What capacity of a flow water heater do I need for a sink, bathtub or wall-mounted faucet
Why the power of a flow heater is not a matter of chance
When a customer comes for the first time with a request "I want a flow heater above the sink", most people think it is enough to choose any model, install a tap, and the problem is solved. In practice, it is not that simple. Over fifteen years of customer experience, I have seen dozens of cases where the heater was installed, water was flowing from it, but people complained about cold water while washing dishes, unpleasant lukewarm baths, or uncomfortable showering. The cause was almost always the same: a poorly chosen power.
The power of a flow heater is a key parameter from which everything depends – the water temperature at the outlet, comfort in everyday use, and also the requirements for the electrical installation. In this article, we will thoroughly examine how many kilowatts you actually need for a sink, a bath, or a wall-mounted tap, and why it is not wise to rely only on gut feeling or a neighbor's advice.
Basic physics: what the heater power actually means
A flow heater works on a simple principle: cold water flows through a heating element, where it is heated and immediately flows out. Unlike a storage water heater, there is no thermal reserve here – the heater must heat exactly as much water as flows through it at that moment. This is where physics comes into play.
The basic formula for calculating the required power looks like this:
P [kW] = Q [l/min] × ΔT [°C] × 0.0698
where Q is the water flow rate in liters per minute and ΔT is the difference between the desired outlet temperature and the temperature of the cold water from the tap. The constant 0.0698 comes from the specific heat capacity of water.
Practical example: water from the tap is 8 °C in winter and you want 38 °C at the outlet. The temperature difference is 30 °C. If the water flows at 4 l/min (a typical comfortable flow when washing hands), the calculation gives: 4 × 30 × 0.0698 = 8.38 kW. That is relatively high. Why then do most people manage with a heater of 3.5–5.5 kW? Because the actual flow rate with flow heaters is significantly lower – the heater itself limits the flow so that it can sufficiently heat the water. The water temperature is therefore the result of a balance between power and flow rate.
Input water temperature: a parameter that people underestimate
This is something that surprises customers. The temperature of the water from the tap changes significantly throughout the year. In summer, it can be 15–18 °C, in winter it drops to 6–8 °C in some regions, and in houses connected to wells, it can be even lower. This difference directly affects the comfort the heater provides.
Specifically: a 3.5 kW heater at a flow rate of 2.5 l/min can raise the water temperature by about 20 °C. In summer (input 16 °C), you get 36 °C at the outlet – pleasant. In winter (input 8 °C), you get 28 °C – lukewarm water that does not satisfy everyone when washing dishes in warm weather, but for washing hands, most people can manage. Therefore: the more northern the location, the older the water pipe buried shallowly, or the longer the pipe from the supply, the higher the power you need to be comfortable even during the cold months.
From the graph it is clear that at a lower inlet water temperature, the difference between 3.5 kW and 5.5 kW is very significant. The comfort threshold for washing is usually around 36–38 °C. The higher-powered model achieves this even in winter, while the weaker heater under the same conditions will end up with just lukewarm water. Therefore, I consistently recommend choosing a more powerful model to customers in detached houses or older buildings with poor pipe insulation.
Basin Mixer: How many kW do I actually need above the kitchen sink
This is the most common installation scenario. A kitchen sink is used for washing dishes, vegetables, and hands – the flow rate is relatively low and the required temperature is 38–45 °C depending on the activity. Compared to showering or filling a bathtub, the demand here is lower.
For a standard kitchen sink with one basin faucet, the following recommendations apply:
- 3.5 kW – sufficient for most households where the water supply is not deeply frozen, the inlet water temperature is at least 10–12 °C, and very hot water is not required (e.g., for washing greasy dishes). It is also suitable where you want only basic comfort and the electrical circuit is limited.
- 4.5 kW – recommended power for most households as a reasonable compromise. It covers the winter period with an inlet temperature around 8 °C, without a significant drop in comfort.
- 5.5 kW – rarely installed above a sink, only exceptionally. Typically in an extension with a long pipe run, or where the owner prefers very hot water (45+ °C) for degreasing dishes.
From practical experience: HAKL PL SET 3,5 kW ohr. s drezovou klas. bat., 20cm IconicLine has proven itself well in panel buildings, where the water supply has a more consistent temperature and the installation is close to the main line. Customers are satisfied even in winter, considering that the inlet temperature in panel buildings rarely drops below 10 °C. In detached houses with very long supply lines from the water meter, I recommend rather HAKL PL SET 4,5 kW ohr. s drezovou klas. bat., 20cm IconicLine – the same shape, the same IconicLine basin mixer with a 20 cm spout, but with significantly higher thermal power.
