How to choose a hot water storage tank – volume, heat exchanger and other criteria
How to choose a hot water storage tank – a complete guide to selection
Selecting a hot water storage tank for domestic hot water (DHW) is one of the decisions that will significantly affect the comfort of heating and hot water preparation for decades to come. A mistake in selection will either result in insufficient capacity – which in practice means cold water in the shower in the morning for a larger family – or an unnecessarily oversized tank that takes up unnecessary space in the boiler room and increases acquisition costs. Let's look at the whole topic systematically, from basic terms to specific decision-making criteria that I personally use when designing systems for customers.
What a hot water storage tank actually solves and why you need it at all
Modern heat sources – especially heat pumps, solar collectors, and to some extent condensing boilers – do not operate efficiently in direct heating mode. A heat pump achieves the highest COP (coefficient of performance) during long, smooth running cycles. When connected directly to DHW preparation without a tank, it starts up every few minutes and runs only briefly – so-called pulsing mode, which dramatically reduces the compressor's lifespan and significantly worsens energy efficiency.
A hot water storage tank elegantly solves this problem: the heat source runs longer and less frequently, heating a large volume of water to the desired temperature, which is then available whenever the user needs it. The same principle applies to solar collectors, where energy comes only during the day and the hot water in the tank is available even in the evening. From an operational perspective, this is therefore not a luxury – in a properly dimensioned system, a hot water storage tank is an essential part of an energy-efficient installation.
In the diagram, you can see the basic principle of stratification – hot water stays in the upper part of the tank, while cold water enters from the bottom. This layering is essential for proper function, and we will return to it when selecting the correct inlets and outlets.
First criterion: The correct volume of the hot water storage tank
Volume is the parameter where mistakes are most commonly made – and in both directions. In practice, I see two main situations: customers with a 200-liter tank connected to a heat pump, where the pump cycles every three minutes and electricity consumption is 30% higher than it should be. The other extreme is 2,000-liter tanks in a two-apartment building, where the water cannot be consumed and legionella has ideal conditions for growth.
The basic rule for single-family homes with a heat pump: calculate with a value of 50–80 liters per person for daily comfort, but at the same time you must also consider the heat source's capacity and the desired cycle length. A 10 kW heat pump should run continuously for at least 30–45 minutes. In 45 minutes, it will deliver: 10 kW × 0.75 h = 7.5 kWh. This corresponds to heating approximately 215 liters of water from 15 °C to 50 °C. Therefore, the minimum tank size for such a pump is around 200–300 liters, but realistically – for a family of four – you will want to have a reserve for at least two showers, which pushes you to over 500 liters.
Specific dimensioning according to the number of people and the heat source:
| Number of people | Heat source | Recommended volume | Note |
|---|---|---|---|
| 2–3 people | Condensing boiler | 200–300 l | the boiler heats quickly, a smaller tank is sufficient |
| 2–3 people | Heat pump 8–10 kW | 500–700 l | longer cycles, higher comfort |
| 4–5 people | Heat pump 10–14 kW | 800–1 000 l | typical single-family home |
| 5–8 people / guesthouse | Solar + boiler/HP | 1 000–1 500 l | solar requires a larger buffer |
| 8+ people / hotel / commercial | Multiple sources or cascade | 2 000–3 000 l and more | individual project solution |
More details about calculating the specific volume for your household can be found in the article What volume of a hot water storage tank do I need for my household, where there are also calculation formulas and examples for different consumption scenarios.
Second criterion: Heat exchanger – number, area and material
The heat exchanger (coil, in English heat exchanger coil) is a tubular coil immersed in the tank, through which the heat transfer fluid from the heat source flows. The water in the tank never mixes with the heat transfer fluid – energy transfer occurs only through the wall of the heat exchanger. This is a crucial safety and hygiene factor, but also a critical parameter for the efficiency of the entire system.
Heat exchanger area – why it is important
The performance of the heat exchanger depends directly on its area. A small heat exchanger with an area of 1–1.5 m² cannot transfer heat quickly enough, and the heat source cannot deliver its full performance – the system underperforms. This is especially critical for heat pumps, as they operate with lower temperature differences than boilers.
Approximate values for selecting a heat exchanger:
- Condensing boiler (> 60 °C): a heat exchanger with an area of 1.5–2 m² is usually sufficient
- Air-to-water heat pump (max. 55 °C): minimum 2–3 m², ideally 3.5–4 m²
- Water-to-water or ground-to-water heat pump: 3–5 m² depending on the power
- Solar collectors: 0.5–1 m² of heat exchanger area for each m² of collector
One or two heat exchangers?
