Electric Water Heaters: Point-of-Use, Under-Sink, and Instant Hot Water Taps — A Trade Guide

When a kitchen or bathroom needs hot water but a full central heating system or unvented cylinder isn't practical — or when an existing hot water supply is simply too far away — a local electric water heater is the answer. Point-of-use (POU) water heaters deliver hot water exactly where it's needed, with no wait for a draw-off from a distant cylinder and no significant pipe heat loss.

This guide covers the two main product families — instantaneous (also called "flow" or "tankless") heaters and stored-volume water heaters — plus the increasingly popular instant boiling water taps. It explains the relevant UK regulations, sizing and wiring requirements, and the installation decisions that matter on the job.

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Why Point-of-Use Heating Makes Commercial Sense

A washing area in a commercial kitchen, a WC cloakroom 20 metres from the boiler plant room, an extension added to a property after the original hot water system was designed — these are all scenarios where running a long hot water secondary return circuit is costly and impractical. A local electric heater installed under the sink or adjacent to the basin is typically cheaper to fit and often more energy efficient for low-demand applications.

POU heaters also eliminate Legionella dead-legs. Where a long pipe run to a remote outlet sits stagnant between uses and the water temperature drops into the 20–45°C range, Legionella bacteria can proliferate. A local heater removes the dead-leg entirely: water is heated fresh at the point of use, so the risk window is closed. This is a genuine selling point when quoting to landlords, care home operators, and commercial premises managers.

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Instantaneous (Unvented Flow) Electric Water Heaters

How they work

An instantaneous water heater contains a heating element (typically 3 kW to 9.5 kW) and a flow switch or pressure switch. When the tap opens and water flows, the switch activates the element and the water is heated as it passes through the heat exchanger. There is no stored volume — water is heated on demand, continuously.

The rise in temperature achievable depends on the power rating and the flow rate:

• A 3 kW heater at 2 L/min will raise cold mains water (at ~10°C in winter) by approximately 21°C, delivering water at ~31°C. That is warm, not hot.
• A 6 kW heater at 2 L/min delivers roughly 43°C — adequate for hand-washing.
• A 9.5 kW heater at 2 L/min can achieve close to 60°C, or at 3 L/min will deliver around 38°C.

The trade takeaway: restrict flow rate to get hotter water from a lower power unit. Most instantaneous heaters are supplied with a flow restrictor or use an open-outlet tap that inherently limits flow. Do not fit a standard mixer tap expecting shower-like flow rates from a 3 kW unit.

Open-outlet vs closed (pressurised)

The vast majority of POU instantaneous heaters are designed for open-outlet use. This means:

• The unit connects to the cold mains on the inlet side.
• The outlet is not closed off — the dedicated open-outlet tap or spout allows water to expand freely as it heats.
• There is no expansion vessel. If you fit a standard mixer tap or any isolating valve downstream, you create a closed system. Pressure builds during heating and can damage the unit or activate a safety device.

The supplied open-outlet tap or the manufacturer-approved tap must be used with open-outlet heaters. This is a common installation error — fitting a standard tap and then blaming the unit for pressure relief valve operation.

Closed (pressurised) instantaneous heaters are designed for use with conventional mixer taps and blending valves. They incorporate an integral expansion vessel or pressure relief valve rated for the inlet pressure. These are more expensive and less common in the UK for small sink applications, but are the right choice where a mixer tap is specified and the cold supply exceeds roughly 0.5 bar dynamic pressure.

Wiring requirements

Instantaneous electric water heaters are fixed appliances and must be wired on a dedicated circuit with appropriate protection:

• 3 kW unit: 13 A circuit — can be fed from a switched Fused Connection Unit (FCU) with 13 A cartridge fuse on the local ring or radial circuit. A 30 mA RCD or RCBO is required (BS 7671 Regulation 701.411.3.3 in bathroom locations).
• 6–7 kW unit: 30–32 A dedicated radial circuit, minimum 4 mm² T&E to a 30 A double-pole switch within 2 m of the unit (or a lockable isolator). 30 mA RCBO protection at the consumer unit.
• 9.5 kW unit: 40–45 A dedicated radial circuit, minimum 6 mm² T&E, 40 A RCBO at the consumer unit.

