What Size Hot Water Cylinder Do I Need for a Low-Temperature Boiler

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What tank volume do I need for a low-temperature boiler?

This question seems simple at first glance, but in practice it hides a whole range of technical and operational aspects that determine whether you'll have enough hot water every day, or whether you'll be fighting the rest of your household over the last 30 litres of hot water in the morning. From my own experience with custom installations, I know that every second customer underestimates the tank selection – they either go for one that's too small (because it's cheaper and takes up less space), or one that's too large (because "more is better"). Both extremes have consequences – energy-related, spatial, and financial. In this article, we'll go through the whole topic systematically, from basic calculations to specific real-world scenarios.

Why the type of boiler matters when choosing tank volume

A low-temperature boiler operates with heating water temperatures typically in the range of 40–75 °C, and in modern designs these boilers are engineered to work efficiently at heating water temperatures of around 55–70 °C. This temperature is key for heating the domestic hot water (DHW) tank.

Condensing boilers can heat the tank faster and work more efficiently at lower return water temperatures. However, low-temperature boilers – both atmospheric and turbo – are robust, easy to set up, and when properly paired with a tank, run reliably for decades. The difference compared to a boiler with an integrated tank (instantaneous heating or combi boiler) is that a separate tank offers greater comfort – you're not limited by the flow rate of water, and several people can shower at the same time without a drop in temperature.

Low-temperature boilers with the option to connect a tank, such as the Vaillant atmoTEC plus VU CZ/SK 200/3-5 or Vaillant turboTEC plus VU CZ/SK 202/3-5, are equipped with a dedicated tank outlet and control logic that prioritizes tank heating – which is essential for everyday comfort.

Diagram: Basic connection of a low-temperature boiler with an external DHW tank. The boiler always prioritizes tank heating over space heating.

Basic calculation rules – from theory to a number

There are several methods for calculating the required tank volume. In common practice, simplified rules are used, but for larger or atypical households, I recommend going deeper.

The 50-litre-per-person rule

The most commonly cited rule states: allow 50 litres of tank volume per household member. This figure is based on average daily hot water consumption of around 40–60 litres per person at a temperature of around 40 °C (including showering, hand washing, dishes, etc.). The tank is heated to a higher temperature (typically 60 °C), so the real usable volume is somewhat larger than the nominal one – water "expands" to a larger volume at a lower temperature when mixed with cold water.

1–2 people: 80–120 litre tank
2–3 people: 120–150 litre tank
3–4 people: 150–200 litre tank
4–5 people: 200–250 litre tank
5 or more people: 250–300+ litre tank

These values apply to standard family homes with one bathroom and typical hot water usage. We'll now discuss when to deviate from this rule.

Calculation based on peak demand

A more precise approach is to calculate based on maximum hourly DHW demand – the amount of water the household consumes during "peak" time (typically morning hours, when everyone is showering and getting ready to leave). A boiler with a connected tank doesn't need to cover the peak instantly with its output – the tank acts as a buffer for this peak.

Shower with flow-control valve: 6–9 litres/minute at 38 °C
Bathtub: 150–200 litres per fill at 40 °C
Sink: 4–6 litres/minute
Dishwasher: 10–15 litres/cycle

When you add this up for a specific household during the morning peak (say, 45 minutes when four people take turns showering), you get a figure of around 180–250 litres at 38 °C – which, for a tank heated to 60 °C, corresponds to an actual tank of 120–160 litres, since mixing hot water with cold water yields a larger volume at a lower temperature.

Chart: Recommended DHW tank volumes for different household sizes. The darker block shows the recommended volume, the lighter block the extended range.

Factors that shift the choice away from the "50-litre rule"

In practice, I've seen dozens of installations where the simple formula failed. Here are specific situations where you need to oversize or, conversely, downsize the tank:

When you need a larger tank than the rule suggests

A bathtub in the household: If the family regularly uses a bathtub (not just a shower), a single fill consumes 150–200 litres at 40 °C. For a tank heated to 60 °C, that's actually 80–100 litres of tank capacity. A 150-litre tank for a 3-person household with a bathtub may be tight.
Multiple bathrooms: In a family home with two bathrooms and multiple toilets and shower cubicles, peak demand multiplies.
Guest accommodation / cottage: If the property is also used for short-term rental or regular overnight visits, plan for higher demand.
Lower boiler output: If the boiler has lower output (e.g. 12–15 kW), it heats the tank more slowly – with higher hot water demand, you need a larger "buffer".
Longer distance from the tank to draw-off points: If the hot water pipes are long (e.g. tank in a basement boiler room, outlet upstairs), you also need to account for pipe losses.
Older, poorly insulated tank: Combined with poorer tank insulation, heat escapes faster, and the boiler must heat more often – a larger volume helps.

