What capacity of a flow water heater do I need for a sink, bathtub or shower
What power of flow heater do I need for a sink, bathtub or shower?
This is one of the most common questions I encounter during consultations with customers. And it's no wonder – the power of the flow heater is a key parameter that determines whether you will have enough hot water for daily use or whether you will suffer from a cold shower in the middle of your morning hygiene. At the same time, it is true that an over-dimensioned heater unnecessarily increases the cost of the electrical installation and the unit itself, while an under-dimensioned one will frustrate you every day.
In this article, I will explain how to correctly calculate the required power for each type of water consumption – sink, bathtub and shower – and I will show you practical examples, including what to watch out for during renovations of older apartments.
Basic physics: how power relates to temperature and water flow
To understand why the power of the heater is so important, you need to know the basic physical relationship between power, flow and temperature difference. A flow heater heats water continuously – this means that it does not have a tank where it preheats the water. It heats it exactly at the moment it flows through. The higher the flow (more liters per minute) and the greater the temperature difference you want to achieve (e.g. from 10°C inlet temperature to 40°C outlet temperature), the more power you need.
An approximate formula looks like this:
P (kW) = Q (l/min) × ΔT (°C) × 0.07
where Q is the water flow in liters per minute and ΔT is the difference between the desired outlet water temperature and the temperature of the cold water from the network. The constant 0.07 includes the specific heat of water and unit conversion. This formula is simplified, but it is sufficiently accurate for practical purposes.
As you can see from the graph, at a higher temperature difference (typically winter, when cold water from the network is around 8–10°C) you need significantly more power than in summer, when cold water is usually 15–18°C. This factor is often underestimated by many customers, and then they are surprised that the heater barely heats the water to a comfortable temperature in January.
Kitchen or bathroom sink: is 4.5 kW enough, or do you need more?
A sink is the least demanding of all water outlets in terms of heater power. The reason is simple: when washing dishes, vegetables or hands, we do not need a high flow, and usually not an extremely high temperature. A typical flow at a sink faucet ranges from 4–6 liters per minute at normal handle opening, and at fully open outlet it can be up to 8 l/min, but that is more of an exception.
Let's calculate specifically: we want water for hand washing at a temperature of 38°C. In winter the cold water is 10°C, so ΔT = 28°C. At a flow of 5 l/min:
P = 5 × 28 × 0.07 = 9.8 kW
Wait – that's more than 4.5 kW! How is it that a 4.5 kW heater works at a sink faucet? The answer is that with a flow heater, we temper the water by mixing hot and cold water. The heater produces water at a maximum temperature (at 4.5 kW and a flow of 3 l/min it is around 55–60°C), and the faucet mixes this hot water with cold to the desired temperature. Therefore, the actual flow of hot water is much lower than the total flow from the faucet.
The following applies to a standard sink in a kitchen or bathroom:
- 4.5 kW – sufficient for a single sink if the heater is mounted directly at the faucet (short pipe, minimal losses). Typically for a kitchen sink with a separate apartment water supply.
- 5.5 kW – a safe choice for a sink where you want to have a reserve even in winter, or if the pipe between the heater and the faucet is longer than 1 meter.
In practice, I have encountered cases where a customer in an old house with longer piping installed a 4.5 kW unit and in December the water barely reached 30°C. When we replaced it with a 5.5 kW unit, the problem disappeared. Therefore, I also recommend for a sink, if you have the option and the electrical supply allows it, to go for HAKL MK SET 5.5kW heater with a sink faucet, 18cm IconicLine, which provides a comfortable reserve without the need to over-dimension the electrical connection.
If your apartment's electrical supply is limited (an older apartment with aluminum electrical wiring, fuses up to 20A), HAKL MK SET 4.5kW heater with a sink faucet, 18cm IconicLine is a solution that fits even on a single-phase circuit with 20A protection and will correctly fulfill its function for a sink.
Shower: the performance boundary between comfort and frustration
A shower is a more interesting case in terms of heater sizing than a sink. Here, two opposing requirements meet: the customer wants a strong, pleasant water flow, but at the same time does not want to invest in an overly expensive electrical installation. And it is precisely here that mistakes are most frequently made.
A standard shower outlet (a classic shower head without massage or rain functions) has a flow rate of about 8–12 liters per minute when the tap is fully open. Modern water-saving shower heads have a flow rate of 6–8 l/min. Rain showers (large heads 30×30 cm) can have a flow rate of up to 15–20 l/min.
