How to choose the right set of a flow water heater + pull switch for your type of water consumption
How to choose the right electric water heater + lever mixer set for your type of water usage
Choosing the right electric water heater is not rocket science – but doing it wrong can mean cold water in winter, blown fuses, or a mixer that simply won't fit your bathtub. Over the years of practical experience, I keep encountering the same mistakes: customers buy a heater based on price, a mixer based on color, and only during installation they find out that the power is insufficient, the flow is weak, or the mixer is actually a sink type and not a bathtub one. That is why this article is about how to choose a complete set at once – heater and mixer – so that it works as one unit from the very first day.
Sets from the HAKL MK with IconicLine mixers are designed exactly with this goal in mind: the heater and mixer are mutually tuned, hydrodynamically matched, and delivered together with compatible accessories. Despite that, the choice of the right power and type of mixer depends on your specific situation – the type of water usage location, the condition of your home's electrical installation, water hardness, and whether it is the primary source of hot water or an emergency solution during central heating failure.
What is a "set" and why to buy it as a whole
An electric water heater + lever mixer set is a combined product where the manufacturer guarantees mechanical, hydraulic, and aesthetic compatibility of both components. This means: the same threaded connection, verified temperature regulation function via the mixer lever (not via the heater's electronics), matched flow rate, and usually also a unified design for a clean installation result.
If you were to buy the heater and mixer separately, you would have to check yourself whether the threads and dimensions match, whether the mixer can handle the pressure the heater generates, and whether the flow through the mixer does not drop below the minimum at which the heater even turns on. In cheaper combinations, it often happens that the heater has an activation flow of 2.5 l/min, but the mixer limits it to 2 l/min at medium opening – and the water remains cold.
Along with functionality, there is also time and administrative savings: one purchase, one invoice, one warranty provider. During installation, you save the plumber's time, as the set comes with a connecting valve and there is no need to improvise with reducers.
Types of water usage and what each of them actually needs
Before we look at specific products, it is important to understand that each type of usage location – sink, bathtub, shower – has completely different requirements for flow rate and power. Hot water for dishwashing at the sink is a completely different world than filling a bathtub.
Kitchen or bathroom sink
At the sink, the average hot water consumption is 3–5 liters per minute. You don't need a large flow, but the water must be at least 38–42 °C with cold water temperature around 10–15 °C. For this usage, a power of 4.5 kW is sufficient at normal summer water temperatures. In winter – when the inlet temperature drops to 5–8 °C – the situation becomes more complicated and 4.5 kW is not enough to achieve the desired temperature at full flow. Here comes 5.5 kW, which comfortably covers a kitchen sink even in January.
Practical example: An apartment building in Bratislava, older panel, cold water in winter at 7 °C. The customer had a 3.5 kW heater (previous installation) – the outlet temperature at full flow was only 30 °C. After replacing it with the set HAKL MK SET 5.5 kW with 18 cm lever mixer IconicLine for sink, the problem was solved. A 18 cm lever mixer is ideal for a standard kitchen sink and a bathroom basin.
Bathtub
A bathtub is the most demanding usage location from an energy point of view. In order to fill the bathtub in a reasonable time (within 15–20 minutes), you need a flow of at least 8–12 l/min. This cannot be covered by standard flow-through heaters – even 9 kW is not enough to heat such an amount of water to 38 °C. A flow-through heater at the bathtub therefore works differently: it is not the primary source of hot water for filling the bathtub, but serves for reheating or combined usage – that is, the water is primarily heated in a storage tank or central preparation, and the flow-through heater covers the rest or serves as a backup.
Another common scenario is a shower bathtub (a shallow bathtub with a shower corner), where the bathtub is primarily used for showering, not for filling. In this case, a flow of 7–9 l/min at 38 °C is achievable with a power of 7–9 kW and the flow-through heater functions as the main source of hot water.
A bathtub lever mixer differs from a sink mixer mainly by having two outlets – one direct (into the bathtub) and one with a switch for the shower hose. The mixer arm is larger and the connection height is different. That is why bathtub sets are delivered with a bathtub lever mixer and not with a sink one.
Shower
A shower is a "golden middle" in terms of flow – a standard shower head has a flow of 7–10 l/min, an economical one 4–6 l/min. For a standard shower, you need a power of 5.5–7 kW in summer and 7–9 kW in winter. A shower cubicle usually has its own shower mixer (not part of this category of MK lever mixers), but in practice, I often encounter a variant where a shower hose is connected to a bathtub mixer from this set. That is why sets with 5.5 kW and 7 kW bathtub mixers are very popular for small showers as well.
