What tank capacity do I need for my household
What tank volume do I need for my household?
This is probably the most common question we get when configuring boiler and tank sets. And honestly – there's no single correct answer that applies to everyone. The volume of a domestic hot water (DHW) tank depends on several variables at once: how many people live in the household, their hygiene habits, when most of them shower, whether you have a bathtub or just a shower cubicle, what output your boiler has, and how the whole system is set up. In this article, we'll break down these variables systematically so you can make an informed estimate – and avoid a mistake we've seen dozens of times in practice: a tank that's either too small (cold water after ten minutes of showering) or unnecessarily large (wasted heat losses, higher price, more space taken up).
Why tank volume matters more than people think
When choosing a boiler and tank set, many customers focus mainly on the boiler's output, brand or price. They tend to view the tank as "just extra metal". That's a mistake. The domestic hot water (DHW) tank is actually the heart of water heating comfort – it determines how much hot water you have available at any given moment, without waiting and without temperature fluctuations.
The boiler heats the water in the tank continuously or at intervals, but the tank works as a storage reservoir that the household "draws from". If the tank is too small, the boiler works at full capacity but can't replenish heat fast enough to cover peak demand. If the tank is too large, you're unnecessarily heating a large volume of water that you only partially use – and the lost heat increases your gas bill every day.
The chart clearly shows that hot water consumption is not evenly distributed throughout the day. Two distinct peaks – morning (morning wash and shower before work) and evening (bathing children, evening shower for adults) – account for the vast majority of daily consumption. The tank must be able to cover at least one of these peaks without interrupting comfort, while the boiler catches up on reheating the next batch of water.
Basic rules for calculating the required volume
In practice, two main methods are used to estimate the required tank volume: a simplified standard method based on the number of people, and a more detailed calculation based on actual daily consumption. For an average family household, the first method with corrections is sufficient.
Method 1: Based on the number of people in the household
This is the quickest way to get reasonable results for average households. It's based on the assumption that each person consumes roughly 40 to 60 liters of hot water per day at a temperature of 45 °C (this is the mixed outlet temperature, not the temperature in the tank, which is usually 60–65 °C).
| Number of people | Minimum tank volume | Recommended volume | Note |
|---|---|---|---|
| 1 – 2 people | 80 l | 100 – 120 l | Recreational properties, apartments |
| 3 – 4 people | 120 l | 150 – 200 l | Family house, common practice |
| 5 – 6 people | 200 l | 200 – 300 l | Larger family house |
| 7 or more people | 300 l | 300 l or more | Possibly two tanks in series |
Note: these figures apply to standard comfort levels. If the household has a large bathtub (180–220 liters of water surface volume), the tank should be sized according to the actual volume of the bathtub, not just the number of people – more on this in the section on special situations.
Method 2: Calculation based on actual consumption
This method is more accurate and takes into account the real habits of a specific household. The procedure is as follows:
- Add up the daily hot water consumption (45 °C) for each member of the household
- Add consumption for dishwashing, laundry (if you use hot water from the boiler)
- Divide the result by time windows – what percentage of consumption falls into the morning and evening peaks
- The tank should cover the larger of the two peaks + a 20% reserve
Example: a family of 4, everyone showers in the morning (2 people) and in the evening (2 people). An adult needs about 40–50 liters of hot water for a shower, a child needs about 60–80 liters for a bath. Morning peak: 2 × 45 l = 90 l. Evening peak: 2 × 70 l = 140 l. The tank should have at least 140 l × 1.2 = 168 l → choose a 200-liter tank.
The effect of boiler output on the required tank volume
This is a connection that many people overlook: the tank and the boiler form a system. A larger boiler can reheat the tank faster after hot water has been drawn off – meaning that with the same household, a more powerful boiler lets you get by with a smaller tank, or conversely, a smaller boiler can be compensated for by a larger tank.
