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.

Main types of DHW system

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

Pros and Cons

The pros and cons of these systems are:

	PRO	CON
Instantaneous	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 & monobloc mixer taps even in attic spaces	Dependant on pressure and continuity of mains water supply
Low pressure stored water	Independant of fluctuations in mains water pressure and brief outages of supply.	Relatively low supply pressure especially at top of building Requires large storage tank in attic or high in building Pumps usually required for showers and some types of taps

Instantaneously heated systems are generally mains pressure systems. The familiar system with a tank in the attic and a cylinder in the airing cupboard is 'stored heat' and 'low pressure stored water'. There are also two main types of stored heat mains pressure systems.

The various permutations of system are shown in this table:

	Instantaneous	Stored Heat
Mains pressure	Combi boilers 'Ascot'-type water heaters ('geysers') Electrically heated showers	Unvented (e.g. 'Megaflow') cylinders Thermal stores and heat banks.
Low pressure stored water	Not generally found in DHW systems	Conventional system: cold water storage tank in attic and hot water cylinder in airing cupboard, usually heated by a gas boiler with electric immersion heater for backup

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Mains pressure stored heat systems: unvented versus thermal store

Although they serve the same purpose - to provide a copious supply of hot water at mains pressure from a storage tank - unvented systems and thermal stores work in radically different ways.

Unvented systems

Like a conventional hot water cylinder fed from a tank in the roof, an unvented cylinder contains hot water (heated by a boiler or immersion heater) which directly supplies the hot taps. However the water in an unvented cylinder comes directly from the cold water main and is at (nearly) mains pressure. Obviously the cyclinder has to be physically stronger to withstand the higher pressure of the mains water, but also various other safety devices have to be incorporated into such systems. More on unvented systems.

Thermal stores and heat banks

In these systems a container of water under low pressure is heated by the boiler or immersion heater etc. Mains pressure cold water passes through a heat exchanger to be heated by water from the store to supply the taps etc. There are various different configurations of thermal stores and heat banks. More on thermal stores and heat banks