Flow heater performance step by step: how to calculate it correctly
Flow heater performance is calculated using a simple physical equation: P [kW] = flow [l/min] × temperature difference [°C] × 0.07. The coefficient 0.07 takes into account the specific heat of water and unit conversion.
Example calculation for a kitchen sink:
- Required flow: 4 l/min
- Inlet water temperature: 10 °C (winter scenario)
- Required outlet temperature: 42 °C
- Temperature difference: 42 − 10 = 32 °C
- Power: 4 × 32 × 0.07 = 8.96 kW
Wait – that would mean that almost 9 kW is needed for a sink in winter? Yes, if you want full flow of 4 l/min at 42 °C and an inlet temperature of 10 °C. In practice, however, this issue is solved by reducing the flow – you open the lever mixer less, the flow drops to 2.5–3 l/min and 5.5 kW is sufficient. This is precisely the advantage of a lever mixer over a thermostatic one: the lever gives you full manual control over the flow and temperature.
If you are interested in a more detailed analysis of the calculation including seasonal variations, please see the article What flow heater power do I need for a sink, bathtub or shower in this Knowledge Center.
Differences between sink and bathtub lever mixers
A mixer is not just about aesthetics – its hydraulic parameters and physical dimensions are key to the correct function of the entire set. Here is an overview of the main differences:
| Parameter | Sink mixer (18 cm) | Bathtub mixer (18/150 cm) |
|---|---|---|
| Height of the spout | ~18 cm from the base | ~18 cm + 150 cm shower hose |
| Outlets | 1 outlet (direct) | 2 outlets (outlet + shower hose) |
| Connection | Undermount or wall-mounted 1-hole | Wall-mounted 2-hole (spacing 150 mm) |
| Typical use | Kitchen sink, bathroom basin | Bathtub, bathtub with shower |
| Max. flow at 3 bar | 8–10 l/min | 10–15 l/min |
| Minimum pressure | 0.5–1 bar | 0.5–1 bar |
For a detailed analysis of the differences including installation details, I recommend reading the article Sink, bathtub or shower lever mixer – what is the difference and which one to choose in this Knowledge Center.
Overview of specific HAKL MK IconicLine sets and for whom they are intended
The HAKL MK IconicLine range offers five sets covering the entire spectrum of common water consumption. I will describe each of them from a practical perspective.
HAKL MK SET 4.5 kW with IconicLine 18 cm sink mixer
HAKL MK SET 4.5 kW with IconicLine 18 cm sink lever mixer is a basic set for a kitchen sink in a residential space. 4.5 kW is a power that can be supplied from a single phase 230 V with a 20 A circuit breaker. This is a key advantage: it does not require any modification of the electrical installation in a standard apartment, where socket circuits are typically protected by 16 A circuit breakers, but a dedicated 20 A circuit is usually reserved for a heater (current at 4.5 kW: I = P/U = 4 500/230 ≈ 19.6 A).
This set is ideal for:
- Kitchen sinks in apartments without central hot water heating
- Cabins, garden houses and recreational buildings where there is no 3-phase connection
- Summer use (inlet water temperature above 15 °C) at the sink
- Supplementary source in the kitchen next to a storage heater in the bathroom
What to keep in mind: at an inlet water temperature below 10 °C (typical winter) and full flow, the outlet temperature will be around 35–37 °C, which may be sufficient for some, but not for others. If you need really hot water (above 45 °C) in January for cooking, go for the 5.5 kW version.
HAKL MK SET 5.5 kW with IconicLine 18 cm sink mixer
HAKL MK SET 5.5 kW with IconicLine 18 cm sink lever mixer is an extended version for year-round use at the sink. 5.5 kW still works on a single phase 230 V (current: 5 500/230 ≈ 23.9 A → circuit breaker 25 A). This is an important detail: it requires a minimum conductor of 2.5 mm² and a 25 A circuit breaker. If you have an older 16 A circuit breaker in the distribution board, it needs to be replaced – this is a job for an electrician, but not a major intervention.
From a practical point of view, the 5.5 kW sink version is the "golden mean" for most apartment kitchens. It can handle year-round heating at the sink, and at low winter temperatures it is sufficient to slightly reduce the flow with the lever and the temperature is comfortable. This is the set I recommend as the first choice for new builds and renovations of apartments without central hot water preparation.
HAKL MK SET 5.5 kW with IconicLine 18/150 cm bathtub mixer
HAKL MK SET 5.5 kW with IconicLine 18/150 cm bathtub lever mixer is an interesting hybrid: the same power as the sink version, but with a bathtub mixer including a 150 cm shower hose. Where does this make sense?
