The Zip InLine ES is a compact electronically controlled instantaneous water heater designed specifically for under-sink handwash installation — available in three outputs: ES3 (3.1kW), ES4 (4.8kW), and ES6 (6.0kW). Hot water flows the instant the tap is opened: there is no stored hot water, no standing heat loss, and no waiting. The unit automatically activates when water flow exceeds the minimum threshold, heats the water as it passes through the bare-wire element, and cuts out when the tap is closed — drawing power only when actively heating. Factory set to deliver 38°C at the outlet, adjustable by a competent installer between 30°C and 50°C. All three models share the same compact 132 × 187 × 80mm body and ½" BSP water connections, making specification and installation straightforward across projects. WRAS approved for connection to wholesome water supply. CE and VDE certified. 1-year manufacturer's warranty.

⚠ Qualified Installation Required — Bare Wire Element Water Quality
Must be installed by a competent electrician via hard-wired dedicated circuit with a double-pole isolating switch (minimum 3mm contact separation in all poles) — not plug-connected. The Zip ES uses a bare-wire (immersed) heating element which requires the supply water to have a minimum specific resistance: ES3: >1300 Ω·cm at 15°C; ES4 and ES6: >1000 Ω·cm at 15°C. Soft water supplies (low mineral content / high conductivity) may fall below this threshold — check with your local water supplier before installation. The unit is not recommended for use with thermostatic mixing valves or thermostatic taps. Install within 0.5m of the outlet to minimise heat loss in the connecting pipework.

ℹ Handwash Only — Not Suitable for Showers or Baths
The Zip ES range is designed and approved for handwashing applications only. Maximum factory flow rate is 3.3 L/min (ES6) — which is not sufficient for a shower (typically 6–12 L/min) or bath filling. It is ideal for commercial washrooms, clinics, schools, offices, and domestic basin applications where a remote or secondary hot water supply is needed without running pipework from a distant boiler or cylinder. For shower or kitchen sink applications, a higher-output unit is required.

3.1 / 4.8 / 6.0 kW

Three models

38°C

Factory set output

IP24

Splash protected

WRAS

Potable water approved

132 × 187mm

Same body, all models

Features & Benefits

✔ Bare-Wire Element — Instant Heat, Zero Standby Loss

The Zip ES uses a bare-wire (immersed) heating element — the element is in direct contact with the water flowing through it rather than heating a metal surface that then heats the water. This eliminates the thermal lag of a conventional sheathed element and gives an almost instantaneous temperature rise as water passes over the wire. Because the element is only energised when water is flowing, there is zero standby energy consumption — the unit draws no electricity between uses, making it significantly more efficient than a stored hot water heater that must maintain temperature around the clock.

✔ Electronic Temperature Regulation — Stable 38°C Output

The electronic controller continuously monitors outlet temperature and modulates power to the element to maintain the set temperature — automatically compensating for variations in inlet water pressure, inlet temperature, and mains voltage. In practice, this means the outlet temperature remains stable at 38°C even when water pressure fluctuates between uses. The factory setting of 38°C is ideal for handwashing and complies with safe temperature guidance for public washrooms. A competent installer can adjust the setpoint inside the unit between 30°C and 50°C — lower for vulnerable user locations, higher for specific applications.

✔ Auto On/Off — No Controls for the User

The Zip ES switches on automatically when the minimum flow rate is exceeded (1.2 L/min for ES3; 1.5 L/min for ES4 and ES6) and switches off automatically when the tap is closed or the flow drops below the cut-off threshold. There are no user controls — no on/off switches, no temperature dials. Hot water at the set temperature is delivered simply by opening the tap. This makes the unit ideal for public washrooms, clinics, and schools where simplified operation is important, and eliminates tampering with temperature settings by unauthorised users.

✔ WRAS Approved — CE & VDE Certified

WRAS (Water Regulations Advisory Scheme) approval confirms the Zip ES complies with UK Water Regulations and Scottish Byelaws for connection to the wholesome (potable) water supply — mandatory for any device connected to the mains water system in the UK. The unit is also CE marked and VDE approved to the Low Voltage and EMC Directives, confirming European electrical safety and electromagnetic compatibility standards. These approvals satisfy the requirements of specifiers, local authorities, and compliance officers across commercial, healthcare, and educational projects.

✔ Compact Under-Sink Design — All Models Same Body

All three ES models share a single compact 132mm tall × 187mm wide × 80mm deep body — small enough to fit inside a standard under-sink cupboard without obstructing other pipework or storage. The snap-on wall bracket system means the unit can be removed for servicing without tools and without disconnecting the plumbing. Selecting a higher kW model for a project does not require a larger enclosure, simplifying specification across mixed-power installations. Water connections are ½" BSP, compatible with standard basin waste fittings and flexible tap connectors.

✔ Solar Pre-Heat Compatible — Up to 60°C Inlet

The Zip ES accepts a maximum inlet temperature of 60°C — significantly higher than most instantaneous heaters, which are typically limited to cold mains temperature only. This makes the ES range compatible with solar thermal pre-heated water systems where the supply water may be warm or hot before entering the heater. When the inlet temperature is high enough that the target outlet temperature is already met, the electronic controller reduces or eliminates element power accordingly — the heater simply passes warm water through without energising the element. Cold water mixing may be required at the outlet to achieve safe handwash temperature when inlet temperature is very high.

