Bosch tankless water heater buying guide
Updated: 17.10.2026
You are about to shortlist a Bosch tankless water heater, and the kilowatt figure in the title is the easy number to misread. On 15 October 2026 the five amazon.co.uk listings sat at £236.63 to £245.74 for the Tronic 4000 units and £662.81 to £706.84 for the Tronic 6100 units. This Bosch tankless water heater buying guide is the method behind the [main comparison](/hub/hot-water/bosch-tankless-water-heater): part number, supply, winter flow, then pipe thread. The Products Compared editorial team read those listings and the public sources named below. We did not laboratory-test the heaters. A qualifying purchase can earn a commission at no extra cost to you.
You are about to shortlist a Bosch tankless water heater, and the kilowatt figure in the title is the easy number to misread.
You are about to shortlist a Bosch tankless water heater, and the kilowatt figure in the title is the easy number to misread. On 15 October 2026 the five amazon.co.uk listings sat at £236.63 to £245.74 for the Tronic 4000 units and £662.81 to £706.84 for the Tronic 6100 units. This Bosch tankless water heater buying guide is the method behind the main comparison: part number, supply, winter flow, then pipe thread. The Products Compared editorial team read those listings and the public sources named below. We did not laboratory-test the heaters. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Power | Supply | Activation | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Bosch Bosch Tronic 4000 10.5 kW | 10.5 kW | Corded electric | 0.55 gal/min | 5.4 | about £239.38 | View offer |
| Alternative |  Bosch Bosch Tronic 4000 6.5 kW | 6.5 kW | Hard-wired | 0.55 gal/min | 4.4 | about £245.74 | View offer |
| Preis-Tipp |  Bosch Bosch Tronic 4000 8.5 kW | 8.5 kW | Corded electric | 0.55 gal/min | 5.1 | about £236.63 | View offer |
On this page
Which Bosch part numbers these listings actually are
The article numbers are 7738007416 (6.5 kW), 7738007417 (8.5 kW), 7738007418 (10.5 kW), 7738007420 (18 kW) and 7738007421 (27 kW). Bosch’s US sheets for the Tronic 4000 C and Tronic 6100 C use those numbers and specify 208/240 V at 60 Hz. Every amazon row here leaves voltage as n/a. Treat the 60 Hz line as a stop-and-check. This page is not claiming a 50 Hz approval.
A European Tronic 4000 ET, such as article 7736504689, is a 230 V unit with a smaller body. If that is the manual in your hand, it is the wrong document for these baskets. The 4000 C listings are non-thermostatic sink heaters. The 6100 C listings are sold on the US sheet as thermostatic whole-house heaters. A UK electric shower is a third product, kept on the shower comparison.
What the kilowatts can heat
Kilowatts describe heat input. Litres per minute describe what you feel, and they fall when the inlet is cold. Stiebel Eltron’s UK sizing FAQ is their rule of thumb for their own range, not a Bosch measurement: divide kilowatts by two for water around 38°C, and by three for water around 50°C. Bosch’s 4000 C sheet, at a rise of about 43°C, is tighter: about 2.2, 2.8 and 3.5 litres per minute for 6.5, 8.5 and 10.5 kW.
Activation on the three 4000 C listings is 0.55 gal/min, about 2.1 litres per minute. If the tap cannot pass that much, the heater does not start. The point-of-use page stays with the single sink.
Supply, plug, and pipe thread
Arithmetic at 230 V is about 28 A, 37 A and 46 A for 6.5, 8.5 and 10.5 kW. That is a sketch of the load, not a cable size. None of these five listings names a BS 1363 plug. The 6.5 kW and 27 kW rows say hard-wired. The other three say Corded Electric and still name no plug.
The US sheet asks for three 30 A double-pole breakers at 18 kW and three 40 A double-pole breakers at 27 kW. Those are US figures, not one spare way in a UK consumer unit. On the same sheets the 4000 C connections are 1/2 inch NPT and the 6100 C connections are 3/4 inch NPT. A UK half-inch tap connector is usually BSP, so the joint needs an adaptor sold as NPT. Book the electrician before you pay. Scotland and Northern Ireland use their own building routes.
What to ignore on the amazon row
Several specification cells contradict the bullets, and the comparison has set them aside. The 6.5 kW heat-output cell says 65 kW, while the title says 6.5 kW. Capacity cells of 55 gallons sit next to bullets that say the heater holds no water. Efficiency is 96% in the bullets and 95% or 98% on the specification line, depending on the listing.
