Frequently asked questions about thermal storage tanks and hot water heaters

We are still preparing products.

But you can look at other categories.

Frequently asked questions about accumulation tanks and hot water heaters – comprehensive technical overview

Over the years of practice in the field of heating technology, we repeatedly come across the same questions – from customers, installers and designers. Some seem simple at first glance, while others hide unexpected depth. We have compiled this article as a practical guide that answers what people are really interested in: what an accumulation tank actually is, how to choose it correctly, what can go wrong, how to take care of it and what to pay attention to during installation. Do not expect a marketing text – below you will find technical information that will really help you in decision-making or solving a specific problem.

What is an accumulation tank and what is its real purpose?

An accumulation tank – in professional jargon also called a "puffer" or "accumulator" – is a thermally insulated tank designed to store hot water, which serves as a thermal energy storage. Its basic principle is simple: when a boiler, heat pump or solar collector produces more heat than is needed at that moment, this heat is accumulated in the form of hot water in the tank. When the need for heat arises (heating, preparation of hot utility water), it is taken from the tank instead of the boiler having to start up again.

This seemingly simple device has a crucial impact on the efficiency of the entire heating system in practice. A solid fuel boiler without accumulation works discontinuously, with short intense cycles and long breaks – this is a catastrophe for any boiler in terms of lifespan and efficiency. An accumulation tank "evens out" these fluctuations.

In the diagram you can see so-called thermal stratification – a phenomenon where the lighter hot water naturally stays at the top of the tank, while the cold water sinks to the bottom. This physical principle is the basis of the proper functioning of an accumulation tank. Therefore, every well-designed tank has inlet and outlet nozzles placed precisely with regard to stratification.

What is the difference between an accumulation tank (puffer) and a hot water storage tank?

This is probably the most common question and at the same time a matter where the most misunderstandings occur. In short: a puffer stores heating water from the central heating circuit and this water never gets into your taps. A hot water storage tank (boiler, heater) stores and heats potable utility water that you will actually use for showering, hand washing, etc.

In practice, we also come across combined tanks – so-called "combination boilers" or tanks with an internal heat exchanger, which serve as a puffer (accumulation of heating water) and as a hot water storage tank (heating of potable water via a heat exchanger) at the same time. These devices are a compromise solution for more confined spaces, where there is no room for two separate devices, but they also have disadvantages – mainly lower heat accumulation capacity and more complex system setup.

A more detailed comparison can be found in the article Puffer vs. hot water storage tank – what is the difference and when to use which in this Knowledge Center.

What volume of an accumulation tank do I really need?

The volume of an accumulation tank is a key parameter and at the same time one where the most mistakes are made – usually too small a volume is chosen. A basic recommendation from practice: for each kW of boiler power, calculate at least 15 – 25 liters of tank volume. For a solid fuel boiler with a power of 20 kW, you therefore need at least 300 – 500 liters of accumulation volume.

For smaller applications – for example, smaller households with a heat pump or as a hydraulic equalizer in combined systems – there are compact solutions. For example, accumulation tank PUFFER PSS 50 (57 liters) is suitable precisely as a hydraulic separator in smaller systems, where the main thing is hydraulic balancing and not long-term heat storage. Similarly, PUFFER PSS 100 (123 liters) finds its application in smaller family houses or as an additional buffer in heat pump systems.

Factors that influence the required volume:

Type of heat source: Solid fuel boilers require a larger volume (min. 20–25 l/kW), gas boilers a smaller one (10–15 l/kW may be sufficient), heat pumps usually 20–50 liters as a hydraulic separator.
Burning time of one fuel load: The longer the boiler burns on one full load, the larger the volume you need, so that all the heat produced can be accumulated.
Thermal losses of the building: A house with low thermal losses (low-energy, passive) needs a proportionally larger puffer, because the heat from the boiler is consumed more slowly.
Combination of heat sources: If you have a boiler + solar collectors, the puffer volume increases – a solar system usually requires 50–80 liters per m² of collector area.
Available space in the boiler room: The physical dimensions of the tank are a limiting factor especially in renovations.

