The best 18kW instantaneous water heaters, compared.
Top pick: WINTEMP YQ-WN18K 18 kW Best value: Not awarded – A lower shelf price on a 240 V, 3/4 inch NPT listing is not a UK value comparison against the 400 V Bosch. The WINTEMP has 4 reviews and no review text. Updated 21.10.2026.
An 18kW instantaneous water heater heats mains water as it flows and does not store a tank. In UK catalogues this rating is the start of the three-phase multipoint class, for a shower plus other outlets, not a plug-in basin unit. A credible unit is 400 V three-phase, about 26 A per phase, with BSP connections and a temperature lock the installer can set. A normal UK house is single-phase, so 18 kW at 230 V is about 78 A and would take almost the whole service. Outlet sizing is in the [18kW instantaneous water heater buying guide](/hub/hot-water/18kw-instantaneous-water-heater/sub/18kw-instantaneous-water-heater-buying-guide). An 18 kW instantaneous water heater heats mains water only while a tap is open. You recognise a credible unit by the supply it is built for. UK says one fuse in the service head is typically single-phase, and three 100 A fuses are three-phase. Stiebel UK says convenience heaters for a bathroom and a kitchen are 400 V, and that its US range is not suitable for UK electrical connections.
An 18kW instantaneous water heater heats mains water as it flows and does not store a tank.
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Personal buying guide
Question 1 of 2
Is 400 V three-phase confirmed?
The WINTEMP YQ-WN18K is listed at £276.46 as an 18 kW heater at 240 V, with a stated maximum of 75 A, two 40 A double-pole switches, two 8/2 AWG cables and a 200 A domestic supply in the bullets. The water inlet is 3/4 inch NPT, the flow line is 4.3 gallons per minute, and the temperature ceiling is 140°F. An LED display and Wi-Fi control are on the page, and the bullets claim two or three points of use plus a 2-year warranty. The star line is 4.4 from 4 reviews, and the listing has no review text, so fit, noise and failures are not described. ETL is the safety mark stated, not a UK water approval, and no BS 1363 plug is mentioned. At this current and thread it follows a North American tankless pattern, which a normal UK single-phase house does not match, even though the title says 18 kW. The score is 88 because the star line is 4.4. That is not best value, and it is not the top pick.
Pros
Cons
Best for: You are comparing a US-pattern 240 V listing and will not treat the star line as a UK fit.
Less suitable: You need a 400 V bathroom group on a UK three-phase supply, or you are choosing from review text.
Specifications
Common questions
What electrical supply does the WINTEMP listing state?
18 kW at 240 V, maximum 75 A, two double-pole switches of 40 A, two 8/2 AWG cables, and a 200 A domestic supply. A BS 1363 plug is not stated.
What pipe thread does it use?
The bullets state a 3/4 inch NPT water inlet. A BSP size is not stated.
Does the listing include UK reviews?
The star line is 4.4 from 4 reviews. The listing has no review text, so praise and complaints are not stated.
The Rheem RTEX-18 is listed at £364.17 as an 18 kW, 240 V tankless heater with 3/4 inch NPT connections and an LED control the page calls accurate to plus or minus 1 degree. The amazon.co.uk line is 4.2 from 2,098 reviews, which is why this comparison marks it as the top pick under a review-count rule of at least 100 reviews. The first 50 review texts are from the United States, Canada and Mexico, not the United Kingdom, so that is not a British verdict. Those texts often praise on-demand hot water and a smaller footprint than a storage tank, and they often report heating elements or controls failing after about one to three years. A second outlet, or a bath filled at full flow, is a repeated limit, and several reviewers found two 40 A circuits harder than the simple-installation line suggested. Amazon states 4.4 gallons per minute while Rheem’s own sheet lists a 7.0 GPM maximum, so the two flow figures stay in conflict. It is the best-documented 240 V listing here, and it is still the wrong supply pattern for a normal UK house.
