What heating systems are used in instantaneous water heaters?

The water in an instantaneous water heater is heated either by a tubular heater system or a bare wire heating system.

Most STIEBEL ELTRON instantaneous water heaters heat water using bare wire heating elements because they offer a number of benefits, namely:

  • More accurate control of the water temperature
  • They prevent temperature fluctutations when water is drawn at another source (e.g. the toilet is flushed)
  • The heating element is resistant to scale because it vibrates and because there is little residual heat when it is turned off, making ideal for hard water areas.

Technical detail on how it works:

The live heating conductors are located directly in the water to be heated. The bare wire heater spirals are set into a pressure-tested insulating block. Pipes in the insulating block house the heater spirals. The insulation resistance is assured by upstream and downstream devices, even if the earth conductor is interrupted. The insulation resistance depends in part on the properties of the water flowing through. For this reason, the electrical resistance and conductivity of the water at the installation location must be taken  into consideration. Bare wire heating elements are also especially suitable for use in hard water. The very small amount of water inside the appliance and the low-mass heating system mean there is hardly any post-heating after the appliance has been switched off – an important factor in the reduction of scale build-up.


The following instantaneous water heaters are equipped with a bare wire heating system:

Instantaneous water heaters

  • Fully electronic closed-loop control with 4i technology (type DHE)
  • Electronic closed-loop control with 3i technology (type DEL, DHB-E)
  • Electronic open-loop control with 2i technology (type DHB ST, HDB-E)

Compact instantaneous water heaters:

  • Electronic closed-loop control with 3i technology (type DCE, DCE … RC)

Mini instantaneous water heaters:

  • Electronic closed-loop control (type DEM)
  • Hydraulic control (type DNM, DHM)
Function diagram for bare wire heating elements

Tubular heaters are just as powerful and compact. Heat transfer takes place on the surface of the copper tubing. The heater spiral is located inside the tube and is surrounded by a filler material for electrical insulation. Tubular heaters are not affected by air in the water and are suitable for soft water.

The following instantaneous water heaters operate with a tubular heater system:

DHC-E, Tempra, Tempra Plus

  • Electronic open-loop control

Instantaneous water heaters

Hydraulic control (type DHF … C, DHC)

Function diagram for tubular heaters