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How to install an 18kW instantaneous water heater

Updated: 21 October 2026

The heater is in the hall and the electrician comes on Thursday. To install an 18kW instantaneous water heater you need a fixed circuit and a plumber on the mains, not a fused spur. A cable size copied from a forum, or power on a pipe that still holds air, is how the first week goes wrong. This page is the handover: who notifies the work, who connects the water, and what you keep before the isolator closes. Models are on the [18kW instantaneous water heater comparison](/hub/hot-water/18kw-instantaneous-water-heater). The notes below come from UK building rules and manufacturer conditions, not a lab test. A purchase through our links can earn a commission at no extra cost to you.

The heater is in the hall and the electrician comes on Thursday. To install an 18kW instantaneous water heater you need a fixed circuit and a plumber on the mains, not a fused spur.

Our top picks

WINTEMP WINTEMP YQ-WN18K 18 kW

Best overall

WINTEMP WINTEMP YQ-WN18K 18 kW
8.8/10about £276.46
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Rheem Rheem RTEX-18

Rank 2

Rheem Rheem RTEX-18
8.4/10about £364.17
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Bosch Bosch Tronic 4000 18 EB

Rank 3

Bosch Bosch Tronic 4000 18 EB
7.0/10about £428.72
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On this page
  1. 1.Why this is not a socket
  2. 2.What an electrician certifies
  3. 3.What a plumber certifies
  4. 4.Before the power goes on

Why this is not a socket

An 18 kW instantaneous heater is permanently wired. There is no 13 A plug. Zip’s sheet calls for permanent wiring, all-pole isolation, and a qualified installer. On 400 V the draw is about 26 A on each phase. The same 18 kW at 230 V is about 78 A. UK Power Networks describes one fuse in the service head as typical single-phase, and three 100 A fuses as three-phase. A 240 V import is a separate mismatch, set out in the three-phase supply guide.

What an electrician certifies

In England a new circuit is notifiable under Part P, and so is work in a bathroom zone. An 18 kW heater needs its own circuit. A registered competent person designs to BS 7671, including Amendment 3:2024, and self-certifies. Elsewhere in the UK, ask which notification applies. Bosch states that installation is for an authorised craftsman. If they hand you the cable to land yourself, stop. The certificate does not promise that every outlet can run together. Flow sits in the buying guide.
WINTEMP WINTEMP YQ-WN18K 18 kW

Testsieger

WINTEMP WINTEMP YQ-WN18K 18 kW
8.8/10about £276.46
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What a plumber certifies

England and Wales use the Water Supply (Water Fittings) Regulations 1999, Scotland the 2014 Byelaws, and Northern Ireland the 2009 Regulations. The plumber’s certificate covers the joint, not the heater in the carton. WRAS approval belongs to a named model and expires. The kept listings do not state one. UK heaters in this class are normally G 1/2 inch BSP, while a 3/4 inch NPT import will not meet that pipe. A 48°C bath cap applies when a dwelling is erected or formed by a material change of use, not automatically when a bathroom is added to an existing home.

Before the power goes on

Rheem’s tankless manual says to purge air before the heater is energised, and the same order belongs on a UK 400 V unit. The plumber connects wholesome water on the correct thread. Air is purged until the flow runs steadily, and only then is the heater switched on. If either trade leaves the last joint for you, the install is not finished. Peter F. drew this handover from UK building rules and the manufacturer conditions named below.

Common questions

Can I wire an 18 kW heater myself?
No. The heater is permanently wired, and there is no 13 A plug. Bosch states that installation is for an authorised craftsman, and a new circuit in England is notifiable under Part P. You can book the trades, clear the wall, and ask whether the service head shows one fuse or three. You should not choose the cable, land it in the consumer unit, or switch the unit on. This is a BS 7671 design, and the person who designs it signs for it.
Is the work notifiable under Part P?
In England, yes for a new circuit and for work in a bathroom zone. An 18 kW heater needs its own circuit, so it sits on that side of the line. A registered competent person usually self-certifies and notifies building control for you. If your electrician is not on a scheme, you need a building-control application before the work starts, and you keep the reference. That form is England’s. Elsewhere in the UK, ask which notification applies at the address.
Do I need both an electrician and a plumber?
Yes on a normal install. The electrician owns the circuit, the isolator, and the electrical notification. The plumber owns the connection to wholesome water and any valve that limits a bath. One person with both competences can cover both, if they certify each side. Two trades who each leave the last joint for the other is how a heater is switched on dry. Agree in writing who purges the air and who may close the isolator.
Does a new bathroom in an old house need a 48°C bath limit?
Not automatically. Requirement G3(4) caps a fixed bath at 48°C when a dwelling is erected or formed by a material change of use. The Part G FAQ says that does not automatically apply to a bathroom added to an existing home. A 1930s house with a new en suite is the usual case outside the trigger, while a new-build flat is inside it. A temperature lock is still sensible if children or older people use the bath. In the older house it is a choice.
What does WRAS approval actually prove?
A WRAS approval belongs to a named model, and it expires. It does not cover every heater of a similar rating, and a directory page for a small open-outlet unit does not approve a different multipoint model. An approved contractor’s certificate is about the installation they carried out, not about the brand in the box. The kept listings here do not state a WRAS approval. If a current approval matters, match the certificate number to the exact model before you buy.
Why must the heater be filled before it is switched on?
The element has to sit in moving water. Switch on while air is still in the chamber and the element heats that pocket, which can wreck it on first use. Rheem’s tankless manual says to purge air before power, and a UK 400 V install should follow the same order. Open an outlet, run water until the stream is steady, and only then let the electrician switch on. A short test burst is not a purge.

Related pages

Best 18kW instantaneous water heater

18kW instantaneous water heater for a shower

18kW instantaneous water heater

What size 18 kW instantaneous heater do I need

240V tankless water heater versus a UK 400 V supply

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)