What Output Do I Need from a Condensing Boiler with DHW for My House

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What power condensing boiler with domestic hot water do I need for my house?

This is one of the most common questions we encounter. And no wonder – buying a condensing boiler with instantaneous domestic hot water (DHW) heating is an investment for 15 to 20 years, and a wrong choice of output will bother you every day. Either the boiler will be oversized, cycling unnecessarily, wearing out faster, and you won't save as much as the salesperson promised. Or it will be undersized – heating won't be enough on freezing days, hot water will run lukewarm, or whoever is first to shower gets the cold water. You don't want either.

In this article, we'll go through the whole topic from the basics. We'll show you how boiler output is actually calculated, what the logic behind the numbers on the labels is, and what the difference between an 18 kW and a 26 kW boiler means in practice for an average family home. We'll go into depth – including things most salespeople don't mention.

Why boiler output isn't one number, but two

When you look at the technical data sheet of a condensing boiler with instantaneous DHW heating, you'll see two numbers for output. For example "18/25 MKV" or "20/26 KKV". This is not a coincidence or a marketing trick – these two numbers have a very specific technical meaning.

The first number is the heating output in kW – i.e. the maximum heat output the boiler is able to deliver to the heating system (radiators, underfloor heating). This number determines whether the boiler can heat your home on the coldest day of winter.

The second number is the output for domestic hot water preparation in instantaneous (flow-through) mode. In instantaneous heating, the boiler briefly switches all its output to heating water – and this is where it needs higher output, because it must heat cold water (typically 10–15 °C) to 45–55 °C in real time, without a storage tank.

These two outputs are therefore independent requirements that must be addressed separately. Most mistakes in boiler selection arise precisely because the customer (or even the salesperson) considers only one of them.

How to calculate the required heating output

The boiler's heating output must cover the house's heat losses at the design outdoor temperature. In Slovakia, the standard calculation uses –12 °C for lowland areas (Bratislava, Nitra) or –15 °C and lower for mountainous regions (Banská Bystrica, Žilina, colder parts of Košice). A precise calculation is made by a designer according to the STN EN 12831 standard, but for practical orientation purposes, it's enough to know a few key values.

Approximate calculation based on specific heat output

The simplest way is to base it on the specific heat output per m² of floor area. This number varies depending on the quality of insulation:

Building type / insulation	Specific output [W/m²]	Example: 150 m² house
Old house without insulation (before 1990)	80–100 W/m²	12–15 kW
House with partial insulation (facade + windows)	50–70 W/m²	7.5–10.5 kW
New building – low-energy standard	30–50 W/m²	4.5–7.5 kW
Passive house or near-zero energy consumption	15–25 W/m²	2.3–3.8 kW

From the table, you can see that most family houses in Slovakia with standard insulation (windows + facade, but not passive standard) and an area of 130–160 m² need a boiler heating output somewhere in the range of 8–14 kW. This is an important number – most condensing boilers with DHW have a heating output of 18–24 kW, which is significantly more than these houses actually need. Nevertheless, this is not a problem, because modern condensing boilers operate with modulation – they can reduce output down to 20–30% of maximum.

What is modulation and why is it important

Burner modulation is the boiler's ability to smoothly regulate output according to current demand. If a boiler can operate for example from 4 to 24 kW, it will run at 6–8 kW on a mild winter day, and only reach maximum on the coldest days. This avoids unnecessary cycling (switching on and off), which wears out the combustion chamber, heat exchanger and other components.

That's why we recommend: don't buy a boiler that's "just right" – for heating, it's better to have a slightly higher output with good modulation than a boiler that has to run at 95% all the time.

How to calculate the required output for instantaneous DHW heating

This involves slightly different math. Instantaneous water heating works on the principle: the boiler must heat the water passing through in real time from the supply temperature (typically 10–15 °C) to the temperature you set (45–55 °C). The higher the water flow rate, the higher the output the boiler needs.

The formula is simple:

P [kW] = flow rate [l/min] × temperature difference [°C] × 0.07

Specific examples:

Shower (8 l/min), heating from 12 to 45 °C → 8 × 33 × 0.07 = 18.5 kW
Bathtub (12 l/min), heating from 12 to 50 °C → 12 × 38 × 0.07 = 31.9 kW
Economical shower (6 l/min), heating from 15 to 40 °C → 6 × 25 × 0.07 = 10.5 kW

This leads to a key point: if you want a comfortable shower without compromises, the boiler needs at least 20–24 kW of output for DHW. For filling a bathtub without a long wait, you need 28–32 kW. That's why most boilers with instantaneous heating have a DHW output of 24–28 kW – this is the "golden middle ground" for an average household with one bathroom.

Effect of inlet water temperature on output

This is a detail that is rarely mentioned but plays a big role in practice. In summer, cold water from the mains in Slovakia has a temperature of 16–18 °C. In January, it can be as low as 6–8 °C. The difference is 10 °C, which means that in winter you need 10–15% more output for the same comfort. A boiler that is sufficient for a comfortable shower in summer may produce somewhat lukewarm water in January. That's why it's wise not to calculate with summer conditions, but with the winter scenario, when choosing a boiler.