Most compact tanks have one coil located in the lower part of the tank. This solution works well if you have one heat source. However, if you combine, for example, a heat pump with solar collectors or with boiler boosting, you need a tank with two heat exchangers. A typical arrangement is: a lower heat exchanger for the solar circuit (lower temperature, but longer heating time), an upper heat exchanger for the boiler or heat pump (quick boosting when energy from the collectors is insufficient).
A detailed comparison of both variants – including connection diagrams and practical recommendations – can be found in the article Hot water storage tank with one or two heat exchangers – what is the difference.
Electric boosting circuit (element)
Most quality hot water storage tanks also have a connection (inlet) for an electric boosting element – a heating element that can heat the water electrically if the primary heat source is insufficient or out of order. This is a very practical backup. When selecting a tank, check whether it has a suitable inlet for the element (usually a G6/4" or G1½" thread) and where it is located – typically in the upper third of the tank, where the hottest water is retained.
Third criterion: Tank material and corrosion protection
Hot water storage tanks for domestic use come into direct contact with drinking water, so material protection is critical from both a hygiene and lifespan perspective.
Steel with enamel coating
The most common and reliable material for drinking water tanks. Enamel (a glass-like coating) is chemically inert, smooth, and bacteria have difficulty adhering to it. The disadvantage of enamel is its brittleness – a mechanical impact can cause micro-cracks, through which the steel wall then corrodes. Therefore, magnesium anodes are essential for enamelled tanks, which protect the tank through sacrificial corrosion.
Stainless steel
A premium option, especially for tanks where there is an increased risk of aggressive water (soft water with low pH, high chlorination). Stainless steel tanks do not require an anode, are lighter, and have practically unlimited lifespan. The disadvantage is the higher price – typically 30–50 % more expensive than enamelled steel.
Magnesium anode – mandatory inspection
For enamelled tanks, the uncompromising rule applies: the anode must be regularly inspected and replaced, usually every 2–4 years (depending on water quality and operating temperature). If the anode is completely depleted and you do not notice it, the tank will start to corrode directly in the steel wall. A customer who came to me with a 10-year-old tank and rusty water – the entire cause was an anode that had not been replaced for almost 8 years.
Fourth criterion: Connections, spacing and hydraulic layout
A hot water storage tank is not just a "water barrel" – it is a hydraulic node of the entire installation. Every properly designed solution must account for where the inlets and outlets are, at what height, in which direction, and what thread they have.
Basic connections
- Cold water inlet (bottom): cold water always enters the lower part of the tank to avoid disturbing the stratification
- Hot water outlet (top): draw the hottest water from the upper part
- Heat exchanger inlet/outlet: heat transfer fluid enters at the bottom and exits at the top to maximize heat transfer
- Circulation outlet: for the circulation pump, which maintains the temperature in the distribution
- Thermometer/thermostat inlet: immersion sleeve for regulation
- Anode inlet: usually at the top of the tank
- Inspection opening: for cleaning the interior, inspection and possible anode replacement
Before ordering, always sketch the layout of your boiler room and verify whether the connections match your distribution routes. A customer from Trenčín once called me a day after installation, saying that he was missing an outlet for the circulation circuit. He had to have a branch welded – unnecessary complications that can be avoided by carefully checking the tank's technical specifications before purchase.
Fifth criterion: Tank insulation
Thermal insulation is a parameter that customers systematically underestimate. A well-insulated tank loses less than 3–5 °C in 24 hours. A poorly insulated cheap tank can lose 10–15 °C overnight, which means the heat source has to reheat several tens of liters every day – and this directly affects operating costs.
Quality insulation for TUV tanks should be at least 80–100 mm thick rigid polyurethane foam. Rigid foam is significantly better than removable fabric covers, as it does not create thermal bridges at the connections and evenly covers the entire tank including the bottom. Some manufacturers offer versions with insulation thickness of 100–120 mm – for longer tank operation (e.g., cottage, seasonal use), this is an investment that pays off.
A more detailed comparison of insulation types and their impact on heat losses is covered in the article Insulation of TUV accumulation tank – why it is important and what to look for when choosing.
Sixth criterion: Maximum operating pressure and temperature
These are safety parameters that must match your system. Standard TUV tanks are dimensioned for an operating pressure of 6–10 bar and a maximum temperature of 90–95 °C. For most standard installations with a pressure tank or direct connection to the water supply, this is sufficient.
However, be cautious with combinations involving a solar collector – a solar circuit can reach temperatures of 160–200 °C and pressures of 10+ bar during stagnation (sunny days, zero consumption). The heat exchanger in the tank must be dimensioned for these conditions, or you must install a safety valve and an expansion vessel with sufficient capacity in the solar circuit.