Detailed calculations and practical examples can be found in the article What volume of buffer tank do I need for my boiler or heat pump?

What are the individual nozzles (connections) on the buffer tank used for?

Modern buffer tanks have several nozzles (connections) at different height levels. Customers often ask why there are so many and what exactly they are used for. Here is a practical overview:

Top nozzle (outlet, usually G 1" or 6/4"): The hottest water leaves from here – to the heating circuit or to a heat exchanger for heating domestic hot water. Always connect the heating circuit outlet from the top.
Bottom nozzle (return, inlet for cold water): Here, the cooled water from the system returns to the tank. Correct connection ensures stratification.
Middle nozzles: Used for connecting a solar heat exchanger, an electric heating element (flange), temperature sensors, or an additional heat source.
Nozzle for thermometer/dip tube: A side nozzle for inserting a submersible thermometer or a controller sensor.
Drain valve: The lowest point of the tank, used for draining and cleaning.
Pressure relief valve / expansion tank: Every buffer tank must be protected by a pressure relief valve set according to the maximum tank pressure (usually 3 bar).

Does a buffer tank need thermal insulation?

Yes, and high-quality thermal insulation is absolutely necessary. Without it, you will lose heat before you can use it. Most buffer tanks are delivered with a removable insulation jacket made of polyurethane foam or mineral wool with a thickness of 50–100 mm. If you buy a tank without insulation (so-called "naked tank"), you should budget for additional insulation.

In practice, we have seen boiler rooms where the buffer tank had no insulation – in winter it did not matter (the heat stayed in the boiler room), but in summer the boiler room became uncomfortably hot and the tank's heat losses over 24 hours reached a temperature drop of 15–20 °C. A properly insulated tank loses only 2–5 °C over 24 hours at an ambient temperature of 20 °C.

What is a sacrificial anode and why is it important?

A sacrificial anode is a rod made of magnesium alloy, which is screwed into a suitable nozzle of the buffer tank or hot water storage tank. It works on the principle of sacrificial corrosion: magnesium is electrochemically more active than steel, so it corrodes preferentially – thus protecting the steel tank from corrosion.

Without a sacrificial anode – or with an exhausted anode that has not been replaced in time – the tank itself corrodes. A corrosion hole in the hot water storage tank means replacing the entire unit. This is several times more expensive than regular anode replacement, which costs only a fraction of the tank's price.

In practice, we recommend checking the anode every 1–2 years. If the anode is worn more than 70 % (the remaining rod is less than 30 % of the original weight), it needs to be replaced. For storage tanks up to 600 liters, for example, a suitable option is a sacrificial anode 5/4" × 400 mm with a galvanized plug and inspection device, for larger volumes an anode 5/4" × 700 mm. For tanks from 300 to 600 liters, also suitable is an anode 5/4" × 400 mm with a galvanized plug.

Warning: not every buffer tank needs an anode. A buffer tank that does not contain potable water (operates only with heating water in a closed circuit) usually does not have and does not need an anode – heating water in a closed system is chemically treated and does not cause the same corrosion as potable water. An anode is mandatory equipment for hot potable water storage tanks.

More about inspection, replacement, and choosing the right anode can be found in the article Sacrificial anode in a buffer tank – what it is, when and how to replace it.

What is the difference between a buffer tank with one and two heat exchangers?

Buffer tanks with heat exchangers (so-called "combined tanks" or "tanks with heat exchanger") are usually intended for more complex systems where there are multiple heat sources or where heating of domestic hot water is handled simultaneously.

Tank without a heat exchanger (naked buffer): Heating water enters directly into the tank and mixes with its contents. Simple, cheap, suitable for systems with one source (e.g. TTP boiler + radiators).

Tank with one heat exchanger (lower spiral coil): The heat exchanger is usually located in the lower part of the tank and is used for heat input from a solar system or another low-temperature source. Primary heating water enters directly (without a heat exchanger).

Tank with two heat exchangers: A lower heat exchanger for solar collectors, an upper heat exchanger for heating domestic hot water (or for a second heat source – e.g. heat pump). The most complex solution, requires precise hydraulic connection and control.

