How to install an instantaneous water heater
Updated: 17.10.2026
You have chosen a point-of-use heater and want to know what happens between the box and hot water at the tap. The safe answer to how to install an instantaneous water heater has two parts: prepare and purge the water path, then have the electrical supply matched to the appliance. Powering an empty element can damage it. Treating a multi-kilowatt heater as an ordinary plug-in appliance can overload the installation. This is the order of work. It does not replace the model manual, a plumber or a competent electrician. If you are still choosing the unit, start with the [tankless water heater comparison](/hub/hot-water/tankless-water-heater). A qualifying purchase can earn a commission at no extra cost to you.
You have chosen a point-of-use heater and want to know what happens between the box and hot water at the tap.
You have chosen a point-of-use heater and want to know what happens between the box and hot water at the tap. The safe answer to how to install an instantaneous water heater has two parts: prepare and purge the water path, then have the electrical supply matched to the appliance. Powering an empty element can damage it. Treating a multi-kilowatt heater as an ordinary plug-in appliance can overload the installation. This is the order of work. It does not replace the model manual, a plumber or a competent electrician. If you are still choosing the unit, start with the tankless water heater comparison. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Power | Supply | Mount | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Stiebel Eltron Stiebel Eltron DCE-X 10/12 | 12 kW | High-current circuit | Wall | 8.8 | about £412.86 | View offer |
| Preis-Tipp |  ThermoMate ThermoMate ET055 5.5 kW | 5.5 kW | 25 A, no plug | Wall | 8.0 | about £109.74 | View offer |
| Alternative |  TopSer TopSer BS-18DC-3 Pro tap | 3 kW | Corded, 220 V | Deck | 8.0 | about £49.99 | View offer |
On this page
Prepare the position and water connection
Read the manual before you drill. Confirm an open-outlet unit that needs its own tap, or a pressure-rated heater that suits the planned fitting. Check orientation, pressure limits, connection size, clearances and IP rating. One brand's instructions do not transfer to another.
Fit it close to the outlet, and keep it accessible for isolation and servicing. Do not hide it in a sealed cupboard if the manual requires ventilation. In a bathroom, agree the position with the electrician before you plumb it in, because the location affects electrical zoning.
Isolate the cold supply and flush the branch before you connect the heater. Hyco and Zip tell installers to clear debris because swarf, solder or dirt can obstruct the small waterways or the flow sensor. Fit the servicing valve and the other water fittings the manual requires. Some manuals prohibit particular jointing compounds.
Run water through it before any power
Keep the electrical supply isolated while you fill and purge. Open the downstream tap or approved outlet and turn the water on slowly. Let it run until the stream is smooth and continuous, with no coughing, spurting or visible air. Hyco's commissioning sequence says to draw water until the flow is smooth before switching on the electrical supply. Zip likewise warns that the heater must be purged before energising.
Watch every joint and the casing while cold water flows. A damp joint needs correction before electricity is introduced, and water from inside the casing is a stop. Do not switch on if pressure is unstable or the heater does not fill as the manual describes. Repeat the purge after any drain. Dry-fire protection is not a reason to skip it. The tankless heater buying guide is where heater type and pressure should already have been checked.
Let an electrician design and test the circuit
The appliance data plate, not an online title, sets the electrical starting point. A 3 kW unit may sit around the load expected of a 13 A connection when its manufacturer explicitly permits one. A 5.5, 6 or 12 kW heater needs a dedicated higher-current arrangement. In this comparison, ThermoMate asks for 25 A or more, Delousiber's bullet asks for at least 40 A, and the Stiebel model class reaches substantially higher currents. There is no universal cable or breaker answer.
A competent electrician assesses supply capacity, earthing, bonding, cable route, protection and isolation. In England and Wales, Part P makes a new circuit notifiable, and work around a bath or shower can fall within special-location rules. Scotland and Northern Ireland use their own building-control arrangements.
Do not fit a 13 A plug to a bare-lead 6 kW heater, and do not copy a cable size from a review of another house. The electrician should test the completed circuit and provide the documentation. If the listing and the data plate disagree, stop until the supplier and installer identify what was delivered. The kilowatts and flow guide helps you check whether the expected outlet performance justifies the circuit before work starts.
Supervise the first hot run and stop on faults
Start the first powered run only after the heater is full, leak-free and the circuit has been tested. Watch the temperature without putting a hand into water that may scald, and check the joints again as it warms.
Isolate the heater if it leaks, smells hot, trips repeatedly, or gives a tingle or shock at the tap. TopSer and Delousiber reviews in this set include leaks, shocks, blown fuses and early failures. Those are not quirks to reset. Call the installer or the supplier.
If performance is safe but inadequate, revisit the outlet demand instead of altering controls beyond the manual. The best tankless water heater picks separate low-flow hand washing from higher-power use.
Common questions
Can I install an instant water heater myself?
You may be able to prepare a suitable location or arrange plumbing if you are competent, but fixed electrical work and any new circuit need the correct design, testing and certification. In England and Wales a new circuit is notifiable under Part P. Bathroom locations and high-current loads are strong reasons to use qualified trades.
What must happen before I switch it on?
The heater must be mounted correctly, connected to the approved outlet arrangement, filled with water and purged until the stream is smooth and free of air. Check every joint for leaks while power remains isolated. Energise only after the electrical installation has been completed and tested for that exact model.
Do I need both a plumber and an electrician?
Often, yes. A plumber can identify the hydraulic type, pressure requirements, valves and leak-free connections. An electrician designs and certifies the supply. Some installers are competent in both areas, but do not assume an electrician will modify pipework or a plumber will approve a high-current circuit.
Should I fit a non-return valve?
Only if the manufacturer's hydraulic scheme and local water requirements call for one. A non-return valve can create a closed section and may require additional pressure or expansion provisions. Follow the exact manual and ask a competent plumber. Do not copy a valve arrangement from a different instantaneous or storage heater.
What should I do if it leaks on first use?
Isolate electrical power and stop the water supply if it is safe to do so. Do not keep running the heater to see whether the leak stops, especially if water reaches the casing. Have the joints and the appliance inspected, and return a faulty unit rather than attempting an internal repair.
Related pages
Sources
Public UK sources used for this comparison, last checked on 17 October 2026.
- Approved Document G (2024 amendments) (retrieved 17.10.2026)
- Approved Document P (retrieved 17.10.2026)
- IET Part P frequently asked questions (retrieved 17.10.2026)
- HSE HSG274 Part 2 (retrieved 17.10.2026)
- Hyco instantaneous water heater chart (retrieved 17.10.2026)
- Stiebel Eltron: what size instantaneous water heater do I need? (retrieved 17.10.2026)
- Stiebel Eltron: instantaneous versus stored water heaters (retrieved 17.10.2026)
- Stiebel Eltron DCE-X 10/12 technical data (retrieved 17.10.2026)
- amazon.co.uk listings for the seven ASINs in this comparison (retrieved 17.10.2026)