In bathroom or kitchen locations, supplementary bonding may be required to extraneous conductive parts (metallic pipework, sinks) if the main equipotential bonding from the consumer unit does not reach the location — refer to Earth Bonding and Equipotential Bonding for the full guidance.

All new circuit work is notifiable under Part P of the Building Regulations unless carried out by a registered competent person scheme member (NICEIC, NAPIT, etc.).

Typical brands and variants

Common UK instantaneous POU heaters include units from Redring, Triton, Stiebel Eltron, and Zip. Most are white plastic units with the element, flow switch, and pressure relief valve housed in a compact case designed to sit under the sink or above a wall-mounted basin. They are supplied with a 1.25 m tail for hard-wiring or, on 3 kW models, sometimes a 13 A plug — though the plug-in option is only appropriate in kitchens, never in bathrooms.

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Stored-Volume Electric Water Heaters (Under-Sink Units)

Vented cistern-type (open-vented storage)

A cistern-type or vented storage water heater has an open cistern that maintains the tank at atmospheric pressure. Hot water is delivered by displacement from a swanneck outlet as cold water fills from below. Because the system is open-vented, no G3 qualification is required and there is no risk of pressurisation.

These are common in commercial applications — office kitchens, pub cloakrooms, school handwash points — where a steady low-rate supply of hot water is needed. Typical capacities: 5 L, 7 L, 10 L, 15 L. Power inputs are usually 2–3 kW. Recovery time for a 10 L unit at 3 kW from cold to 65°C is approximately 25–35 minutes.

Installation considerations:

• Must be mounted above the tap outlet to maintain gravity flow. The cistern must be accessible for inspection and cleaning under Water Supply (Water Fittings) Regulations 1999.
• No pressure vessel required; no expansion relief needed.
• Inlet must incorporate a servicing valve — refer to Isolation Valves and Service Valves.
• Wiring: 13 A FCU (switched) for 2–3 kW units is typically adequate. RCD protection required in all locations near water.

Pressurised (unvented) stored-volume heaters up to 15 L

Where a pressurised supply at mains pressure is required from a storage unit, an unvented stored-volume heater is fitted. These are sealed vessels connected directly to the cold mains. The Water Industry Act and the Unvented Hot Water Cylinders and G3 regulations apply — however, vessels up to 15 litres capacity are exempt from the G3 competent person requirement (Schedule 1 paragraph G3 2(b) of the Building Regulations 2010).

Heaters up to 15 L can therefore be installed by competent plumbers without G3 certification, as long as the unit is self-contained (factory-fitted safety devices including thermostat, thermal cut-out, and pressure relief valve) and installed in accordance with the manufacturer's instructions and Water Fittings Regulations.

However, the following are still mandatory regardless of capacity:

• A pressure reducing valve (PRV) set to a maximum of 3 bar (check manufacturer's specification) on the cold inlet — see Pressure Reducing Valves.
• An expansion vessel or expansion relief valve to accommodate thermal expansion in the closed system. Many small units include this; on those that don't, fit an in-line expansion vessel on the cold inlet.
• A tundish and discharge pipe (D1/D2) to drain safely if the T&P valve operates — sized per BS 6700 / BS EN 806.
• A double check valve (or single check valve integral to the pressure reducing valve body) to prevent backflow — see Backflow Prevention.

Units larger than 15 L fall fully under G3 and require a G3-qualified engineer.

Sizing stored-volume POU heaters

The key sizing question is peak demand, not average demand. A 5 L unit is adequate for hand-washing at a single basin (typical draw: 0.5–1 L per wash). A 10 L unit handles a commercial hand-wash basin with higher traffic. A 15 L unit can serve a small kitchen sink for intermittent use.

If the application is more demanding — bar or café wash-up, multiple outlet supply — step up to a larger commercial cylinder or consider whether a local electric heater is the right solution versus a central supply.

