What size DHW cylinder do I need for a family house or apartment

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Introduction: Why the DHW tank capacity is a key decision

When a customer decides on a boiler with a built-in tank, attention naturally focuses on the boiler's output, its energy class or price. The tank capacity tends to be a secondary consideration – and that's exactly the mistake that is hard to fix later. The tank is a fixed part of the boiler; it cannot be replaced without replacing the whole unit, and if it's too small, everyday comfort in the household suffers every single day. On the other hand, if it's unnecessarily large, you'll pay more for the boiler, consume more energy heating it, and the tank will take up space that could have been used for something else.

This topic seems simple – in practice, we encounter it differently in every job. A family of three in a 70 m² apartment has completely different needs than a household of six in a family house with two bathrooms and a sauna. It also matters how people live: whether everyone showers at the same time in the morning, or the household takes turns throughout the day, whether they use a shower or a bathtub, whether they plan solar preheating, and dozens of other factors.

In this article, we cover the topic comprehensively – from basic physical concepts, through specific calculations and real-life scenarios, to a comparison of available solutions. The goal is that after reading it, you will know exactly what tank capacity you actually need.

Basic concepts: what a DHW tank is and how it works in a boiler with a built-in tank

DHW stands for "domestic hot water" – i.e. water you use for washing hands, showering, bathing and running the household. A DHW tank is a container in which this water is heated and kept ready for immediate use.

In boilers with a built-in tank (so-called combi boilers with tanks, or low-temperature boilers with a tank), the tank is physically built directly into the boiler's casing. The tank is heated by a heat exchanger connected to the boiler's primary circuit. In practice, this means the boiler doesn't heat DHW instantaneously (as with a standard combi boiler), but pre-heats the entire volume of water in the tank and keeps it at an adjustable temperature – typically 55 to 65 °C.

Compared to instantaneous heating, this principle has key advantages: DHW pressure and temperature remain stable even when drawing water at multiple points simultaneously, you're not limited by the exchanger's output, and the tank acts as a thermal buffer for the whole household. The disadvantage is that the tank has a fixed capacity – once you use it up, you have to wait for it to reheat.

How to calculate a household's actual DHW consumption

The tank capacity directly depends on how much hot water the household consumes during the so-called peak draw-off period – that is, the time when everyone showers or bathes in quick succession. This is the decisive factor, not the average daily consumption.

For reference, here are typical water volumes for common uses (water at approx. 40 °C, i.e. mixed with cold water):

Quick shower (5–6 minutes): 40–60 litres of mixed water, of which approx. 25–35 litres of hot water from the tank (60 °C)
Longer shower (8–10 minutes): 65–90 litres of mixed water, approx. 40–55 litres of hot water from the tank
Standard bath (2/3 full): 120–150 litres of mixed water, of which 70–90 litres hot
Large bath (full): 160–200 litres of mixed water, approx. 95–120 litres hot
Washing dishes in the sink: 10–20 litres of hot water from the tank
Morning routine (washing face, hands, shaving): 5–10 litres of hot water

The mixing coefficient is important: the tank is heated to 60 °C, but when drawing water, you mix it with cold water (15 °C in summer, 8–10 °C in winter) to a pleasant temperature of 38–42 °C. This means that from every litre of water in the tank (60 °C), you get about 1.9 litres of mixed water (40 °C) in summer, and about 1.6 litres in winter. For simple calculations, a coefficient of 1.7–1.8 is used as a year-round average.

So the tank doesn't need to be sized for the volume of mixed water, but for the volume of hot water (60 °C) that you consume during the peak period, before the tank manages to reheat.

Reference table: what tank capacity for how many people

From dozens of installations, we've learned that the fastest reference indicator is the number of people in the household combined with the type of use (shower vs. bath) and timing (simultaneous vs. spread throughout the day). Here is the table we use for initial consultations:

Number of people	Mostly showers	Mix of shower/bath	Bath + showers	Typical tank
1–2 people	50–80 l	80–100 l	100–120 l	80 l
3–4 people	100–120 l	120–150 l	150–180 l	120–150 l
5–6 people	150–180 l	180–220 l	220–280 l	180–200 l
7 or more people	200+ l	250+ l	300+ l	200–300 l (external tank)

Note: these values apply to standard tanks heated to 60 °C with a mixing coefficient of approx. 1.7. If your tank is heated to only 50 °C (for example to save energy), you need a tank 20–30% larger, because the temperature difference against cold water is smaller.

