3.5kW instant water heater buying guide
Updated: 17.10.2026
You are looking at a small electric tankless heater because a cloakroom basin takes too long to run hot from a distant cylinder. A 3.5kW instant water heater buying guide has one decision before the price: whether 3.5 kW is the right class for that one basin, and whether the circuit can feed it. At 230 V, 3,500 W is about 15.2 A, already above a BS 1363 13 A plug. The flow has to stay low or the water stays lukewarm. Prices sit on the [tankless water heater 3.5kW comparison](/hub/hot-water/tankless-water-heater-3-5kw). We used five amazon.co.uk listings and public UK sources, with no lab test. A qualifying purchase can earn a commission at no extra cost to you.
You are looking at a small electric tankless heater because a cloakroom basin takes too long to run hot from a distant cylinder.
You are looking at a small electric tankless heater because a cloakroom basin takes too long to run hot from a distant cylinder. A 3.5kW instant water heater buying guide has one decision before the price: whether 3.5 kW is the right class for that one basin, and whether the circuit can feed it. At 230 V, 3,500 W is about 15.2 A, already above a BS 1363 13 A plug. The flow has to stay low or the water stays lukewarm. Prices sit on the tankless water heater 3.5kW comparison. We used five amazon.co.uk listings and public UK sources, with no lab test. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Stated power | Supply in the listing | Stated flow | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Axelrivet Axelrivet 3.0 kW under-sink | 3,000 W | UK plug in the title | 2.9 L/min | 5.2 | about £44.99 | View offer |
| Alternative |  Builrand Builrand 3.0 kW under-sink | 3,000 W | 220 V in the title | Not stated | 4.8 | about £44.99 | View offer |
 MRNIU MRNIU 3.5 kW under-sink | 3,500 W | 230 V, about 15.2 A | 4 L/min | 6.8 | about £42.00 | View offer |
On this page
What 3.5 kW means on a UK supply
A 3.5 kW instantaneous heater has no cylinder. It heats only while the tap runs. Divide 3,500 by 230 and you get about 15.2 A. The Plugs and Sockets etc. (Safety) Regulations 1994 cover appliances designed for not more than 13 A, and 230 V times 13 A is 2,990 W. A fitted 13 A plug does not make a 3,500 W load acceptable.
Seller pages disagree between 1.5 mm² and 2.5 mm², so this is not a wiring instruction. An electrician sizes the circuit to BS 7671. A 3.0 kW neighbour is about 13.0 A, and Hyco’s 13 A connection on some of those heaters does not transfer to 3.5 kW. In this set only the MRNIU technical details state 3,500 W. The Builrand and the Axelrivet state 3,000 W.
What temperature you can expect
The number on the display is a target. Hyco’s rule of thumb is temperature rise in °C equals kilowatts times 14.3, divided by litres per minute. At 3.5 kW and 2 L/min that rise is about 25°C. From a 10°C winter inlet the outlet is about 35°C if you keep the stream narrow. At the MRNIU’s stated 4 L/min the rise is about 12.5°C, so a 4 L/min claim and a 55°C claim cannot both hold from a cold inlet. That is arithmetic, not a laboratory test.
Ascot’s chart, at a 15°C inlet, marks its own 3.5 kW models at about 40°C at 2 L/min and marks a kitchen sink and a shower as unsuitable. On the MRNIU, reviewers report water around 18°C to 30°C with the control at 55°C. The jug check is on how to use a 3.5kW instant water heater.
Rules before you order
In England and Wales a new circuit, a consumer-unit change, or work in a bathroom special location is notifiable under Part P unless a scheme member self-certifies. Scotland and Northern Ireland use their own building rules. The IET’s Part P FAQ is the England and Wales note. A legal install does not create heat the element does not have.
Under BS 7671 section 701 the heater stays out of zone 0. In zone 1 it has to be fixed, permanently connected, at least IPX4, and allowed by the maker. A 13 A socket has to sit at least 3 m from the zone 1 boundary. The MRNIU listing does not state an IP rating. Schedule 2 excepts an instantaneous heater of not more than 15 litres from unvented relief-valve duty, so this is not G3 work. The 48°C cap in Approved Document G is a bath rule.
