Buying, selling or running a home – everything you need to know about your home and garden!

What size of cold water tank do I need?

John asked:

We have a large 5–bedroom house with central heating/hot water system and three shower/bathrooms. What size of cold water tanks do we need? Will 82–gallon tanks be enough?

John, after a bit of researching it seems that this is something of a “How long is a piece of string?” question: it depends on the reliability and pressure of your mains water supply.

Many houses (both very old and very recent ones) don’t have a cold water tank at all and simply take all their cold water direct from the mains. This is great if you like your showers to have a good strong supply of water—no need for a pump or so-called power shower!

It also saves loft space. Put it this way: a 220–litre/50–gallon tank typically measures about 1175 mm x 600 mm x 500 mm, or 46″ x 24″ x 20″. That’s taking up 0.7 square metres of space (8 sq ft) which you might want or need for other purposes. And if you’ve had a loft conversion, you won’t have much space left to store a water tank!

The downside is that if your water supply fails at all, then you’re without cold water, which means you’ve only got one flush on each of your toilets. If you’ve had no notice of the outage, and no time to fill buckets or baths, then you’ve got trouble!

Another problem with a direct cold-water system is that if the pipes have tight bends in them, the pressure of the water can lead to “water hammer” when you turn the taps off.

An indirect water supply does have the advantages of being quieter and of giving you some reserves if your water supply’s at all unreliable. But if your showers are gravity-fed, you’ll need a minimum of about 220 litres/50 gallons. For a larger household, you’ll probably want something of the size you suggest—about 360 litres/80 gallons—or conceivably more than one tank, although that’ll take a bit more ingenuity from whoever does your plumbing.

Your tank should be fitted with a By-law 30 kit to ensure that the water stays clean inside the tank.

Screwfix do a range of shapes and sizes of cold water tanks:

Cold water tanks from Screwfix Direct

Bear in mind, too, that if your showers are gravity-fed they’ll need a minimum of 900 mm/3 ft head (the head is the vertical distance between the bottom of the cold water tank and the shower sprayhead).

Last of all, water is heavy! The loft floor should be strong enough to support the load of your tank, which will be approaching 400 kg/8 cwt for a 360–litre/80–gallon tank.