The Consumer Rights Act 2015 requires goods to match the description. Use the wattage the title and the wattage line share. Price order is not quality order: the 6.5 kW unit is the dearest of the small three, at £245.74, and the 27 kW unit is cheaper than the 18 kW unit in this scrape.
A shortlist, not a winner
Two listings show 3.1 out of 5 from two reviews. Three have no Customer Reviews line. That is not a review winner, and this page does not award one. A single basin, once the activation flow is real and an electrician has accepted a dedicated circuit, can sit with the 6.5 kW hard-wired listing at £245.74. If you need more flow at the sink and the supply can take it, the 8.5 kW listing at £236.63 and the 10.5 kW listing at £239.38 are the other point-of-use rows. Keep the 18 kW and 27 kW listings off the shortlist until the supply can feed three high-current circuits.
Leave the basket if the only spare point is a 13 A socket, if a January inlet cannot support the litres you want, or if the joint you own is a generic BSP connector.
Common questions
Is the Tronic 4000 on amazon.co.uk the small European 230 V model?
The five part numbers, 7738007416 to 7738007421, match Bosch’s US Tronic 4000 C and 6100 C sheets, which state 208/240 V at 60 Hz. A European Tronic 4000 ET, such as 7736504689, is a smaller 230 V unit. If that ET manual is the one in front of you, it belongs to a different heater. The amazon rows themselves leave voltage as n/a.
Will a 13 A plug run any of these?
No listing names a BS 1363 plug. The 6.5 kW and 27 kW rows say hard-wired. The other three say Corded Electric and still name no plug. At 230 V the arithmetic is about 28 A for 6.5 kW, already above a 13 A fuse, and about 37 A and 46 A for 8.5 kW and 10.5 kW. Aqua Hot’s single-phase range says heaters over 3 kW are not a ring-main job.
How many litres per minute should I expect in a UK winter?
Bosch’s 4000 C sheet, at a rise of about 43°C, falls to about 2.2, 2.8 and 3.5 litres per minute for 6.5, 8.5 and 10.5 kW. That is the US manufacturer sheet, not an amazon cell and not a test we ran. Stiebel Eltron’s UK rule of thumb is kilowatts divided by two near 38°C and by three near 50°C. Measure your tap. A rise that large is a cold inlet.
Why does one listing say 65 kW and another say 55 gallons?
The 6.5 kW heat-output cell says 65 kW, while the title says 6.5 kW. Several capacity cells say 55 gallons beside bullets that say the heater holds no water. The 27 kW flow cell says 0.14 cubic metres per minute against a 0.55 gal/min bullet. Those cells are set aside. Use the wattage the title and the wattage line share.
Is the cheaper 27 kW unit the better buy?
In this scrape the 27 kW listing is £662.81 and the 18 kW listing is £706.84. The US sheet asks for three 40 A double-pole breakers at 27 kW and three 30 A breakers at 18 kW. The electrical job gets larger as the price falls. Neither listing has a review line. Check the supply before you treat the lower price as a saving.
Do I need an electrician before I order?
Yes, if you do not already know the circuit can take the load. The US installation manual requires a qualified installer and hard-wiring, and a new circuit in a dwelling in England is notifiable. Finding out after you have paid that the consumer unit has no spare way is the expensive version of this page. Fitting and freeze risk come after that answer.
Related pages
Sources
Sources from the research digest. amazon.co.uk prices in this comparison were captured on 15 October 2026.
- Bosch Tronic 4000 C engineering submittal (Feb 2023) (retrieved 15 October 2026)
- Bosch Tronic 4000 C installation manual (Nov 2025) (retrieved 15 October 2026)
- Bosch Tronic 6100 C engineering submittal (Feb 2023) (retrieved 15 October 2026)
- Approved Document G (England, 2024 amendments) (retrieved 15 October 2026)
- Building Regulations &c. (Amendment) Regulations 2012, regulation 6 (retrieved 15 October 2026)
- Consumer Rights Act 2015, goods terms (retrieved 15 October 2026)
- Stiebel Eltron UK sizing FAQ (retrieved 15 October 2026)
- IET Part P FAQ (England and Wales) (retrieved 15 October 2026)