How to install a Bosch tankless water heater
Updated: 17.10.2026
You are booking an electrician for a Bosch tankless water heater, not stripping a consumer unit yourself. The five amazon.co.uk listings are high-current US part numbers, two of them marked hard-wired, and a new circuit in a dwelling in England has to be notified. This page is what must be true of the circuit, the paperwork and the freeze risk before anyone powers the unit. Kilowatt choice sits on the [buying guide](/hub/hot-water/bosch-tankless-water-heater/sub/bosch-tankless-water-heater-buying-guide). The [main comparison](/hub/hot-water/bosch-tankless-water-heater) holds the five rows. The Products Compared editorial team used those listings, the US manual and the UK rules cited below. We did not install a unit. A qualifying purchase can earn a commission at no extra cost to you.
You are booking an electrician for a Bosch tankless water heater, not stripping a consumer unit yourself.
You are booking an electrician for a Bosch tankless water heater, not stripping a consumer unit yourself. The five amazon.co.uk listings are high-current US part numbers, two of them marked hard-wired, and a new circuit in a dwelling in England has to be notified. This page is what must be true of the circuit, the paperwork and the freeze risk before anyone powers the unit. Kilowatt choice sits on the buying guide. The main comparison holds the five rows. The Products Compared editorial team used those listings, the US manual and the UK rules cited below. We did not install a unit. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Supply | US sheet | Power | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Bosch Bosch Tronic 4000 10.5 kW | Corded electric | Dedicated circuit | 10.5 kW | 5.4 | about £239.38 | View offer |
| Preis-Tipp |  Bosch Bosch Tronic 4000 8.5 kW | Corded electric | Dedicated circuit | 8.5 kW | 5.1 | about £236.63 | View offer |
| Alternative |  Bosch Bosch Tronic 6100 27 kW | Hard-wired | Three 40 A double-pole | 27 kW | 3.6 | about £662.81 | View offer |
On this page
What the listing’s power line does and does not mean
The 6.5 kW Tronic 4000 and the 27 kW Tronic 6100 say hard-wired. The 8.5 kW, 10.5 kW and 18 kW listings say Corded Electric. None of the five names a 13 A plug or a BS 1363 socket. Corded on those rows is a catalogue phrase, not a lead you can test in the kitchen. Bosch’s Tronic 4000 C installation manual, the US edition dated November 2025, tells a qualified installer to hard-wire the heater and to keep the unit indoors.
A dedicated circuit is the usual shape of the job because of the load. A NICEIC comment on instantaneous heaters makes the same general point, and it is not a line of BS 7671 that names Bosch. At 230 V the arithmetic is about 28 A, 37 A and 46 A for 6.5, 8.5 and 10.5 kW. That sketch is why a socket is the wrong picture. It is not a cable size. Voltage on every display row is n/a. The US sheets say 208/240 V at 60 Hz. Hand both facts to the electrician.
Notification in England
Three kinds of electrical work in a dwelling in England are notifiable under the 2012 amendment to the Building Regulations. A new circuit is one. Replacing a consumer unit is another. An alteration in a bathroom special location is the third. If the job needs a new circuit, it is notifiable because it is a new circuit, including in a kitchen.
The usual route is a competent person who self-certifies through a scheme. The other route is to tell building control yourself before the work starts. Approved Document P sits with that duty, and the design follows BS 7671. Scotland and Northern Ireland use their own building rules. The IET’s Part P FAQ describes the England and Wales routes. It is not a licence to wire the heater.
Why 18 kW and 27 kW are a different job
The Tronic 6100 bullets talk about an under-sink unit. The US engineering sheet does not. Part 7738007420, the 18 kW model, is specified with three 30 A double-pole breakers. Part 7738007421, the 27 kW model, is specified with three 40 A double-pole breakers. Those are three circuits in the US document, not one breaker in a UK board. In this scrape the 27 kW listing is £662.81 and the 18 kW listing is £706.84, so the cheaper box is the larger electrical job.
Get the supply checked before either is ordered. A board with no spare way, or a quote that assumes one 32 A way, is a reason to stop. The 4000 C models are still a dedicated-circuit conversation, at the single-circuit currents above. They are the heaters on the point-of-use page. Thermostatic modulation on the 6100 C sheet does not turn the listing into a UK electric shower.
Water temperature, storage rules, and frost
Approved Document G in England limits water delivered to a fixed bath to 48°C. The storage-vessel clauses apply only if a vessel is there. These listings are tankless, the bullets say they hold no water, and a discarded 55-gallon cell does not create a cylinder. You still meet the bath limit if a bath is on the pipe. Electric instantaneous heaters are the type that document points towards BS EN 60335-2-35.
The 4000 C is non-thermostatic, and the outlet cools if the flow rises. The 6100 C sheet talks about a modulated outlet around 30–52°C. A bath fed from either unit still has to meet 48°C at the bath. Frost is the other stop. The US 4000 C manual excludes freeze damage from the warranty and says not to energise a unit that may have frozen. A cupboard that can drop below 0°C is a reason to pause. Water fittings in England and Wales also sit under the Water Supply (Water Fittings) Regulations 1999.
What to stop doing yourself
Stop at the consumer unit, the cable and the terminals. Tell the electrician the part number, the kilowatts from the title, and whether that listing says hard-wired or Corded Electric. Say whether the heater will sit in a kitchen, a cloakroom or a bathroom, whether a new circuit is likely, whether a bath is downstream, and whether the cupboard can freeze.
Ask who will notify building control before the old circuit is disturbed. If the person quoting cannot self-certify, the building-control route has to be open first. Do not energise a unit that may have frozen. Do not convert American wire gauge from the US manual into a UK cable and treat that as a design. Do not put a BSP tap connector on an NPT thread.
Common questions
Can I plug it into a socket?
The listings do not offer that. Two say hard-wired. Three say Corded Electric and name no plug. The US 4000 C manual tells a qualified installer to hard-wire the heater. A 13 A plug is fused at 13 A, and 6.5 kW at 230 V is already about 28 A. If the only point you have is a socket, the job has not started.
Is a new circuit notifiable in a kitchen?
Yes, in England, because a new circuit is notifiable on its own. The bathroom rule is an extra case: an alteration in a special location is also notifiable. A kitchen does not have to be a bathroom for the new-circuit duty to apply. Replacing the consumer unit is notifiable as well. Scotland and Northern Ireland have their own routes.
Who can sign the work off?
A registered competent person can do the work and self-certify it to building control. If you cannot find one, you notify building control before the work begins. The IET’s Part P FAQ describes those England and Wales paths. The US manual’s qualified-installer line is the manufacturer’s bar. It does not replace the British notification duty.
Do the 18 kW and 27 kW models use one breaker?
The US sheet specifies three 30 A double-pole breakers for 18 kW and three 40 A double-pole breakers for 27 kW. Those figures are American. A UK electrician still has to design the supply. An amazon bullet about fitting under a sink does not change the breaker count on the sheet.
Does tankless mean I can skip cylinder paperwork?
The storage-vessel parts of Approved Document G apply when a vessel is present. These heaters are listed as tankless, and the bullets say they hold no water. Water to a fixed bath in England must still not exceed 48°C at delivery. A new circuit is still notifiable. Tankless is not a shortcut around Building Regulations.
What if the cupboard can freeze?
Do not power the heater. The US 4000 C manual treats freeze damage as outside the warranty and says not to energise a unit that may have frozen. If the space can fall below 0°C, sort the location with the installer before the supply is closed. A quick electrical test on a frozen heat exchanger is not a result you want.
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)