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Best 18kW instantaneous water heater for a UK three-phase supply

Updated: 21 October 2026

You already know the building needs about 18 kW, and the supply is three-phase. The search still misleads you. On amazon.co.uk the row with thousands of stars is often a 240 V tankless heater, and the title still says 18 kW. The best 18kW instantaneous water heater, on this page, is a 400 V multipoint unit for a shower plus a basin or a kitchen tap, with the limit that two large outlets at once are too much for 18 kW. 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 UK laboratory ranking in this set.

You already know the building needs about 18 kW, and the supply is three-phase. The search still misleads you.

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.What best means once the supply is three-phase
  2. 2.The Bosch Tronic, the only 400 V listing in the set
  3. 3.Stiebel and Zip, outside this Amazon set
  4. 4.When the star rating points at the wrong heater

What best means once the supply is three-phase

Best is a condition, not a trophy from a star sort. The heater has to be a three-phase electronic multipoint unit at 400 V, about 26 A on each phase, with BSP threads a UK plumber can connect. Among the ASINs we kept, only the Bosch Tronic 4000 18 EB is that 400 V class. The Rheem RTEX-18 and the WINTEMP YQ-WN18K are 240 V, and both state 3/4 inch NPT. On the main comparison the Rheem is the top pick because it has 2,098 ratings. Those texts are not British opinion. If the service head is still unknown, settle that in the 18kW instantaneous water heater buying guide.

The Bosch Tronic, the only 400 V listing in the set

The Bosch Tronic 4000 18 EB, ASIN B07GCYDDLL, is a fixed 18 kW heater on 400 V, marked energy class A. On 19 October 2026 the live price was £385.10, with £77 for delivery. The price used here is about £428.72. Bosch states that an authorised installer should fit it. The listing shows 6 litres per second, and that is not a usable flow. The score was 3.5 from four ratings, and the one written review is a German text about a failed element. It suits a three-phase building that wants one fixed 18 kW plate from amazon.co.uk. Look past it if you need a published litre-per-minute figure or a WRAS approval on the listing. Who wires it is on the installation page.
WINTEMP WINTEMP YQ-WN18K 18 kW

Testsieger

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

Stiebel and Zip, outside this Amazon set

Two catalogue heaters do the same job and are not in the kept ASIN set. The Stiebel Eltron DHE 18/21/24 is switchable, with 29 A on the 18 kW step and a set range of 20–60°C. Heater Shop listed it at £808.62 including VAT. It can move to 21 kW or 24 kW only if the supply can carry the higher step. The Zip DBX18 lists 26 A, a WRAS approval, IP25, about 9.8 litres per minute at 38°C and about 6.8 litres per minute at 50°C. A trade price we saw was £564 including VAT, and UKES showed the model unavailable. The Zip gives a flow the Bosch listing does not, and it has no higher step.

When the star rating points at the wrong heater

Skip the Rheem RTEX-18 if you wanted a British 400 V multipoint heater. Its sheet describes 240 V at about 75 A, two 40 A double-pole breakers and NPT threads. On 19 October 2026 it was about £364.17, at 4.2 from 2,098 ratings that are not UK installations. The WINTEMP YQ-WN18K was about £276.46, at 4.4 from four ratings and no review text. A lower price does not create a 400 V heater. Best value is not awarded. If you only have single-phase, leave this 18 kW class. A purchase from the comparison can earn us a commission at no extra cost to you.

Common questions

Which kept Amazon unit is actually 400 V?
Only the Bosch Tronic 4000 18 EB, ASIN B07GCYDDLL, among the ASINs we kept. It is a fixed 18 kW heater at 400 V, marked energy class A on the amazon.co.uk listing. The Rheem RTEX-18 and the WINTEMP YQ-WN18K are 240 V tankless heaters with NPT threads, even when the title says 18 kW. A matching kilowatt figure does not mean a matching supply.
Why is the Rheem not the UK three-phase pick despite 2,098 reviews?
The 4.2 score from 2,098 ratings is on the amazon.co.uk listing, and those reviews are not British installations. The Rheem RTEX-18 is a 240 V design at about 75 A, with two 40 A double-pole breakers and 3/4 inch NPT threads. The main comparison marks it as the top pick because of that review count. For a UK three-phase multipoint supply it is the wrong voltage and the wrong pipe thread.
Is energy class A the same as 99% efficiency?
No. Energy class A is the label on the Bosch Tronic 4000 18 EB listing. It is not the same claim as 99% efficient, and this set has no UK laboratory measurement that quotes 99% for the Bosch, the Stiebel DHE, or the Zip DBX18. Class A also does not rank those three against each other. Electric instantaneous heaters have little stored-water loss, but that general point is not a test result for a named model.
Does a German test on the CLAGE listing count here?
No. Stiftung Warentest is a German consumer test. A line of it on the CLAGE DSX Touch listing is not a UK ranking, and we do not use it to call any heater the best. On 19 October 2026 an amazon.co.uk total for that CLAGE was £748.17, including an import charge. Import charges and stock both move. Judge the listing on voltage, pipe thread, and whether you can get it supported in the UK, not on a German score.
What should I skip if I only have single-phase?
Skip every 18 kW heater on this page: the Bosch, the Stiebel DHE, and the Zip DBX18, and also the Rheem and WINTEMP 240 V listings. At 230 V an 18 kW load is about 78 A, which is most of a typical domestic service. The Rheem’s own design is about 75 A with two 40 A breakers, and that is not an ordinary UK single-phase consumer unit. It will not stand in for a 400 V multipoint unit, and it is not called illegal.

Related pages

18kW instantaneous water heater on three-phase

18kW instantaneous water heater for a shower

18kW instantaneous water heater

What size 18 kW instantaneous heater do I need

How to install an 18kW instantaneous heater in the UK

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)