In family homes with a combined system of TTP boiler + solar panels + hot water heating, a two-exchanger tank is almost standard. This saves the installation of a separate solar storage tank and space in the boiler room, but you should expect a higher purchase price and more complex control.

Can I install an electric heating element in a buffer tank?

Yes, most buffer tanks have a flanged connection for this purpose – it is usually a flanged opening G 6/4" or directly a 5/4" connection on the side of the tank. An electric heating element (so-called "electric flange" or "immersion heater") with a power of typically 2, 3, 4 or 6 kW is inserted into it.

Electric element is used as:

Standby heat source: When the boiler is not working (service, power outage, summer period), the electric element keeps the water warm.
Boost heating of domestic hot water: In tanks where the primary source is solar or heat pump, the electric element boosts the water temperature when the sun is not enough.
Primary source in transitional period: In spring and autumn, when it is not cost-effective to operate the boiler, but heat is needed.
Legionella protection: Regular heating to 70 °C electrically kills Legionella pneumophila bacteria.

Important warning: an electric heating element must be installed by a person with a valid electrical qualification. In tanks for potable water, the flange must have the correct IP protection and must be compatible with the existing tank neck.

How to correctly connect an accumulator tank to the heating system?

The connection depends on what the heat source is and what type of heating circuit you have. There are two basic schemes:

1. Series connection (boiler → accumulator tank → heating circuit): The boiler heats the water, which first goes to the top part of the buffer tank. From the top part of the buffer tank, it goes to the heating circuit. The return from the circuit goes to the bottom part of the buffer tank, from where the boiler draws cold water for heating. This is the classic connection for a solid fuel boiler.

2. Connection via a hydraulic separator: The buffer tank serves as a hydraulic bridge between the primary circuit (boiler or heat pump) and the secondary circuit (heating, hot water). Both circuits are hydraulically separated, which is advantageous when the flow rates and temperature levels differ.

We recommend always connecting a safety valve at the boiler outlet, an expansion tank, and a drain valve during installation. A detailed installation procedure can be found in the article Installation of an accumulator tank – procedure, connection, and placement in the boiler room.

Accumulator tank and heat pump – what is the correct procedure?

Heat pumps (air/water, water/water, ground/water) have specific hydraulic requirements. Modern heat pumps with inverter compressors actually do not need a large buffer tank for "storing" heat – but they need a sufficient volume of water in the system to prevent the compressor from cycling too often (protection against short cycling).

The minimum water volume in the system for heat pumps is usually specified by the manufacturer in the technical documentation. If the water volume in the pipes and heating elements is insufficient (which is common in underfloor heating with low water content or in fan coil units), a small buffer is added as a hydraulic separator – not for accumulation, but to provide a sufficient water volume.

More technical details about the combination of a buffer tank with a heat pump are covered in the article Accumulator tank in a system with a heat pump or solar collector.

What is the maximum operating pressure of an accumulator tank?

Most standard accumulator tanks for family homes are designed for a maximum operating pressure of 3 bar (300 kPa). Some hot water storage tanks are pressurized to 6 or even 10 bar (so they can be connected directly to the water supply network without a pressure-reducing valve).

It is important that the safety valve in the system is correctly set and functioning. The safety valve must be set to a pressure equal to or lower than the maximum operating pressure of the tank. For example, if the tank is designed for 3 bar, set the safety valve to 2.5 or 3 bar. Never block the safety valve and check it regularly (at least once a year).

What is the lifespan of an accumulator tank and what does it depend on?

A quality accumulator tank should last 20 – 30 years. In practice, however, we have seen tanks that failed after 5 years, as well as those that have been operating reliably for 35 years. The lifespan depends mainly on:

Water quality: Hard water with high calcium and magnesium content forms limescale inside the tank, which reduces the efficiency of heat exchangers and can damage the walls. Water softening or regular cleaning extends the lifespan.
Functionality of the protective anode: A depleted or missing anode = rapid corrosion of the potable water tank.
Quality of the internal tank surface: Tanks with an internal glazed layer are more resistant to corrosion than tanks with just steel. There are also stainless steel tanks (much more expensive, but no anode is needed).
Operating temperature: Long-term operation at maximum temperatures (above 90 °C) accelerates degradation. We recommend a maximum operating temperature of 80 °C in normal operation.
Regular maintenance: Anode inspection, tank interior inspection, descaling of heat exchangers.

