What tank capacity do I need for my home
What tank volume do I need for my home?
This is one of the first questions customers ask themselves when choosing a condensing boiler with a built-in tank – and at the same time one of those without a single correct answer valid for everyone. The volume of the domestic hot water (DHW) tank directly determines how much hot water you have available without waiting for another heating cycle. Choose too small a tank – you'll run out of hot water in the middle of a shower. Choose too large a tank – you're paying for volume you'll never use, plus its daily standby energy losses.
In this article, we'll go through all the factors that determine the correct choice of volume – from household size and usage patterns, through tank type and placement, to specific calculations and real-life examples. If you're also interested in choosing a specific boiler model, check out our article How to choose a condensing boiler with a built-in tank: what to look out for in this Knowledge Center.
Why tank volume matters more than people think
When people buy a boiler, they usually focus on power in kilowatts, energy class, or price. Tank volume is usually addressed afterwards, almost last. That's a mistake. The hot water tank is essentially a "buffer" between the heat source (burner) and your actual water consumption. Its volume determines the comfort of hot water draw-off – especially during the morning peak, when the whole family showers one after another (or at the same time).
A boiler with a small tank must reheat water with every larger draw-off, which takes several minutes. During this time, lukewarm or cold water flows from the tap. Conversely, a boiler with an adequate tank always has a reserve of hot water ready for use – no waiting, no temperature fluctuations.
Basic calculation: how much hot water do you actually consume?
Before we move on to specific recommendations, it's good to know real consumption figures. These values are proven from practice and correspond to typical usage in Slovak households:
- Shower (5–8 minutes): 35–60 liters of total water, of which hot water (60 °C) is 12–20 liters (the rest is cold water when mixing)
- Bath: 100–150 liters of total water, hot water (60 °C) 40–55 liters
- Washing dishes by hand: 8–12 liters
- Washing up, hand washing and basic hygiene: 3–6 liters per day per person
- Dishwasher: 10–15 liters (but usually heats its own water)
This means that one adult consumes roughly 40–70 liters of hot water daily when showering and performing basic hygiene (in mixed form, not pure hot water). Converted to pure hot water at 60 °C, this represents about 15–25 liters per day per person. The boiler's tank is of course replenished and reheated several times during the day – it's not that the tank has to cover the entire day's consumption at once. The key is to handle the morning and evening draw-off peaks.
Main factors that determine the required tank volume
1. Number of people in the household
This is the most significant factor. The more people there are, the more simultaneous or consecutive hot water draw-offs occur in a short time. Reference table of volumes for typical family households:
These figures are indicative and assume typical usage – i.e. that most household members shower in quick succession in the morning or evening. If the family showers throughout the day at longer intervals, the tank has time to reheat and a smaller volume is sufficient.
2. Usage pattern: showers vs. baths
Households that regularly use a bathtub (not just a shower) need a significantly larger tank. Filling a bathtub uses 40–55 liters of pure hot water (60 °C), which is almost the entire volume of a 42-liter tank. So if you plan to take baths as well, we recommend considering a tank of at least 60 liters, or combining the boiler with an external tank – we discuss this decision in more detail in the article Condensing boiler with tank vs. boiler with external tank: which pays off more.
3. Number of bathrooms and distance from the boiler
If a house has two bathrooms, it's likely they'll sometimes be in use at the same time. The distance of the bathroom from the boiler also plays a role – with a long pipe run, all the cooled water in the pipe must be "pushed out" before each draw-off, wasting both water and heat liters. In such cases, a larger tank is an advantage, because hot water reaches the draw-off points faster and with less heat loss in the piping.
4. Morning and evening peak times
In practice, the tank is sized exactly for these peak draw-offs. If an entire four-member family showers in the morning within 45 minutes, the tank must cover these 4 draw-offs with minimal reheating in between. A 42-liter tank handles this very well with a boiler output of 20–26 kW – the boiler manages to reheat the water between showers quickly (typically 10–15 minutes to heat 42 liters from 15 °C to 60 °C at 26 kW).
5. Boiler output vs. tank volume – mutual relationship
Tank volume and boiler output are not independent variables – they complement each other. A higher-output boiler heats the tank faster, compensating for its smaller volume. Conversely, lower boiler output requires a larger tank to achieve comparable comfort. In the category of condensing boilers with a built-in tank, boilers typically have an output of 20–35 kW and tanks of 21 or 42 liters.