An important note to conclude this section: the basin mixer in these sets is dimensioned for one point of use. If you plan to connect another outlet (e.g., a dishwasher or a second faucet), the flow heater for hot water will not solve it – you need a different solution (a storage tank or a central hot water system).
Bathtub Mixer: here the power really matters
Filling a bathtub is the most demanding scenario in terms of power. Why? Because a bathtub has a volume of 150–200 liters and most people want to fill it in a reasonable time – not in an hour, but in 20–30 minutes. This means a higher flow rate and a longer operating time for the heater.
Let's calculate a specific example: you want to fill a 150-liter bathtub to 40 °C. The inlet water temperature is 10 °C. The required thermal power at a flow rate of 6 l/min (a decent filling speed, bathtub in ~25 minutes) would be: 6 × 30 × 0.0698 = 12.56 kW. This is much more than a standard flow heater can provide at 230 V.
This is the basic limitation of flow heaters when filling a bathtub: they are not an ideal solution for filling an entire bathtub with hot water under normal grid power. They work at a lower flow rate (2–3.5 l/min), which prolongs the filling time, or they fill the bathtub to a lower temperature and then top it up with cold water. Despite this, these sets are very popular in practice where there is no central hot water system and a storage water heater is not possible (due to space or construction limitations).
For bathtubs, two power levels are available:
- 4.5 kW – an acceptable choice when the inlet water temperature is 14 °C or higher, or if a slightly longer filling time or lower water temperature (37–39 °C) is acceptable. Suitable for summer periods and warmer regions, or for apartments where the water supply runs through heated areas. Take a look at HAKL PL SET 4,5 kW ohr. s vaňovou klas. bat., 18/150cm IconicLine.
- 5.5 kW – I recommend this as the first choice for a bathtub whenever it is electrically feasible. It provides significantly better results in winter, greater comfort, and shorter filling times. The installation requirements are higher (a suitable circuit breaker and cable cross-section are required), but the difference in daily comfort is immediately noticeable. HAKL PL SET 5,5 kW ohr. s vaňovou klas. bat., 18/150cm IconicLine is the most popular model in this category in practice.
One important thing from customer experience: with a bathtub mixer, the spout is positioned low above the edge of the tub and the water flows slowly for a longer period. Customers accept this as normal if they are aware of it beforehand. Problems arise when the customer expects the bathtub to fill as quickly as from a central system – this is realistically not possible. Properly setting expectations before installation prevents 90 % of complaints.
Standing mixer: a specific case that is often underestimated
A standing mixer (also called free-standing, free-standing or long-arm) is installed next to the bathtub or on the bathroom floor. In terms of heater performance, it usually has the same water volume as a sink mixer, but under different usage conditions. A standing mixer is used either for filling the bathtub (slower flow, longer time) or for showering with a handheld shower head.
Interesting fact: a standing mixer usually has a lower flow rate than a bathtub drain, as it is structurally designed for a gentler flow. In practice, this means that a 3.5 kW heater is able to heat the flowing water to an acceptable temperature even at an inlet temperature of 10 °C.
For example, you can find the HAKL PL SET 3.5 kW heater with a standing classic mixer, 22 cm IconicLine in our range – a set specifically designed for this type of application. The 22 cm arm of the mixer is longer than in the sink version, which suits larger sinks or free-standing bathtubs.
Practical tip: a standing mixer is often installed next to renovated or retro bathtubs, where the customer wants to preserve the aesthetics. In this case, a 3.5 kW power is fully sufficient if the bathtub is not filled entirely through the standing mixer, but only topped up. If the customer wants to fill the bathtub exclusively through the standing mixer with 150 liters of hot water, I recommend choosing the 4.5 kW version (if available) or warn the customer in advance about the longer filling time.
Overview table: power vs. installation location
| Installation location | Min. power | Recommended power | Note |
|---|---|---|---|
| Kitchen sink (electric hob) | 3.5 kW | 3.5–4.5 kW | Water inlet usually above 10 °C even in winter |
| Kitchen sink (family house, well) | 4.5 kW | 4.5 kW | Cold water can be 6–8 °C even in summer |
| Bathroom sink | 3.5 kW | 3.5–4.5 kW | Low flow, short-term use |
| Bathtub (summer period / warmer region) | 4.5 kW | 4.5 kW | Expect slower filling |
| Bathtub (year-round use, northern Slovakia) | 5.5 kW | 5.5 kW | Requires a 25 A circuit breaker and 4 mm² cable |
| Standing mixer (retro bathtub, sink) | 3.5 kW | 3.5 kW | Lower mixer flow, smaller water consumption |
Electrical requirements: what your installation must handle
This is the second side of the coin that customers often overlook. The power of the heater is not only a matter of comfort, but also of real electrical installation requirements. Most older apartments have an electrical wiring system designed for appliances up to 3.5–4 kW on one circuit. If you want a 5.5 kW heater, you must check in advance:
- The circuit breaker must be properly dimensioned – for 3.5 kW (230 V) a 16 A single-phase circuit breaker is sufficient, for 4.5 kW we recommend 20 A, for 5.5 kW a 25 A circuit breaker and a separate circuit are required.
