Bosch point-of-use tankless water heater
Updated: 17.10.2026
You have one basin or one sink the main system does not reach, and three amazon.co.uk listings look as if they were written for the cupboard underneath. A Bosch point-of-use tankless water heater only earns that job if the tap can start it and a plumber can make an NPT joint. The three candidates are the Tronic 4000 units at 6.5, 8.5 and 10.5 kW, priced at £245.74, £236.63 and £239.38 on 15 October 2026. The [main comparison](/hub/hot-water/bosch-tankless-water-heater) holds every row, including the two larger models this page leaves out. We did not fit a heater. A qualifying purchase can earn a commission at no extra cost to you.
You have one basin or one sink the main system does not reach, and three amazon.co.uk listings look as if they were written for the cupboard underneath.
You have one basin or one sink the main system does not reach, and three amazon.co.uk listings look as if they were written for the cupboard underneath. A Bosch point-of-use tankless water heater only earns that job if the tap can start it and a plumber can make an NPT joint. The three candidates are the Tronic 4000 units at 6.5, 8.5 and 10.5 kW, priced at £245.74, £236.63 and £239.38 on 15 October 2026. The main comparison holds every row, including the two larger models this page leaves out. We did not fit a heater. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Supply | Listed flow | Case | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Bosch Bosch Tronic 4000 10.5 kW | Corded electric | n/a | 1.59 kg | 5.4 | about £239.38 | View offer |
| Alternative |  Bosch Bosch Tronic 4000 6.5 kW | Hard-wired | n/a | 21.6 × 33 cm, 2.39 kg | 4.4 | about £245.74 | View offer |
| Preis-Tipp |  Bosch Bosch Tronic 4000 8.5 kW | Corded electric | 1.5 gal/min | 33 × 11.4 cm, 2.41 kg | 5.1 | about £236.63 | View offer |
On this page
The three listings that are actually point of use
The 6.5 kW Tronic 4000 (B0BWKBQ1TY, part 7738007416) is £245.74, listed as hard-wired, with 3.1 out of 5 from two reviews. The listed body is about 21.6 cm wide by 33 cm high, 11.4 cm deep, at 2.39 kg. The 8.5 kW listing (B0BWK7W2YK, part 7738007417) is £236.63, with a stated flow of 1.5 gal/min and no Customer Reviews line. Its dimension block reads 33 cm by 11.4 cm, at 2.41 kg. The 10.5 kW listing (B0BWKDBT8F, part 7738007418) is £239.38, also 3.1 from two ratings, and the page lists it at 1.59 kg.
All three bullets say point of use, under a sink, wall-mounted, plastic, activation 0.55 gal/min. They do not share one footprint, so measure the cupboard against the row you are buying. The 18 kW and 27 kW listings repeat the under-sink sentence. Those part numbers are the whole-house sheet. The buying guide is where that supply question belongs. None of the three small rows states a voltage. The US sheet says 208/240 V at 60 Hz.
The flow that switches the heater on
Activation is 0.55 gal/min, about 2.1 litres per minute. Bosch’s Tronic 4000 C troubleshooting guide says the heater will not activate correctly below that flow. The check in that US guide is to measure the flow and, if it is short, to look at a clogged aerator and a valve that is not fully open. It is not an instruction to dismantle a tap on a live circuit. Electrical work belongs on the installation page.
A jug test is only a rule of thumb. On the hot outlet alone, about 2 litres per minute fills a one-litre jug in roughly half a minute. If a spray tap takes a minute to deliver that litre, it is under the threshold on purpose. A tap sold to save water can be too low for this heater. Confirm the litres before you confirm the basket.
Hotter or faster, not both
The 4000 C is non-thermostatic. Power is selected in steps, and the outlet cools when you ask for more water. The troubleshooting guide’s lukewarm note points at flow that is too high, or a unit left off its higher setting. At a rise of about 43°C the US sheet falls to about 2.2, 2.8 and 3.5 litres per minute for 6.5, 8.5 and 10.5 kW. Open the tap to fill faster and you spend that temperature.
A washing-up bowl shows the limit. Six litres at about 2.8 litres per minute is a little over two minutes, and only if you stay near the flow that keeps the rise. Open up and the water arrives cooler. Stiebel Eltron’s UK guidance, for their own heaters, is that a small store is often the better way to fill a kitchen bowl. A hand basin that needs a short warm run is the fairer job for these three.
The pipe under the sink
The US sheet for the 4000 C specifies 1/2 inch NPT connections and a pressure window of 7–115 psi. The amazon cells do not state the thread, so this follows the manufacturer sheet. A UK 15 mm to half-inch compression adaptor is commonly BSP. NPT and BSP are different thread forms. The adaptor has to be sold as NPT. A generic half-inch tap connector is the wrong fitting, including one that almost starts.
Ask the plumber whether the cold feed sits inside that pressure window, and to make the NPT joint, before the heater goes on the wall. Do not plumb these as a tiny tank, and do not size a cupboard for 55 gallons. The body on the row is a wall-mounted unit of about 1.6–2.4 kg. If the joint, the flow or the supply fails, the under-sink bullet has not made the job.
Common questions
Will it run a spray tap?
Only if that tap can pass about 0.55 gal/min, roughly 2.1 litres per minute. Many spray taps are designed to sit at or below that region. A one-litre jug that takes about a minute on the hot outlet is under the start threshold. The US troubleshooting guide treats flow below 0.55 gal/min as a failure to activate. Measure the tap you have.
How do I know if 0.55 gal/min is reaching the heater?
Time the hot outlet with a one-litre jug. About 2 litres per minute fills it in roughly half a minute, which is the scale of the activation figure. This timing is a rule of thumb, not a Bosch test procedure. The US guide’s own check is a flow measurement, with a clogged aerator as one reason the figure comes out low. Leave electrical work to the installer.
Can I fill a washing-up bowl?
You can run water into it. You should not expect a quick bowl of hot water. At about 2.8 litres per minute, six litres takes a little over two minutes, and opening the tap further on a non-thermostatic 4000 C cools the outlet. Stiebel Eltron’s UK guidance often points a kitchen bowl towards a small store. A short handwash is the closer match.
Which listing is the under-sink one?
All three say under-sink point of use. The 6.5 kW row says hard-wired, at £245.74, with two written reviews. The 8.5 kW row is £236.63, the only one with a stated flow of 1.5 gal/min, and it has no reviews. The 10.5 kW row is £239.38, with one review mentioning a kitchen sink. Choose by supply and by the litres you measured. The 18 kW and 27 kW bullets also say under the sink. They are not this shortlist.
Will a normal 15 mm tap connector fit?
A normal UK 15 mm to half-inch connector is usually BSP. The 4000 C sheet specifies 1/2 inch NPT. The amazon cells do not print the thread, so this answer rests on the manufacturer sheet. Buy an adaptor sold as NPT. A fitting that almost starts is not a completed joint.
Why does the water cool when I open the tap further?
The 4000 C is non-thermostatic. The same kilowatts spread across more litres produce a smaller rise. The US guide lists flow that is too high, or a unit left off its higher setting, as reasons the outlet is lukewarm. At a large rise the sheet’s flows are about 2.2–3.5 litres per minute. Faster than that, on a cold inlet, means cooler water.
Is the 18 kW model a bigger under-sink unit?
The bullets say it fits under sinks. Part 7738007420 is the 18 kW Tronic 6100 C, which the US sheet specifies on three 30 A double-pole breakers, at £706.84 in this scrape. The 27 kW listing copies the same under-sink wording. Use the three Tronic 4000 rows for a single sink. Use the larger two only after a supply check those breakers imply.
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)