Pros
Cons
Best for: You are reading the high-count 240 V listing and will treat the reviews as North American experience.
Less suitable: You want a UK 400 V multipoint heater, or you would read 2,098 stars as British opinion.
Specifications
Common questions
Is the Rheem RTEX-18 a UK three-phase heater?
The listing states 240 V and 18 kW, with 3/4 inch NPT connections. It does not state 400 V or a BSP thread. Review text says this model needs two 40 A breakers.
Where do the reviews come from?
The amazon.co.uk line is 4.2 out of 5 from 2,098 reviews. The first 50 review texts are from the United States, Canada and Mexico, not the United Kingdom.
What do reviewers say goes wrong?
Repeated themes are heating elements or controls failing after about one to three years, a temperature drop when a second outlet opens, and heavy wiring. Praise centres on on-demand hot water and a smaller cupboard than a tank.
The Bosch Tronic 4000 18 EB is listed at 18 kW and 400 V, which is the UK three-phase pattern in this set, at £428.72 in the price used here and £385.10 plus £77 delivery on the live amazon.co.uk page on 19 October 2026. The card is energy class A, 483 × 236 × 115 mm, and the listing says only an authorised craftsman may install it. Across 4 reviews the star line is 3.5, and the only review body is marked Reviewed in Germany: the heating element failed after seasonal use, described as about 17 months of running time across three years. One text cannot show a pattern. The flow cell is not a credible heated-flow figure, so you cannot size a shower from it. A Bosch comparison block lists the 18 kW step at 32 A, while UK peers such as the Zip DBX18 are specified nearer 26 A, so the electrician should use the manual rather than that block alone. It is the 400 V option among the kept listings, and a mismatch if your house is a normal single-phase supply. The score is 70, from 3.5 stars, so it is not the top pick.
Pros
Cons
Best for: You already have a three-phase supply and want the 400 V 18 kW unit in this Amazon set.
Less suitable: Your house is a normal single-phase supply, or you need a stated litres-per-minute flow.
Specifications
Common questions
What supply does the Bosch Tronic 4000 18 EB need?
The listing states 400 V and 18 kW, with a fixed connection. It does not state a BS 1363 plug. A comparison block on the Bosch family lists the 18 kW step at 32 A.
Can it feed a shower?
The bullets say it is an 18 kW heater for showers and bathrooms and for a multiple supply. A bath flow and a UK water-pressure range are not stated. The flow cell is not a figure you can size from.
Who may install it?
The listing says it can only be installed by an authorised craftsman. Clickfix plus is described as a quick mounting system, not as a DIY plug-in.
The score on each card is the amazon.co.uk star line multiplied by 20, from the three listings kept on 19 October 2026. Bosch 3.5 gives 70. Rheem 4.2 gives 84. WINTEMP 4.4 gives 88. Nothing else is added or subtracted, because a UK-supply adjustment would hide the star line you still see on the page. A higher score is a higher star line. It is not a judgement that the heater suits a UK service. The top pick is the Rheem, the only kept listing with at least 100 reviews. Best value is not awarded. This is not a laboratory test.
Amazon star line
100%The whole score is the amazon.co.uk star line times 20. Bosch is 3.5 from 4 reviews, score 70. Rheem is 4.2 from 2,098 reviews, score 84. WINTEMP is 4.4 from 4 reviews, score 88. A higher score is only a higher star line.
Confirm the supply before you compare prices. In the UK, 18 kW instantaneous water heating is a 400 V three-phase multipoint, about 26 A per phase, not a 13 A basin heater. A normal single-phase house would see about 78 A at 230 V. A 240 V title with an NPT thread is a different product.
An 18 kW instantaneous water heater heats mains water only while a tap is open. It does not store a cylinder of hot water. In UK catalogues this rating is the start of the three-phase multipoint class: a shower plus other outlets, not a plug-in basin unit. You recognise a credible unit by the supply it is built for. That is 400 V three-phase, about 26 A per phase, BSP connections, and a temperature lock the installer can set. A normal UK house is single-phase.