Specific products and for whom they are suitable
In the category of TUV accumulation tanks, you will find several proven solutions for different situations. Here are specific recommendations based on the scenario:
For a family house with 4–5 people and a heat pump up to 14 kW
ZGIB 850-liter accumulation tank is a typical choice for a larger family house or a small guesthouse, where it combines daily comfort with sufficient buffer for the heat pump. An 850-liter volume provides a sufficient supply of hot water for 4–5 people with some reserve, and the heat pump has enough space for long, economical cycles. A similar role is played by the ZGIB 1000-liter accumulation tank, which I recommend wherever the family is larger or increased TUV consumption is expected (house with showers and a bathtub, frequent visits).
For larger buildings, combined heat sources or solar
Where there is a combined heat source (solar + boiler or solar + heat pump) and a higher number of people, I recommend the ZGIB 1500-liter accumulation tank. A solar installation with a collector area of 10–15 m² requires precisely this volume so that the solar energy from a long summer day has somewhere to go and does not stagnate unnecessarily. This volume also covers double-shift consumption in a guesthouse for up to 8 people.
For guesthouses, hotels or larger operations
In the case of commercial use with higher and less predictable hot water consumption, volumes of 2 000+ liters are suitable. ZGIB 2 200-liter accumulation tanks are specifically designed for these applications – residential buildings, smaller hotels, rehabilitation centers. For the most demanding operations or in systems with high-performance heat pumps, there is also the solution HPWB3000 accumulation tank with a volume of 3 000 liters, optimized for systems with higher-performance heat pumps.
Space requirements and installation conditions
Before making a binding choice of volume, check whether the selected tank physically fits into the space of the boiler room. A tank with a height of 2 200 mm plus a top inlet (approximately 150–200 mm for the anode and pressure gauge) actually requires a room with a minimum height of 2 500 mm. With a ceiling height of 2 400 mm, which is common in basemented family homes, this can be a problem.
Measure the width of doorways and corridors as well. A 1 000-liter tank typically has a diameter of 790–850 mm and a height of 1 700–1 900 mm. Some products are available in flatter variants with a larger diameter – check the exact dimensions in the technical specifications. Comprehensive overviews of dimensions can be found in the article Dimensions and space requirements of TUV accumulation tanks 500–3 000 liters.
An empty tank weighs 80–300 kg depending on the volume, a full tank is of course a multiple of that. For a volume of 1,000 liters, expect a total floor load of approximately 1,100–1,200 kg. If you install it in a basement on a concrete floor, it is not a problem. On wooden beams of an upper floor, it would be a problem – a structural assessment is required there.
Connection to a heat pump or boiler
The hydraulic connection of the tank to the heat source significantly affects the efficiency of the entire system. The most common mistake: too small a pipe diameter between the HP and the tank, or the reverse connection of the inlet/outlet of the heat exchanger. If the hot water inlet from the HP goes to the top part of the heat exchanger and the cold water outlet returns to the bottom, the heat exchanger is working correctly in a counterflow. If you have it the other way around, the efficiency of heat transfer drops by 15–25 %.
A detailed description of the correct connection including diagrams for various configurations can be found in the article Connecting a hot water storage tank to a heat pump or boiler – how to do it. I recommend reading it before ordering materials, so that you have the correct pipe dimensions and fittings properly selected.
Protection against legionella – thermal disinfection
Legionella pneumophila is a bacterium that thrives at temperatures of 25–50 °C and forms dangerous aerosols, for example, during showering. In hot water storage tanks, it is a real risk if the water remains in this temperature range for a long time.
Prevention is simple: regularly (at least once a week, ideally automatically) heat the tank to at least 60 °C and let this temperature act for at least one hour. At 60 °C, legionella is destroyed within a few minutes, at 70 °C practically instantly. Modern controls for heat pumps have a "anti-legionella" function that automates this cycle. Make sure your controller has this function and is properly configured – it is a real safety requirement, not just a marketing feature.
For air-to-water heat pumps, which typically operate up to 55 °C, this means you must use an electric element or a combined source (boiler) for disinfection. Set the cycle, for example, every Sunday at 2:00 AM – outside of peak hours and outside of showering times.