Primarily for small bathrooms with a bathtub, where the bathtub is not used for filling, but exclusively as a shower cubicle (which is very common in older apartment buildings). A shower with a flow of 5–6 l/min and an inlet water temperature of 15–20 °C (spring and summer months) is comfortable with 5.5 kW. In winter at an inlet temperature of 8 °C and a flow of 5 l/min: P = 5 × (38 − 8) × 0.07 = 10.5 kW – this is clearly not covered by 5.5 kW at full flow. Solution: a water-saving shower head with a flow of 4 l/min reduces the requirement to P = 4 × 30 × 0.07 = 8.4 kW – still above 5.5, but the reality is that most people do not shower at 38 °C, but at 36–37 °C, which reduces the requirement to manageable ~6 kW. In practice: the 5.5 kW bathtub version works as a shower in the transitional period and in summer, in winter a compromise with a lower flow is needed.
HAKL MK SET 7 kW with IconicLine 18/150 cm bathtub mixer
HAKL MK SET 7 kW with IconicLine 18/150 cm bathtub lever mixer is the most common choice for bathroom renovations with a shower in family homes. 7 kW is the threshold where a switch to 3-phase power supply 400 V is usually made or it remains on a single phase with a 32 A circuit breaker and a 4 mm² conductor (I = 7 000/230 ≈ 30.4 A).
With 7 kW and an inlet water temperature of 10 °C you can expect:
- Flow 4 l/min → outlet 35 °C
- Flow 5 l/min → outlet 30 °C
- Flow 3 l/min → outlet 43 °C (comfortable shower with a water-saving head)
This is a set that can handle a year-round shower in a family home. The electrical requirement is a bit higher – if you don't have it, you need to discuss it with an electrician. More about electrical requirements can be found in the article Electrical connection of 4.5 kW, 5.5 kW, 7 kW and 9 kW flow heater – requirements for wiring.
HAKL MK SET 9 kW with IconicLine 18/150 cm bathtub mixer
HAKL MK SET 9 kW with IconicLine 18/150 cm bathtub lever mixer is the most powerful set in this series. 9 kW can handle a comfortable shower in winter at a normal flow of 6–7 l/min. On a single phase 230 V it is a current of ~39 A, which requires a 40 A circuit breaker and a 6 mm² conductor – such an installation is very unusual in panel buildings. In practice, the 9 kW heater is powered either from a 3-phase network 400 V (3× ~13 A per phase), or in family homes from a dedicated power circuit.
This set makes sense for:
- Single-family homes with their own electrical connection and higher capacity
- Situations where the flow heater is the only source of hot water for the entire bathroom
- Mountain properties and cottages with cold well water (inlet temperature 5–8 °C throughout the year)
- Bathrooms where the bathtub is actually filled (even if you don't fill the tub completely with the flow heater, 9 kW will at least give you warm water faster)
Electricity: what you need to have ready before purchasing
This is the point where many projects fail in practice. The electrical installation must match the heater's power – not the other way around. Here are the basic requirements:
| Set power | Voltage | Current (approx.) | Circuit breaker | Conductor cross-section |
|---|---|---|---|---|
| 4,5 kW | 1× 230 V | ~20 A | 20 A (C) | 2,5 mm² |
| 5,5 kW | 1× 230 V | ~24 A | 25 A (C) | 2,5–4 mm² |
| 7 kW | 1× 230 V / 3× 400 V | ~30 A / 3×10 A | 32 A / 3×16 A | 4 mm² / 1,5 mm² |
| 9 kW | 3× 400 V | 3×13 A | 3×16 A (C) | 1,5–2,5 mm² |
Important note: every flow heater must be protected by a residual current device (RCD) with a sensitivity of 30 mA, which is mandatory in bathrooms according to STN 33 2000-7-701. If you are unsure, read the article Electrical connection of flow heaters 4,5 kW, 5,5 kW, 7 kW and 9 kW – requirements for wiring in this Knowledge Center.
Water pressure and its influence on set selection
Besides electricity, the second critical parameter is pressure and flow in the piping. The HAKL MK flow heater will only start when the water flow reaches the activation threshold – usually 2.0–2.5 l/min. If the pressure in the piping is too low (less than 0.5 bar), the heater will not start at all or will start irregularly.
Common pressure problems and their solutions:
- Pressure below 0.5 bar (old apartment building, top floor): heater does not start → solution: small pressure pump at the inlet, or selection of a heater with a lower activation flow.
- Pressure above 6 bar (new buildings with direct connection to the grid): risk of damaging the battery → solution: pressure reducing valve before the set.
- Fluctuating pressure (garden wells, boreholes): irregular heater activation → solution: pressure tank or hydrofor before the heater.
All HAKL MK IconicLine sets are designed for operating pressure of 0.5–6 bar, which covers the vast majority of household piping systems. If you have doubts about pressure, open the tap and fill a 10-liter bucket – if it takes more than 2 minutes, you have a flow problem.