A detailed guide on extending the lifespan can be found in the article How to extend the lifespan of an accumulator tank – maintenance and regular service.

What temperature should be in the hot water storage tank?

The temperature of the hot water tank is a compromise between two risks: too low temperature (below 55 °C) promotes the growth of Legionella bacteria, while too high temperature (above 65 °C) accelerates limescale buildup and increases energy consumption.

The recommended operating temperature of the hot water tank is 55 – 60 °C. If the tank is out of use for a longer period (holiday, weekend), we recommend heating the water to 70 °C for at least 30 minutes upon return – this kills any possible Legionella bacteria (so-called thermal disinfection, Legionella protection).

Some controllers and tanks have an automatic "anti-Legionella" function that regularly (e.g., once a week) increases the temperature to 70 °C for a set period. This function should be activated if the tank is not in daily intensive use.

Why does the water in the tank smell and what to do about it?

The smell of hot water (typically "sulfur-like" – like rotten eggs or swamp) is a fairly common complaint and usually has two causes:

1. Sulfates in drinking water + magnesium anode: Sulfate-reducing bacteria in the presence of a magnesium anode produce hydrogen sulfide (H₂S). Solution: replace the magnesium anode with an aluminum anode (if the sulfate content in the water is high) or install an anode with a zinc core.

2. Legionella and other bacteria: Long-term water stagnation at low temperature (below 50 °C) causes the growth of various bacteria. Solution: thermal disinfection (heating to 70 °C), regular operation of the tank at a minimum of 55 °C.

If the smell appears after replacement or with a new tank, it is usually the first cause. After thermal disinfection and possibly changing the type of anode, the problem usually disappears.

How do I know if the accumulator tank or water heater is damaged?

Failures of accumulator tanks and water heaters have typical symptoms:

Reddish water from taps: Corrosion of the inner wall of the tank – either a missing or depleted anode, or a damaged enamel layer.
Leaking or leaking tank: Corrosion has pierced the wall (end of the tank's life – usually replacement, not repair).
Long heating time: Limescale buildup on the heat exchanger (insulates, slows down heating). Solution: descaling.
Increased energy consumption without change in usage: Again, limescale or poor insulation.
Noise – cracking, bubbling: Typical for electric water heaters – limescale on the heating element. The element needs to be cleaned or replaced.
Leaking from the safety valve: Either the system pressure is too high, or the expansion tank has lost pressure and the safety valve is taking over its function. The expansion tank needs to be checked and inflated.

A systematic overview of faults, causes and solutions can be found in the article Common faults of accumulator tanks and water heaters – causes and solutions.

Can I place an accumulator tank on its side – horizontally?

Most accumulator tanks are designed for vertical installation. Horizontal installation disrupts thermal stratification (hot and cold water mix more quickly), which significantly reduces the efficiency of the accumulation. In addition, in a horizontal position, the protective anode and the air vent are not properly positioned (air accumulates in a different place).

There are special tanks designed for horizontal installation, but in standard practice it is not recommended to place a standard vertical tank on its side. If the height of the space in the boiler room is an issue, look for a tank with a smaller diameter and greater height (slender design), or install multiple smaller tanks in parallel.

Can I connect two accumulator tanks in parallel?

Yes, parallel connection of two (or more) tanks is a common solution when one tank is not sufficient in volume, but the space does not allow installation of one large tank. The condition is proper hydraulic balancing – both tanks must have the same inlet and outlet pressure, otherwise one tank will be filled preferentially.

With a parallel connection, it is necessary for the supply and return to be connected symmetrically (same length of branches, same pipe dimensions) or to use a distributor (header). An unbalanced parallel connection causes uneven heat distribution and reduces the efficiency of the entire system.