Price vs. quality – where is the real difference
You can find tanks with the same marked volume on the market from a price of 400 € to 1,500 € and more. What is the difference? Based on experience, I list the most important differences:
- Thickness of the enamel coating: cheaper tanks have a thinner enamel, which is more prone to micro-cracks due to temperature fluctuations
- Surface area of the heat exchanger: cheap tanks save on the coil area – a shorter coil = less heat transfer = lower heating performance
- Thickness of the insulation: cheap variants have 50 mm foam, quality ones have 80–100 mm
- Quality of the welded joints: in cheap tanks, the welds around the connections corrode sooner than the rest of the tank
- Certifications and warranties: reputable manufacturers have CE, DVGW or SKZ certification, which guarantees that the materials are approved for contact with drinking water
An investment in a quality tank with a wall thickness of 3–4 mm, quality enamel, and strong insulation pays back in lower operating costs and a lifespan of 20–25 years. A cheap tank with a wall thickness of 1.5–2 mm lasts 8–12 years, after which problems begin – rusty water, leaks, replacement of the entire tank.
Installation and assembly – what to watch out for
The installation of a hot water storage tank should be carried out by a certified installer, as it is a pressure system connected to drinking water. In addition to the correct hydraulic connection, it is necessary to install a safety valve (set to the maximum operating pressure of the tank), an expansion vessel (if the system does not have a central one), and a thermometer with visual control.
The entire procedure including commissioning, first filling, and bleeding is described in the article Installation of a hot water storage tank – procedure, connection and commissioning. I recommend reading it, even if you hand the installation to a professional – as a customer, you can then check that it was done correctly.
Frequently asked questions (FAQ)
Can I use a hot water storage tank for heating as well?
Not directly. Hot water storage tanks for TUV are designed for direct contact with drinking water and have an enameled surface. Combined tanks (buffer-tank) are a special type, where the hot water tank is integrated into a larger heating accumulator tank using a "tank in tank" system or via a heat exchanger. If you want one tank for both functions, look for the explicit designation "combined tank" or "combi-tank".
How long will hot water stay in the tank without reheating?
With good insulation (100 mm PU foam), the tank can maintain the temperature by a maximum of 3–5 °C lower after 24 hours. In practice: if you set the night heating to 65 °C, you will have water at 60–62 °C in the morning – which is still a comfortable temperature for a shower. With cheap or damaged insulation, the loss can be 10–15 °C overnight, which significantly increases operating costs.
Do I need an expansion vessel for the tank?
Yes, always. When heating water, its volume increases (e.g., 1,000 liters of cold water expands by about 17 liters when heated to 60 °C). This excess must go somewhere – into the expansion vessel. Without it, the pressure in the system would rise until the safety valve opens, which is not normal operation. The size of the expansion vessel depends on the volume of the tank and the operating pressure – for a 1,000-liter tank, count on an expansion vessel of 25–35 liters.
Can I fill the tank with hard water from the tap?
Yes, normal water from the public water supply is fine. Problems arise with very hard water (over 400 mg/l total dissolved solids or hardness over 25 °dH) – repeated heating causes limescale to deposit on the tank walls and the heat exchanger, reducing heat transfer efficiency. With hard water, I recommend installing a water softener or at least checking the condition of the heat exchanger regularly (every 2–3 years) and possibly flushing the tank with citric acid.
When is it necessary to replace the magnesium anode?
The anode is checked during every annual service and replaced when it is worn down to less than 1/3 of its original thickness – usually every 2–4 years. In soft or chemically aggressive water, it wears out faster. If the wear is repeated quickly, consider switching to an active (electric) anode, which does not need to be replaced. Neglecting the anode is the most common cause of premature destruction of an enameled tank – more on this topic can be found in the article Maintenance and service of a hot water storage tank – what and how often to check.
What is the correct temperature setting for the tank?
The recommended operating temperature is 55–60 °C. A lower temperature (below 50 °C) increases the risk of legionella growth. A higher temperature (above 65 °C) unnecessarily increases energy consumption and accelerates limescale formation. For heat pumps that have difficulty reaching 60 °C at low outdoor temperatures, set the minimum temperature to 55 °C and the automatic anti-legionella cycle to 60 °C once a week (with an electric element if needed).
Conclusion: How to proceed with the selection
Selecting a hot water storage tank deserves more attention than most customers give it. In short: start with consumption (number of people, habits), then from the heat source (type, power, temperature range), consider the number of circuits (solar, boiler, HP), check the space conditions in the boiler room, and only then choose a specific product. Do not save on insulation or materials – this investment pays back in decades of reliable operation.
If you are unsure about any parameter, the article Common questions about hot water storage tanks will answer most typical doubts. For specific model solutions – from 850-liter tanks for family homes to 3,000-liter tanks for guesthouses – you can compare available products directly in the category hot water storage tanks.
Do you have a question about this topic?
Not sure how to decide or dealing with a specific situation in your home? Write to us – we are happy to help.