The chart shows that with a 25 kW boiler and a 120-liter tank, the heating time is about 40–45 minutes, whereas with a 15 kW boiler and the same tank, it can take as long as 60–75 minutes. That's why specific tank volumes are typically combined with specific boiler outputs in ready-made sets. For example, the set Protherm Aqua Complet Panther 25KOO + FE 120 BM combines a 25-kilowatt boiler with a 120-liter tank – a combination suitable for 2–4 people with average hygiene requirements, where the boiler can reheat the tank quickly enough after the morning peak to cover the evening one. If your household has more people or higher demands, opt for a set with a larger tank – we cover the topic of correctly pairing boiler output with tank volume in detail in the article Correct sizing of boiler output to tank volume.
Tank types and how they affect the choice of volume
Before you decide on a specific tank volume, it's important to understand that not all tanks of the same volume perform the same way. The key differences lie in the number of heat exchangers (coils), the quality of insulation, and the way the boiler heats the tank.
Single-coil tank
A standard tank with a single coil (heat exchanger) heats the water in the tank from just one source – the boiler. Such tanks are cheaper, more compact, and completely sufficient for most households with a gas boiler. An example is the VIH CK 70 tank, which you can find, for instance, in the set Vaillant atmoTEC plus VU 200/3-5 + VIH CK 70 or Vaillant turboTEC plus VU 202/3-5 + VIH CK 70.
Dual-coil tank
A tank with two coils allows heating from two sources – for example, from a boiler and solar collectors, from a boiler and a heat pump, or from a boiler and a fireplace with a heat exchanger. If you're planning to combine with renewable sources, you need to choose a dual-coil tank from the start – and a larger volume (from 200 liters upward), since the solar source is slower and the tank needs to store more energy. We cover this topic in more detail in the article Single-coil or dual-coil tank – which to choose.
The effect of insulation on effective volume
A tank with weaker insulation loses heat faster, which means the water in it cools down sooner – effectively "shrinking" its usable volume over time. Quality tanks from Protherm or Vaillant have rigid PU insulation with heat losses typically of 1–1.8 kWh per 24 hours for a 120-liter tank. Cheap tanks can have two to three times higher losses. So when choosing, pay attention to this parameter too, not just the volume in liters.
A tank with poor insulation effectively "acts" as a tank 20–30% smaller of the same volume, because the water cools down before the household manages to use it. The boiler therefore has to heat the tank much more often, which increases gas consumption and burner wear.
Special situations that change the calculation
Large bathtub or whirlpool
A standard bathtub holds roughly 150–180 liters of water. If you want to fill the bathtub once a day (for example, for children), you need to size the tank to cover filling the bathtub plus a regular shower for adults at the same time. For a family with a bathtub and two children, we recommend at least a 200-liter tank. A whirlpool bath (150–400 liters of water) may require a 300-liter tank or a combination of two tanks.
Cottage, recreational property
Different rules apply to cottages: consumption is irregular – zero during the week, sudden with multiple guests over the weekend. Here, a smaller tank with a powerful boiler that can quickly reheat water is better than a large tank, in which the water would sit for a whole week and unnecessarily lose temperature (and you'd have to deal with hygiene safety – legionella at temperatures below 60 °C).
Household with multiple bathrooms
Multiple bathrooms don't automatically mean higher daily consumption, but they do increase the risk of simultaneous consumption – two people showering at the same time in different places. The tank must be able to cover simultaneous consumption without a significant drop in temperature. As a rule of thumb: each additional bathroom that is actually used simultaneously adds 30–50 liters to the required tank volume.
Household with electric backup heating (bivalent tank)
Some tanks have a built-in electric resistance heater (immersion heater) that serves as a backup in case of boiler failure or to cover peaks. In this case, you can opt for a slightly smaller tank, since electricity will cover any gas outage. On the other hand – electric heating is significantly more expensive (cost of electricity vs. gas), so it should really only serve as a backup, not as a regular supplement.
Real-world examples – what we've seen with orders
Over the years of selling and installing systems, we've encountered several recurring scenarios that illustrate well why choosing the right tank volume isn't a given.