Most frequently asked questions (FAQ)

Do I have to install an accumulator tank to a solid fuel boiler?

It depends on the country and current regulations. In Slovakia, installation of an accumulator tank is recommended by standards and boiler manufacturers for solid fuel boilers with a power output above 15 kW – and for some types of boilers (gasification boilers, boilers with forced combustion), it is even mandatory for proper operation and safety. A boiler without accumulation would smolder at low load (inadequate combustion), which increases emissions and soot buildup in the chimney. An accumulator tank allows the boiler to burn at full power at all times (which is optimal for combustion) and the heat is stored in the tank. Heat is then drawn from the tank continuously according to demand.

What is the difference between a steel and a stainless steel accumulator tank?

Steel tanks (carbon steel with internal protection – enameling or epoxy coating) are significantly cheaper, but require a protective anode and are more prone to corrosion if the anode is depleted or the enamel layer is damaged. Stainless steel tanks (AISI 316L or similar alloy) are more resistant to corrosion, do not require an anode, have better hygienic neutrality (suitable even for more aggressive water) and a longer lifespan. The price is 3–5 times higher. Investment in a stainless steel tank is worthwhile especially where hard or aggressive water is present, or where anode maintenance is difficult.

Can an accumulator tank freeze if the boiler room is not heated?

Yes, if the temperature in the boiler room drops below 0 °C, freezing of the water in the tank is possible. This is fatal for any steel tank – ice has a larger volume than water, and the tank walls crack. In an unheated building during the winter period, the tank must be drained or freezing prevented in some other way (minimum temperature in the boiler room at least +3 °C). If freezing is a risk, for example due to a power or heating outage, the addition of an antifreeze solution (propylene glycol) to the heating circuit – but not to the potable water tank – may be considered.

Why does hot water in the house only reach a maximum of 40 – 45 °C, even though the tank is heated to 60 °C?

This is a classic problem of a thermostatic mixing valve (thermostatic valve or TVV). This valve is installed after the tank and automatically mixes hot water from the tank with cold water to ensure the output temperature does not exceed the set value (a safety measure against scalding). If the valve is incorrectly set or faulty, the hot water is cooler than it should be. Check the valve setting (usually 38–50 °C) or clean/replace it.

How long will hot water stay hot in an accumulator tank without further heating?

It depends on the quality of thermal insulation, the tank volume, the ambient temperature, and the required output temperature. Approximately: a well-insulated tank (100 mm PUR insulation) loses about 3–7 °C in 24 hours at an ambient temperature of 20 °C and a content of 500 liters. With poorer insulation (or if the tank is in a cold environment), losses can reach 10–15 °C in 24 hours. Therefore, tank insulation is key – a well-insulated tank heated in the morning to 80 °C will still be above 70 °C in the evening (18 hours later).

What is the price of an accumulator tank and what does it depend on?

The prices of accumulator tanks vary widely – from several tens of euros for small hydraulic separators (50–100 liters) to several thousand euros for large combined tanks with heat exchangers (1 000 – 2 000 liters). Price is influenced by: volume (directly proportional), material (steel vs. stainless steel), number and type of heat exchangers, thickness of thermal insulation, certifications, and country of origin. Remember: a cheaper tank that will need replacement in 8 years may be more expensive overall than a high-quality unit with a 25-year lifespan.

Conclusion: what to take from this article into practice

Accumulator tanks and hot water storage tanks are devices that do not draw attention to themselves when functioning properly. Problems arise when the correct volume is not selected, when the protective anode is neglected, or when the installation is done without understanding the principle of stratification and hydraulics. An investment in a properly dimensioned, high-quality, and regularly maintained tank pays off in energy savings, comfort, and the long life of the entire heating system.

If you are unsure about the selection, see other professional articles in this Knowledge Center – for example, How to choose an accumulator tank – volume, type and connection to the system or Electric heating element in an accumulator tank – installation and selection of the flange. You will find specific procedures, calculations, and practical recommendations verified in real installation practice.

Do you have a question on this topic?

Having trouble making a decision or dealing with a specific situation in your household? Write to us - we'll be happy to help.