Practical examples: what boiler for what house

Theory is nice, but it's best understood with specific examples. These are the types of situations we encounter repeatedly:

Case 1: Older apartment building, 2-room flat, 55 m², 2 people, 1 bathroom

Heat losses with standard insulation of an older flat: approx. 3.5–5 kW. Shower once a day, or bath 2× a week. Here a boiler with a heating output of 12–15 kW and a DHW output of 20–24 kW would be sufficient. In practice, for example, the Protherm Puma Condens 18/24 MKV is installed in flats – its 18 kW for heating is oversized for a flat, but modulation lets it drop down to 4–5 kW, so it operates efficiently. The 24 kW DHW output easily handles a comfortable shower.

Case 2: Family house from the 1990s, 130 m², 4 people, 2 bathrooms

This is probably the most common scenario in Slovakia. An older house with replaced windows and an insulated facade has a specific output of around 55–65 W/m², which for 130 m² gives 7.2–8.5 kW. But watch out – we have 4 people and 2 bathrooms here. A situation where two people shower almost simultaneously in the morning is not unusual. However, an instantaneous boiler usually supplies DHW from a single point – it can't fully supply two taps at once. For such households, we recommend either a boiler with a DHW output of 28 kW and accepting some pressure drop during simultaneous draw-off, or considering storage-tank heating (more on that in the article Condensing boiler with instantaneous DHW heating vs. storage tank DHW heating). A solid choice for this house is the Protherm Gepard Condens 18/25 MKV – 18 kW for heating with modulation from 4 kW, 25 kW for DHW.

Case 3: New low-energy-standard building, 160 m², 4 people, 2 bathrooms

The specific output of a low-energy new building is 30–40 W/m², which for 160 m² is 4.8–6.4 kW. Heating demands are therefore minimal. Here, boiler selection is largely determined by the DHW requirement. For 4 people with two bathrooms, the Vaillant VUW 236/5-3 ecoTEC pro or Vaillant VUW 26CS/1-5 ecoTEC plus IoniDetect is a good fit. Both have a DHW output of 26 kW, which is optimal for this type of household. The Vaillant VUW 26CS/1-5 also features IoniDetect technology – ionization-based monitoring of the combustion process, ensuring higher reliability and a longer service life.

Case 4: Larger family house, 200 m², partial insulation, 5 people, 2 bathrooms + shower cubicle

Heat losses: 200 m² × 60 W/m² = 12 kW. DHW: 5 people, 3 draw-off points. Here a boiler with a heating output of 20–24 kW and a DHW output of 26–28 kW is suitable. The Protherm Panther Condens 20/26 KKV is exactly right for this segment – 20 kW heating, 26 kW DHW. Modulation from 4 kW ensures efficient operation even in transitional periods.

What else affects the choice of output – less obvious factors

Type of heating system: radiators vs. underfloor heating

Underfloor heating operates with low temperatures (30–40 °C), which is ideal for a condensing boiler – condensation works most efficiently precisely at low return temperatures (below 57 °C). For a radiator system, it depends on the type of radiators: old cast-iron radiators need 70–80 °C, which is not favorable for condensation. New panel radiators only need 55–60 °C. The lower the temperature of the heating system, the higher the condensation effect and the lower the gas consumption.

For boiler output, this means: with underfloor heating, you can go with a slightly lower boiler output, because lower temperature gradients allow more efficient heat transfer through the floor area. With old radiators requiring high temperatures, it's wise to have a reserve in output.

Ceiling height and room volume

The approximate calculation based on m² of floor area works well for a standard ceiling height of 2.5–2.7 m. If you have high ceilings (3 m or more, for example in old Art Nouveau flats or in halls), you need to recalculate based on m³ of room volume and add a correction factor. For a 3 m ceiling, increase the output by at least 15% compared to the calculation based on area.

Orientation and glazing of the house

North-facing facades, large glazed areas, skylights in winter – all of this increases heat losses. A house with an extensive north-facing glazed wall can have heat losses 20–30% higher than a simple calculation based on area and specific output would suggest. This is exactly the case where a professional thermotechnical calculation from a designer is necessary.

Boiler room vs. installation in a flat – impact on DHW comfort

The length of hot water piping from the boiler to the draw-off points significantly affects comfort. If the boiler is in a basement boiler room and the bathroom is on the third floor, cold water runs from the pipes for the first 10–30 seconds before hot water arrives. This is not a matter of boiler output, but of hydraulics. The solution is a DHW circulation pump (circulation loop), which keeps hot water in the pipe permanently. Most quality condensing boilers have an input for a DHW circulation pump.

