Hot Water Cylinder Sizing - Dimensioning

Sizing up a hot water cylinder is not and should not be complicated, particularly for a domestic installation. There are however a few things that the installer should take into account when selecting a cylinder size


The most important rule of thumb is that a cylinder should be sized to a property, not to who lives there. A single person living in a 5 bedroom,3 bathroom property will not use much hot water, but the property should have at least a 250 litre cylinder to allow for a large family to move in at a later date. The exception to this rule , obviously, is that if a 6 person household lives in a 3 bedroom property, the cylinder should be selected larger than the house would normally require.

Domestic cylinders are typically available in 6 sizes from 120 to 300 litres. If we take the most “standard” installation, a 4 bedroom house with gas central heating and a bathroom and ensuite shower, a 180 litre would typically be suitable. A very rough rule of thumb is when you add a bedroom or a bathroom/ensuite, increase it by one size.

Modern hot water cylinders heat up quickly and cool down very slowly. Mid-sized cylinders will heat up in about half an hour, and take days to lose heat. If you heat a cylinder up on Monday morning and don’t use the hot water, it will still be useably hot on Wednesday or Thursday.


For Commercial sizing advice please contact OSO hotwater