Environmental Information Exchange

Hot water

The heating of water for washing, cleaning, etc. uses energy.  Some methods are more efficient than others depending on technology, demand and other factors.  Hot water may need to comply with legislation, including the prevention of Legionnaires Disease.  The overarching aim of hot water efficiency is to use the least amount of energy to provide the hot watered needed.

Hot water systems

There are two ways to deliver hot water:

See our renewables page for information about solar water heating.

Hot water can be heated by electricity or gas.  Hot water temperatures are often 60 to 80°C when generated, however, a comfortable temperature for a bath or shower is 38-44°C, usually by mixing cooler water with the hot water.

Types of hot water system




Point of use
(includes combi-boilers and electric point of use)

Only uses energy when water is running

Useful for relatively small uses of hot water such as washing hand or some dishes

Hot water tank
(includes small emersion heaters and larger tanks; some systems use the building’s heating system to create hot water)

Provides enough hot water for high frequency use

Stored hot water cools, so constant energy for reheating is required

Hot water boilers for drinks

Hot water boilers are used for making hot drinks; they provide water at about 100°C.  Ideally these devices will be off when a premises is not occupied or the device is not needed.  However some manufacturer specifications for boilers advise against repeatedly switching off the unit for fear of damaging internal parts (contracting and expanding of materials when cooling down and heating up).


Hot water is controlled in a number of ways that can be modified to reduce wasted energy.  Ideally tanks of any sized are switched off when the building is not in use.  This can be manually or via a timer.


Point of use

Hot water tank

Water temperature settings (such as a pictorial dial or numerical  thermostat located on the unit)

Set to comfortable temperature

Generally set to 60°C.  For tanks of 250L or more, seek advice regarding Legionnaires Disease

On / off switch

Does not use power when on if tap is closed

Switch on when needed, switch off when not needed

Taps (flow of water activates)

Only heats water when tap is on

Tank will refill if enough water is removed through tap

Timer (hot water is on only during these times)


30 minutes is usually enough to heat a tank of water from cold.  Controls may be located on the unit.

Building management system controls hot water times and temperature


Times and temperatures can be set via BMS software