What Boiler Output with DHW Heating Do I Need for My House

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What boiler output with domestic hot water heating do I actually need for my house?

This is one of the questions every second customer asks when choosing a new combination boiler. And it's no wonder – output is a number you see in every specification, but few people explain what it really means and how to estimate it correctly for your specific case. Practical experience shows: most people have an oversized boiler. Paradoxically, they buy more "just to be safe" and thus complicate their life and wallet.

In this article, we will examine the topic in detail – from the physical basics through calculation methods to specific recommendations for different types of households. Read it all the way through, because understanding the relationships will save you money when buying and in daily operation.

Two independent output demands of one boiler

A combination boiler with flow-through domestic hot water (DHW) heating must cover two completely different functions. It is important to understand that these functions compete for the boiler's output and each has different characteristics – different duration, different output requirements and different load characteristics.

First function: heating. This is a long-term, slow delivery of heat to the heating system over hours and entire days. The output required for heating depends on the building's heat loss – that is, how much heat escapes through walls, roof, windows and floor to the outside.

Second function: DHW heating. This is a fast, intensive delivery of heat over minutes – when someone turns on the hot water tap. Flow-through heating is short-term, but very output-intensive. The boiler must be able to immediately and continuously heat the water from 10 °C (inlet temperature of cold water) to 40–45 °C at the required flow rate.

Most combination boilers therefore have two different numerical values: the rated output for heating (e.g. 20 kW) and the maximum output for DHW heating (e.g. 24 kW or 28 kW). When choosing, both must be considered.

Calculation of required heating output: the basis is the building's heat loss

The boiler output for heating is correctly determined based on the building's heat loss. This value indicates how many kilowatts of heat the boiler must continuously supply on the coldest day of the year (design outdoor temperature) to maintain 20–21 °C inside. In Slovak conditions, we calculate with a design outdoor temperature of −12 °C to −15 °C depending on the region.

Approximate calculation according to area and building insulation standard

The simplest way – although not the most accurate – is to estimate the heat loss according to the usable area and the insulation category of the house. In practice, we use these approximate specific heat losses:

House category	Specific heat loss (W/m²)	Example: 120 m²	Example: 180 m²
New construction (2015+), low-energy house	30–45 W/m²	3.6 – 5.4 kW	5.4 – 8.1 kW
New construction (2000–2015), well insulated	50–70 W/m²	6 – 8.4 kW	9 – 12.6 kW
Older building, retrofitted	70–100 W/m²	8.4 – 12 kW	12.6 – 18 kW
Older building, minimally or not insulated	100–150 W/m²	12 – 18 kW	18 – 27 kW

From the table it is clear that a modern new construction of 120 m² needs only 4–6 kW for heating. This is a number that surprises many customers – they believe that at least 20 kW is needed for a family house. But 20 kW is an output that applies to DHW heating, not to heating a typical modern building.

Practical example No. 1 – new construction, family of four

The customer has a family house from 2018, usable area 148 m², 14 cm EPS external insulation, triple glazing, attic with 20 cm mineral wool. Approximate heat loss: 148 × 45 = 6 660 W, so about 7 kW. For heating, a boiler with an output of 7–8 kW is sufficient. Since the combination boiler also has a DHW heating function, we choose a boiler with an output of typically 20–24 kW – this number is determined by the DHW requirement, not the heating requirement.

Practical example No. 2 – older brick house, partial renovation

The customer has a house from 1975, brick masonry 44 cm without external insulation, windows replaced with double glazing, only the ground floor insulated. Area 165 m². Heat loss: 165 × 110 = 18 150 W, so about 18 kW. Here, a 20 kW boiler for heating is insufficient with a large margin, at least 20 kW is needed and practically a combination of a 24 kW boiler or a plan for insulation before boiler replacement.

Output for DHW heating: flow rate and number of taps decide

For flow-through heating of hot water, simple physics applies. The output required to heat the water is calculated as:

P (kW) = flow rate (l/min) × temperature difference (°C) × 0.07

Example: we want to heat water at a flow rate of 10 l/min from 10 °C to 45 °C, the temperature difference is 35 °C:

P = 10 × 35 × 0.07 = 24.5 kW

This is a key figure that determines the minimum boiler power required for DHW. In most households, the flow rate during a shower is between 7–10 l/min, and when filling a bathtub it is 12–15 l/min.