Specific scenarios from practice: how we decide on real orders

Scenario 1: Two-person household in an apartment (60–75 m²)

This is a situation we deal with very often – an older, renovated apartment, two adults, each showering in the morning before leaving for work, i.e. practically at the same time. One showers for 6 minutes, the other for 8 minutes. Together that's approx. 60–70 litres of hot water from the tank (60 °C). If the tank manages to reheat during the first person's shower, the second one has no problem. With an 80-litre tank the situation is comfortable; a 60-litre tank would be on the edge. We recommend 80 litres as an absolute minimum, preferably 100 litres if the customer doesn't want to worry about it.

For this kind of use, the Thermona THERM PRO 14 KX is suitable, for example, which has an 80-litre built-in tank – a reasonable choice for a couple who shower, with a compact footprint suitable even for a smaller apartment boiler room.

Scenario 2: Four people in a family house, one bathroom

Parent + two teenagers + another adult. The teenagers typically take a bath every day, the parents shower. Morning peak: a bath (approx. 80 litres of hot water), then two showers of 35 litres each. Total draw-off during the morning peak: 150 litres of hot water from the tank. A 120-litre tank isn't enough without topping up from the boiler, meaning waiting. A 150-litre tank is on the edge. For this profile, we recommend at least 150–160 litres, ideally 180 litres.

Scenario 3: Five-member family, two bathrooms, family house

This is a typical case where a built-in tank starts to become a limiting factor. Staggered bathroom times, simultaneous showers, or once a week a bath for the children together – draw-off during the evening peak can reach 200–250 litres of hot water within 40–50 minutes. Here we reach the limit of standard built-in tanks, which usually don't exceed 200 litres. The solution can be a boiler with a larger built-in tank, or a combination of a boiler and an external tank (which, however, is a different product category).

Scenario 4: Single-person apartment, home office

The customer works from home, showers once a day, washes dishes by hand. Total daily hot water consumption doesn't exceed 60–70 litres, but it's spread throughout the day. For them, a 60–80-litre tank is completely sufficient. The rule here is: when consumption isn't concentrated in a peak but spread out, a smaller tank works great, because the boiler manages to reheat between draw-offs.

The effect of the tank's comfort temperature on its effective capacity

One of the most important – and least discussed – variables is the tank's set temperature. Many households set the tank to 55 °C for hygiene reasons (protection against Legionella) or to 65 °C for a larger thermal reserve. The difference is quite significant:

100 l tank at 55 °C with 10 °C cold water → effective volume of hot water (40 °C): approx. 136 l
100 l tank at 65 °C with 10 °C cold water → effective volume of hot water (40 °C): approx. 182 l

That's a 34% difference with the same physical tank volume! So if you want to get the most out of a smaller tank, heat it to a higher temperature – but at the same time, don't forget a safety thermostatic mixing valve on the outlet (i.e. a mixing valve set to a protective limit of 45 °C), so you don't injure the family with hot water.

Conversely, if you think you'll set the tank to 45 °C to save energy, you have to reckon with the fact that the effective usable volume of hot water (40 °C) will be minimal – practically the tank has no temperature reserve left for mixing. A 100 l tank at 45 °C will give you, with 10 °C water, only about 78 litres of mixed 40 °C water. Additionally, there's a risk of Legionella growth below 55 °C.

Built-in tank vs. external tank: where's the limit

Boilers with a built-in tank have a practical ceiling of around 100–200 litres, because the tank must be physically carried by the boiler's casing. For larger families or houses with higher comfort demands (sauna, multiple bathrooms), the solution is an external tank connected to a boiler without a tank, or a combination of a boiler with a smaller built-in tank and an external tank for solar heating.

From practical experience: most customers with a 4–5 member family who take comfort seriously choose a 200–300 litre external tank with solar panels. But if they're not interested in solar collectors and want a compact solution, a built-in tank up to 150–160 litres is usually enough, with somewhat more compromising behaviour (not everyone showers at the same time).

Tank construction in Thermona boilers: what to watch out for

If we look specifically at the range of Thermona low-temperature boilers with a built-in tank, we find several different solutions. Thermona offers tank solutions mostly ranging from 80 to 130–160 litres, depending on the model.

The Thermona THERM PRO 14 KX and its variant THERM PRO 14 KX (second version) are compact low-temperature boilers with a built-in tank designed for smaller households (1–3 people) or apartments where boiler room space is limited. The tank in these models is glass-enamelled, ensuring corrosion protection and long service life. A zinc or magnesium anode protects the tank electrochemically – you can read more about this topic in the article Rust, limescale and the tank anode – how to extend the boiler's service life.

For larger households, the Thermona THERM 28 LXZ.A 5 is worth considering, offering higher output and a larger tank capacity, suitable for 4–5 member families with higher comfort demands. The TKX models (for example the Thermona THERM PRO 14 TKX or the THERM PRO 14 TKX (second version)) are variants with connected preheating equipment – typically a solar heat exchanger or another added preheating device, which in practice means the effective usable tank volume increases thanks to an external heat source.