Oversizing vs. undersizing: real consequences

We've mentioned oversizing – a boiler with modulation handles it better than an old on/off boiler. But there are also situations where oversizing really does cause harm:

Passive and low-energy houses: If the house needs 3 kW and the boiler can't go below 7 kW, it will cycle even at minimum output. This increases wear, reduces service life and ruins the condensation effect.
Underfloor heating with low temperatures: Excessive output causes rapid overheating and frequent shutdowns.
Small flats: In a 50 m² flat with modern insulation, a 24 kW boiler has no business being there – wasted money on purchase and potentially higher failure rate.

Undersizing is the other extreme. Symptoms include:

Hot water is too lukewarm, especially with higher flow rates (bathtub, two taps simultaneously).
The house doesn't heat up to the required temperature on freezing days (below –10 °C).
The boiler runs at full output almost continuously – the heat exchanger and burner wear out faster.

Practical recommendations for selection – step-by-step approach

To conclude this section, let's summarize the procedure we recommend to customers:

Measure the floor area of the house (heated spaces, not attic/unheated basements).
Determine the insulation condition: Old house without insulation, partial insulation (windows + facade), low-energy standard.
Calculate the approximate heat losses using the table above. Add a 10–15% reserve.
Determine DHW requirements: Number of people, number of bathrooms, preferred flow rate (shower vs. bath).
Take winter into account: Calculate with an inlet water temperature of 8–10 °C, not summer conditions.
Compare the result with the outputs of specific models and verify that the boiler has sufficient downward modulation.
If in doubt, have a thermotechnical assessment done by a designer – an investment of €50–100 pays off with the right boiler choice.

The article How to choose a condensing boiler with instantaneous DHW heating: what to focus on covers the selection in more detail, including other parameters such as gas type, NOx emission class, or smart home compatibility.

Frequently Asked Questions (FAQ)

The boiler has an 18 kW heating output – is that also its DHW output?

No. During instantaneous DHW heating, the boiler switches to a higher output – typically 24–28 kW. This number is always the second one in the boiler designation (e.g. 18/25 = 18 kW heating, 25 kW DHW). The heating and DHW outputs are separate from each other and operate alternately (not simultaneously).

Can I use an 18/24 kW boiler to heat a 200 m² house?

It depends on the insulation. A low-energy 200 m² house only needs 6–8 kW for heating, so an 18 kW boiler with sufficient modulation is more than sufficient. An old uninsulated 200 m² house may need up to 16–20 kW – in that case, an 18 kW boiler would be undersized. You should always base your decision on the actual heat losses, not just the area.

Why is my water lukewarm even though the boiler has 24 kW?

There can be several causes: the DHW temperature set too low on the boiler (check the setting, we recommend 45–50 °C), a secondary heat exchanger contaminated with limescale, too high a water flow rate (tap fully open with low inlet pressure), or cold inlet water in winter. You'll find a more detailed diagnostic guide in the article Common faults of condensing boilers with instantaneous DHW heating and their solutions.

Does the correct boiler size affect actual gas savings?

Yes, significantly. A correctly sized boiler operates within the optimal modulation range and achieves maximum condensation effect. An oversized boiler that cycles on and off loses the condensation effect with every start-up. According to various measurements, a correctly set up and sized condensing system can save 10–20% of gas annually compared to an incorrectly sized one.

Do I need a higher-output boiler for a 150 m² new building because of filling the bathtub?

For a bathtub, you need a DHW output of at least 26–30 kW (at a flow rate of 10–14 l/min). Most boilers in this segment have a DHW output of 24–28 kW. A 24 kW boiler will fill the bathtub more slowly and at a lower temperature; 28 kW is more comfortable. If filling the bathtub is a priority, choose a model with a DHW output of at least 26 kW. For heating a 150 m² new building, 15 kW is sufficient, so heating output is not the limiting factor here.

Is it possible to install a boiler now and increase its output later?

No – the output of a condensing boiler is determined by the hardware (the size of the heat exchanger, burner, flow components). It cannot be changed without replacing the entire boiler. That's why it's wise to think ahead when choosing: are you planning an extension? Will grandparents be living here? Are you considering a bigger bathroom? Take this into account when choosing the boiler now, not in five years.

Conclusion: the right output is a balance, not a maximum

Choosing the output of a condensing boiler with instantaneous DHW heating is not a numbers race. It's not true that "the higher the output, the better" – but neither is "the lower, the cheaper to run." The right choice is one where the heating output matches the actual heat losses of the house (with a slight reserve), the DHW output matches your habits and number of draw-off points, and the boiler has sufficient downward modulation to operate efficiently even in transitional periods.

For most family houses in Slovakia, a boiler with a heating output of 18–20 kW and a DHW output of 25–26 kW is the right choice. For smaller flats and low-energy new buildings, 15–18 kW is enough. For larger houses or houses with higher DHW demands, go for 24/28 kW. And if you're not sure – call us or have a heat calculation done. It's much cheaper than replacing the boiler in five years.

Take a look at our full range of condensing boilers with instantaneous DHW heating and compare the technical parameters of individual models. Every boiler's data sheet also states the minimum modulation output – don't forget to check this number when choosing.

Do you have a question about this topic?

Can't decide or dealing with a specific situation in your household? Write to us - we'll be happy to help.