From the graph it is evident that for a comfortable shower (8–9 l/min) with heating from 10 °C to 45 °C, the boiler consumes approximately 20–22 kW. This is precisely why combined boilers are typically dimensioned for 20–28 kW – not for heating, but for DHW.

Influence of the incoming water temperature

An important factor that is often underestimated: the temperature of cold water from the tap is not the same throughout the year. In summer it can be 15–18 °C, while in winter it drops to 6–8 °C. In winter, the boiler must overcome a larger temperature difference and the power demand is higher. When dimensioning, always calculate for the most unfavourable scenario – incoming temperature 8 °C, desired output 45 °C, temperature difference 37 °C. If you want a flow rate of 10 l/min in this scenario, you need: 10 × 37 × 0.07 = 25.9 kW.

What impact does the number of bathrooms and people in the household have?

For flow heating of DHW, a simple but important rule applies: the flow boiler supplies the consumption points sequentially, not simultaneously. If someone is showering while another person opens a tap in the kitchen, the boiler's power is divided between both consumption points and the temperature or flow rate at each point decreases.

In practice, this means:

1–2 people, 1 bathroom: Simultaneous consumption is rare. A 20 kW boiler is sufficient.
3–4 people, 1–2 bathrooms: Occasional simultaneous consumption. We recommend 24 kW.
5+ people, 2 bathrooms and more: Frequent simultaneous consumption. 24–28 kW is needed and it is advisable to consider a storage solution or a combined storage system.

A big mistake is to think that a combined boiler with flow heating can handle several simultaneous showers without reducing comfort. It can – but only if it is properly dimensioned for this scenario, which is practically not possible with standard domestic boilers rated at 20–24 kW for two parallel showers.

More on this topic can be found in the article What flow of hot water does a boiler with flow DHW heating provide in the Knowledge Centre, where there are also specific measured flow values for individual models.

Specific models and their performance parameters

Let's look at the practical parameters of boilers available in this category. The choice of a specific model depends on a combination of several factors – power, type of flue gas exhaust, energy efficiency class, and the desired DHW comfort.

Vaillant atmoTEC pro VUW SK 200/3-3 a 240/3-3

Vaillant atmoTEC pro VUW SK 200/3-3 is an atmospheric combined boiler with a rated power for heating of 20 kW and a power for DHW heating of 23.3 kW. Suitable for houses with a heat loss up to 18–20 kW, i.e., for older medium-sized buildings (120–160 m²) or modern larger houses (160–200 m²). For a family of 3–4 people with one bathroom, this model comfortably meets the needs.

Vaillant atmoTEC pro VUW SK 240/3-3 offers 24 kW for heating and a higher power for DHW. It is suitable for larger houses or where there are higher comfort requirements for hot water – for example, households with two bathrooms, where simultaneous use is not common, but comfort during single use is important.

Vaillant turboTEC pro VUW SK 202/3-3 a 242/3-3

Turbo boilers differ from atmospheric boilers in the way flue gases are exhausted and air is supplied – they have a sealed combustion chamber and flue gases are exhausted by forced draft through a coaxial chimney. This brings greater flexibility in installation, even where a traditional chimney is not available. Vaillant turboTEC pro VUW SK 202/3-3 has comparable performance parameters to the atmospheric model 200, but with a higher energy class. Vaillant turboTEC pro VUW SK 242/3-3 is a 24 kW version with a turbo chimney – suitable for new builds or renovations, where a new chimney insert is being built or a façade outlet is used.

A detailed comparison of atmospheric and turbo boilers can be found in the article Atmospheric vs. turbo boiler with DHW preparation: which type is suitable for you in the Knowledge Centre.

Vaillant atmoTEC plus VUW CZ/SK 240/3-5

Vaillant atmoTEC plus VUW CZ/SK 240/3-5 is a higher-tier model in the range. Compared to the "pro" model, it offers extended features – higher comfort, better regulation, and a higher energy class. The power outputs are comparable, but the overall user experience is different. If you are planning a long-term investment and comfort is important to you, this model makes sense. More about the differences between the pro and plus models can be found in the article Vaillant atmoTEC pro vs. atmoTEC plus: what they differ in and which to choose.

Typical scenarios and recommendations based on the size and age of the house

Instead of abstract numbers, let's look at specific scenarios we encounter most frequently in practice:

Scenario A: Apartment or small house up to 80 m², new build

Heat loss: 2.5–4 kW. DHW demand: shower, 1 bathroom, 2–3 people. DHW performance required: 18–20 kW. Recommended boiler: 20 kW. A 24 kW boiler would be unnecessarily oversized for heating, and frequent cycling would reduce its lifespan.

