How to install an undersink water heater
Updated: 17.10.2026
You have chosen a type and you are about to drill under the sink. The useful version of how to install an undersink water heater is a sequence: confirm the tap, fit the water, fill the vessel, and only then think about power. A vented tank that you cap, or a 6 kW box pushed into a 13 A plug, is the failure this page is meant to stop. Product choice stays on the [point-of-use water heater comparison](/hub/hot-water/point-of-use-water-heater). We used maker notes, Approved Documents and the amazon.co.uk listings of 15 October 2026. We did not fit a heater. A qualifying purchase can earn a commission at no extra cost to you.
You have chosen a type and you are about to drill under the sink.
You have chosen a type and you are about to drill under the sink. The useful version of how to install an undersink water heater is a sequence: confirm the tap, fit the water, fill the vessel, and only then think about power. A vented tank that you cap, or a 6 kW box pushed into a 13 A plug, is the failure this page is meant to stop. Product choice stays on the point-of-use water heater comparison. We used maker notes, Approved Documents and the amazon.co.uk listings of 15 October 2026. We did not fit a heater. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Before power | Supply | Mount | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  COMFEE' COMFEE' 10 L undersink | Fill the 10 litre tank | 2 kW, corded | Undersink, 30 cm deep | 9.6 | about £79.99 | View offer |
| Preis-Tipp |  Triton Showers Triton T30i handwash | No tank to prefill | 3 kW, corded | At the basin | 9.0 | about £55.99 | View offer |
| Alternative |  Stiebel Eltron Stiebel Eltron DCE-X 10/12 Premium | Own circuit first | 10/12 kW, not a plug | Oversink, 109 mm deep | 9.0 | about £412.86 | View offer |
On this page
Confirm the tap before you buy pipe
The tap is part of the heater. An open-outlet store uses one vented tap, and that outlet is the vent. Hyco’s Handyflow instructions put the pipes at the top, allow only the open-outlet tap, and tell you to fill before power. Heatrae Sadia’s notes say the same: one outlet, and do not over-tighten the spout to stop a drip. A standard mixer on that tank closes the vent.
An unvented store can feed a normal tap if the maker’s safety kit is on the inlet and the discharge is visible. England and Wales water fittings rules expect a servicing valve. A store over 15 litres that is unvented also needs temperature relief. The Ariston Andris R includes a pressure relief valve and dielectric junctions, and lists installation kits as extra. The Triton T30i is a handwash, not a vented tank. If you are unsure which family you bought, settle it on the buying guide before you solder.
Mount, fill, then power
Fix the heater close to the outlet. A long pipe throws away heat you have just paid for. The COMFEE' 10 litre store is 32.4 cm by 32.4 cm by 30 cm deep, and one reviewer removed a shelf to clear that depth. The Andris R is 34.6 cm wide and 38.5 cm high, and the listing does not state the depth. The Stiebel DCE-X is about 109 mm deep. Follow the sheet for orientation. Undersink stores in the maker PDFs want the connections at the top.
Fill a storage heater before you switch it on. Hyco states that order because a dry element fails. Open the tap, run water until the air has gone, and only then energise. On a vented heater the spout drips while the tank heats. That drip is expansion, and Heatrae tells you not to over-tighten it away. If the drip continues when the heater is cold, or water appears at a joint, that is a leak. An instant heater has no tank to prefill. You still run water through before you expect heat, and you still isolate the supply until the pipes are made.
What the electrics will actually allow
At 230 V, 3 kW is about 13 A, 6 kW about 26 A, and 12 kW about 52 A. Those figures size the conversation. They are not a cable design, and this page will not give you one. The IET, in Wiring Matters for September 2025, says a long 3 kW immersion is a poor match for a BS 1363 connection. A handwash that only draws power while water flows is a shorter duty, so it is not honest to forbid every 3 kW plug. The Axelrivet and the Mulcort state a UK plug at 3,000 W. A UK-plug sentence next to 6,000 W is a contradiction.