When you can choose a smaller tank

Small household of 1–2 people with a shower (no bathtub): An 80-litre tank is sufficient, and real savings in purchase price, space, and energy for maintaining temperature are achievable.
Boiler with higher output relative to demand: If you have a more powerful boiler (e.g. 24 kW) and a small household, the tank recharges quickly and doesn't need to be large.
Combined DHW preparation with another source: If the tank is equipped with an electric heating element or a solar connection, the boiler only acts as a booster and the tank can be smaller.

The effect of low-temperature boiler output on tank selection

Boiler output directly determines how quickly the tank can be reheated. This is very important in practice – a tank that heats up in 45 minutes gives you much greater comfort than one that takes the boiler 2 hours.

An approximate formula for calculating tank heating time:

Heating time (h) = (Tank volume × 1.16 × ΔT) / (Boiler output × 1000)

Where ΔT is the temperature difference (from 15 °C to 60 °C = 45 K), and 1.16 is the specific heat capacity of water (Wh/l·K).

Example: a 150-litre tank, a 20 kW boiler, heating from 15 °C to 60 °C:
Time = (150 × 1.16 × 45) / 20,000 = 7,830 / 20,000 = 0.39 hours = approx. 23 minutes

Example for a weaker 12 kW boiler and a 200-litre tank:
Time = (200 × 1.16 × 45) / 12,000 = 10,440 / 12,000 = 0.87 hours = approx. 52 minutes

In practice, the actual time is 20–30% longer due to transfer losses (the tank's heat exchanger efficiency is not 100%), but the formula gives you a good estimate. For boilers like the Vaillant atmoTEC plus VU CZ/SK 240/3-5 with an output of around 24 kW, tank heating is fast even with larger volumes (200–250 l).

Table of approximate tank heating times at different boiler outputs and tank volumes. Actual time may be 20–30% longer depending on the quality of the tank's heat exchanger.

Specific real-world scenarios

To make these recommendations as useful as possible, here are several typical real-world installations, including the most common mistakes made when choosing a tank.

Scenario 1: Two-room apartment, 2 adults, shower only

The customer wanted a large 200-litre tank "with a margin." After discussion, we found out: no bathtub, both shower one after the other in the morning, each for about 7 minutes, and they work different shifts – the boiler always manages to reheat the water in time. We chose an 80-litre tank. Result: lower gas consumption (smaller volume of water to maintain at temperature), the tank fits in a smaller utility room, and they've never had a problem with hot water.

Scenario 2: Family house, 4 people, bathtub + shower, morning peak

Four people – two parents, two children. Morning routine: father showers (8 min), mother takes a bath (approx. 160 l), children each shower for 6 min. Boiler 20 kW. A 150 l tank was too small – after the bath, cold water mixed with the remaining hot water and the mother had to wait for reheating. The correct solution was a 200-litre tank or a change in bathing order (bath last, after overnight tank recharging). The customer solved it both organizationally and with a 200 l tank.

Scenario 3: Older family house, Vaillant atmoTEC 280/3 gas boiler, 5 people

The Vaillant atmoTEC plus VU CZ/SK 280/3-5 with an output of 26 kW and a 200-litre tank. The family felt the tank wasn't enough. Upon inspection, we found that the tank temperature was set to only 50 °C (originally set by a previous service technician), which reduced the effective usable volume. After raising the tank setting to 60 °C and activating the anti-legionella program, comfort improved significantly – without replacing the tank.

Scenario 4: Cottage with irregular use

A property occupied on weekends, intensively in summer (6–8 people), only occasionally in winter. A 20 kW turboTEC boiler. The customer wanted a 300-litre tank. Our recommendation: a 200-litre tank with an electric booster heater (backup 2 kW heating element) to top up during summer peaks when the boiler isn't enough. A cheaper solution than a 300-litre tank, with no heating issues.

Atmospheric vs. turbo boiler – the difference for tank sizing

This topic is covered in more detail in the article Atmospheric vs. turbo low-temperature boiler – which to choose with a tank, but from a tank perspective, it's important to know the following:

Atmospheric boilers (atmoTEC) generally have lower output and slower tank heating at the same nominal rating compared to turbo boilers. The tank's heat exchanger must be correctly sized – if the tank has too small a heat exchanger (small exchange surface area), heating will be slower regardless of boiler output. For the Vaillant turboTEC plus VU CZ/SK 122/3-5 with an output of 12 kW, we recommend tanks up to 120 litres for a typical household of 2–3 people to maintain heating comfort.

