including a large break tank on low demand, holds water longer, so it warms and stagnates. Match the volume to realistic use so it turns over.
• In a gravity system the outlet pressure comes from the vertical distance between the cistern's water level and the outlet, not from the volume stored: roughly every 10 m of head gives 1 bar of static pressure. Allow for pipe and fitting losses when checking performance during draw-off.
• In England the water company must keep at least 7 metres of static head, about 0.7 bar, in the communication pipe serving a property. That is a low figure, so measure the pressure and flow actually available before relying on mains-fed appliances.