Taps and showers

Instant water heater: when is it worth it and when is it not

An instant water heater heats water the moment it flows – it does not require a boiler or hot water supply. However, at around 3.3 kW, it only provides about 1 to 2 litres of hot water per minute, while a standard basin tap delivers around 5 litres. Therefore, it is worth it for washing hands in the toilet, garage, cottage, or workshop, where the hot water supply is far away. It is insufficient for a shower, bath, or sink in a family kitchen. It requires a separate electrical circuit, which should be designed and installed by an electrician.

Prepared by: the TGB Kúpeľne team, bathroom showroom in Senec since 1991updated 8. 11. 20266 min read

In a toilet at the end of the corridor, in a garage or cottage, hot water is hard to access – the supply is far away and by the time hot water arrives, litres of cold water have already run out. An instant water heater solves this directly at the sink: inside the tap is a heating element that heats the water the moment it flows. No boiler is needed, no hot water supply is required, just a cold water supply and electricity. However, it has clear limits that salespeople often remain silent about. In this article, you will find out how much hot water an instant water heater actually provides, what it needs in terms of electricity, and when a small boiler is better.

How an instant water heater works

When you open the tap and water starts to flow, the flow switch activates the heating element. Water is heated as it passes through the tap, and the heating turns off after closing. This leads to two conclusions:

• Hot water is immediate – there is no waiting for cold water to run out from a long pipe.
• Power is limited – the more water flows, the less it heats. At low flow, it is warmer; at high flow, it is lukewarm.

Heating only activates at a certain flow rate. For example, Stiebel Eltron states that for the small flow heater EIL 3 Trend, the activation flow is more than 1.0 l/min, and for EIL 3 Premium, more than 1.5 l/min. At a weak stream, heating does not activate at all.

How much hot water it provides

This is the most important number. The manufacturer HAKL states for the OB 500 tap with a power of 3.3 kW, measured at an inlet water temperature of 12 °C and a pressure of 3 bars:

For comparison: a standard basin tap delivers around 5 l/min – for example, Hansgrohe states a flow rate of 5 l/min for the Metris tap. Therefore, an instant water heater provides hot water at a lower flow rate. It is sufficient for washing hands, but not for filling a basin or showering. In winter, when the water from the tap is colder, the hot water is even weaker.

When is it worth it and when is it not

In cottages where there is no heating in winter, remember that the water in the mixer must not freeze – it should be drained before winter according to the manufacturer's instructions.

What is needed for electricity

A flow mixer is an appliance with high power consumption – at 3.3 kW it draws similar to a powerful kettle, but for a longer time. The technical sheets from manufacturers state:

In practice, this means a separate electrical circuit with protection according to the technical sheet, a residual current device of no more than 30 mA, and placement according to zones in the bathroom. A standard socket, which also has a washing machine or dryer, is not suitable for a flow mixer. The supply will be designed and connected by a qualified electrician who will issue a revision report – more in the chapter Electricity and zones.

Water pressure and quality

• Minimum pressure: HAKL at OB 500 states a minimum working pressure of 0.1 MPa (1 bar). With low pressure, for example from your own home waterworks during fluctuations, the heating may not activate.
• Limescale: with hard water, the heating element and aerator can become clogged. Regularly descale the aerator – how to do this is described in the article Dripping mixer.
• Thermal protection: quality mixers have a safety feature against overheating – HAKL at OB 500 states thermal operational protection and a service fuse on the spiral.

Flow mixer, small boiler or hot water supply?

For example, the small 10-litre heater Tatramat EO 10 has a power of 2 kW and is available in versions for installation above or below the sink. For a low-pressure connection of the heater, you need a mixer designed for low-pressure heaters – more in the article Bath and sink mixer. If you are planning a WC renovation, it is most convenient to extend the hot water supply – tips for a small sink can be found in the article Small sink for WC.

Installation in brief

1. An electrician will assess whether the distribution board can handle another circuit with the necessary protection and will prepare a separate supply with a residual current device.
2. A plumber will prepare the cold water supply with a shut-off valve.
3. The mixer is installed on the sink or basin according to the instructions and is filled with water first – only then is the electricity turned on.
4. During the first start-up, let the water run through and check the temperature and tightness.

An appliance with a heating element must not be turned on dry – always water first, then electricity. Therefore, leave the installation of the flow mixer to a professional.

If you are dealing with hot water in a WC or small bathroom, we can advise you at the studio in Senec whether a flow mixer, small tank or supply is better. Schedule a visit.

Related: Bath and sink mixer · Small sink for WC · Small WC · Electricity and zones · Concealed sink mixer

Sources: product sheet HAKL OB 500, technical sheet HAKL MK-1, technical sheets Stiebel Eltron EIL 3 Trend, EIL 3 Premium and EIL 7 Premium, Tatramat EO 10 N/P, Hansgrohe Metris 31086000 (flow 5 l/min), STN 33 2000-7-701 (zones) according to the chapter Electricity and zones.

Terms in the glossary: IP rating · Aerator