3 phase water heating for apartments

45% of new build properties in the UK are city centre flats and apartments. For most of these electricity is the preferred fuel. And for many years the default solution for hot water for each new appartment has been an unvented cylinder heated by 2 x 3kW electric immersion heaters. By its very nature this solution takes up a lot of space.

As  global market leaders in instantaneous water heaters, Stiebel Eltron have have developed alternative solutions to this space-hungry model, which take up much less space and offer additional benefits. Used extensively in overseas markets for almost 70 years, these solutions are tried and tested. Why have they never been used here, I hear you ask? The answer is simple; they do require 3 phase electric supply, which has not traditionally been run to individual domestic units in the UK.

But market conditions in the UK are now changing. Space is more than ever at a premium. Manufacturers from all areas of product design are reflecting this trend by producing, space saving and slimline products. Add to this that the volumes of hot water demanded by consumers continues to grow. Instantaneous water heaters on a 3 phase supply give the kind of powerful, limitless hot water supply that young city dwellers look for. 

Instantaneous water heaters provide answers to all of those problems and offer a whole host of other benefits that more than compensate for the small additional cost of 3 phase electricity being run to each unit.




The Benefits of Instantaneous water heaters vs stored water

Instantantenous water heaters Stored water heaters
No cylinder cupboard required Cylinder cupboard required
H 478, W 225, D 105mm H 1,384, Diameter 579mm
A never-ending supply of hot water Limited to cylinder volume
No reheat time 1 ½ hrs reheat time
Cheap running costs £60 per year cost (due to insulation losses)
Zero standing losses (better SAP score) Standing losses = 0.23 CO2 tonnes per year

Maintenance free

*No scale build up (bare wire technology)

*No discharge or blow off valve meaning the unit is inherently safe with no expansion dangers.

Annual maintenance required

*Regular descaling required

*Regular maintenance of valves and G3 safety equipment

Managing electrical loading

Your immediate concern with this system might be the capability of the electrical supply required to support the simultaneous use of showers and baths.

However, this is not such an issue as it first might appear. Regulation 311-01-01 does require the maximum demand of an installation to be assessed, but it permits diversity to be taken into account. And long history has shown that the greater the number of flats, the greater the diversity in the timing of usage. This makes this way of working particularly attractive for larger developments, where the maximum load can be as little as 10% of the connected load.

Diversity = maximum demand divided by connected load x 100


Instantaneous V’s Stored – Electrical load for water heating

Here are two examples of how this was calculated and installed. Both were subsequently monitored and proved the efficacy of the model.

First Moda Melton – 77 apartments with DHE 21 instantaneous water heaters

Monitored between November 2011 to February 2013.

Diversity = max demand of 165.24 divided by a connected load of 1617 x 100 = 10.2



Bay View – 42 apartments with DHE 27 instantaneous water heaters

Monitored between November 2011 to February 2013

Diversity = maximum demand of 138.96 divided by connected load of 1134 x 100 = 12.2

3 Phase instantaneous installation requirements

What’s required:

  • 15mm cold feed to water heater
  • 400V electric supply

Currently UK apartments are each typically fed by a single phase 230V supply.  

So how can a 400V water heater be installed into a UK apartment?

New build apartments have 400V provided to each floor in the riser. Currently this is then split down into a phase per apartment. By taking the 400V to each apartment the instantaneous solution becomes possible.

Cost of 3 phase instant solution vs stored water

Taking into acount the costs of annual maintenance and greater unit and installation costs of stored water solutions the result of installing the 3 phase electrical supply and choosing instantaneous water heating is COST NEUTRAL for developers.

We can provide fully costed examples of this, from actual UK developments. If you would like to discuss this with us further please do call our sales office.

Stiebel Eltron instantaneous range

We have a wide range of models suitable for 3 phase installations.