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Install an instantaneous water heater in the UK

Updated: 21 October 2026

The box is open on the kitchen floor and the flex ends in something that looks like a 13 A plug. Before you drill the wall, you need to know whether that plug is an installation method or a reason to stop. To install an instantaneous water heater in the UK, from this set, you are choosing between a like-for-like replacement on an existing suitable circuit and a new circuit that an electrician should notify. The [main comparison](/hub/hot-water/instantaneous-water-heater-with-remote-control) shows which listings name a breaker. A qualifying purchase through our links can earn us a commission at no extra cost to you.

The box is open on the kitchen floor and the flex ends in something that looks like a 13 A plug. Before you drill the wall, you need to know whether that plug is an installation method or a reason to stop.

Our top picks

Stiebel Eltron Stiebel Eltron DCE-X 6/8 Premium

Best overall

Stiebel Eltron Stiebel Eltron DCE-X 6/8 Premium
6.9/10about £344.99
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Mulcort Mulcort LF-001 3000 W

Best value

Mulcort Mulcort LF-001 3000 W
5.1/10about £46.99
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Delousiber Delousiber 6000 W tankless

Rank 3

Delousiber Delousiber 6000 W tankless
3.3/10about £46.99
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On this page
  1. 1.What you are allowed to do yourself
  2. 2.Why 3 kW is already the edge of a plug
  3. 3.Bathroom zones and the water side
  4. 4.Where this set is specific

What you are allowed to do yourself

In England, Approved Document P makes a new circuit, a new consumer unit, or work around a bath or shower notifiable, unless a registered competent person self-certifies. Adding a dedicated way for a heater is that new circuit. DIY is not outside the rules, and a new circuit is still notifiable. You can usually mount a bracket and offer up pipe tails you can isolate. You should not design the cable, choose the breaker from a forum, or cut a moulded plug off to make it fit. If the listing is above a 13 A circuit, book the electrician before you plumb the heater in and lose the return window. The buying guide sizes the kilowatts. It does not replace Part P.

Why 3 kW is already the edge of a plug

A normal UK house is single-phase, around 230 V. A 13 A BS 1363 plug is fused at 13 A, which is about 3 kW at that voltage. The IET’s September 2025 note is already uneasy about a 3 kW immersion on a 13 A fused connection unit when the load runs for a long time. An instantaneous heater at 3 kW can sit on for as long as the tap is open. A stated 6 kW load at 230 V is about 26 A, which is why a ring final is the wrong place. The only kept listing that states a UK plug is the Mulcort at about £57.99, and it is 3000 W. Read that as a warning. Two heaters must not share that socket.
Stiebel Eltron Stiebel Eltron DCE-X 6/8 Premium

Testsieger

Stiebel Eltron Stiebel Eltron DCE-X 6/8 Premium
6.9/10about £344.99
View offer

Bathroom zones and the water side

IPX4 is the splash rating often cited for bathroom zones 1 and 2. Several listings in this set print IPX4. That line does not mean the heater may be plugged into a bathroom socket, and it does not put a 13 A socket inside the zone. The water side is separate from the breaker. The Water Fittings Regulations 1999 require backflow protection, with a narrow exception if expansion returns to a pipe at or below 25°C. Unvented-cylinder paperwork under Approved Document G is the wrong form for a tankless unit. You still owe a safe connection, an isolator you can reach, and an outlet that will not scald. Where Approved Document G applies, a fixed bath is limited to 48°C. None of the mini listings is a bath filler, so that limit is a reason to stop.

Where this set is specific

The Stiebel DCE-X 6/8 Premium does not state a BS 1363 plug. UK reviews say it is not an ordinary socket, and one describes cable in the region of 6 to 10 mm² back to the consumer unit. That is an owner’s report, not a recipe to copy. The Briwellna bullets ask for a 25 A double-pole breaker for the 5.5 kW unit. One VexNook review says a 16 A caravan supply was not enough and that the heater needs 32 A. The Delousiber bullets ask for a fuse of at least 40 A, while UK reviews say a 13 A style plug was fitted. Do not cut the plug off.

Common questions

Do I need an electrician if the heater has a plug?
If the stated power is 3000 W, a plug is already the edge of what BS 1363 is comfortable with for a long load, and the IET has said as much about a 3 kW immersion. If the stated power is 5.5 kW or 6 kW, a plug in the photo does not make it a plug-in appliance. The Delousiber reviews describe exactly that mismatch. Book an electrician before you rely on the socket, and do not cut the plug off as a home remedy.
Is a new circuit notifiable in England?
Yes. Approved Document P treats a new circuit as notifiable work in a dwelling, unless a registered competent person self-certifies through a scheme. A new consumer unit, and electrical work in the special location around a bath or shower, sit in the same bracket. Scotland and Northern Ireland use their own building-control routes. Ask the electrician which route covers your address before the work starts.
What is different in a bathroom?
The room is a special location. Zones, IP ratings and RCD protection apply, and the work is notifiable unless a competent person self-certifies. IPX4 on a listing is a splash claim, not a permit to use a bathroom socket. A 13 A socket stays outside the immediate bath or shower zone. Say if the job is only a cloakroom basin, so the electrician is not pricing a shower room.
Who connects the water, and who connects the electrics?
A plumber makes the mains connection, the isolator and the backflow protection the Water Fittings Regulations require. An electrician owns the circuit, the breaker and the test certificate. One tradesperson may do both if they are qualified for both. A bullet that says you can fit the heater without a plumber does not cover a 25 A breaker or a leak onto live parts.
What should I do if a 6 kW unit arrives with a 13 A plug?
Stop. Do not cut the plug off, do not replace the fuse with a larger one, and do not run a shower just to test it. Photograph the plug and the 6000 W claim, then return the unit while you still can. A proper 6 kW installation is a dedicated circuit in the mid-20 A range or higher, designed by an electrician. The Delousiber reviews are the worked example of why the plug and the bullet cannot both be right.

Related pages

Instantaneous water heater for shower

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Instantaneous water heater with remote control

Best instantaneous water heater with remote control

What size instantaneous water heater do I need in the UK

Sources

Public UK sources and manufacturer pages named below. amazon.co.uk prices, stars and review counts on these seven listings were read on 20 October 2026. Page updated 21 October 2026.

  1. Stiebel Eltron UK sizing FAQ (retrieved 20 October 2026)
  2. Stiebel Eltron UK, instantaneous versus stored (retrieved 20 October 2026)
  3. Stiebel Eltron UK counter mat (retrieved 20 October 2026)
  4. Stiebel DCE-X 6/8 Premium (retrieved 20 October 2026)
  5. Approved Document P (2013) (retrieved 20 October 2026)
  6. Approved Document G (retrieved 20 October 2026)
  7. IET Wiring Matters, September 2025, 3 kW and a 13 A connection (retrieved 20 October 2026)
  8. Water Fittings Regulations 1999, Schedule 2 paragraph 15 (retrieved 20 October 2026)