Example 1 – Undersized tank, a married couple with two children, apartment in Bratislava: The customer bought a set with an 80-liter tank because "it was cheaper and we only have a small space". Result: after the two adults' morning shower, the tank was practically empty, and the children had to wash with cold water. After six months, we replaced the tank with a 120-liter one – the problem was solved, but the customer paid for the replacement twice.
Example 2 – Oversized tank, retired couple, family house: The architect designed a 300-liter tank "just to be safe". A household of two older people used about 60–70 liters of hot water per day. The rest of the tank was unnecessarily heated and cooled, and the tank's annual heat losses exceeded 200 kWh – which, at a gas price of €0.10/kWh, represents €20 a year just for the excess tank losses. That doesn't sound like much, but over 15 years, it's €300 just for an unnecessarily large tank.
Example 3 – Correct choice, family of 4 + one bathroom with a bathtub: The customer chose Vaillant turboTEC plus VU 202/3-5 + VIH R 120 with a 120-liter tank and a 20 kW boiler. The family showers in two waves (two adults in the morning, two children bathing in the evening). The tank covers the evening peak, and the boiler manages to reheat the water by morning. No issues, fully satisfied.
Example 4 – Solar combination, family of 5: The customer wanted to combine a gas boiler with solar collectors. There was significant pressure to use the existing single-coil 150 l tank. We insisted on replacing it with a 200 l bivalent tank with two coils. Otherwise, the solar collectors would have had nowhere to feed their energy. The customer understood and got the right equipment – during the summer months, the boiler barely runs at all.
Tank volume and hygiene safety – legionella
This is a topic that sellers often avoid, but it's important in practice. The bacterium Legionella pneumophila multiplies in stagnant water at temperatures of 25–50 °C. Hot water tanks are therefore a potential risk if not set up correctly.
Basic protective rules:
- The tank should be heated to at least 60 °C at least once every 24 hours – modern boilers have this anti-legionella heating function built into their controls
- The water temperature in the tank should never drop below 55 °C for a prolonged period
- If the tank is out of operation for several days (cottage, holiday), let it heat up to 70 °C upon return and flush all water outlets
- Large tank with low consumption = higher risk, because the water stays without heating for longer
That's precisely why "bigger tank = better tank" isn't always true. When sizing, always balance comfort (sufficient hot water) against safety (water is exchanged and heated quickly enough).
Boiler and tank set – how it works together in practice
If you've been wondering whether to buy a boiler + tank combination or a combi boiler (with integrated DHW heating), there's a fundamental difference: a combi boiler heats water on-the-fly – i.e. it has no tank and only heats water at the moment of draw-off. Tank-based systems store hot water, which is an advantage for larger households: with simultaneous draw-off at multiple outlets, output and temperature don't drop as dramatically as with a combi boiler.
A set with a separate boiler and tank is the standard solution for family houses. The boiler (e.g. Protherm Aqua Complet Panther 25KTO + FE 120 BM) heats the tank via a heat exchanger, while the boiler's control monitors the tank's temperature and starts heating when it drops below the set value (usually 55–60 °C). The boiler thus works in intervals rather than continuously – which is energy efficient and extends the equipment's lifespan. You can find out more about how these sets work and how to connect them correctly in the articles How to choose a boiler and tank set – what to watch out for and How to connect a boiler to a tank – installation procedure.
A practical three-step calculator
If you don't want to do complicated calculations, use this quick three-step procedure to get a reliable estimate:
Step 1 – Basic volume: Number of people × 50 liters = basic consumption (e.g. 4 people × 50 l = 200 l per day)
Step 2 – Adjustment for habits: Do you have a large bathtub? Add 50 l. Does everyone shower at once in the morning? Don't split into two waves, the tank must cover everyone at once. No bathtub, only showers? You can subtract 20–30 l from the base.
Step 3 – Tank = 70–80% of daily consumption: The tank doesn't need to cover the entire daily consumption, the boiler will make up the rest during the day. Typically: 200 l daily consumption × 0.75 = 150 l tank. Round up to the nearest standard volume (160 l → choose 200 l).
