How to choose a combination of a flow water heater + traditional tap: what to pay attention to

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How to choose a flow water heater + classic tap set: what to pay attention to

The combination of a flow water heater with a classic tap is one of the simplest and most reliable solutions for preparing hot water directly at the point of use. Despite its apparent simplicity, practice shows that an incorrect choice of set can cause a whole range of problems – from water that is too cold at higher flow rates, through insufficient electrical supply capacity, to mechanical incompatibility of the tap with a specific sink or bathtub. In this article, we will look at all the key parameters to consider when choosing, as if we were discussing them with the customer directly in the store – objectively, with specific numbers and examples from real installations.

What is a flow water heater + classic tap set and why it is worth buying as a whole

A flow water heater (also known as PL – flow, instant) heats water directly during the flow – it has no tank, it works electrically and delivers hot water instantly, as long as it has sufficient power. A classic tap in this context means a standard lever or knob tap with a fixed spout and a faucet, as opposed to a thermostatic or mixer tap.

If you buy the heater and the tap separately, you may encounter a whole range of pitfalls: different thread dimensions of connecting hoses, unsuitable length of the tap spout for the depth of the sink, too low flow capacity of the tap in combination with the heater's power, or an aesthetic mismatch between the tap and the heater cover. Sets such as those found in the category SETS Flow water heaters + taps – PL classic taps on Atria.sk are designed so that the heater and the tap are technically and visually compatible, the connections fit and there is no need for any adjustments.

From a practical point of view: we have seen customers who bought a cheap heater and a "random tap from a hypermarket". The result was either cold water (because the flow of the tap was too high for the heater's power) or water leaks at the connections (because the seals and threads did not fit exactly). A set eliminates these risks from the start.

Heater power: the key parameter that determines everything else

The power of the flow water heater is an absolutely essential parameter. It is given in watts or kilowatts (kW) and directly affects by how many degrees the water is heated during the flow. Basic physics: the higher the power, the warmer the water at the same flow rate – or the same water temperature at a higher flow rate.

For standard use, the following approximate rules apply:

3.5 kW – sufficient for a sink tap when washing hands, dishes or cleaning vegetables. The flow is lower, the temperature is sufficient. Typical scenario: kitchen sink, where you do not want to pull a long hot water line from a central tank.
4.5 kW – a more flexible option, suitable for a sink and a bathtub tap with a slightly higher flow. A good compromise for most households.
5.5 kW – mainly intended for a bathtub or a wall-mounted tap, where the customer expects sufficient flow for filling the bathtub or comfortable showering. At a lower power, the water from the bathtub tap would come out lukewarm or cold.

More on how to correctly calculate the required power for a specific type of tap and point of use can be found in the separate article What power flow water heater do I need for a sink, bathtub or wall-mounted tap in our Knowledge Center.

Types of classic taps in sets: basin, bath, and wall-mounted

A classic tap in the sense of this category is a single-lever (towel) mixer without a thermostatic head, with a fixed spout. They differ in the way of mounting and purpose:

Basin tap

It is mounted directly into the hole in the sink or the countertop. The spout points over the sink and has a standard height so that a typical pot can fit comfortably underneath. The typical connection spacing is 1/2", and the spout length is 20–22 cm. In HAKL sets, you will find, for example, HAKL PL SET 3.5 kW with a classic basin tap, 20 cm spout IconicLine or the more powerful HAKL PL SET 4.5 kW with a classic basin tap, 20 cm spout IconicLine, which is appreciated by customers who need a slightly higher flow rate for their sink.

Bath tap

It is mounted either on the edge of the bath (into prepared holes) or on the wall. It has a shorter body with a flexible hose and a handheld shower. The connection spacing is usually 15 cm (for mounting on the bath) or 150 mm on the wall. The spout is swivel, allowing it to be adjusted both above and below the bath. For this purpose, we offer, for example, HAKL PL SET 4.5 kW with a classic bath tap, 18/150 cm IconicLine or for greater comfort HAKL PL SET 5.5 kW with a classic bath tap, 18/150 cm IconicLine.