A practical example: a 42-liter tank is heated by a 26 kW boiler (at a temperature difference of 45 °C) in roughly 10–14 minutes. This means that if the first person showers and uses 10–12 liters of hot water, the tank is reheated back to full temperature before the second person showers. This cycle works well for up to 3–4 people with typical time spacing between showers.
Specific models and their tank volumes
In the condensing boilers with a built-in tank category on atria.sk, boilers are available with two basic tank volumes: 21 liters and 42 liters. Let's look at who each one suits.
21-liter tank – who is it for?
A 21-liter tank is a compact solution primarily suitable for single or two-person households with low DHW demands. It's also a suitable solution for cottages and weekend houses, where hot water is used only occasionally. A boiler with a 21-liter tank has smaller dimensions, is easier to install, and consumes less energy to maintain the tank temperature.
Among the available products, this category includes the Protherm Tiger Condens 20/26 KKZ 21 – a model with modulating output of 4–26 kW and a built-in tank of exactly 21 liters. For a couple living in an apartment or small house without demands for simultaneous draw-offs, this solution is economically advantageous and space-saving.
42-liter tank – who is it for?
The 42-liter tank is the golden mean in this product category – it covers most typical family households with 2–4 people. It's the recommended volume for homes where hot water is drawn off in several consecutive showers in the morning or evening.
Atria.sk offers several boilers with this tank. The Protherm Tiger Condens 20/26 KKZ 42 offers an output of 4–26 kW with a 42-liter tank – suitable for a standard family home with 3–4 people. If you're looking for higher output for a larger home or larger heating areas, the Protherm Tiger Condens 30/35 KKZ 42 offers up to 35 kW with the same 42-liter tank – here the boiler's output shortens the tank reheating time, increasing comfort even with a higher number of draw-offs.
For those who want a combination of comfort and smart control, the Protherm Tiger Condens 20/26 KKZ 42 + smart controller set is also interesting – the smart controller allows you to set time programs for tank heating so that hot water is always ready exactly when you need it (for example, tank heating at 5:30 a.m. before the family wakes up).
An interesting alternative from another manufacturer is the Vaillant VUI 26CS/1-5 ecoTEC plus IoniDetect + sensoCOMFORT 720 – a premium model with advanced combustion diagnostics and smart control. This boiler combines output with advanced electronics and is suitable for households that value comfort of use and remote management via app in addition to output.
Practical scenarios from real orders
Scenario 1: Apartment in a block of flats, 2 people, no bathtub
A couple living in a 3-room apartment in a block of flats with one bathroom (shower cubicle only, no bathtub). Both shower in the morning within a 20-minute window. DHW consumption is minimal, with a maximum of one simultaneous draw-off in the morning. For this case, a 21-liter tank is completely sufficient. The boiler manages to replenish the tank between showers without any problems. Another advantage of the smaller tank is lower heat loss during the day (the tank holds less water at temperature).
Scenario 2: Family house, 4 people, 1 bathroom
A family with two children in a family house. All four shower in the morning before school and work within 45–60 minutes. One draw-off is usually a bath (the younger child). In this case, the 42-liter tank is at the edge of comfort – the tank covers the first two showers without reheating, but for the third and fourth it depends on whether the boiler managed to reheat in the meantime. With an output of 26 kW, this is borderline. In such a case, we recommend considering a 60-liter tank (external) or a boiler with higher output, or at least smart pre-heating of the tank.
Scenario 3: Recreational cottage, 2–6 people irregularly
A cottage that is occupied irregularly – sometimes 2 people for the weekend, sometimes the whole family with children for 14 days. Here the situation is interesting: the tank must handle occasional peak demand at full occupancy, yet most of the time it's not used at all. A 42-liter tank is a compromise choice here – when the cottage is empty, an unnecessarily large volume isn't heated, and when the cottage is fully occupied, reheating between draw-offs is sufficient. It's important to set the boiler so the tank doesn't heat automatically when the cottage is empty (a smart controller handles this very elegantly).
Scenario 4: Modern family house, 5 people, 2 bathrooms
A five-member family in a new-build house with two bathrooms. Both bathrooms are in use simultaneously in the morning. This is a case where a built-in 42-liter tank isn't enough, and the client should consider a combined solution – a condensing boiler with a built-in tank plus an external instantaneous water heater or an external tank of 80–100 liters. An alternative is a direct-heating boiler (without a tank) and an external tank of 100+ liters. This topic is discussed in more detail in the article Condensing boiler with tank vs. boiler with external tank: which pays off more.
