Electric instantaneous water heaters are energy-saving on demand instant water heaters and are also called an electric tankless water heater - from 9kW to 27kW output from Flexiheat UKFlexiheat UK

Electric instantaneous water heaters – 9kW to 36kW Output

Electric instantaneous water heaters are energy-saving on demand instant water heaters and are also called an electric tankless water heater - from 9kW to 27kW output from Flexiheat UK

Instant electric water heaters

Instantaneous electric water heaters, or electric instantaneous water heaters, are energy efficient instant hot water heaters that heat domestic hot water on demand.

They are sometimes called instant electric water heaters, tankless electric water heaters, flow-through, electric instantaneous inline water heaters, or electric continuous water heaters, as all these terms refer to instant water heaters that heat water on demand and are tankless water heaters, i.e., have no hot water storage.

On demand water heaters are installed everywhere, from domestic residential properties to commercial settings, depending on the hot water flow rate supply required.

With wide output power ratings ranging from 9 kW to 36 kW, we have the instantaneous water heater range to suit all your flow rate requirements, be they for commercial applications such as a shower block with numerous showers or multiple sinks.

Instant Hot Water Heaters

Instantaneous electric water heaters reduce energy consumption and energy bills as they provide hot water only when there’s a demand. Because there’s no storage element, they are often called tankless water heaters or in line electric water heaters, and this allows the water heaters to have a compact design and are easy to install in suitable locations with limited access thanks to their space-saving design.

Providing instant hot water means that you don’t have to heat water to 60 degrees C, which you would have to in an electric hot water tank for anti-legionella purposes. This fact makes instantaneous water heaters very cost effective and energy efficient as generally your provide hot water at 38C to 40C.

Energy-efficient instantaneous water heaters

Instantaneous electric water heaters are 100% energy efficient in that they covert all the electric energy supply to the instant hot water heaters into the water with no energy losses.

Our instant electric water heaters use bare wire heating elements which are particularly well-suited for use in hard water areas. The relatively small volume of water within the instant water heater, along with the low-mass heating process, means that there is little post-heating after the instant hot water heater is turned off, which contributes significantly to the decrease in scale build-up.

This innovative water heating technology leads to significant energy savings, increased product life and continued performance of these instant water heaters, even in hard-water locations.

  • Higher energy efficiency for instant hot water with an electric water heater
  • Faster heat-up time which reduces energy consumption
  • Reduced likelihood of scale accumulation in hard water areas when you heat water
Water hardness map of the UK and Ireland

Water hardness map of the UK and Ireland – a lot of the country is Hard to Very hard in terms of calcium levels, which effects scale potential.

  • Reacts faster to changes in voltage and water pressure.
  • To feed the instant water heater, you can use preheated water.

Electronic control of the hot water temperature

Our electric instant water heaters allow precise and easy adjustment of the hot water temperature through the electronic control. You can set the hot water temperature between 30°C and 60°C, with a hot water temperature control accuracy of 1°C for these instant electric water heaters.

Internal picture of our electric instant water heaters with electronic control - Flexiheat UK

Instant water heaters 9kW to 27kW with space saving design

Electric instant hot water heaters product range

These instant water heaters have four power ratings or output ranges in “one” instant water heater. Except for the 27kW electric multipoint water heater which only has one power setting of 27kW.

For example, our 9kW to 15kW instant electric water heaters can be set to either 9kW, 11kW, 12kW, or 15kW power outputs.

This effects the draw-off point flow rate; thus, at 9 kW, it will produce 4.3 litres, at 11 kW, it will produce 5.2 litres, at the standard temperature rise of 30 °C for daily use in winter period – as that’s when you want hot water the most, shown as Δt  , as you can see in the following data.


Power output of the instantaneous water heaters9kW to 15kW power outputs17kW to 24kW power outputs27kW
Energy efficiency classAAA
Heat capacity ( power setting options)9/11/12/15 kW17/18/21/24 kW27 kW
Power supply – All require a 3 Phase power supply 400V 3N~400V 3N~400V 3N~
Mains water supply pressure (Bar) min – maximum1 to 101 to 101 to 10
Hot water flow rate at  Δt=30 in litres per minute4.3/5.2/5.8/7.28.1/8.7/10.1/11.613
Rated current (Amps)3×13/15.9/17.3/21.73×24.6/26/30.3/34.63 x 39
Dimensions – W x D x H (in mm)245 x 126 x 440245 x 126 x 440245 x 126 x 440

Please note that this range of instantaneous water heaters requires a minimum flow rate of 2.5 l/min to start heating the water. They will not operate if the flow rate is lower than this point.

Download the EIWH5 Series instant electric water heaters manual – Flexiheat UK

These compact water heaters should be mounted in such a way as to enable easy access for service and repair. The install requires a minimum distance of 100mm from the walls and ceiling from these instantaneous water heaters.

