What size instantaneous water heater do I need?

The first step is to check the current flow rate of your supply if you wish to replace it with a like-for-like product.

To do this simply use a stop watch for one minute whilst filling (and emptying, and refilling, if necessary) a container of known volume. Once you know how many litres a minute your supply is, you are ready to choose your instantenous heater using the following rules of thumb:

The kW rating of a heater divided by 2 = the flow rate per minute at a showering temperature of 38 degrees C

The kW rating of a heater divided by 3 = the flow rate per minute at a washing up temperature of 50  degrees C

Be sure to confirm with your installer that your water and electric supply are suitable for the model selected.

See our FAQ on instant vs stored water heaters for examples of typical applications of instant water heaters of various sizes.