When choosing a model in this category, we also recommend checking the comparison article How to choose a low-temperature boiler with a built-in tank: capacity, output and operating type, which covers this topic from a broader perspective, including output parameters.

The effect of solar collectors on the required tank capacity

If you're planning solar collectors – whether now or in the future – the tank capacity needs to be sized differently. Solar systems work most efficiently when the tank has enough capacity to accumulate the heat collected during sunny hours. If the tank is too small, the solar system will quickly overheat it and shut down – you lose the return on your investment in solar panels.

The basic rule for sizing a tank with a solar system is: 50–80 litres per m² of collector. For a typical family house with 4–6 m² of solar collector area, this means a tank of 200–400 litres. These are values at which the built-in tank of a standard boiler (80–160 l) isn't enough, and an external tank with two heat exchangers is needed (one for the boiler, the other for the solar circuit).

However, if you're only planning a small solar system (2–3 m², for example just for preheating water), a 120–160 litre tank with a single solar heat exchanger may suffice. The TKX series boilers are designed precisely for this case – they contain a tank with a solar heat exchanger prepared in the lower section, where preheated water from the solar collectors enters. You can find more about this configuration in the article How to connect solar collectors to Thermona low-temperature boilers with a tank.

Tank reheating time: a practical matter often overlooked

When the tank is depleted – i.e. you've used up the supply of hot water – the boiler has to reheat the entire volume. This time is crucial for comfort: if reheating takes 30 minutes, the next shower can happen half an hour later. If it takes 60–80 minutes, that starts to become unpleasant.

Reheating time depends on the boiler's output and the tank's capacity. A rough formula:

t [min] = (V × ΔT × 1.163) / (P × 60)

where V is the tank capacity in litres, ΔT is the temperature difference (e.g. 10 °C incoming cold water → 60 °C, so ΔT = 50 K), P is the boiler's thermal output in kW. The constant 1.163 converts Wh into kilocalories, i.e. it accounts for the specific heat of water.

Example: a 120-litre tank, a 14 kW boiler, ΔT = 50 K:

t = (120 × 50 × 1.163) / (14 × 60) = 6,978 / 840 ≈ 8.3 minutes

A 120-litre tank with a 14 kW boiler reheats from cold water in about 8–10 minutes – that's great. A 200-litre tank with the same boiler: about 14 minutes. These are realistic values for boilers in the 14–20 kW category. A 28 kW boiler reheats a 200-litre tank in about 7–8 minutes. So the boiler's output really affects how long you wait for reheated water – it's not just a question of tank capacity.

Special situations: accommodation, home office, holiday property

A few less common but practically relevant scenarios:

Rented apartments or rooms: Here, consumption is unpredictable. Holiday guests tend to bathe mostly in the evening hours. We recommend sizing the tank at 1.5 times the standard need for the given number of people, or choosing an external tank with sufficient capacity.

Home office with children at home: If someone works from home and children are also at home, DHW consumption is more evenly spread throughout the day, which favours a smaller tank, because the boiler manages to reheat between the spread-out draw-offs. 100–120 litres for 4 people in this mode of operation is usually enough.

Holiday cottage or cabin: Use is irregular, weekend-based. Here it's important to set up the boiler so that, in the absence of occupants, the tank doesn't drop below a hygienically safe temperature (anti-Legionella program). We choose the tank capacity based on the maximum number of people during the peak stay (a summer weekend, Christmas), not based on average usage.

Most common mistakes when choosing tank capacity

From experience, we know the recurring mistakes customers make when deciding:

Underestimating the peak: The customer calculates based on average consumption, not the fact that three people will shower one after another within 20 minutes in the morning.
Ignoring family growth: Today's three-person family may become a five-person family in 5 years. A 100-litre tank that was sufficient may suddenly become tight.
Forgetting about the bathtub: The customer says "we only have shower cubicles" – and buys a bathtub a year later. A bathtub changes the whole calculation.
Underestimating the tank's heat loss: If the tank isn't well insulated or stands in a cold boiler room, it cools down faster and the boiler has to work harder. That's unnecessary cost.
Choosing based on the tank's price, not the actual need: A cheaper model with a smaller tank may seem advantageous – but everyday discomfort and a possible replacement in a few years are far more expensive.
Not accounting for cold winter water: In winter, water enters the tank at 8–10 °C instead of the summer 15–18 °C. The effective usable tank volume is always 10–15% smaller in winter than in summer.

Practical step-by-step calculation for your specific case

Here is the procedure we use when consulting with a customer. Go through it step by step:

Step 1: Determine the number of people. Not the current number, but a realistic outlook for the next 10 years.

Step 2: Determine the type of use. Mostly showers → count 30–40 l of hot water (60 °C) per person for peak draw-off. Bath → 80–100 l of hot water (60 °C) per bath.

