Domestic Hot Water Systems

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  • Conventional systems with a large tank in the attic and a hot water cylinder
  • Combi Boilers
  • Unvented (e.g. Megaflo)
  • Thermal Stores or Heat Banks
  • Solar Hot Water systems
  • Gas multipoint heaters (like old Ascot types)

and more ....

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 - unvented and thermal stores/heat banks - becoming increasingly common in the UK are discussed, as are solar and other renewable systems.

This article describes the main features of DHW systems and explains terms used in describing such systems. Further articles 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.

Main types of DHW system

The main differences between types of DHW systems are:

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


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


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


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

The pros and cons of these systems are:

  • Continual supply of hot water.
  • Rate of supply limited by fuel supply/consumption. No backup when heater (e.g. combi) fails.
Stored heat
  • Potentially very high rate of supply of hot water.
  • Can accomodate backup heating (e.g. electric immersion for gas boiler) and multiple heat sources (e.g. solar gas).
  • Supply runs cold when stored heat exhausted: takes time to re-heat.
  • Heat lost from store while not in use.
Mains pressure
  • 'Dry loft': no need for tank in attic.
  • Good pressure for showers