For a comfortable shower at 39°C and a flow rate of 8 l/min in winter (ΔT = 29°C):
P = 8 × 29 × 0,07 = 16,2 kW
Does this mean that we need a 16 kW heater for a comfortable shower? Of course not – again, the heater produces hot water, which is mixed with cold water in the tap. The actual load on the heater depends on the mixing ratio. If the heater produces water at 55°C and you want 39°C, you will mix approximately 60% hot and 40% cold water. Therefore, the real flow rate of the hot branch is only 4.8 l/min.
Practical result: for a standard shower with a classic head, a heater with a power of 5.5 to 7 kW is sufficient, with the following considerations:
- 5.5 kW – suitable if you use a water-saving shower head (total flow of 6–7 l/min), or if you are in an area with warmer tap water (preheated in housing estates, summer months)
- 7 kW – a comfortable choice for a standard shower with a normal head, reliable even in winter with cold water from 8°C
- 9 kW and more – necessary for rain showers with large heads, or if you want a really strong water flow without any restrictions
In practice, I helped a family who renovated their bathroom and installed a large 25×25 cm rain shower. The electrical installation was pre-prepared for 7 kW. The result was that when the shower was fully open, the water was only lukewarm – the flow was too high for a 7 kW heater to effectively heat. We either had to overdimension the electrical system to 9 kW, or the customer had to limit the flow with a regulator. Lesson learned: if you plan a shower with a large head, plan for 9 kW and adjust the electrical system accordingly before tiling the bathroom.
Bathtub: the most demanding case in terms of power and time
A bathtub is by far the most demanding of all water outlets – and this is due to two reasons. First is the flow rate: when filling a bathtub, you want to fill 150–200 liters of water as quickly as possible, which means a maximum flow rate from the tap, typically 12–18 liters per minute. The second problem is temperature: water in the bathtub cools down quickly, so you fill it at a higher temperature (40–45°C), which increases the demands on the heater.
Let's calculate for a bathtub: flow rate 15 l/min, desired temperature 42°C, cold water in winter 10°C. Again, we are talking about the total flow of mixed water. A heater producing 55°C water must provide about 65% of the total flow, i.e., 9.75 l/min of hot water. To heat this water from 10°C to 55°C (ΔT = 45°C), we need:
P = 9,75 × 45 × 0,07 = 30,7 kW
This clearly shows why a flow heater is not an ideal solution for filling a bathtub at full flow. A 9 kW flow heater when filling a bathtub will operate at maximum capacity, but the flow of hot water will be limited – the tap will have to be set so that most of the water comes from the hot branch, which limits the total flow.
What does this mean in practice?
- 7 kW – it will fill the bathtub, but slowly. With a flow of about 8–10 l/min with the tap set significantly to the hot side. For a 150-liter bathtub, you will wait 15–20 minutes.
- 9 kW – significantly more comfortable. The flow will increase, and the bathtub will fill in 10–14 minutes. It is still not as fast as with a 80-liter storage heater, but it is sufficient for normal use.
- 9 kW is the practical limit for a single-phase connection (230V, 40A) in a standard apartment. For higher power, you would have to use a three-phase connection, which requires completely different wiring.
Therefore, I recommend at least 7 kW for a bathtub – and this is the real minimum for comfortable use. For more comfortable filling, go for the HAKL MK SET 9kW ohr. s vaňovou pák. bat., 18/150cm IconicLine, where the set includes a bathtub tap with a long filling spout optimized for filling the bathtub.
Seasonal influence: why winter is critical for sizing
One of the most important, yet least mentioned factors when choosing the power of a heater is the seasonal influence of tap water temperature. In Slovakia, the temperature of cold water from the network ranges between:
- Winter (December – February): 6–12°C, in older panel buildings with longer piping even lower
- Spring/autumn: 12–16°C
- Summer: 16–22°C, in some areas even up to 24°C
The temperature differential difference between summer and winter can be up to 15°C, which directly affects the heater's effective performance. A 5.5kW heater that heats water to 40°C without any problems in July may barely reach 33°C in January under the same flow rate. Therefore, I recommend dimensioning always according to winter conditions, not according to summer comfort.
Practical tip: if you are unsure about the temperature of your tap water in winter, simply measure it with a thermometer after 30 seconds of cold water flow – this is your real input temperature for winter conditions.
Water pressure and its influence on heater selection
Another factor that receives less attention during consultations is the water pressure in the network. Flow heaters have a defined operating pressure range – typically 0.1 to 0.8 MPa (1 to 8 bar). The minimum pressure is important: if the pressure is too low, the heater will not even turn on (it has a thermostatic switch that requires a certain minimum flow).
On the other hand, high pressure (over 0.6 MPa) increases the flow through the tap, which at the same power reduces the output water temperature. In apartment buildings in Slovakia, pressure typically ranges between 2–4 bar, while in single-family homes it depends on the connection to the network or the height of the water tank.