- Cable cross-section – 3.5 kW: min. 2.5 mm², 4.5 kW: min. 2.5 mm² (for shorter distances), 5.5 kW: min. 4 mm² for longer distances.
- A separate circuit without other appliances – the flow heater must not be connected to the same circuit as the washing machine, electric stove or other high-power appliances.
- Earthing and protective conductor – mandatory for all powers, without exception.
We will discuss this topic in more detail in the article What electrical requirements does a 3.5–5.5 kW flow heater have and what must your home installation meet in the Knowledge Center. If you have doubts about the capacity of your installation, always consult an electrician before ordering the heater – changing the circuit breaker and cable costs a few extra euros, but it avoids disappointment or, worse, a safety risk.
Water pressure and flow heater: another factor that affects the result
Along with the inlet temperature and power, the water pressure in the network also plays a role. Flow heaters have a minimum starting pressure – usually 0.1–0.15 MPa (1–1.5 bar). If the pressure is lower, the heater will not turn on at all. On the contrary, very high pressure (over 0.6 MPa) can damage internal components or cause unpleasant noise. In practice, most city water supplies have a pressure of 2.5–4 bar, which is ideal for these heaters.
Low pressure issues typically occur in older homes with outdated plumbing, in mountainous areas, or at the end of a water supply branch. In such cases, the heater either does not work at all or turns on and off unstably. The solution is either to increase the pressure (pressure tank, pumping system) or to choose a model with a lower starting pressure.
If you are in doubt, measuring the pressure at home is a simple task – a manometer for 10 EUR from any hardware store can be attached to a garden hose connection or to the washing machine drain valve. Customers who verified this in advance had no surprises after installation.
Why a set (heater + tap) is more advantageous than buying separately
Practical question: why buy a set when I can buy the heater and the tap separately? There are several reasons, and they are not just marketing:
- Hydraulic compatibility – the tap in the set is pre-tested and adjusted for the heater's power. The flow and resistance of the tap correspond to the heater's heating power so that the resulting water temperature is comfortable at a normal tap opening.
- Mechanical compatibility – the tap connections (thread, placement) are dimensioned precisely for the respective heater series. There is no risk that the tap will not fit or that a reducing bushing will be needed.
- Visual consistency – the tap and heater are from the same series (here IconicLine), so they visually match. With separate purchase, it is a possibility, but not a certainty.
- Warranty service – a complaint procedure is simpler when both parts are from one order and from one manufacturer.
In practice, I have seen several cases where a customer bought heater X and tap Y, and the water was either too hot (tap with low flow resistance) or too cold (tap with high resistance). A set eliminates this problem.
Seasonality and intermittent use: when a heater may not be enough
Flow heaters are excellent as a year-round solution where it is the only source of hot water. However, you should keep in mind that in winter, the power at the same flow rate is not enough to produce the same hot water as in summer. This is not a product flaw – it is a physical reality. Customers who are aware of this have no problem: it is enough to slightly reduce the flow from the tap in winter so that the water has more time to heat up.
Regarding seasonality, here is another practical tip: if you plan to use the heater only seasonally (for example, in a cottage only in summer), a lower power is sufficient, as the inlet water temperature will be higher. A cottage owner who opens the cottage in July after winter has 14–16 °C in the well, and a 3.5 kW heater will provide pleasant water without problems. The same customer in a permanently occupied house would be disappointed with lukewarm water in December with a 3.5 kW heater.
More on how to choose a specific set and what to compare before purchasing can be found in the article How to choose a flow heater + classic tap set: what to pay attention to in the Knowledge Center, as well as 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 examples from real customer cases
Case 1 – Panel apartment, Bratislava, 2-room: The customer wanted to solve the issue of cold water at the kitchen sink after replacing the central system. The apartment is on the sixth floor, with an inlet water temperature of about 12 °C in winter. We installed a 3.5 kW sink set. Result: satisfaction, water temperature of 34–36 °C at full tap opening in winter, which is sufficient for dishwashing. The customer would have also been satisfied with 4.5 kW, but 3.5 kW fully met the need without the need to change the circuit breaker.