Technical Specifications — ES3, ES4 & ES6 Compared

Specification	ES3	ES4	ES6
Power Rating	3.1 kW	4.8 kW	6.0 kW
Rated Current	13A	20A	25A
Flow Rate @ 38°C Factory Set *	1.7 L/min	2.6 L/min	3.3 L/min
Switch-On Flow Rate	1.2 L/min	1.5 L/min	1.5 L/min
Min. Diff. Pressure @ Factory Flow	0.25 bar	0.50 bar	0.75 bar
Min. Specific Water Resistance	>1300 Ω·cm @ 15°C	>1000 Ω·cm @ 15°C	>1000 Ω·cm @ 15°C
Power Supply	240V ~ 1/N/PE
Factory Set Temperature	38°C
Installer Adjustable Range	30°C – 50°C
Max. Inlet Temperature	60°C
Water Connection	½" BSP
Max. Working Pressure	10 bar (1.0 MPa)
Recommended Operating Pressure	2–4 bar (0.2–0.4 MPa)
Rated Volume	0.2 litres
Element Type	Bare wire (immersed)
IP Rating	IP24
Dimensions (H × W × D)	132 × 187 × 80 mm
Weight	1.5 kg
Colour	White
Approvals	WRAS · CE · VDE (LVD & EMC)
Warranty	1 year — Zip Heaters (UK)

* Flow rates based on 12°C supply temperature, 3 bar dynamic pressure, 240V supply.

Frequently Asked Questions

Which model — ES3, ES4 or ES6 — should I choose?

The key variable is flow rate at the desired outlet temperature, which depends on your incoming cold water temperature. The quoted flow rates (1.7 / 2.6 / 3.3 L/min at 38°C) assume a 12°C inlet temperature at 3 bar. In summer or on mains supplies that are naturally warmer, the same models will deliver slightly higher flow rates at 38°C. In cold winter conditions or at high altitude, the heater works harder and flow rate may be marginally lower at the same temperature setting. For a normal basin handwash, all three models produce adequate flow — the ES3 is the most economical where a lower flow is acceptable; the ES6 gives the most comfortable flow rate and suits busier installations or locations where users expect higher flow. The ES4 is the most commonly specified middle option for commercial washrooms.

What is the water quality requirement and how do I check?

Because the bare-wire element is in direct electrical contact with the water, the water must be a sufficient electrical insulator. Zip specifies a minimum specific resistance of >1300 Ω·cm at 15°C for ES3 and >1000 Ω·cm at 15°C for ES4 and ES6. Very soft water (low in dissolved minerals, as found in some upland/rural areas of the UK, particularly Scotland and Wales) can have lower specific resistance than this and would not be suitable. Ordinary mains water in most UK towns and cities easily exceeds these thresholds. To check: contact your local water supplier and ask for the specific resistance or conductivity figure for your area, or your local council water quality report will often list it. Water with specific conductivity above 100–150 µS/cm at 25°C may require checking against Zip's threshold.

Can I use this with a mixer tap or thermostatic tap?

The ES range is not recommended for use with thermostatic mixing valves or thermostatic taps. This is because thermostatic valves blend hot and cold to a fixed temperature — but the Zip ES already delivers water at a pre-set temperature (38°C), so adding a thermostatic valve creates a conflict between two temperature-regulating devices. For non-thermostatic mixer taps (standard hot/cold basin mixer), a non-return valve must be fitted on the hot water outlet to prevent cold water back-flowing into the heater when both hot and cold sides are used simultaneously. A flow control valve may also be needed on the cold supply to balance pressure. The unit is supplied with a jet regulator for M22 or M24 threaded tap outlets, which gives optimum flow control at low rates.

Is there a Legionella risk with this type of heater?

Legionella pneumophila multiplies in stored water held between approximately 20–45°C. Because the Zip ES is an instantaneous heater with only 0.2 litres of rated volume (the water in the unit at any time) and heats that water to 38°C immediately on demand — with no stored volume held warm between uses — the conditions for Legionella growth are fundamentally different from a stored hot water cylinder or calorifier. The HSE's L8 ACOP and HSG274 guidance specifically recognises instantaneous heaters as inherently lower risk than stored systems. However, the upstream cold supply pipe and any dead-legs between the mains connection and the unit must be managed in a full water safety risk assessment, particularly in healthcare and care home settings. Consult your water hygiene consultant for site-specific advice.

How is the unit serviced, and what spare parts are available?

The Zip ES snaps off its wall bracket without tools or plumbing disconnection — making access for inspection and cleaning straightforward. The main serviceable components are: the bare-wire heating spiral (model-specific — ES3, ES4, and ES6 have different spirals), the front cover, the filter sieve, water connection seals, the flow adjustment screw with seal, the temperature sensor, and the safety thermal cut-out. All are available as Zip spare parts via Zip Heaters (UK) Ltd. The filter sieve should be checked and cleaned periodically, particularly in hard water areas. A full fault finding guide is provided in the installation manual — the most common issues are low flow rate (blocked filter or jet regulator) and failure to switch on (flow below the minimum threshold).

Instant Water Heaters