Scenario B: Family house 100–150 m², new build 2005–2015

Heat loss: 6–10 kW. Family of 4, 1–2 bathrooms, occasional simultaneous DHW use. DHW performance required: 20–24 kW. Recommended boiler: 20 kW or 24 kW depending on DHW comfort. If the house has a bathtub, choose 24 kW.

Scenario C: Older house 150–200 m², solid brick without insulation

Heat loss: 15–20 kW. DHW performance required: 20–24 kW. Recommended boiler: 24 kW. Here, the boiler performance is mainly determined by heating needs, not DHW. If the house is not insulated, consider insulation first – after that, you can opt for a smaller and more cost-effective boiler in operation.

Scenario D: Older house 200+ m², solid brick, without insulation

Heat loss: 20–30 kW or more. A combination boiler with direct DHW heating reaches its performance limits here – most models have a maximum of 24–28 kW, which may not cover heating needs. In this case, we recommend either insulating before replacing the boiler, or installing a larger boiler without direct DHW heating and an external storage tank. This dilemma is discussed in more detail in the article Direct DHW heating vs. storage water heater: advantages and disadvantages with a low-temperature boiler.

Why an oversized boiler is harmful

The same mistake is repeated in practice: the customer buys a boiler that is 8–10 kW more powerful than needed "just to be sure." They think it won't hurt. The opposite is true.

Cycling: An oversized boiler reaches the set temperature very quickly and turns off. After a short while, it turns on again. This short cycling wears out the combustion chamber, heat exchanger, and pump disproportionately quickly.
Lower efficiency: The boiler achieves the highest efficiency in the medium to high performance range, not during starts and stops. Frequent cycling means higher gas consumption in reality.
Higher purchase price: A larger boiler simply costs more money – and that money will never be justified.
Worse regulation: Modern boilers have modulating performance, but even that has a lower limit (typically 30–40 % of the nominal performance). If this limit is still higher than the house's needs, modulation won't help.

Modulating performance: how the boiler adapts to actual demand

Modern condensing and low-temperature boilers – including Vaillant atmoTEC and turboTEC models – have modulating burners. This means the boiler's performance is not fixed, but smoothly adapts to the current heat demand. A 24 kW boiler does not have to run at 24 kW all the time – it can operate at 8, 12, or 18 kW depending on what the heating system needs at the moment.

The modulation range is typically 1 : 4 to 1 : 5. A 24 kW boiler can therefore operate from 5–6 kW to 24 kW. For a house with a heat loss of 7 kW, this means the boiler will operate mostly at minimum or medium performance during the heating season – which is energy-efficient and kind to the equipment.

From the diagram, it is evident that during the transitional period (spring, autumn), the heating power required is lower than the minimum modulation power of the boiler. In this period, cycling occurs – and it is precisely here that an oversized boiler has negative effects. A properly dimensioned boiler has a smaller problem, as its minimum modulation power is also lower.

Gas consumption: how boiler power affects operating costs

Example of annual gas consumption calculation for a house with a heat loss of 8 kW (new build 140 m²):

Annual heating energy requirement: approx. 16,000–18,000 kWh (depends on region and habits)
Annual hot water energy requirement: approx. 2,500–3,500 kWh
Total approx. 19,000–21,000 kWh
Efficiency of a low-temperature boiler: approx. 85–90 %
Natural gas requirement: approx. 2,000–2,300 m³/year (heating value of gas ~10 kWh/m³)

If the same household had an unnecessarily oversized boiler with worse efficiency at partial load (e.g. 80 % instead of 87 %), the annual gas consumption difference could be 200–400 m³ – at current gas prices, this represents tens to hundreds of euros per year.

Practical approach: how to choose the right power step by step

If you want to determine the power as accurately as possible, proceed as follows:

Determine the heat loss of the house – ideally from the energy certificate or project documentation. If you don't have these documents, use an approximate table according to the area and house category above.
Determine the required hot water flow – how many taps and showers can be open at the same time? For most households, it is sufficient to calculate for one draw at a time.
Calculate the required power for hot water – flow (l/min) × temperature difference (°C) × 0.07.
Select the higher of the two values – heating power vs. hot water power. This is the minimum boiler power.
Check the modulation – the minimum modulation power of the boiler should be as close as possible to the power required during mild frosts (typically 30–50 % of the nominal power).
Consider the type of flue gas exhaust – an atmospheric boiler requires a functional chimney, while a turbo boiler can lead a coaxial chimney through the façade. This limitation may determine the type of boiler regardless of power.