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Instant Boiling Water Taps

What they are

Instant boiling water taps (sometimes called "boiling water taps" or "hot water taps") deliver near-boiling or 100°C water on demand from a dedicated spout or a multi-function mixer tap. They have become increasingly popular in high-specification domestic kitchens and commercial office environments.

The system consists of:

• A tank unit stored in the sink cabinet, typically 2.4–4 L capacity, that maintains water at ~98°C (near-boiling) under slight pressure.
• A dedicated spout or combination tap with a child-safety mechanism (typically requiring simultaneous push-and-rotate action) to prevent accidental scalding.
• A cold water filter (usually under-sink inline carbon) to improve taste and remove chlorine.

Some systems also provide chilled and sparkling water from the same tap, with an additional compressor unit.

Regulations and safety

Boiling water taps deliver water at temperatures that cause immediate full-thickness burns. The child-safety activation mechanism is not optional — specify only units with a compliant mechanism and ensure it is functioning at commissioning.

Water Fittings Regulations require a double check valve on the cold inlet (Fluid Category 2 risk — elevated temperature). Backflow protection is typically built into the filter assembly but verify on site.

The tank unit is a small unvented stored water heater. At 2.4–4 L it falls within the sub-15 L G3 exemption, but the mandatory safety devices (T&P valve, thermal cut-out, tundish, PRV) still apply. Most proprietary systems include these in the tank unit as factory-fitted components; confirm before installation and ensure the tundish discharge is routed correctly.

Wiring

Tank units typically draw 2–3 kW. A dedicated 13 A switched FCU with 30 mA RCD protection is adequate for most units. Do not share the circuit with other high-draw appliances. The tank should be accessible for periodic descaling (particularly in hard water areas).

Water hardness and scale

Instant boiling water taps are particularly vulnerable to limescale in hard water areas (above ~200 mg/L CaCO₃ — most of southern England). The supplied inline filter provides minimal scale protection. In hard water areas:

• Specify or recommend a dedicated inline scale inhibitor or polyphosphate dosing unit on the cold inlet to the tank.
• Advise the customer to descale the tank at manufacturer-recommended intervals (typically 6–12 months in hard water).
• A whole-house water softener upstream is the most effective long-term solution.

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Thermostatic Mixing Valves and Scalding Prevention

Where a stored-volume heater is set to 60–65°C (as required to prevent Legionella growth under L8 guidance), water delivered at the outlet at that temperature will cause scalding in under 5 seconds. In any location where vulnerable users may be present — care homes, schools, sheltered housing, premises covered by the Care Quality Commission — a thermostatic mixing valve (TMV2 or TMV3) must be fitted at each outlet to blend the stored water with cold and limit the delivered temperature to 41–44°C (hand-wash) or 38°C (accessible bath).

See Thermostatic Mixing Valves: Scalding Prevention and Installation Guide for full requirements. Instantaneous heaters set to deliver at approximately 40–43°C at the tap do not typically require a TMV at that outlet, but confirm the output temperature under all operating conditions.

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Comparison Summary: Which Type to Specify?

Type	G3 required?	Best for	Key limitation
Instantaneous open-outlet (3–9.5 kW)	No	Single basin, low flow rate, hand-wash	Dedicated open-outlet tap required; temperature varies with flow
Vented cistern storage (5–15 L)	No	Commercial hand-wash, low-pressure applications	Must be mounted above tap; gravity feed only
Unvented storage ≤15 L	No (but safety devices mandatory)	Mains-pressure hot water from a standard tap	PRV + expansion + tundish required; TMV if vulnerable users
Unvented storage >15 L	Yes	Multiple outlets, higher demand	G3 engineer required; full unvented safety equipment pack
Instant boiling water tap	No (≤4 L tank)	High-spec domestic kitchen, commercial office	Scalding risk; hard water descaling; safety activation mechanism

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Get it from APM Electricals, Acton

We stock what UK trades need — same-day collection from our trade counter in Acton. No minimum order. Trade accounts welcome.

Browse our water heater range, or visit us at 24 Western Avenue, Acton, London W3 7TZ. Call 020 8702 8080 to check stock.