Energy efficiency – a larger tank isn't always economical

A larger tank holds more hot water – but it also "loses" more energy through the tank walls (so-called standing heat loss). Modern tanks with polyurethane insulation have heat losses of around 0.5–1.5 kWh/day at a temperature of 60 °C. Older tanks without good insulation can lose as much as 2–4 kWh/day.

Example: a 300-litre tank with a loss of 1.5 kWh/day versus a 120-litre tank with a loss of 0.6 kWh/day. The difference is 0.9 kWh/day = approx. 328 kWh/year. At a gas price of €0.08/kWh, that's a saving of about €26/year. Not dramatic, but an unnecessary loss if the tank is unnecessarily oversized.

This aspect is covered in more detail in the article Energy savings when combining a low-temperature boiler and a hot water tank.

Chart: Comparison of daily tank heat losses with modern polyurethane insulation versus old/poor insulation. The difference is significant with larger volumes.

Legionella and minimum tank operating temperature

This is an aspect that customers tend to underestimate, but it has a direct effect on properly sizing both the tank and the boiler. Legionella pneumophila bacteria multiply in water at temperatures of 25–50 °C. The tank must therefore be regularly heated to at least 60 °C, ideally once a week to 70 °C (anti-legionella program).

For a low-temperature boiler, this means: the tank must be sized so the boiler can heat it to 60–65 °C within a reasonable time (within 60–90 minutes under normal daily demand). If the tank is too large relative to boiler output, the anti-legionella heating takes too long and the boiler operates in a suboptimal mode. Setting heating priority and the anti-legionella program is described in detail in the article Setting tank heating priority on a low-temperature boiler.

Space and installation constraints

In addition to calculations, there are also practical constraints that narrow down the tank choice:

Height of the utility room or boiler room: A standing 200-litre tank is typically 160–175 cm tall, and a 300-litre tank 180–200 cm. If the ceiling is low (e.g. 200 cm), a 300-litre tank including accessories (valves, expansion vessels) may not fit.
Tank diameter: Larger volume means larger diameter. A 200-litre tank has a diameter of about 55–60 cm, a 300-litre tank 65–75 cm. The utility room door must be wide enough for handling during installation.
Horizontal tanks: An alternative for low spaces are horizontal tanks – these are mounted under the ceiling or under stairs, but have poorer water stratification (less efficient use of hot water at the top of the tank).
Weight of the tank with water: A 200-litre tank + water = approx. 240–260 kg. The utility room floor must be able to bear this; on upper floors or above a basement, this needs structural assessment.
Connection to existing piping: The tank must be positioned so that the hot water supply and return pipes aren't too long – long DHW runs cause heat loss and long waits for hot water at outlets.

More on physical installation and connection can be found in the article Installing a low-temperature boiler with a tank connection step by step and also in the article Connecting a Vaillant atmoTEC and turboTEC boiler to a hot water tank.

Tank with one or two heat exchangers?

Standard tanks for low-temperature boilers are equipped with a single (lower) heat exchanger, through which heating water from the boiler passes and heats the tank water. Tanks with two heat exchangers additionally have an upper exchanger – this serves a second heat source, typically solar collectors or a heat pump.

When combined with a low-temperature boiler and no solar setup, a single-exchanger tank is sufficient. If you plan to add solar collectors or another source in the future, invest right away in a dual-exchanger tank – a later modification is expensive and complicated. Tanks with a solar exchanger typically cost 10–20% more upfront, but the investment pays off in the medium term.

Enamel or stainless steel tank interior?

This is another practical decision point. Enamelled tanks are cheaper, common, but require regular inspection and replacement of the magnesium anode (every 2–3 years), which protects the interior from corrosion. Stainless steel tanks are more expensive, don't require an anode, and are more resistant to hard water and aggressive water. In areas with hard water (most of Slovakia), a stainless steel tank usually pays off through service savings and extended lifespan.

More on anode inspection and overall tank maintenance can be found in the article Maintenance and servicing of a low-temperature boiler with a tank – what and when to check.

The most common mistakes when choosing tank volume

Based on real-world installations, I've identified these recurring mistakes:

Choosing a tank based on available space rather than actual need: "Only an 80-litre tank will fit" – the customer then doesn't have enough hot water. Sometimes it's better to enlarge the utility room or place the tank elsewhere.
Ignoring the bathtub in calculations: The bathtub is the biggest hot water consumer in the household. Without accounting for it, the tank is always undersized.
Overestimating boiler output: Customers think a more powerful boiler will make a 300-litre tank hot in 15 minutes. The reality is different – even a 28 kW boiler needs about 44 minutes for a 300-litre tank.
Underestimating pipe losses: Long, uninsulated DHW pipes can lose 20–40% of heat in winter – the tank thus depletes sooner than expected.
Choosing a tank without a magnesium anode or with a non-functioning one: An enamelled tank without a working anode will rust through in 5–8 years. Regular checks are essential.

