Hot Water Cylinders

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Sizing a hot water cylinder

Many people remain under the impression that hot water storage tanks mean that they will run out of hot water, and therefore have to wait for the tank to heat up. If the tank is sized correctly according to the needs of the household, this should not happen. Here’s a quick guide to sizing

Obviously larger houses will have larger tanks. When selecting the size of the hot water tank, a simple rule of thumb is that for a typical domestic household, you should allow between 35 and 45 litres for every occupant. That said a mains pressure system can use about 18 litres of water per minute at 40 degrees Celsius if a decent quality shower head is used. Certain brands of shower can use up to 25 litres of water per minute.

However, it must be noted that personal habits also play a big part in total hot water use. Two households of the same size can use completely different amounts of hot water, with one of them using twice as much as the other!In determining how much hot water you require you should always consult with the occupants, and consider the following:

• A bath uses 100 litres of hot water at 40 degrees Celsius (equating to 60 litres at 60 degrees Celsius)

• Showers can use about 18 litres of hot water per minute at 40 degrees Celsius (equating to 11 litres at 60 degrees Celsius)


The following average consumption values can be used as a general rule (hot water requirements per person per day):

• Low Consumption = 20 – 30 litres

• Average Consumption = 30 – 50 litres

• High Consumption = 50 – 70 litres


On this basis a typical four person household would often use around 200 litres of hot water a day and this is the figure that will generally be used under the new EU energy labelling scheme.This does not necessarily mean that a 200 litre tank is required as dependent on the heating system the tank may be partially reheated during the day. It is up to the installer to match the correct size of tank relative to boiler (or electrical) input to avoid running out of hot water.

Driving to improve standards within the domestic hot water industry