3.5kW under sink water heater
Updated: 17.10.2026
The cupboard under a cloakroom basin looks like the obvious place for a small tankless box. A 3.5kW under sink water heater only works there if the job is one outlet, a short pipe, and a tap that cannot run away to a washing-up flow. A fully open kitchen mixer will feel like a failed heater. This page is from the Products Compared editorial team on 15 October 2026. We used amazon.co.uk listings and trade charts. We did not measure your cupboard. Prices stay on the [tankless water heater 3.5kW comparison](/hub/hot-water/tankless-water-heater-3-5kw). We may earn a commission if you buy through our links, at no extra cost to you.
The cupboard under a cloakroom basin looks like the obvious place for a small tankless box.
The cupboard under a cloakroom basin looks like the obvious place for a small tankless box. A 3.5kW under sink water heater only works there if the job is one outlet, a short pipe, and a tap that cannot run away to a washing-up flow. A fully open kitchen mixer will feel like a failed heater. This page is from the Products Compared editorial team on 15 October 2026. We used amazon.co.uk listings and trade charts. We did not measure your cupboard. Prices stay on the tankless water heater 3.5kW comparison. We may earn a commission if you buy through our links, at no extra cost to you.
Our top picks
| Badge | Product | Printed size | Stated use | Mount | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Axelrivet Axelrivet 3.0 kW under-sink | 20 × 23.5 cm, 1.25 kg | Small basin | Above or under a sink | 5.2 | about £44.99 | View offer |
| Alternative |  Builrand Builrand 3.0 kW under-sink | 11 × 19 cm, 1 kg | Hand washing | Small wall-mounted box | 4.8 | about £44.99 | View offer |
 MRNIU MRNIU 3.5 kW under-sink | 26 × 19 cm, 1.4 kg | Sinks, not a shower | Under-sink, wall mounted | 6.8 | about £42.00 | View offer |
On this page
The job under the basin
Under-sink, in this power class, means hand washing at one basin close to the heater. Ascot’s chart marks its own 3.5 kW models as suitable for a washbasin and unsuitable for a kitchen sink and a shower. A Perfect 35 seller page puts the same class under a sink for a cloakroom basin at about 2 L/min, and it is not a shower.
Reviews split the same way: hand washing on the MRNIU, no dishes, and a small basin only on the Axelrivet. ManoMano’s retailer guide puts a kitchen sink near 5.5 kW. That is sales guidance, not a legal minimum. Read the MRNIU kitchen line as light rinsing at a low flow.
The tap and the pipe
The usual fix is one outlet, a short pipe, and a spray head or restrictor. Hyco’s 3.0 kW Rho comes with a 1.6 L/min restrictor and a spray-head tap, and is not for a single-lever mixer. It connects at 13 A because it is 3.0 kW. That is not permission for a 3.5 kW element on a plug. At 3.5 kW and 4 L/min the rise is about 12.5°C. At 2 L/min it is about 25°C.
A kitchen monobloc opened fully can flow well above 4 L/min, and a mixer can blend cold around the heater. If the instructions are silent, assume you need a spray head. None of these listings prints a maximum pipe length. The jug check is on how to use a 3.5kW instant water heater.
What fits in the cupboard
Three listings print a size. The MRNIU is 26 cm by 19 cm and 1.4 kg, wall mounted. Depth is not stated. The Builrand is 11 cm by 19 cm and 1 kg. The Axelrivet is 20 cm by 23.5 cm and 1.25 kg, and its bullets say above or under a sink. Do not assume every unit can sit above the basin because one bullet says so.
B0G64LCBWG and B0H8Q2SG5H do not state mount or size. The MRNIU does not state an IP rating. IPX4 in a title is not a zone 1 certificate. Pipework is plumbing. A new circuit is electrical, and in England and Wales it is notifiable unless a scheme member self-certifies. The 15.2 A sum is in the 3.5kW instant water heater buying guide. If the space is a cubicle, read 3.5kW water heater vs electric shower.
3.0 kW plug-in versus 3.5 kW
A plug-in 3 kW neighbour is a different circuit from a true 3.5 kW load. 3,000 W at 230 V is about 13.0 A. 230 times 13 is 2,990 W, the line in the 1994 plug regulations. 3,500 W is about 15.2 A. Sellers of specified 3.5 kW basin heaters forbid a 13 A plug, and their cable notes disagree, so the circuit is designed to BS 7671.
The Axelrivet says UK Plug in the title and 3,000 W in the heat-output field. Lucas reports about 4,200 W. The Builrand is also 3,000 W. If you want a 13 A-class basin heater, you are shopping that neighbour, with the review limits still attached. A three-pin plug reported on the MRNIU does not convert 3,500 W into a 13 A appliance.
Common questions
Will a 3.5 kW under-sink heater wash dishes?
Not in the way a kitchen sink is normally used. Ascot marks its own 3.5 kW models unsuitable for a kitchen sink. gosia’s MRNIU review says the unit would not wash dishes. Shelley used a kitchen tap for hand washing. The listing does mention kitchen sinks, and it excludes showers and baths. Read that as light rinsing at a low flow. A retailer guide puts a kitchen sink nearer 5.5 kW, which is sales guidance, not a regulation.
Does the heater have to live under the basin?
The MRNIU is described as under-sink and wall mounted. Axelrivet bullets say above or under a sink. The two Generic listings do not state a mount. Do not treat above the basin as true of every ASIN. Whichever side you use, keep the pipe short. None of these files prints a maximum length. A long pipe leaves a cold slug at the spout.
Can I keep my existing mixer tap?
Maybe, and assume it will fight a 3–3.5 kW element if you open it fully. Hyco’s 3.0 kW Rho is not for a single-lever mixer. If your instructions are silent, a spray head is how you hold the flow near 2 L/min, where 3.5 kW can add about 25°C. A mixer that blends cold around the heater feels cooler at the spout.
How close does the heater need to be to the tap?
Close enough that you are heating one outlet, not storing hot water in a long pipe. No listing in this set states a maximum length. If a long run of cold water arrives before any warmth, the pipe is doing too much of the job. Two basins are a second outlet. This power class is single point. A second tap is a different heater.
What size are the units that actually print dimensions?
The MRNIU is 26 cm wide by 19 cm high and 1.4 kg. Depth is not stated. The Builrand is 11 cm by 19 cm and 1 kg. The Axelrivet is 20 cm by 23.5 cm and 1.25 kg. B0G64LCBWG and B0H8Q2SG5H do not state size or weight, so you cannot plan a cupboard around them. Measure the void, including the pipes.
Who plumbs it, and who connects the electrics?
A plumber can deal with the pipe, the spray tap and a regulating valve. An electrician deals with the circuit. A true 3.5 kW load is about 15.2 A at 230 V, above a 13 A plug. In England and Wales a new circuit is notifiable unless a scheme member self-certifies. A 3.0 kW neighbour sits near 13.0 A. Water on the electrics, a hot plug, or a burning smell is a stop.
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)