Water hardness and limescale: how they affect the lifespan of the set
Flow water heaters are more sensitive to hard water than storage heaters – because the water is heated as it flows through a small volume, the temperature is higher and calcium carbonate precipitates quickly on the heat exchanger and on the outlet parts of the faucet. In Slovakia, water hardness varies greatly: in Bratislava around 20–25 °dH (hard), in some parts of Orava under 10 °dH (soft).
With hardness above 20 °dH I recommend:
- Set the heater temperature to max. 45–50 °C (higher temperature = faster limescale buildup)
- Clean the inlet filter regularly (every 6–12 months)
- Clean the aerators and outlet parts of the faucet (descale with vinegar or a special cleaner)
- Consider a water softener or a polyphosphate doser before the heater
A detailed guide on cleaning and maintenance can be found in the article Maintenance and Cleaning of IconicLine Lever Faucets – How to Extend Their Lifespan.
Installation and space requirements of the set
Before ordering the set, also check the practical space requirements. The HAKL MK flow heater has compact dimensions – most models are smaller than a shoebox – but it must be mounted in an accessible position, not behind drywall. The reason: the resettable thermal fuse (thermostat protection at 75 °C) may require manual reset, and the mesh filter must be regularly removed and cleaned.
For a sink faucet (arm height 18 cm), the standard placement of the heater is under the sink or on the wall next to the sink. The recommended distance between the heater and the faucet should be as short as possible – every additional meter of pipe between them means greater heat loss (cold water standing in the pipe since the last use).
A bathtub faucet is mounted on the wall next to the tub, usually 60–80 cm from the floor, with a standard distance of 150 mm between the connecting holes. The heater can be in an adjacent room (e.g., behind the tub in a technical shaft), but again: the shorter the connection, the faster and more energy-efficient the hot water.
The complete step-by-step installation process is detailed in the article Installation of a flow heater set with a lever faucet step by step.
HAKL MK sets vs. custom combination: when it pays off and when it doesn't
This question arises almost with every order. The answer depends on your situation:
The set is worth it when:
- You are installing for the first time and have no experience with component compatibility
- You want a unified design of the faucet and heater
- You need a quick solution (set = everything in stock, one supplier)
- You want to minimize installation risks (everything designed to fit together)
A custom combination may make sense when:
- You have a specific faucet (e.g., thermostatic or a particular design) that you want to keep
- You are replacing only the heater and the existing faucet is functional and compatible
- You need non-standard dimensions or special accessories
A more detailed comparison of costs and benefits can be found in the article HAKL MK sets vs. separate heater and faucet – what is more cost-effective.
Frequently asked questions (FAQ)
Can I use the bathtub faucet from this set in a shower without a tub?
Yes, it is technically possible. The HAKL MK IconicLine bathtub faucet has a shower hose with a length of 150 cm, which you can use with a shower head even without a tub. It is mounted on the wall next to the shower. It works just as well functionally, although a shower faucet may be more aesthetically suitable. If you want to save money while keeping the shower function, a set with a 7 kW bathtub faucet is a legitimate choice for a shower area as well.
Why is cold water flowing even when the heater is turned on?
There are several common reasons: the heater did not start (flow below the activation threshold), the thermal fuse has tripped and is waiting for a reset, the heater is new and has not yet heated the water in the pipe (normal after the first use after a period of inactivity), or there is an electrical problem (circuit breaker in the distribution board). A detailed guide on diagnostics can be found in the article Why is cold water flowing even when the flow heater is turned on – causes and solutions.
What temperature should be set on the heater for regular home use?
For the kitchen sink, I recommend setting the heater regulation to maximum (or unlimited) and controlling the temperature exclusively via the tap lever – this is the philosophy of the HAKL MK set. The heater heats the water to the maximum possible temperature for the given flow rate, and the lever adjusts (closes) the flow to regulate the final temperature. The thermostat on the heater is used only as a protection against scalding (the recommended maximum temperature for a household with children is 45 °C).
What to do if the heater does not work after installation?
Step one: check the circuit breaker and residual current device in the distribution board. Step two: fully open the tap lever and check if water is flowing (the flow rate must be at least 2–2.5 l/min). Step three: check whether the thermostat protection on the heater has been reset (a small red button on the device body, accessible after opening the cover). Step four: verify the voltage at the heater terminals using a multimeter. A detailed troubleshooting procedure can be found in the article Common faults of thermostatic mixing valves with lever taps and how to eliminate them.
What is the lifespan of the HAKL MK IconicLine set?
Do you have a question on this topic?
Having trouble deciding or dealing with a specific situation in your household? Write to us – we are happy to help.