Domestic Hot Water Systems

There is a wide and potentially bewildering variety of systems for supplying hot water in houses and flats ('DHW': Domestic Hot Water). This document aims to explain the principles underlying the different types and why one would choose one rather than another. In particular the various types of mains pressure systems becoming increasingly common and popular in the UK are discussed, as are solar and other renewable systems.

This page describes the main features of DHW systems and explain terms used in describing such systems. Further pages will give more details on particular varieties of system, including some hybrid systems combining elements of the basic types discussed here, discussion of some commercial and other systems available, and links to suppliers' web sites and other resources.

The main differences between types of DHW systems are:

• Instantaneously heated e.g. combi boilers, 'Ascot'-type water heaters, instantaneous electric water heaters

versus

• Stored heat e.g. systems with some sort of hot water cylinder

and

• Mains pressure water e.g. combi boilers, unvented ('Megaflow' type) cylinders

versus

• Low pressure with stored water e.g. conventional: tank in attic and hot water cylinder in airing cupboard