How that sorts the listings in front of you
Only the MRNIU spec table states 3,500 W, and the listing limits it to sinks and basins. The Builrand and the Axelrivet state 3,000 W. The Generic listings put 3.5 kW and shower in the title and have no wattage in the technical details. On 15 October 2026 a Bosch Tronic Comfort Compact 3.5 kW was near £167 on amazon.co.uk. That ASIN is not in this comparison.
If the job is a cubicle, use 3.5kW water heater vs electric shower. Which listing is least incomplete is on best 3.5kW tankless water heater.
Common questions
Can I use a normal 13 A socket for a 3.5 kW instant water heater?
No. 3,500 W at 230 V is about 15.2 A. The 1994 plug regulations cover appliances designed for not more than 13 A, and 230 times 13 is 2,990 W. A 3.0 kW heater sits near that line. A 3.5 kW heater sits above it. A three-pin plug reported on the MRNIU does not change the arithmetic. An electrician designs the circuit to BS 7671. Seller pages disagree between 1.5 mm² and 2.5 mm².
Is a 3 kW tankless heater the same purchase as a 3.5 kW one?
No. At 230 V, 3,000 W is about 13.0 A and 3,500 W is about 15.2 A. The Builrand and the Axelrivet state 3,000 W. Only the MRNIU states 3,500 W in the technical details. The rise is smaller at the same flow. Hyco’s 13 A connection on some 3.0 kW basin heaters does not carry across. One Axelrivet review reported about 4,200 W, so a UK Plug title is not a measured current.
What flow still gives hand-wash temperature in winter?
Keep the flow near 1.5–2 L/min. Hyco’s formula gives about 25°C of rise at 2 L/min, about 35°C from a 10°C inlet. Ascot’s chart, at a 15°C inlet, shows about 40°C at 2 L/min and about 48°C at 1.5 L/min. At 4 L/min the rise is only about 12.5°C, so a cold inlet stays in the low twenties. Those figures are charts and arithmetic, not a lab test of these ASINs.
Does a new circuit have to be notified?
In England and Wales, yes, unless a scheme member self-certifies. The IET’s Part P FAQ treats a new circuit, a consumer-unit change, and work in a bathroom special location as notifiable. Scotland and Northern Ireland use their own building rules. Notification does not add kilowatts. If the cable route is unknown, that is an electrician’s job before you assume a plug will do.
Can this heater sit in a bathroom?
Only within BS 7671 section 701, and only if the maker allows it. It stays out of zone 0. In zone 1 it has to be fixed, permanently connected, and at least IPX4. A 13 A socket has to be at least 3 m from the zone 1 boundary. The MRNIU file has no IP rating, and IPX4 in a title is not a certificate.
Do I need a G3 unvented installer for a 3.5 kW tankless heater?
No. Schedule 2 of the Water Supply (Water Fittings) Regulations 1999 excepts an instantaneous heater of not more than 15 litres from the unvented relief-valve duty. This is not G3 cylinder work. Approved Document G still points these heaters at BS EN 60335-2-35:2002. The 48°C figure in paragraph 3.65 is a bath outlet limit, not a promise that this heater can fill a bath. The MRNIU listing excludes baths and showers.
Related pages
Sources
Public UK sources used with the amazon.co.uk listings in this set; shelf prices from 15 October 2026.
- Approved Document G, sanitation, hot water safety and water efficiency (retrieved 15 October 2026)
- Water Supply (Water Fittings) Regulations 1999, Schedule 2 (retrieved 15 October 2026)
- IET Part P frequently asked questions, England and Wales (retrieved 15 October 2026)
- SELECT guidance on bathrooms and BS 7671 section 701 (retrieved 15 October 2026)
- IET Wiring Matters, BS 7671:2018 bathroom socket questions (retrieved 15 October 2026)
- Plugs and Sockets etc. (Safety) Regulations 1994 (retrieved 15 October 2026)
- Hyco instantaneous heater charts (retrieved 15 October 2026)
- Ascot technical chart for instantaneous heaters (retrieved 15 October 2026)