Case 2 – Family house, Orava, new bathroom: The customer wanted to provide hot water for the bathtub in the house without a central boiler (as the boiler was only for heating, not for hot water). The inlet water temperature from the well in December was measured at 7 °C. We set up a 5.5 kW bathtub set. Result: the bathtub fills slowly (~35 minutes for 120 liters), temperature 37–38 °C. The customer was warned in advance about the filling time, which he accepted. He was not satisfied when we initially recommended 4.5 kW – at this inlet temperature, it would have been a borderline situation.
Case 3 – Bathroom renovation, retro bathtub with a wall-mounted tap: The customer had a freestanding bathtub and wanted a wall-mounted tap with a flow heater. The requirements were mainly aesthetic, functionally it was sufficient for handwashing and occasional filling of the bathtub (not always). A 3.5 kW set with a 22 cm wall-mounted tap was an ideal solution – quiet operation, neutral aesthetics, sufficient power for regular use.
Most frequently asked questions (FAQ)
Is a 3.5 kW heater sufficient for a kitchen sink, or is a 4.5 kW better?
A 3.5 kW heater is sufficient for a kitchen sink in an apartment building in most cases. If you live in a family house where the well water can be 6–8 °C in winter, or if you require really hot water for degreasing dishes, go for 4.5 kW. The price difference of the set is minimal, but the comfort difference in cold weather is felt every day. In case of doubts, always choose a higher power.
Can I fill a 150-liter bathtub with hot water using a 4.5 kW flow heater?
Yes, but expect a longer filling time – typically 35–50 minutes depending on the inlet water temperature. The heater works at a lower flow rate to allow the water to reach the desired temperature. If you want to fill the bathtub quickly (within 20 minutes), a flow heater is not an ideal solution – you would need a power of over 10 kW, which requires three-phase power. A 4.5–5.5 kW flow heater is, however, a full-featured solution where there is no other option.
What if I have only 16 A circuit breakers in my electrical panel? Can I install a 5.5 kW heater?
Not directly. A 5.5 kW heater at 230 V draws a current of ~23.9 A, which a 16 A circuit breaker would trip – and that is even a conservative calculation (in the inrush phase, the current may be higher). For 5.5 kW, you need at least a 25 A circuit breaker and also verify the cable cross-section (at least 4 mm²). If the panel allows the replacement of the circuit breaker, it is solvable – but only by an electrician. If your home installation does not allow a 25 A circuit, consider a 4.5 kW (20 A) or 3.5 kW (16 A) solution.
Is it worth buying a higher power heater "for the future" in case the lower one is not enough?
In principle, yes, but only if the electricity is dimensioned. Buying a 5.5 kW heater for an apartment with a 16 A circuit breaker does not make sense – you will never run it at full power. On the contrary, if you have a 25 A circuit available or can create one at a reasonable cost, a 5.5 kW heater is a wise choice even for a sink, as it ensures comfortable water temperature even in extreme cold and provides room for adding another outlet in the future.
What is the difference between a sink, bathtub, and wall-mounted tap in a set – can they be interchanged?
Each type of tap is structurally designed for a specific installation location. A sink tap has a shorter spout (20 cm) and a different type of outlet than a bathtub tap (different flow, different type of aerator). A wall-mounted tap has a longer neck and a special base. Interchangeability is limited – the connections may be the same (1/2" threads), but the spacing, lengths, and design are different. Always buy a set intended for the specific place of use. For more detailed information on tap types and their dimensions, read the article Sink, bathtub, or wall-mounted tap: which set type is suitable for my bathroom or kitchen.
How long does a flow heater last and what shortens its lifespan?
With proper installation and regular maintenance, the lifespan ranges from 8–15 years. The most common killer of flow heaters is hard water – lime deposits settle on the heating element and reduce its efficiency until it burns out. Regular descaling (once a year for hard water) significantly extends the lifespan. More information on prevention and care can be found in the article Maintenance and cleaning of a flow heater with a classic tap: how to extend the lifespan of the set.
Conclusion: Choose the power with care, not based on price
The power of a flow heater is ultimately a simple equation: the more water you need to heat, the colder the incoming water is, and the higher the temperature you require, the higher the power you need. For a kitchen sink in an apartment, 3.5–4.5 kW is sufficient. For a bathtub, I recommend 5.5 kW whenever electricity allows, while a shower unit usually requires only 3.5 kW.
Do not decide based on price alone. The difference between a 3.5 kW and a 5.5 kW set in the HAKL PL range is just a few dozen euros, but the difference in comfort during daily use in the colder months is huge. Also, definitely check your electrical installation before placing an order – this is a step that will save you a lot of trouble after installation.
If you are unsure about the selection, take a look at the specific product sheets for the 3.5 kW sink set, 5.5 kW bathtub set, or 3.5 kW set with a shower unit, where you will find exact technical specifications including flow rates, connection spacing, and electrical requirements for each model.
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