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.

A useful visual guide to chooing the right model is also provided below as a picture or can be downloaded as a PDF.

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