How to install an instantaneous water heater in the UK
Updated: 17.10.2026
You can make an installation visit efficient without opening the consumer unit or copying a cable size from a product page. The useful preparation is to identify the exact heater, its rating plate, the outlets it will serve and the route between the water supply, the appliance and the electrical intake. This guide explains how to install an instantaneous water heater in the UK by separating safe preparation from regulated electrical work. Compare the product class on the [21 kW instantaneous water heater comparison](/hub/hot-water/21kw-instantaneous-water-heater) before you book anyone, and treat a marketplace wiring bullet as a prompt for questions, not as a design.
You can make an installation visit efficient without opening the consumer unit or copying a cable size from a product page.
You can make an installation visit efficient without opening the consumer unit or copying a cable size from a product page. The useful preparation is to identify the exact heater, its rating plate, the outlets it will serve and the route between the water supply, the appliance and the electrical intake. This guide explains how to install an instantaneous water heater in the UK by separating safe preparation from regulated electrical work. Compare the product class on the 21 kW instantaneous water heater comparison before you book anyone, and treat a marketplace wiring bullet as a prompt for questions, not as a design.
Our top picks
| Badge | Product | Power | Connection | Shower use | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Stiebel Eltron Stiebel Eltron DCE-X 10/12 | 10/12 kW | Hard-wired | Mixed reviews | 8.2 | about £412.86 | View offer |
| Alternative |  ThermoMate ThermoMate ET055 5.5 kW | 5.5 kW | No plug | Sinks only | 6.6 | about £109.74 | View offer |
| Preis-Tipp |  Triton Showers Triton Instaflow 10.1 kW | 10.1 kW | Fixed install | Not a shower | 7.0 | about £154.99 | View offer |
On this page
Identify what you are installing
An electric instantaneous heater warms cold water as it passes, without keeping a cylinder hot. A 3 kW basin heater may approach a 13 A socket. The captured 5.5–12 kW units need fixed wiring and a dedicated circuit. A genuine 21 kW model is normally a 400 V three-phase appliance. Do not start from a generic tankless diagram. What that supply asks for is in the three-phase supply guide.
Photograph the rating plate and note the full model number before the visit. Compare voltage, phases and power settings with the manual for that revision. A marketplace title is not a design document.
Decide whether the product is an electric heater, a dedicated electric shower or a gas appliance. Triton says its multi-point Instaflow is not for use as an electric shower, even at 10.1 kW. Gas work belongs with a Gas Safe registered engineer. This page does not give gas installation steps.
Understand what is notifiable
In England, Approved Document P identifies a new circuit, a replacement consumer unit and work in a bath or shower special location as notifiable. A registered competent person can normally self-certify within their scheme. Otherwise agree the building-control route before work starts. The design still follows BS 7671 where an alteration is not notifiable.
A new circuit remains notifiable, and bathroom zones have separate location rules. Wales, Scotland and Northern Ireland use their own building-control frameworks, so do not apply England's wording across the UK.
IP25 describes enclosure protection under specified tests. It does not mean the heater can be mounted anywhere in a bathroom. The electrician applies the zone rules and the manufacturer's location limits to the actual room.
Prepare the information before the visit
List every outlet, the distance to each one and which outlets may run together. Note whether the job is hand washing, a kitchen, one shower, or a bathroom and kitchen used close together. How those outlets map to kilowatts is in the buying guide.
Ask the plumber to check pressure, flow and valves against the manual's activation flow and maximum pressure before you buy fittings.
Give the electrician the manual, a rating-plate photo, the proposed position and the approximate cable route. Open access to the meter, service head and consumer unit, and mention other large loads such as an electric shower, induction hob, EV charger or heat pump. Do not remove covers from the service head or consumer unit yourself.
Know where DIY stops and who to call
DIY stops before circuit design, protective-device selection and electrical connection. A fixed-wired heater needs a competent electrician to assess and test the installation. A 21 kW appliance also needs confirmation that 400 V three-phase is available. The distribution network operator controls any upgrade. Changing the consumer unit does not create three-phase.
Plumbing must follow the Water Supply (Water Fittings) Regulations and the manufacturer's instructions. Ask whether WRAS approval or another accepted route is required. An ingress rating is not water-fittings approval.
No stored cylinder is not the same G3 unvented-cylinder project, but Part G still matters. Temperature, scald risk and outlet controls should suit children, older people and anyone with reduced sensitivity.
Common questions
Can I wire a 21 kW instantaneous heater myself?
No DIY wiring instructions are appropriate for this load. A 21 kW electric model normally requires 400 V three-phase, a dedicated circuit and model-specific protective design. In England, a new circuit is notifiable work unless handled through an applicable self-certification route. Use a competent electrician and confirm the incoming service first.
Does no cylinder mean Part P does not apply?
No. Removing stored hot water changes the plumbing system, not the electrical rules. A new circuit and work in a bath or shower special location can still be notifiable in England, and all electrical work must be safe. BS 7671, Part G and the applicable Water Fittings Regulations remain relevant.
What should I have ready for the electrician?
Provide the exact manual, a photograph of the rating plate, the proposed mounting point, access to the meter and consumer unit, and a list of major electrical loads. Explain which outlets must run together. Do not supply a cable-size guess from an online listing. The electrician must design for the actual route and conditions.
Is a plumber enough?
Not when the heater requires a new or altered electrical circuit. A plumber can assess pipework, pressure, valves and water compliance, while a competent electrician designs, connects and tests the circuit. Some installers cover both scopes through qualified staff, but confirm credentials and who will certify each part before accepting the quote.
Does IP25 allow installation in any bathroom zone?
No. IP25 is an enclosure-protection rating, not blanket permission for every bathroom position. The actual zone, distance from water sources, circuit protection and manufacturer's mounting restrictions all matter. Ask the electrician to assess the proposed location against BS 7671 and the exact appliance instructions.
Who arranges a three-phase upgrade?
Start with an electrician's load assessment, then contact the local distribution network operator for availability and a supply-upgrade process. The network operator controls the incoming service. Your electrician designs the installation after the offered supply is known. Changing internal equipment alone does not convert a single-phase home.
Related pages
Sources
Public UK sources from the research digest, plus the six amazon.co.uk listings in this comparison. The Veito amazon.co.uk offer was checked on 16 October 2026.
- Approved Document P (retrieved 16 October 2026)
- Building Regulations 2010 (retrieved 16 October 2026)
- GOV.UK repairs and competent persons (retrieved 16 October 2026)
- Quality Heating instant-heater guide (retrieved 16 October 2026)
- Flexiheat UK tankless range (retrieved 16 October 2026)
- Veito Blue S at Quality Heating (retrieved 16 October 2026)
- Veito UK Blue S (retrieved 16 October 2026)
- B&Q Veito Blue S (retrieved 16 October 2026)