Wall-mounted tap

It is mounted on a stand separately, mostly on free-standing bath tubs (so-called clawfoot tubs) or on sinks without a hole, where the tap is placed on the floor or on a special stand. The spout is usually significantly longer (22 cm or more) to reach over the edge of the bath. An example is HAKL PL SET 3.5 kW with a classic wall-mounted tap, 22 cm spout IconicLine.

For more information on the differences between these types and which one is suitable for your bathroom or kitchen, see the article Basin, bath, or wall-mounted tap: which type of set is suitable for my bathroom or kitchen.

Electrical requirements and home installation conditions

This is the point where customers most often get burned – literally and figuratively. A flow heater with a power of 3.5–5.5 kW is not like a regular appliance plugged into an outlet. It requires the fulfillment of specific electrical installation conditions; otherwise, you may end up with a blown fuse, a burnt cable, or – in the worst case – a damaged heater.

Minimum requirements for each power

Heater power	Minimum cable cross-sections (Cu)	Circuit breaker	Socket / direct connection
3.5 kW / 230 V	2.5 mm²	16 A (B/C)	Direct connection (not an extension cord!)
4.5 kW / 230 V	2.5 mm²	20 A (C)	Direct connection, dedicated circuit
5.5 kW / 230 V	4 mm²	25 A (C)	Direct connection, dedicated circuit, RCD 30 mA

Important note: in the bathroom, every electrical appliance near water must be protected by a residual current device (RCD) with a sensitivity of 30 mA. This is not a recommendation, but a legal requirement derived from the standard STN 33 2000-7-701. If your bathroom installation does not have an RCD, it must be installed before the heater is mounted.

Another practical matter: the heater must be on its own electrical circuit, i.e., on its own circuit breaker. If you connect a hair dryer, washing machine, or another appliance to the same circuit, the circuit will trip. From experience, we know that this is one of the most common reasons for complaints – the customer claims the heater does not work, but in reality, the circuit breaker has tripped due to multiple appliances being connected.

A more detailed description of the electrical requirements and installation conditions can be found in the article What electrical requirements does a 3.5–5.5 kW flow heater have and what must a home installation meet.

Dimensions, spacing, and mechanical compatibility of the tap

One of the most common mistakes when choosing a set (and when choosing a tap separately) is neglecting mechanical compatibility. The tap must physically fit into the mounting location – and that is not a given.

What to measure before purchasing

Diameter of the hole in the sink or basin – the standard size is 35–38 mm. Most modern taps have a body with a diameter of 35 mm, which is compatible. Older basins may have holes of 25–30 mm or even larger.
Thickness of the sink or bath rim – the tap mounting nut must be long enough to be tightened through a thick rim. In HAKL IconicLine sets, the mounting kits are dimensioned for standard thicknesses of 20–60 mm.
Spacing for a bath tap – 15 cm spacing (center-to-center) is the European standard for mounting on the edge of a bath, 150 mm is the standard spacing for wall mounting. Before ordering, check the spacing of the holes in your bath or wall console.
Length of the tap spout – for sinks, it is important that the spout extends sufficiently deep over the center of the drain. A 20 cm spout is suitable for most kitchen sinks, 22 cm for wall-mounted installation on a deeper bath.
Height of the spout – for a basin tap, make sure the height of the outlet is sufficient to allow a standard pot to fit underneath (at least 15 cm above the bottom of the drain).

For a detailed analysis of the dimensions and compatibility of IconicLine taps with various types of sinks and bathtubs, see the article Classic IconicLine tap: dimensions, spacing and compatibility with various types of sinks and bathtubs.

Water pressure in the network: why it matters for flow water heaters

A flow water heater is very sensitive to water pressure. Unlike a storage heater, which heats water in a tank and the pressure depends on the water supply, a flow heater requires a minimum flow rate to activate at all. Most modern flow heaters (including HAKL PL) have electronic control that turns on the heating at a flow rate of about 0.5–1 l/min – and this is also the minimum, which must not be exceeded.

Problems arise in two situations:

Too low pressure – the heater does not start or starts and stops (the so-called "cold – warm – cold water" effect). This typically occurs in houses with old piping, in prefabricated houses, or when supplied from a private well with a low-pressure pump. The minimum operating pressure for most flow heaters is 0.05–0.1 MPa (0.5–1 bar).
Too high pressure – not only does it increase the risk of water leakage at the connections, but it can also mechanically stress the heater body. The maximum pressure is usually 0.6–0.8 MPa (6–8 bar). In a network with pressure above this limit, a pressure-reducing valve is mandatory.