Frequently asked questions (FAQ)

Is a 20 kW or 24 kW boiler better for a new build of 130 m²?

For a new build of 130 m², the heat loss is typically 5–7 kW – from the perspective of heating, even a 10 kW boiler would be sufficient. The choice between 20 kW and 24 kW therefore depends entirely on hot water heating. If you have one bathroom, a shower and 3–4 people, a 20 kW boiler is sufficient and a better choice – it cycles less and is more efficient. A 24 kW boiler is worth considering if you want more comfort when filling a bathtub or if you have two bathrooms and need a higher flow rate.

Can a boiler with flow hot water heating be used to fill a bathtub?

Yes, but you need to consider real limitations. A 20 kW flow boiler heats water at a flow rate of 7–8 l/min with a temperature difference of 35 °C. A 150-liter bathtub will be filled in 20–22 minutes. If you want to fill the bathtub faster or at a higher temperature, you need a boiler with higher power (24 kW) or you need to lower the desired temperature. A storage water heater fills the bathtub faster, as it has the entire volume of hot water available at once – more on this topic in the article Flow hot water heating vs. storage water heater.

Why is a 20 kW boiler sometimes not sufficient to supply hot water in a larger house?

A combined boiler functions as a flow heater during hot water heating – it heats water only at the moment it is flowing. If the flow at the tap is higher than the boiler can heat to the desired temperature, the temperature of the outgoing water will drop. For example, a 20 kW boiler at an incoming temperature of 8 °C and a desired temperature of 45 °C can heat a maximum of 8 l/min. If you open a shower with a flow rate of 10 l/min, the outgoing temperature will be lower – the boiler simply does not have the power for more. Solution: lower the flow at the tap or use a more powerful boiler.

Is it worth buying a more powerful boiler if you plan to build an extension in the future?

This line of thinking is logical at first glance, but problematic in practice. A larger boiler will cycle for years before the extension and have worse efficiency. Moreover, dimensioning a boiler for a hypothetical future need is unreliable – building regulations change, modern extensions have low heat loss, and by the time the extension is built, you may have to replace the boiler anyway. A better approach: buy a boiler for your current needs, and do a new calculation when the extension is built.

What impact does water quality have on the performance and lifespan of a boiler with flow hot water heating?

Significant. In areas with hard water (calcium content above 2.5 mmol/l), limescale forms in the hot water heat exchanger. This reduces heat transfer and consequently lowers the hot water flow at the same boiler power. A neglected boiler with a thick layer of limescale can lose up to 20–30 % of its flow performance. We recommend regular maintenance and, in hard water areas, consider a water softener. More in the article Maintenance and service of combined low-temperature boilers: what and how often.

Can I install the boiler myself, or do I need a professional?

Installation of a gas boiler in Slovakia requires professional installation performed by a person with the appropriate certification for gas work. DIY installation is not only dangerous (risk of gas leak and carbon monoxide), but also illegal – in the event of damage, your insurance will not cover the costs. Details on installation can be found in the article Installation of a combined low-temperature boiler with flow hot water preparation in the Knowledge Center.

Conclusion: choose the boiler power according to actual needs, not fear

Correctly dimensioning a boiler with flow hot water heating is one of the most important decisions when purchasing. It is not just a matter of comfort – it is a matter of lifetime operating costs, equipment lifespan, and real comfort in the household.

Key conclusions to take away from this article:

For medium-sized modern new builds (100–160 m²), a 20 kW boiler power is sufficient for heating – a higher power is only relevant for hot water.
Hot water power depends on the required flow and temperature difference – for a standard shower, 18–22 kW is sufficient, for a bathtub or higher comfort, 24–28 kW.
An oversized boiler = higher costs, worse efficiency, shorter lifespan.
Power modulation is your friend – but only if the boiler is correctly dimensioned so that modulation can work in the right range.
For old, poorly insulated houses, the heat loss may be higher than the power of standard combined boilers – consider insulation first.

If you are unsure, always consult a professional – an experienced technician with a few simple questions about your house can determine the correct power more accurately than any online calculator. More information on selecting a specific model can be found in the article How to choose a low-temperature boiler with flow hot water preparation: what to pay attention to, where we also cover other technical parameters that you should not forget when choosing.

Do you have a question about this topic?

Can't decide or are you dealing with a specific situation in your household? Write to us – we are happy to help.