What this means for power selection: if you know that you have high pressure in the house (over 4 bar), you should consider installing a pressure-reducing valve and at the same time choose a heater with higher power, because higher flow at high pressure requires more energy for heating.
Comparison of HAKL MK set performances for different scenarios
For better orientation, I provide a clear table of recommended sets according to the installation location and typical scenario:
| Point of use | Scenario | Power | Recommended set |
|---|---|---|---|
| Kitchen sink | 1 person, summer apartment, limited electricity | 4.5 kW | HAKL MK SET 4.5kW sink |
| Kitchen/bathroom sink | 2–4 people, year-round use | 5.5 kW | HAKL MK SET 5.5kW sink |
| Shower | Economy head, 1–2 people | 5.5 kW | – |
| Shower + bathtub | Standard use, 2–4 people | 7 kW | HAKL MK SET 7kW bathtub |
| Bathtub | Comfort filling, 3–5 people, winter | 9 kW | HAKL MK SET 9kW bathtub |
| Rain shower / bathtub | Large head, maximum comfort | 9 kW+ | HAKL MK SET 9kW bathtub |
Electrical distribution and protection – what must the apartment wiring meet
The power of the heater is closely related to the requirements for the electrical installation. This is a very important aspect, as many customers first buy a heater and only then find out that their electrical installation cannot handle the load. This topic is more thoroughly covered in the article Electrical connection of flow heaters 4.5 kW, 5.5 kW, 7 kW and 9 kW – requirements for wiring, but the basic rules are as follows:
- 4.5 kW / 230V: single-phase circuit, 25A circuit breaker, cable min. 4 mm² Cu, socket or fixed connection
- 5.5 kW / 230V: single-phase circuit, 25A circuit breaker (recommended 32A), cable min. 4 mm² Cu
- 7 kW / 230V: single-phase circuit, 32–40A circuit breaker, cable min. 6 mm² Cu – this is the limit of single-phase connection
- 9 kW / 230V: single-phase circuit, 40A circuit breaker, cable min. 6 mm² Cu; alternatively three-phase 400V with lower load per phase
Attention: in old panel buildings and apartment buildings from the 60s–80s, there may be aluminum conductors with lower conductivity and an older electrical distribution board that does not allow fitting a 40A circuit breaker without replacement. This should be verified before ordering the heater, ideally with an electrician.
Combination of multiple draw-off points: what if I want to supply two outlets from one heater?
This scenario is more common than it might seem. A customer has a small apartment, a bathroom with a shower and a utility room with a sink. They want one heater to supply both locations. Or: the kitchen and bathroom are right next to each other, and they want to supply both sinks with one heater.
The basic rule is simple: a flow heater supplies as much water as its power allows. When you open two outlets at the same time, the capacity is divided between both, which means lower temperature or lower flow from each of them.
Therefore, for the combination of draw-off points the following applies:
- Sink + sink (simultaneous use): minimum 7 kW, ideally 9 kW
- Shower + sink (parallel use): minimum 9 kW, for a strong rainfall shower I do not recommend it at all without significant overdimensioning
- Bathtub + shower (parallel filling): unrealistic with a flow heater, I recommend a storage heater or a central source
If you are unsure whether your scenario can be handled by a flow heater, read the article How to choose the right set of flow heater + lever mixer for your type of water draw-off, where you will find a more detailed analysis according to various situations.
Why buy a heater + mixer set as a whole?
You might be wondering why not buy the heater and the mixer separately and save money. The reason lies in technical and practical aspects of compatibility. The lever mixers in HAKL MK sets are designed with consideration for the hydraulic requirements of flow heaters – they have the correct internal diameter, correct seating and seals compatible with the heater. At the same time, they solve the installation issue: the heater and the mixer are designed to be installed on the same surface (typically the wall above the sink or next to the bathtub) without the need for extension hoses and complicated adapters.
More about the advantages of sets compared to separate components can be found in the article HAKL MK sets vs. standalone heater with a mixer – what is more cost-effective.
A practical advantage is also in the case of complaints and warranty service: you have one manufacturer, one seller, clearly defined compatibility. From dozens of customer cases, I know that the most frequent problems arise precisely when combining different manufacturers – incorrect opening pressure, leaks at the joints, or a mixer with too high a flow that the heater cannot heat up.
Typical mistakes when choosing power – a lesson from practice
When I retrospectively evaluate hundreds of installations, I identify several recurring mistakes:
- Underestimating the temperature of the incoming cold water in winter: the customer tests the heater in summer and is satisfied. In January, the water barely runs warm. Solution: always dimension according to winter conditions.
- Choosing the cheapest (weakest) heater without considering the flow of the mixer: a standard bathtub mixer has a flow of 15+ l/min at full opening. A 5.5 kW heater cannot comfortably heat this.