Technical parameters affecting the actual usable tank volume
The nominal tank volume (e.g. 42 liters) is not the same as the usable volume of hot water. Several technical facts are important to understand:
- Water stratification in the tank: The temperature in the tank doesn't change abruptly – the top part is hottest (60 °C), the bottom cooler (incoming cold water). The usable volume of hot water is therefore only a part of the total tank volume.
- Tank temperature and mixing: A tank heated to 60 °C provides a larger volume when mixed with cold water (15 °C) to an outlet temperature of 40 °C – specifically, at a 1:1 mixing ratio, you get roughly 60–70 liters of "usable" water at 40 °C from a 42-liter tank. That's why a higher tank temperature is advantageous (within the norm for Legionella prevention – at least 60 °C).
- Tank heat loss: The tank loses part of its heat through insulation every hour. Modern tanks in condensing boilers have good thermal insulation (loss of 1–2 °C per hour), but with longer standing time (e.g. overnight), the temperature can drop by 5–10 °C. Smart controllers address this with morning pre-heating before the expected draw-off.
- Antibacterial heating (anti-Legionella): The standard recommends heating the tank to 70 °C for 1 hour at least once a week to eliminate Legionella bacteria. Modern boilers do this automatically. This cycle consumes some extra energy – a larger tank consumes more.
Tank volume and energy efficiency – dependence on energy prices
A large hot water tank isn't free – every liter of water you keep at 60 °C costs energy even when no one is drawing from it. This "standby heat loss" is low in modern boilers (typically 0.5–1.5 kWh per 24 hours for a 42-liter tank with good insulation), but at a natural gas price of around €0.07–0.10/kWh, this represents an annual cost of €12–55 just to maintain the tank temperature.
This energy loss increases with a larger tank volume. So the rule is: a larger tank means higher comfort, but also somewhat higher standby energy consumption. For households with low DHW consumption (1–2 people), a smaller tank is therefore a better economic choice. For larger families, the savings from comfort prevail – they don't have to heat water separately or wait for reheating.
Another factor is heating time. Boilers with an output of 20–26 kW heat a 42-liter tank from 15 °C to 60 °C in 10–14 minutes (depending on the specific boiler and temperature difference). Boilers with an output of 30–35 kW manage this in 7–10 minutes. Faster heating = shorter time without hot water, but higher instantaneous gas consumption. You can find more about boiler outputs in the article What output of a condensing boiler with a tank is enough for my home in this Knowledge Center.
Comparison of built-in 21 l vs. 42 l tank – overview table
| Parameter | 21-liter tank | 42-liter tank |
|---|---|---|
| Recommended number of people | 1–2 | 2–4 |
| Number of showers without reheating | 1–2 | 3–4 |
| Suitability for a bathtub | No (insufficient) | Borderline (1 bath) |
| Heating time (26 kW boiler) | 5–7 minutes | 10–14 minutes |
| Heat loss/day | 0.3–0.7 kWh | 0.6–1.4 kWh |
| Suitability for 2 bathrooms | No | Conditionally |
| Boiler size | More compact | Larger/heavier |
| Typical use | Apartment, 1–2 people, cottage | House, 3–4 people, standard |
What to do if the built-in tank isn't enough?
If your needs exceed what a built-in 42-liter tank can cover, you have several solutions:
- External DHW tank (80–200 l): A condensing boiler can be connected as a heat source for an external tank. This solution is suitable for large families or homes with high DHW consumption. The tank is usually installed next to the boiler or in a utility room.
- Combination of a tank and an instantaneous water heater: Less common, but a functional solution. The instantaneous heater covers peak draw-offs beyond the tank's capacity.
- Dual-tank solution: In some projects, two boilers are installed, or one boiler with a large external tank (100–200 l), for really large homes or small guesthouses.
- Solar preheating: A solar collector preheats the water going into the tank, significantly reducing the energy demand of DHW heating. The boiler then only "tops up" to the required temperature, while the tank covers a larger volume.
This decision is discussed in more detail in the article Condensing boiler with tank vs. boiler with external tank: which pays off more in this Knowledge Center.
How tank volume affects DHW comfort in practice – what customers usually overlook
Over years of practical experience, we repeatedly encounter customers who underestimate tank volume in their initial choice and later regret it. A typical scenario: a customer buys a boiler with a 21-liter tank because it's cheaper and more compact. Half a year later, they call to say they're running out of hot water during a shower in winter. In summer it's not a problem, because showers are shorter, but in winter at full heating output (the boiler can't efficiently heat and reheat the tank at the same time), comfort drops.