Step 3: Estimate the length of the peak draw-off window. If everyone bathes within 30 minutes – the tank must cover everyone at once (the boiler manages to partially reheat, but not the entire volume). If the window is 60 minutes or more – the tank needs to cover about 70% of the total draw-off, the rest is reheated by the boiler.

Step 4: Calculate the required tank capacity. Add up the draw-off of all people during the peak and subtract about 15–20% (partial reheating during the draw-off). The result is the minimum tank capacity in litres of hot water (60 °C).

Step 5: Include a reserve. We recommend adding a 20–30% reserve for winter conditions, visitors, and future family growth.

Example calculation for 4 people, mix of shower/bath, 30-minute peak:
2 showers × 35 l + 1 bath × 90 l + morning routine for 2 people × 8 l = 70 + 90 + 16 = 176 litres of hot water (60 °C) needed. Minus 15% reheating = 150 litres. Plus 25% reserve = 188 litres. Result: minimum tank of 180–200 litres.

Frequently Asked Questions (FAQ)

Is an 80-litre tank sufficient for 3 people?

It depends on the household's habits. If everyone showers briefly (5–6 minutes) and not at the same time, 80 litres will handle three people without problems – the boiler manages to reheat between draw-offs. But if two people shower one right after the other and a third adds a bath, 80 litres isn't enough and you'll have to wait 10–15 minutes for reheated water. For a three-person family with more comfortable requirements, we recommend a 100–120-litre tank.

Can I later replace the tank with a bigger one without replacing the boiler?

With boilers featuring a built-in tank, it's not possible to replace the tank separately – the tank is an integral part of the boiler and is replaced together with it. That's why it's important to choose the right capacity from the start. One option for future capacity expansion is connecting an external tank (if the boiler model technically allows it), but not all models support this feature. Discuss this when choosing your boiler.

Why did my tank cool down quickly even though I set it to 60 °C?

Rapid tank cooling has several causes: the boiler room is very cold (below 10 °C), the tank's insulation jacket is damaged or insufficient, the tank is too small and gets depleted quickly, or the tank's anode is worn and the tank is affected by corrosion. We recommend regularly checking the anode – you can find more information in the article Rust, limescale and the tank anode – how to extend the boiler's service life.

Does it make sense to choose a bigger tank than I need, for reserve?

Yes, but only to a reasonable extent. A 20–30% reserve above the calculated need is reasonable and recommended. However, an unnecessarily large tank (for example 200 litres for a couple) increases investment costs, takes up more space, and above all increases heat loss – the tank has to constantly keep a larger volume at temperature, which shows up in energy consumption. That's why we don't choose a tank "to be safe" without an upper limit, but size it rationally.

How does the tank help me during a power or gas outage?

The tank acts as a thermal battery – it stores thermal energy. After a gas or power outage, you have a supply of hot water in the tank for several hours (depending on capacity and insulation) before the tank cools down. For a typical 1–2 hour outage, this is comfortable. A well-insulated 120-litre tank typically loses 1–3 °C per hour in a normal boiler room – so after an hour you still have usable hot water.

What if I'm planning solar collectors – how does this affect the tank choice now?

If you know you'll add solar collectors in a year or two, choose a boiler with solar circuit preparation (Thermona's TKX series). The tank in these models has a lower heat exchanger for preheating from solar panels, which in practice means a smaller tank with solar preparation covers greater consumption. Choosing a boiler without solar preparation and later dealing with an external tank is technically possible, but it means an additional installation and extra costs. The topic of solar connection is covered in the article How to connect solar collectors to Thermona low-temperature boilers with a tank.

Conclusion: a correctly sized tank is the foundation of a comfortable household

Choosing the DHW tank capacity is not a routine decision that can be settled with a single sentence. It's a technical sizing task that depends on the number of people, their habits, the draw-off profile, the planned development of the household, and the technical solution of the boiler room. A poorly chosen tank will bother you every day – too small won't give you comfort, too large will unnecessarily burden your budget and energy consumption.

That's why it's important to go through a specific calculation for your conditions – whether on your own following the procedure in this article, or in consultation with an expert when choosing a boiler. On the boilers with a built-in tank page, you'll find an overview of available models with specific tank parameters, where you can compare these values with your calculation.

For a complete decision on a suitable boiler, we also recommend reading other articles in the Knowledge Center – especially How to choose a low-temperature boiler with a built-in tank: capacity, output and operating type, where we also cover output sizing and choosing the right type of operation for your specific heating system, and Low-temperature boiler with a tank – boiler room requirements, dimensions and space, where you'll find out what spatial requirements a boiler with a tank places on the boiler room or utility room.

Do you have a question on this topic?

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