- Installation with too long a pipe between the heater and the mixer: every meter of pipe means losses. The heater should be as close as possible to the draw-off point – ideally directly next to the mixer.
- Ignoring water pressure: in a pressurized water supply system (e.g., with a pump), the flow can be unexpectedly high and the heater cannot keep up.
- Purchasing a set without checking the condition of the electrical installation: a 9 kW heater in an apartment with an old 25 A fuse panel simply does not work.
Installation and mounting: where to place the heater for the best effect
The correct position of the heater relative to the draw-off point is not just an aesthetic matter – it directly affects the efficiency of heating. Basic rule: the shorter the route of the hot water pipe from the heater to the mixer, the lower the heat losses and the faster the availability of hot water.
For a sink: install the heater directly above or next to the sink, at most 30–50 cm from the mixer. A longer pipe causes you to have to flush out the cold water in the pipe each time you open the mixer before hot water arrives – which is a waste of water and energy.
For a bathtub or shower: the heater should be mounted on the wall next to the bathtub, ideally at a height of 160–180 cm, so that it is accessible and at the same time out of the reach of the shower. Bathtub sets have a longer connecting hose (150 cm), which provides some flexibility in placement.
A detailed installation procedure can be found in the article Installation of a flow heater set with a lever mixer step by step.
Most frequently asked questions (FAQ)
Is a 4.5 kW heater sufficient for daily showering?
For showering, 4.5 kW is usually not sufficient for a comfortable experience if you use a standard shower head. 4.5 kW is primarily dimensioned for sink draw-off points with a flow of up to 5–6 l/min. For a shower, where you want a stronger flow and a temperature of at least 38°C even in winter, I recommend at least 5.5 kW and ideally 7 kW. With 4.5 kW and a shower, you will have to significantly limit the flow by opening the mixer only slightly, which is not comfortable.
Why does the heater not suffice in winter, even though it works well in summer?
The reason is the temperature differential. The power of the heater is fixed (e.g., 7 kW), but winter cold water has only 8–10°C, while summer water has 18–22°C. With the same power and the same flow in winter, you will achieve an output temperature that is 10–12°C lower than in summer. The solution is either to reduce the flow of the mixer (partially close the lever) or to invest in a heater with higher power. More on this topic can be found in the article Why cold water flows even with a flow heater turned on – causes and solutions.
Can I connect one 9 kW heater to the bathroom and kitchen at the same time?
Technically yes – one heater can supply multiple draw-off points. The problem is simultaneous use: if you open the mixer in the kitchen and the bathroom at the same time, the flow is divided and the output temperature drops from both points. For separate use (one outlet at a time, the other at another time) it works without problems, if the piping is properly dimensioned and the heater is close to both points. For parallel simultaneous use, one 9 kW heater is not an ideal solution.
Is a 5.5 kW bathtub set sufficient, or should I go for 7 kW?
For filling a bathtub, 5.5 kW is on the edge of usability. You will be able to fill the bathtub, but it will take a long time – with a weaker flow (to achieve sufficient temperature) even 25–30 minutes for a 150-liter bathtub. For greater comfort and appropriate filling speed, I recommend at least 7 kW, best the HAKL MK SET 7kW heater with bathtub lever mixer, 18/150cm IconicLine. If the electricity allows, 9 kW is a comfortable choice that you will notice every day.
What is the difference between a sink set and a bathtub set besides the power?
The difference is also in the type of mixer. Sink sets (4.5 kW and 5.5 kW) include a compact sink lever mixer with a standard outlet arm, which is mounted directly on the sink or on the kitchen countertop. Bathtub sets (5.5, 7 and 9 kW) have a bathtub mixer with a longer connecting hose (150 cm) and a different outlet system adapted for filling the bathtub and a shower hose. They are not interchangeable. A more detailed analysis can be found in the article Sink, bathtub or shower lever mixer – what is the difference and which one to choose.
Is it worth buying a heater with a higher power than you currently need, "for the future"?
Generally speaking: if the electricity and budget allow, a slight overdimensioning (e.g., 7 kW instead of 5.5 kW) is a reasonable investment. A heater with higher power works with less load (it does not always operate at 100% power), which prolongs its lifespan and reduces the risk of overheating the heating element. At the same time, you have a reserve for future changes – for example, installing an additional shower, changing the equipment, etc. On the other hand, buying a significantly overdimensioned heater just for that, without a real need, is not worth it – it unnecessarily burdens the electrical installation and increases the purchase costs.
Conclusion: How to choose the right performance without errors
Selecting the correct performance of a flow heater is not rocket science, but it does require answering a few basic questions before purchasing: Where will I install the heater? What is the maximum flow rate of the battery at that location? What temperature
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