Nothing at 6 kW or 10–12 kW belongs on a 13 A plug. The Stiebel listing prepares a cable, and reviewers say it cannot go into a socket. Some describe a 50 A or 60 A breaker on their own installs. Do not copy those reports. In England, Approved Document P makes a new circuit, a consumer-unit change, and work in a bathroom special location notifiable. That zone reaches 0.6 metres from the bath or tray and 2.25 metres up. A like-for-like swap outside a bathroom may not be notifiable, and the work still has to meet BS 7671. Scotland and Northern Ireland differ. The store-or-instant circuit split is on the undersink or instant page.
When you stop and call someone
Stop before the last joint when the job is a new circuit, a consumer-unit change, or work inside a bathroom special location. That work is notifiable in England and needs a competent electrician. An unvented safety kit still needs a discharge you can see, even when a store of 15 litres or less is not itself notifiable. A 9–12 kW heater, which here means the Stiebel DCE-X, waits until an electrician has looked at the consumer unit. A relief valve that runs in normal use, or a plug that smells hot, is a fault to isolate, not a drip to cap.
Do not cap an open-outlet drip that only happens during heat-up. Do not fit a 6 kW heater because the price was about £50 and the bullets mentioned a shower. If the kilowatts are still undecided, match the room on the basin or kitchen page before you drill.
Common questions
Do I have to use a special tap?
On an open-outlet store, yes. The maker’s vented tap is the safety outlet, and a normal mixer closes it. An unvented store uses a normal tap only with the safety kit fitted. A handwash such as the Triton usually has its own spout. Read the sheet for the unit you bought rather than reuse a mixer that is already on the sink.
Why does the spout drip when the tank is heating?
On a vented undersink heater the drip is water expanding as it heats, leaving through the open outlet. Hyco treats that as normal, and Heatrae says not to over-tighten the tap to cure it. The drip should tie to heat-up, not to a cold tank or a wet cupboard floor. Water from a joint, or a relief valve that runs all the time, is a leak. Isolate the heater.
Should I fill the heater before I turn it on?
Yes for a storage heater. Fill it, run the tap until the air has cleared, and only then switch on. That is Hyco’s order, and a dry element is a failed element. An instant heater has no store to prefill in the same way. You still want water flowing before you expect heat, and you still leave the supply isolated until the pipes are finished.
Can I plug in a 3 kW heater?
Sometimes, and not as a rule for every listing. A short-duty 3 kW handwash is a different load from a 3 kW immersion left on for hours, which the IET says sits poorly on a BS 1363 connection. Confirm a UK plug or a fused spur, a circuit that can take it, and that you are outside a notifiable bathroom zone. A listing that says both “UK plug” and “6,000 W” has not earned a plug-in install.
Who installs a 6 kW or 12 kW unit?
An electrician, on a dedicated circuit, after looking at the consumer unit. In this set that is the Stiebel DCE-X and the 6 kW listings, none of which are 13 A appliances. Reviewer stories about 50 A or 60 A breakers describe those houses. They are not a specification you copy. The heater price does not include that work.
Is a new circuit notifiable?
In England, yes. Approved Document P classes a new circuit, a replacement consumer unit, and work in a bathroom special location as notifiable. Replacing an existing fused connection outside that zone often is not, and the result still has to comply with BS 7671. A registered competent person can self-certify notifiable work. Otherwise you notify building control first. Scotland and Northern Ireland have their own routes.
Where does the discharge from a relief valve go?
To a visible, safe point, as the maker’s kit describes, not into a closed pipe you cannot see. The Andris R includes a pressure relief valve, and the listing says installation kits are separate, so the termination is still your job. An unvented store above 15 litres needs temperature relief under the water fittings rules. If the valve runs when the tank is not heating, stop and get a plumber.
Related pages
Sources
Public UK sources used for this comparison. Listing prices, stars, and review counts are from amazon.co.uk extracts dated 15 October 2026.
- Approved Document G, with 2024 amendments (retrieved 15 October 2026)
- Approved Document P, 2013 (retrieved 15 October 2026)
- Water Supply (Water Fittings) Regulations 1999 (retrieved 15 October 2026)
- HSE HSG274 Part 2 (retrieved 15 October 2026)
- IET Wiring Matters, September 2025 — 3 kW and BS 1363 (retrieved 15 October 2026)
- Retained regulation 812/2013, Annex II (retrieved 15 October 2026)
- Ariston Andris Lux, UK (retrieved 15 October 2026)
- Stiebel Eltron — instantaneous versus stored water heaters (retrieved 15 October 2026)