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 |