Product features

This instantaneous water heaters range has the following features:

  • Four selectable power outputs per instantaneous water heater (except the 27 kW, which is one only), which can be set during installation
  • Memory function: With the touch of a button, users can store and call up frequently required water temperatures.
  • An electronic safety system equipped with a WTC safety switch
  • Electronically controlled, so the water temperature is always accurate to the degree
  • Maximum water temperature lock (for scald protection).
  • A user-friendly LCD display showing the instant water heaters set temperature

    Digital display on our instantaneous electric water heaters from 9kw to 27kW from Flexiheat UK

    The digital display on our instant water heaters , showing the hot water temperature set on the water heater

  • IP25 protection rating
  • Can be used for providing hot water top-up heating, i.e., suitable for reheating preheated water up to a maximum of 60 °C ,such as solar water systems or heat pump DHW systems.
  • Our units come with a 2-year warranty.

Multipoint electric water heaters

The quantity of hot water that the instant water heater will generate, which is directly proportional to the temperature rise that the water heater has to achieve. You simply need to cover the flow rates of your hot water fixtures.

For domestic hot water heating, the normal temperature you require is 40 °C; this is a comfortable temperature for bathing, be it for baths or showers. This temperature is also a good sink temperature for wash hand basins, although for a kitchen sink tap, you really should have a hot water temperature of 50 °C to remove grease from kitchen utensils and dishes.

Cold water incoming temperatures

For sizing instant hot water heaters, we always look at the “coldest” cold mains water supply temperature that we get in the UK, which is in the winter period—when you want hot water the most!. This temperature varies for different regions of the UK, but generally, the coldest incoming water temperature we expect is 10 degrees C. Please see the following chart for more detail.

mains cold water temperature for the UK

Mains cold water inlet temperatures for the UK by region-  Source – Energy Saving Trust – 2008

Flow Rates for hot water fixtures 


Standard shower head 7 to 8 litres a minute flow rate

Standard shower head 7 to 8 litres a minute flow rate

  • A standard shower head has a hot water demand of 7 or 8 litres per minute, while European Water Label “green band” water-saving shower heads have a demand of 6 litres per minute. Think of this standard as an “ErP” rating for hot water bathroom products, taps, and so on.
  • Bath mixer taps: modern bath taps have an approximate demand rate of 12 litres per minute.
  • Kitchen sink taps: 4 to 6 litres per minute
  • Basin/cloakroom “spray taps”: from 2 litres to 4 litres per minute ** be careful to check the flow rates on your spray taps – as all our instant water heater range require a minimum flow rate of 2.5 l/min to start these instant electric water heaters.

Please note –

Please be aware that drench showers can have some huge flow rates, from 15 litres to pretty much any flow rate; 30 litres per minute is not uncommon.

Drench showers with hot water flow rates from 15 to 30 litres a minute

A drench shower

So by adding all your potential hot water outlets and then applying a diversity factor (the fact that not all the hot water outlets will be used at the same time), you can calculate your hot water demand.

What if I have a different required hot water temperature rise?

Lots of industrial or commercial applications may require an application to heat water via a higher or lower temperature rise, so how can you calculate the flow rate that these instantaneous electric water heaters / 3 phase electric instant water heater can produce? You use the following formula:

Volume (L/min) = (kW rating x 60 minutes/hour) / (Specific Heat Capacity of Water x ΔT)


  • Volume: Volume of water heated in litres (L) per minute (min)
  • kW rating: Power rating of the heater in kilowatts (kW)
  • Specific Heat Capacity of Water: Approximately 4.18 kJ/kg°C
  • ΔT: Desired temperature rise in degrees Celsius (°C)

So an example

Our electric 27 kw instant electric water heaters, with a required hot water temperature rise of 50 °C, would produce

= 27 (kw rating) x 60 (seconds in a minute) divided by 4.18 (Specific Heat Capacity of Water) x 50 degrees C (temperature rise)

result = 7.75 litres per minute.

36kW Instant hot water heater

36kW electric instant water heater from Flexiheat UK

36 kW Electric instant water heater

Our largest 3 phase electric instantaneous water heater has an output of 36kW. This is a commercial electric instantaneous water heater that can produce 17.3 Litres a minute with a 30C temperature rise of energy efficient hot water for commercial settings. This 36kW instant hot water heater also benefits from a compact design making them easy to install for point of use applications such as multiple kitchen sinks etc.


Rated powerkW36
Rated voltage400V 3~
Rated currentA3 x 52
Rated voltage380V 3~
Rated currentA3 x 55
Rated voltage415V 3~
Rated currentA3 x 39.9
Supply mains water pressure – min to maximumBar1 to 6
Minimum Operating flowl/min2.5
Flow rate at Δt = 30°C @ 6 Bar water pressurel/min17.3
Maximum water flow ratel/min22
Temperature range set by knobNORMAL mode°C30 to 60
SHOWER mode°C30 to 55
Overall dimensions – H x B X Dmm492 x 315 x 140
Fuse rated currentA63
Connecting wires sectionmm24 x (10 ÷ 25)
Safety classIP 24
The maximum allowed network impedance0.09
Water inlet and outlet connections1/2” Male

Full details on this 36kw tankless water heater,  / instant water heater are in the following manual, which you can download here –

EIWH36 – 36kW Electric instantaneous water heater manual from Flexiheat UK

Contact us

Should you require further assistance, check stock, or get a price on our energy efficient and cost-effective electric instantaneous water heater or electric multipoint water heater range, with temperature control then please don’t hesitate to contact our sales team on 01202 822221 or click here for our email contact form.