Get to know your Hot Water Cylinder

The choice of water heaters and their installation will be influenced by:

• availability of fuel – only gas and electricity are covered by this guide
• capital cost
• running cost
• available water pressure
• the chemical composition of the water.

Basic types of water heating installations include:

• storage heaters – at low pressure or mains pressure (electric or gas)
• push through water heaters – not covered by this guide
• boiling water units
• solar water heaters – not covered by this guide
• solid fuel heaters with a 'wetback' – not covered by this guide
• heat pump water heaters – not covered by this guide
• instantaneous:
• electric or gas

Storage Water Heaters

Storage water heaters hold hot water in a thermally insulated container until it is required. Hot water which is less dense than cold water, rises to the top of the cylinder where it is drawn off and replaced by cold water entering the cylinder at the bottom.

Cylinders must be large enough to meet the draw-off demand at peak periods and must have sufficient recovery capacity to be ready for th next peak draw-off

Cut-away diagram of electric mains pressure hot water storage heater.

Diagram showing principles of gas hot water storage heater.

The water pressure at which a storage heater operates may be:

• low pressure – open-vented systems where the pressure is provided by a cold water storage header tank (3-12 metre head – 30 to 120 kPa) (typically 3-7 metre head)
• low pressure – systems which rely on a pressure-reducing valve to reduce the pressure below the mains pressure (3-12 metre head – 30 to 120 kPa) ( typically 3-7 metre head)
• mains pressure – with a working pressure equal to the incoming supply ( ranging from 120 kPa to 1400 kPa) (typically 350-700 kPa)
  

Mains pressure water heaters are the predominant type of new installation in Australia and New Zealand.

This table gives an indication of some characteristics of some storage water heaters.