Planning and design

Instant water heater or boiler: comparison, power and water flow

A flow heater heats water only when it is being used and has no storage, while a boiler heats it in advance and keeps it in a tank. It depends on how much hot water you need at once and what electrical supply you have. A small heater on 230 V is sufficient for a sink, while a powerful three-phase is needed for a shower – we will illustrate this with numbers from technical sheets.

Prepared by: TGB Kúpeľne team, bathroom showroom in Senec since 1991updated 9. 11. 20265 min read

Illustrative image: small instantaneous water heater under the sink and boiler in the technical room

We compare types of water heaters, boiler volumes, and their placement in the article Boiler for the bathroom. Here we address one specific question: when is a flow heater sufficient and when is a storage tank better? The answer can largely be calculated.

Difference in brief

Performance and flow: how much hot water a flow heater provides

A flow heater must heat water at the moment it flows through it. From the specific heat capacity of water (4.186 kJ per kg and degree), a simple relationship arises: flow in litres per minute is approximately power in kW times 14.3 divided by the temperature increase you are heating the water by.

The calculation matches the technical sheets. Stiebel Eltron states that for the DHB-E 18/21/24 model, at a heating of 28 °C, the flow is 9.2, 10.7, and 12.3 l/min. Small non-pressurised flow heaters for sinks have in their manuals around 1.9 to 2.5 l/min when heating by 25 °C. In winter, when the water in the pipes is colder, the same heater will provide less hot water.

Additionally, the flow heater only activates at a certain flow – for the DHB-E, this is more than 2.5 l/min. With a very weak flow, the water will remain cold.

Electricity: what a flow heater needs

• Shower heater – Stiebel Eltron DHB-E 18/21/24 connects to 400 V (three phases), rated current is 29, 31 or 35 A and protection is 32, 32 or 35 A depending on the set power. The DHB-E model has a power of 27 kW with a current of 39 A and protection of 40 A.
• Main circuit breaker of the flat – a powerful flow heater needs a reserve on the main circuit breaker. An electrician will assess if you have it.
• Small heater for the sink – connects to 230 V. The supply, protection and installation will be designed by an electrician according to the instructions and zones in the bathroom.

The supply for a powerful flow heater is an intervention in the electrical installation, which is carried out by a qualified electrician and ends with an inspection.

Small flow heater for the sink

For one sink, such as in the WC or garage, a small flow heater is often sufficient. It is usually non-pressurised – water flows out of it through an open outlet and requires a special tap provided or specified by the manufacturer. We explain why a standard tap is not sufficient in the article Non-pressurised tap.

On vkupelni.sk, for example, the Mereo flow heater 3.5 / 4.5 / 5.5 kW, low-pressure, is available for €115.00 (as of November 2026). Even simpler is a sink with a tap that has heating built-in – we describe it in the article Flow tap with heating.

When is a boiler better

• More people and more draws at once – shower and kitchen simultaneously. The flow heater would divide the power between both locations.
• Bathtub – filling it with a flow heater takes a long time, a boiler has water ready.
• Weaker electrical supply – a boiler with a power of around 2 kW fits into a standard flat without a three-phase supply.
• Low tariff – a boiler can heat during cheaper times, while a flow heater consumes electricity just when you are showering.

If you want to heat water more economically, check the article Boiler with heat pump. You can find boilers and flow heaters in the e-shop under the category Water heaters and boilers.

Proven combinations

• Boiler for the bathroom and small heater for a distant sink – there is no need to run a long hot water line to the WC or kitchen.
• Flow heater at the cottage – heats only when you are there, and there is no need to wait for the heating of the supply. More in the article Bathroom at the cottage and summer house.
• Boiler with a small flow heater for top-up – some models are designed for top-up of preheated water, for example, DHB-E up to an inlet temperature of 55 °C.

How to decide

1. Calculate simultaneous draws – one shower, or shower and kitchen at the same time?
2. Check the electrical supply – do you have a three-phase supply and a reserve on the main circuit breaker?
3. Estimate the flow rate – according to the table above and the head you want to use.
4. Consider the space – will a tank fit, or is there only space for a small unit?
5. Consult with an electrician and plumber before purchasing.

Sources: technical sheets for Stiebel Eltron DHB-E (voltage, current, protection, flow at heating at 28 and 50 °C, switching quantity, top-up to 55 °C) and PER 18/21/24, instructions for small non-pressurised flow heaters (flow at heating at 25 °C), calculation from the specific heat capacity of water, offer on vkupelni.sk (November 2026); our articles Boiler for the bathroom and Flow tap with heating.