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A 240V tankless water heater on a UK supply

Updated: 21 October 2026

You have two amazon.co.uk pages open, both headed around 18 kW, and you need to know which one a British house can run. One pattern is a three-phase instantaneous heater at 400 V. The other is a 240V tankless water heater built for a North American supply, and the UK listing still uses that title. The kilowatts match. The service head, the breakers and the pipe thread do not. This page separates those two machines so you do not treat a high star count as a British fit. Live prices and the review-count rule are on the [18kW instantaneous water heater comparison](/hub/hot-water/18kw-instantaneous-water-heater). Peter F. checked those listings on 19 October 2026. There is no laboratory ranking in this set.

You have two amazon.co.uk pages open, both headed around 18 kW, and you need to know which one a British house can run. One pattern is a three-phase instantaneous heater at 400 V.

Our top picks

WINTEMP WINTEMP YQ-WN18K 18 kW

Best overall

WINTEMP WINTEMP YQ-WN18K 18 kW
8.8/10about £276.46
View offer
Rheem Rheem RTEX-18

Rank 2

Rheem Rheem RTEX-18
8.4/10about £364.17
View offer
Bosch Bosch Tronic 4000 18 EB

Rank 3

Bosch Bosch Tronic 4000 18 EB
7.0/10about £428.72
View offer
On this page
  1. 1.Two different 18 kW heaters
  2. 2.What a UK three-phase unit draws
  3. 3.What the Amazon 240 V listings require
  4. 4.What an adapter does not fix

Two different 18 kW heaters

An 18 kW title is not a supply. UK Power Networks says one fuse in the service head is typically single-phase, and three 100 A fuses are three-phase. A UK multipoint heater at this rating is 400 V, about 26 A on each phase, with BSP connections. Stiebel Eltron UK says its US Tempra range is not suitable for UK electrical connections. That sentence is about Tempra, not a test of these listings. The Bosch Tronic 4000 18 EB is the only kept listing at 400 V. The Rheem RTEX-18 and the WINTEMP YQ-WN18K are 240 V with 3/4 inch NPT. On 19 October 2026 those 240 V rows were about £364.17 and about £276.46. A lower price does not create a 400 V heater. Phase and flow sit in the 18kW instantaneous water heater buying guide.

What a UK three-phase unit draws

On 400 V, 18 kW is a circuit an electrician can design. Zip’s DBX18 sheet lists 3/PE 400 V and 26 A, with 1/2 inch BSP. Stiebel’s DHE 18/21/24 lists 29 A on the 18 kW step. Neither figure is a cable size, and this page does not turn them into a wiring instruction. The Bosch listing is 18 kW at 400 V and energy class A. A family comparison block lists 32 A, which is higher than those two sheets, so the electrician uses the manual. The same 18 kW at 230 V is about 78 A. One fuse in the service head rules the class out, Bosch included. Once three fuses are confirmed, the best 18 kW heater page applies that 400 V condition.
WINTEMP WINTEMP YQ-WN18K 18 kW

Testsieger

WINTEMP WINTEMP YQ-WN18K 18 kW
8.8/10about £276.46
View offer

What the Amazon 240 V listings require

Rheem’s RTEX sheet describes 18 kW at 240 V and 75 A, with two 40 A double-pole breakers and 3/4 inch NPT. The sheet lists 7.0 gallons per minute and amazon.co.uk says 4.4, so those figures conflict. The sampled texts are from the United States, Canada and Mexico. The WINTEMP bullets state 75 A, two 40 A switches, a 200 A supply and 3/4 inch NPT, at 4.4 from four ratings with no review text. A one-fuse house is not that service.

What an adapter does not fix

NPT is the thread on the Rheem and WINTEMP sheets. The Zip DBX18 uses 1/2 inch BSP. A thread adapter joins a pipe. It does not change 75 A at 240 V into 400 V at about 26 A. Stiebel’s Tempra answer is the parallel, not a test of the Rheem: the US range is refused for UK electrical connections, and these listings are that pattern. They are not called illegal here. They still will not run as a normal single-phase consumer-unit job. Best value is not awarded. A purchase through our links can earn a commission at no extra cost to you.

Common questions

Is US 240 V the same as UK 230 V?
No. A normal UK house is about 230 V single-phase, with one fuse in the service head on the UK Power Networks description. The Rheem RTEX-18 is listed at 240 V and 75 A, with two 40 A double-pole breakers. Those voltages look close, and the wiring is a different system. A UK 18 kW multipoint heater is 400 V, about 26 A per phase on the Zip DBX18 sheet. The 240 V listing is the wrong machine for a one-fuse house.
What is NPT, and why does it matter?
NPT is the pipe thread on the Rheem and WINTEMP listings, 3/4 inch in both cases. UK multipoint heaters in this class use BSP. The Zip DBX18 sheet uses 1/2 inch BSP. An NPT tail does not mate with BSP pipe without an adapter, and that adapter does not repair the electrical mismatch.
Can an electrician just adapt a US heater?
An adapter can join NPT to BSP pipe. It cannot turn a 75 A, two-breaker, 240 V design into a 400 V heater at about 26 A, or put that load on a normal UK single-phase consumer unit. Stiebel Eltron UK refuses its own Tempra range for UK electrical connections for this pattern. These listings are not called illegal here. Where you do have three-phase, the kept 400 V unit is the Bosch Tronic 4000 18 EB, and the circuit remains the electrician’s design.
What current does a UK 18 kW three-phase unit draw?
About 26 A per phase is the Zip DBX18 figure at 3/PE 400 V. Stiebel lists 29 A on the 18 kW step of the DHE 18/21/24. The Bosch listing is 400 V and 18 kW, and a family comparison block mentions 32 A, which is not those two sheets. None of this is a cable to buy from this page. The 75 A figure is the Rheem and WINTEMP 240 V pattern. The electrician uses the manual of the heater being fitted.
Which kept ASIN is the 400 V one?
Only the Bosch Tronic 4000 18 EB, ASIN B07GCYDDLL. It is fixed 18 kW at 400 V, and the amazon.co.uk listing marks it energy class A. The Rheem RTEX-18 and the WINTEMP YQ-WN18K are 240 V with 3/4 inch NPT. On 19 October 2026 the Bosch price used here was about £428.72, and the live page showed £385.10 plus £77 delivery. The cell that prints 6 litres per second is not a shower flow.

Related pages

18kW instantaneous water heater installation

18kW instantaneous water heater

Best 18kW instantaneous water heater on three-phase

What size 18 kW instantaneous heater do I need

18kW instantaneous water heater for one standard shower

Sources

Public UK sources named below. amazon.co.uk prices, stars and review counts on these three listings were read on 19 October 2026. Page updated 21 October 2026.

  1. Approved Document G, with 2024 amendments (retrieved 19 October 2026)
  2. Part G FAQ, bath temperature limit (retrieved 19 October 2026)
  3. Approved Document P, 2013 (retrieved 19 October 2026)
  4. UK Power Networks, single-phase and three-phase (retrieved 19 October 2026)
  5. Stiebel Eltron UK, what size instantaneous water heater (retrieved 19 October 2026)
  6. Stiebel Eltron UK, Tempra products in the UK (retrieved 19 October 2026)
  7. Zip DBX18 installation sheet (retrieved 19 October 2026)
  8. Rheem RTEX product sheet (retrieved 19 October 2026)
  9. Flexiheat UK, electric multipoint water heaters (retrieved 19 October 2026)
  10. amazon.co.uk, Bosch Tronic 4000 18 EB (retrieved 19 October 2026)