Practical recommendations by property type

Summary of recommended combinations for common property types:

Property type	Number of people	Recommended volume	Note
Apartment, shower only	1–2	80 l	Also 100 l if occasional guests
Apartment, shower + bathtub	2–3	120–150 l	150 l recommended
Family house, shower	3–4	150–200 l	200 l standard
Family house, bathtub + shower	4–5	200–250 l	Consider 2 exchangers for solar
Larger family house / 2 bathrooms	5–7	250–300 l	Boiler output min. 20 kW
Cottage / recreational property	variable	150–200 l + electric booster	Depends on max. occupancy

Frequently Asked Questions (FAQ)

Can I connect any tank to a Vaillant low-temperature boiler?

Technically yes, if the tank has an exchanger compatible with the boiler's temperatures and the connection dimensions match the boiler connections (usually G ¾" or G 1"). Vaillant recommends its own tanks from the VIH series or certified compatible tanks from other manufacturers. When using a third-party tank, you need to verify that the boiler's control system properly manages heating via the appropriate tank thermostat. More detailed connection information is described in the article Connecting a Vaillant atmoTEC and turboTEC boiler to a hot water tank.

Why isn't the tank enough even though I sized it correctly?

The most common causes are: the tank temperature is set too low (below 55 °C), the magnesium anode is non-functional, the tank is fouled with scale (reducing exchanger performance), or the boiler is faulty and not reheating the tank. Another common reason is a change in household consumption (a new family member, a bathtub was installed). Solutions to these problems can be found in the article Common faults of low-temperature boilers with a connected tank and their solutions.

Is a 200-litre tank always the right choice for a 4-person family?

In most cases, yes – 200 litres is a proven "sweet spot" for a 4-person family with shower habits. The exception is if the family regularly uses a bathtub or has two bathrooms with active simultaneous use. In that case, I recommend a 250-litre tank or a 200-litre tank with the option of electric boosting. Remember that not only volume matters, but also boiler output and tank recharge speed.

Does a 300-litre tank make sense with only a 12 kW boiler?

A combination of a 300-litre tank + a 12 kW boiler is unbalanced. The boiler will take over 100 minutes to heat the tank from cold, which is impractical and energy-inefficient for everyday use. For a 300-litre tank, we recommend at least a 20 kW boiler. If you have a 12 kW boiler, stick to tanks up to 150 litres (for a 3-person household).

Is it worth getting a tank with two heat exchangers if I don't have solar collectors yet?

If you plan to add solar collectors or a heat pump within the next 5 years, a dual-exchanger tank is definitely worth getting right away. The additional cost is 10–20% compared to a simple tank, but retrofitting a second exchanger (i.e. replacing the tank) would cost several times more. If you don't plan on solar or another source at all, a simple single-exchanger tank is sufficient.

How long does a tank last with proper maintenance?

An enamelled tank, with regular replacement of the magnesium anode (every 2–3 years) and descaling (every 5 years), lasts 15–20 years. A stainless steel tank, which doesn't need an anode, can last 25–30 years. The key is regular servicing – neglecting the anode on an enamelled tank shortens its lifespan to 5–8 years. The full maintenance plan is described in the article Maintenance and servicing of a low-temperature boiler with a tank – what and when to check.

Conclusion – how to proceed with your choice

Choosing the right volume for a hot water tank for a low-temperature boiler is something that shouldn't be underestimated or overestimated. The basic procedure is straightforward: determine the number of people in the household, find out whether you have a bathtub or just a shower, assess the boiler output and the space available in the boiler room, and choose a tank within the appropriate volume range accordingly.

The 50-litres-per-person rule is a good starting point, but always adjust it based on real conditions – bathroom type, boiler output, water hardness, and a planned second heat source. The most energy-efficient combination is a tank properly sized to the boiler output, with modern polyurethane insulation, a temperature setting of 60 °C, and a functioning anti-legionella program.

If you're unsure which combination of boiler and tank is optimal for you, start with the article How to choose a low-temperature boiler with the option to connect a tank and Frequently asked questions about low-temperature boilers with the option to connect a tank, where you'll find further practical advice for specific situations.

Do you have a question about this topic?

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