Before installation, we recommend measuring the water pressure in the network with a simple pressure gauge, which you can buy for a few euros at any plumbing wholesaler. It is a five-minute job that can save you hours of problem-solving later.

Material and surface finish of the tap: aesthetics and durability

Classic taps in the HAKL IconicLine sets are made of brass with a satin chrome or glossy finish. Brass is an ideal material for taps in terms of resistance to corrosion and mechanical stress – much better than zinc alloy (Zamak), which is used in cheaper taps and which begins to crack and corrode from the inside after prolonged contact with water and moisture.

What to pay attention to when comparing taps:

Weight – a brass tap is noticeably heavier than a zinc one. If the tap seems too light, it is a bad sign.
Surface finish – galvanic chrome should be even, without visible edges, excess material or dull spots. These problems appear on cheap taps after a few months.
Tap mechanism – turn the lever – the movement should be smooth and not too loose. The cartridge (the heart of the tap) should be ceramic, not plastic. A ceramic cartridge will last for tens of thousands of turns without dripping, while a plastic one wears out much faster.
Compression nut and sealing rings – check whether all sealing elements and mounting parts are included in the packaging. A good set includes all of them, including compression nuts, gaskets and an installation kit.

What to pay attention to during the installation itself

The installation of a set consisting of a flow heater and a classic tap is not an electrician or plumber task that must be solved exclusively by a professional – but a few rules must be followed. A detailed installation procedure including water and electricity connection can be found in the article Installation of a flow heater + classic tap set: procedure, water and electricity connection. Here we will mention only the most important warnings:

Installation order – first, mount the tap mechanically (plumbing part), then connect the water heater to the water supply, and finally connect the heater to the electrical grid. Never in the opposite order.
Depressurization – before turning on the heater, it must be completely filled with water and air must be purged. Procedure: open the tap, let the water run and allow it to flow until a continuous stream without air bubbles is achieved. Only then turn on the electricity.
Thread sealing – use Teflon tape or hemp sealing with paste, not just rubber seals. On pressurized water supply lines, rubber seals alone are not reliable in the long term.
Electrical connection in the bathroom – in the bathroom, protection zones apply according to the STN 33 2000-7-701 standard. An electrical socket must not be located within the protective zone within 60 cm of the edge of the bathtub or shower. The heater must have a fixed connection (not a plug into a socket) and must be protected by an RCD.

Lifespan of the set and regular maintenance

A flow water heater with a traditional tap is relatively low-maintenance. Despite this, there are a few preventive measures that can significantly extend the lifespan of the entire set:

Water inlet filter

Most flow heaters have a fine mesh filter at the water inlet to catch impurities from the pipes. This filter should be inspected and cleaned every 6–12 months, or more frequently in areas with hard or polluted water. A clogged filter reduces flow and the heater may start "cycling" (turning on and off repeatedly).

Descaling

In areas with hard water (hardness above 15 °dH), limescale accumulates on the heating element. This acts as an insulator, reduces heating efficiency, and shortens the lifespan of the heating element. It is recommended to use a water softening filter at the water inlet or to perform professional descaling regularly (every 2–3 years). A detailed guide can be found in the article Maintenance and cleaning of a flow heater with a traditional tap: how to extend the lifespan of the set.

Tap cartridge

The ceramic cartridge is a mechanical component that gradually wears out. If the tap starts dripping when the handle is closed, the cartridge is likely worn or clogged with limescale. Replacing the cartridge is a simple and inexpensive repair that even a manually skilled person can handle – and it will extend the life of the tap by several more years.

Most common problems and their solutions

From practice, we know several typical problems that customers report after installing the set. A comprehensive overview can be found in the article Common faults of flow heater and tap sets: low pressure, cold water, dripping. Here is a brief summary:

Water is not sufficiently hot – most common causes: the heater's power is too low for the flow rate, the tap flow is too high, the heater is not properly adjusted (temperature regulator), or the incoming water temperature is too low (winter, well water).
Heater does not start – water pressure is too low, the filter is clogged, the electrical circuit has no voltage (blown fuse or RCD).
Dripping from the tap aerator – worn or dirty cartridge, loose compression nut.
Dripping at the connections – insufficiently sealed threads, loose compression nut.
Noisy operation of the heater – limescale deposits on the heating element, air in the system.