Second thing customers overlook: during the transitional season (spring, autumn), the boiler operates at low output and heats the tank very quickly. In winter, in freezing weather, the boiler works mainly on heating and reheats the tank with lower priority. Some boilers have an adjustable priority for tank vs. heating – with the right setting, this significantly improves comfort. More on this in the article How long does it take to heat water in the tank and what does DHW comfort depend on.
Third thing: water quality. In waters with high hardness (which is common in Slovakia, especially in the south of the country), the tank scales up with limescale deposits faster. This reduces heating efficiency and the actual usable tank volume. Regular descaling (1–2 times a year) is essential in hard water areas. More in the article Maintenance and servicing of a condensing boiler with a tank: what and how often to check.
Frequently asked questions (FAQ)
What tank is enough for a 3-room apartment with two people?
For a couple living in an apartment and showering normally, a 21-liter tank is completely sufficient. The boiler manages to quickly replenish the tank between or after showers. If you have a bathtub and like to bathe, consider a 42-liter tank instead, even though it's oversized capacity for a couple – comfort will be better and you won't hear angry comments when someone runs out of hot water.
Can a 42-liter tank handle the morning peak for a 4-person family?
Yes, but with conditions. If all four shower one after another within 40–50 minutes (not simultaneously), a 42-liter tank with a 26 kW boiler can handle it. The boiler replenishes the tank between showers in 10–12 minutes. The critical factor is boiler output – at 35 kW, tank recovery is even faster. Problems arise if two people shower at the same time (the tank empties faster) or if someone prefers long showers of 15+ minutes.
Is a 42-liter tank worth it for solo living?
From a comfort perspective, yes – you'll never run out of hot water. From an economic perspective, it's not ideal, because you're keeping hot water in a volume you mostly don't need. The annual loss in standby energy with a 42-liter tank is €15–30 more than with a 21-liter tank. If you live alone in an apartment, a 21-liter tank is the more economically sensible choice.
Can I connect a boiler with a built-in 42 l tank to an external tank to increase DHW capacity?
It depends on the specific boiler model. Most condensing boilers with a built-in tank aren't primarily designed to be connected to an external tank – the built-in tank is an integral part of the boiler's hydraulics. If you need greater DHW capacity, it's usually more advantageous to choose a boiler without a built-in tank directly (i.e. a combination with an external tank). Consult a technician before any such modification and check it in the specific model's documentation.
How long does the water temperature in the tank hold without reheating?
With modern boilers with a well-insulated built-in tank, the temperature drops by roughly 8–15 °C over 24 hours (depending on ambient room temperature and insulation quality). This means that if you heat the tank to 60 °C in the evening, it will still be around 48–52 °C in the morning – which is still a comfortably usable temperature for showering. The boiler then only "tops up" the tank to the required temperature within a few minutes. That's why setting the tank to preheat at night (before the morning peak) is a very effective solution.
Does water hardness affect the choice of tank volume?
Indirectly, yes. In areas with hard water, limescale and calcium deposits build up faster in the tank, reducing heating efficiency and gradually shrinking the actual usable tank volume. For this reason, it's recommended to install a water softener or magnetic filter before the boiler inlet in hard water areas. Regular tank descaling (recommended once a year) is an essential part of maintenance. A larger tank in hard water means a bigger limescale problem – this is another argument for choosing a tank that is "just the right size" rather than unnecessarily large.
Conclusion: how to decide correctly
Choosing the right tank volume is basically simple if you honestly answer a few questions before buying: How many people will use the boiler? Do they shower or also take baths? What time window does the morning/evening draw-off peak cover? Do you have one or two bathrooms?
For most Slovak households, this rough rule applies: a 21-liter tank for 1–2 people without a bathtub, a 42-liter tank for 2–4 people or for anyone who wants a comfortable reserve. If a family exceeds 4 people or has two bathrooms in simultaneous use, it's time to consider solutions with greater DHW capacity – either via an external tank or a combination of sources.
If you're not sure which model matches your specific situation, check out the entire condensing boilers with built-in tank category and compare available models. If in doubt, the best step is to consult a technician before installation – a correctly sized tank will save you nerves and money for years to come. Find out more about the whole selection and installation process in the article Installing a condensing boiler with a built-in tank: procedure and requirements.
Have a question on this topic?
Can't decide or dealing with a specific situation in your household? Write to us - we're happy to help.