Domestic Hot Water Systems

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There is a wide and potentially bewildering variety of systems for supplying hot water in houses and flats (Domestic Hot Water or "DHW"). These include

  • 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:

Advantages Disadvantages
  • 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 temporary loss 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 also 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: Unvented and Thermal Stores & Heat Banks.

Combining these factors we have:

Instantaneous Stored Heat
Mains pressure
  • Combi boilers
  • Gas "multipoint" water heaters
  • "Ascot"-type water heaters ("geysers")
  • Electrically heated showers
  • Unvented (e.g. "Megaflow") cylinders
  • Thermal Stores & 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

Mains pressure stored heat systems: Unvented versus Thermal Store/Heat Bank

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 cylinder 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


In a vessel containing water at different temperatures the hotter water will tend to accumulate at the top with progessively cooler water further down. This phenomenon is called stratification and is generally desirable in DHW systems. In the relatively tall and narrow cylinders used in DHW systems this stratification is more pronounced, allowing a wide range of temperatures from top to bottom e.g. 60C nearer the top and 10C lower down.

Direct and Indirect systems

Conventional gravity-fed cylinders, thermal stores and heat banks have two different arrangements for heating the water contained in them.

  • In Direct systems the water in the cylinder is heated directly, either by an electric imersion heater in the cylinder or by the water being circulated around a boiler. This was a common arrangement with back-boilers behind open fireplaces and ranges such as Agas and Rayburns, but is not generally used with central heating boilers.
  • In Indirect systems the cylinder has a heat exchanger coil to transfer heat from the primary heat source (e.g. boiler) to the water in the cylinder. This keeps the water in the boiler separate from the water in the cylinder, allowing the water in the boiler and central heating radiators to be treated with chemicals and kept free of dissolved oxygen to prevent corrosion of iron or steel radiators and boilers. All unvented systems are indirect.

Direct Indirect
DHW direct cylinder.gif DHW indirect cylinder.gif

Primary and Secondary

In indirect systems there are two separate bodies of water. Primary water circulates around the primary heat source (e.g. boiler) and transfers heat (via the heat exchanger coil in the Indirect diagram above) to the Secondary water (in the body of the cylinder, above) which supplies DHW. In some systems there are more than two separate bodies of water but there is no clearly agreed nomenclature for such systems (e.g. 'tertiary' water).

Primatic cylinders

A type of system which is still sometimes found in older installations uses a cylinder (usually known as a "Primatic") in which the primary (boiler) water and secondary (DHW) water are supplied from the same (DHW) tank but has an arrangement within the cylinder which uses trapped air to keep the two bodies of water separate during operation. These types of cylinder have poor heat transfer from primary to secondary water, and must not be used with corrosion inhibitor or other chemicals in the primary water. They are not used for new DHW systems.


(Re-heating performance)

In Direct systems hot water from a boiler can circulate directly through the cylinder so the rate at which the cylinder water heats up is limited only by the heat output of the boiler. In Indirect systems the rate of heating is usually limited by the rate of transfer across the heat exchanger coil. The time taken to heat (or re-heat) the cylinder is its recovery time. Poor recovery can result in the boiler cycling - switching off and on instead of running continously - as well as slow re-heating of the water. The Building Regulations [citation needed] covering cylinders now call for better performance from modern cylinders than was acceptable from older designs. For example for a common size of cylinder (450mm diameter x 900mm height) a standard size coil will have a surface area of about 0.6 square metres and will heat the water from 10C to 60C in about 25 minutes (with a primary flow of about 15 litres per minute at 82C). A fast-recovery cylinder of the same size will have approximately twice the surface area of coil and a recovery time of about 15 minutes (with 18 litres/minute primary flow).

See Also