Comparison of HAKL PL sets by power: which one is best for your case

The HAKL PL SET IconicLine series covers three power levels and three types of taps. The following table summarizes when to choose which set:

Set	Power	Tap type	Typical use
HAKL PL SET 3.5 kW basin	3.5 kW	Basin tap, 20 cm spout	Kitchen sink, washbasin in a workshop, office kitchenette
HAKL PL SET 4.5 kW basin	4.5 kW	Basin tap, 20 cm spout	Kitchen sink with higher flow demand, washbasin in a bathroom
HAKL PL SET 4.5 kW bath	4.5 kW	Bath tap, 18/150 cm	Smaller bathtub, low-flow shower, bathroom without a central tank
HAKL PL SET 5.5 kW bath	5.5 kW	Bath tap, 18/150 cm	Larger bathtub, high-flow shower, more demanding conditions
HAKL PL SET 3.5 kW freestanding	3.5 kW	Freestanding tap, 22 cm spout	Freestanding bathtub, washbasin without a hole, design bathroom

A detailed comparison of the power versions in terms of flow, energy, and suitability for different scenarios can be found in the article Differences between HAKL PL 3.5 kW, 4.5 kW and 5.5 kW sets: when a lower power is sufficient.

Practical scenarios from practice: when to choose which solution

To go beyond theory, let's look at a few typical situations we encounter when selecting sets:

Scenario 1: Kitchen sink in an apartment without central hot water

The customer has an apartment from the 80s, where hot water is irregular or non-existent. They want a solution for the kitchen sink – washing dishes, vegetables, and hands. The flow rate is not high, and there is no sink in the kitchen. Ideal solution: HAKL PL SET 3.5 kW with a basin tap. Electrical installation: they must check whether they have a dedicated 16 A circuit in the kitchen, or install one if not.

Scenario 2: Cabin without central water heating

The cabin has only cold water from a well, a bathroom with a bathtub, and a storage heater was removed during renovation. The customer wants a fast and affordable solution for a shower and filling the bathtub. The right choice: HAKL PL SET 5.5 kW with a bath tap – the higher power compensates for the lower incoming temperature of well water (8–12 °C). It is important to check the capacity of the electrical connection at the cabin – old cabins often have only a 25 A main circuit breaker, which must be carefully calculated in combination with other appliances for a 5.5 kW heater.

Scenario 3: Design Bathroom with Freestanding Bathtub

The customer is renovating the bathroom and plans to install a freestanding bathtub. The sink does not have a faucet opening. Solution: A freestanding faucet on its own stand next to the tub, connected to a flow heater. The HAKL PL SET 3.5 kW with a freestanding faucet is a functional solution if the customer plans to fill the tub slowly or primarily uses it for bathing (not for high-flow showering).

Scenario 4: Supplementary Heater for Bathroom Sink

The house has a central water heater, but it takes 2–3 minutes for hot water to reach the sink in the guest bathroom, which is far from the heater. The customer wants to eliminate water waste while waiting. Solution: A small flow heater directly at the sink – 3.5 kW with a sink or bathtub faucet is sufficient. In this case, the heater acts as a "booster" – if hot water from the heater arrives, the flow heater turns off (no need to pay for additional heating), if not, it provides hot water instantly.

Most Frequently Asked Questions (FAQ)

Can I use a flow heater for a bathtub and a sink at the same time (two points of use)?

No, a flow heater is designed for one point of use at a time. If you were to supply two faucets simultaneously from it, the flow would be divided and the temperature would drop significantly. For two points of use, you need either two separate heaters or a storage heater with sufficient capacity for both outlets.

Must the installation of the set be performed by a certified electrician?

The plumbing part (installation of the faucet, connecting the hoses) can be done by a skilled person without certification. The electrical connection of the heater (especially in the bathroom) should be performed by a certified electrician. Only a certified electrician is authorized to perform electrical work in bathrooms due to safety regulations.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.