Instantaneous vs stored water heaters - which to choose?

Instant water heaters are smaller and easier to fit and install then stored water heaters. They provide hot water on demand, heating the water as and when required, making them economical with power. These units are more limited in their flow rate and the temperatures of water available than stored water heaters.
Stored water heaters are bigger, and can provide much better flow rates and higher temperatures than instant water heaters. However once you have used all the stored water, you will have to wait for a new store of water to be heated before it can be used again.

Examples of how instant water heaters might be used

This will depend on what electricity supply you have, either 3 phase or single phase
3 phase example:
As a guide, the DHE 27kw units would be capable of supplying enough water for two to six basins, one to two sinks or a shower/bath. It would struggle to do more than one application at the same time – e.g. whilst running a bath or having a shower, if hot water is called for at the sink, it may affect the shower/bath.  

Typical applications for a 3 phase supply:

  • Kitchen – supplying two sinks
  • Home – Shower, Bath, Basin Sink
  • Hairdressers salon or gym – supplying two to three showers at the same time
  • Toilet block – supplying 6-8 hand basins

Single phase example:

On single phase, the biggest unit available are capable of supplying hot water to a shower and a basin/sink. We would not recommend using this unit to fill a bath.

Please contact us if you need advice on which model is most suitable for your needs. Also see our FAQ on rules of thumb for choosing the right size instantaneous water heater

Typical applications for larger single phase instantaneous water heaters:

  • Home – Shower and basin/sink
  • Hairdresser's salon or gym – Either one shower or two to three basins

Typical applications for smaller single phase instantaneous water heater:

  • One to three hand basins or for one shower when the demands for a shower are not too high (flow rate between 3 to 5 litres per minute max)

Examples of how stored water heaters might be used

Small vented water heaters

  • Known as open or unpressurised units. Cannot be fitted directly to mains pressure
  • SN/SNU 5 to 15 litres. Only suitable to supply one outlet. A basin or kitchen sink. Must be fitted as a open/vented system with either the valve/spout supplied for over sink installation or to a ‘special’ open tap, which will have three pipes (one pipe to cold water supply, two pipes to the unit).
  • These units can be fitted to more than one outlet or with a standard tap in conjunction with a pressure relief safety kit

Small unvented water heater:

  • Sometimes known as sealed or pressurised units. Will be fitted to mains pressure so therefore must comply with G3 building regulations (fit unit with supplied safety kit)
  • Available in our SH/SHU range from 5 to 15 litres. Suitable to supply more than one outlet and must be fitted to in accordance with G3 building regulations (unvented safety kits)