The Rheem line is 4.2 from 2,098 reviews, and the first 50 texts are from the United States, Canada and Mexico. The WINTEMP line is 4.4 from 4 reviews with no text. Neither star line is British opinion, and a higher star line is not a UK-supply judgement.
The Bosch row is 400 V. The Rheem and WINTEMP rows are 240 V with 3/4 inch NPT, about 75 A and two 40 A breakers. A normal UK house is single-phase. Do not assume a US 240 V heater will run on that consumer unit, and do not treat it as a 13 A plug-in.
A standard shower is about 7–8 L/min. Stiebel’s rule of thumb at 18 kW is about 9 L/min at 38°C and about 6 L/min at 50°C. A bath mixer near 12 L/min, or a drench head, is beyond that duty. The Bosch flow cell is not a figure you can size from.
Supply comes first. UK Power Networks says one fuse in the service head is typically single-phase, and three 100 A fuses are three-phase. Stiebel Eltron UK says convenience heaters for a bathroom and a kitchen are 400 V, and that its US Tempra range is not suitable for UK electrical connections. A credible 18 kW unit is 400 V three-phase, about 26 A per phase, with BSP connections. Zip DBX18 is specified at 26 A.
Read litres per minute at a stated rise, not a title that only says 18 kW. Stiebel’s rule of thumb is kilowatts divided by 2 at about 38°C and divided by 3 at about 50°C, so about 9 L/min and 6 L/min at 18 kW. Flexiheat prints 8.7 L/min at a 30°C rise. A standard shower is about 7–8 L/min. A bath mixer near 12 L/min, or a drench head, is beyond a comfortable 18 kW duty. Electronic modulation holds a set temperature until the element is saturated.
Three typical situations, each with a recommendation from this comparison.
You already have three-phase and you want the 400 V 18 kW unit in this Amazon set. The Bosch Tronic 4000 18 EB is the only kept listing stated at 400 V, energy class A, and the page says an authorised craftsman must install it. The star line is 3.5 from 4 reviews.
You are reading the 240 V listing with the long review file, and you will treat those texts as North American experience. The Rheem RTEX-18 is the top pick under the review-count rule, at 4.2 from 2,098. The sampled texts are from the United States, Canada and Mexico, not the United Kingdom.
You want the £276.46 row written out so a cheap 18 kW title is not mistaken for a UK 400 V multipoint. The WINTEMP YQ-WN18K states 240 V, 75 A and a 3/4 inch NPT inlet. The star line is 4.4 from 4 reviews, with no review text. It is not best value for this category.
The top pick is the Rheem RTEX-18 at about £364.17. It scores 84, from 4.2 and 2,098 reviews, and it is the only kept listing with at least 100 reviews. The sampled texts are from the United States, Canada and Mexico, so that is not a British verdict, and the supply is 240 V with 3/4 inch NPT. Best value is not awarded. The WINTEMP is cheaper, at about £276.46, with 4.4 from 4 reviews and no review text, on the same North American pattern. The Bosch Tronic 4000 18 EB at about £428.72 is the only 400 V row. Skip the Rheem if you would read 2,098 stars as British opinion. Skip the WINTEMP if you need a UK multipoint. Skip the Bosch if the house is a normal single-phase supply.
Not awarded
A lower shelf price on a 240 V, 3/4 inch NPT listing is not a UK value comparison against the 400 V Bosch. The WINTEMP has 4 reviews and no review text.
Editorial
Peter F. is the editor for DIY & tools at productscompared.co.uk. Comparisons in this category are checked against amazon.co.uk listings, review evidence and public UK sources.
Guides on what to look for, how to use a product, and common mistakes.
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.
This page contains affiliate links to amazon.co.uk (tag pcuk26-21). If you buy through them, we may earn a commission at no extra cost to you. Prices, stars and review counts are taken from those listings. We did not run a lab test.