Compare the result with the table in the Method 1 section – both methods should give a similar result. If they differ significantly, have it assessed by a technician.
Frequently Asked Questions (FAQ)
Can I keep my old tank and just buy a new boiler?
This depends on the condition of the tank, its volume, and technical compatibility with the new boiler. If the tank is older than 10–12 years, has corrosion on the anode, or a damaged enamel layer, replacing both units at once pays off more than keeping the old tank. A new boiler may be technically compatible with an older tank, but if the tank loses efficiency or starts rusting, the cost of replacing the tank alone in two years will be wasted money. Buying a boiler + tank set together also saves you on installation and gives you certainty that the components are matched to each other.
Is a 120-liter tank enough for a family of 4?
For 4 people, a 120-liter tank is the minimum – it works if showers are spread across two time windows (morning and evening) and if no one fills a large bathtub every day. For greater comfort, and if you have a larger bathtub or everyone showers at once in the morning, a 150 to 200-liter tank is a safer choice. With a 25 kW boiler and a 120 l tank (such as in the set Protherm Aqua Complet Panther 25KOO + FE 120 BM), the time to recharge the tank after the morning peak is about 30–40 minutes, which suits most families.
Does it make sense to buy a bigger tank "just in case"?
Only to a certain extent. A tank 30–40% larger than you need is enough to cover visits and special occasions. A tank 100% larger than you need will unnecessarily cost you money (higher purchase price, higher heat losses, larger footprint). Moreover, with low consumption, there's a hygiene risk – the water isn't refreshed sufficiently, and the tank needs to be heated to the anti-legionella cycle even though it doesn't make energy sense. A healthy reserve is 20–30% above the calculated minimum, not more.
What if my tank is in an unheated room (boiler room, garage)?
In an unheated room, the tank's heat losses are significantly higher, especially in winter. With a tank in a garage at a winter temperature of 5–8 °C, losses can be 1.5–2 times higher than in a warm boiler room. In this case, either choose a tank with thicker insulation (min. 100 mm PU foam), or place the tank as close to the boiler as possible and shorten the length of the piping, where heat is also lost.
What is the lifespan of a tank and when should it be replaced?
A quality enameled tank has a lifespan of 15–20 years with proper maintenance. The key factor is regularly replacing the magnesium anode (a protective rod that sacrifices itself to corrosion instead of the tank's shell) – usually every 2–3 years, or according to its condition at service. If the anode is worn down to less than 1/3 of its original diameter, it needs to be replaced. Neglecting the anode is the most common cause of premature tank corrosion and early replacement. We cover the topic of service maintenance in the article Service and maintenance of a boiler and tank set – what and how often.
What if I have low water pressure in the supply – does it affect the choice of tank?
Water pressure directly affects outlet performance, but not the tank volume as such. However, low water pressure (below 1.5 bar) will slow down filling the bathtub or shower, which extends the draw-off time and thus the load on the tank. In practice, this means that with low pressure, you should consider a pressure booster pump at the inlet, not a larger tank. Size the tank according to consumption, not pressure.
Conclusion – key criteria in brief
Choosing the right tank volume isn't rocket science, but it does require a moment's thought about your household's actual habits. If you take away just one thing from this article, let it be this: don't start with the tank's price, start with the number of people, their hygiene habits, and whether you have a bathtub or just a shower cubicle. Calculate the required volume, compare it with the boiler's output, check that the tank meets hygiene requirements, and that it fits the space conditions of your boiler room.
If you're not sure, feel free to contact us or check out other articles in the Knowledge Center – specifically How to choose a boiler and tank set – what to watch out for, Protherm Panther vs. Vaillant turboTEC – comparison of sets, or Frequently asked questions about boiler and tank sets. A correctly configured set will serve you for 15–20 years without problems – it's worth paying appropriate attention to the choice right from the start.
Do you 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.