Frequently Asked Questions About Solar Storage Tanks
Frequently asked questions about solar storage tanks – answers from practice
Solar storage tanks are among those devices that raise a lot of questions from customers both before and after purchase. Some are completely basic – what it actually is and what it is used for – while others are very specific and technical, such as what thread size the manholes of the heat exchanger have, or what happens if the tank stands without use in summer. From years of customer experience, I know that the same questions repeat over and over, regardless of whether it is a family home with a few people or a cabin with a large family. This article collects them all in one place and answers them as we would explain on site – clearly, with numbers and without unnecessary fluff.
What is a solar storage tank and how does it differ from a regular water heater?
A standard pressure water heater is essentially an insulated tank with an electric heating element or with one heat exchanger (pipe or jacket type). It is designed to draw energy from one source – either electricity directly or heat from a boiler via the heat exchanger. A solar storage tank is structurally different in several ways at once.
First, in most cases, it has two heat exchangers – one in the lower third of the tank for the solar circuit, and another in the upper part for heating from the boiler or heat pump. The area of the lower heat exchanger is usually larger, as the solar system works with lower temperature differences than the boiler. Second, the tank wall is thicker and the thermal insulation is more pronounced, as the tank accumulates heat for a longer period – sometimes up to 24 hours, while a regular water heater assumes a faster turnover. Third, the connection is adapted to the specific pressures and temperatures of the solar medium – that is, a water-glycol mixture.
In practice, this means that if you were to connect only an electric heating element to a solar storage tank and not use any collector circuit, it would work technically – but you would be wasting money on an unnecessarily large tank. On the other hand, a regular tank without a lower heat exchanger cannot realistically be connected to a solar system, as you simply have nowhere to connect the solar medium without completely modifying the entire unit.
How many liters of tank capacity do I need for my household?
This is probably the most common question and also the one where mistakes are most frequently made – in both directions. People buy a tank that is too small and are disappointed that hot water is not sufficient in summer without additional heating, or they buy a tank that is too large, where the energy from the collectors is not enough to heat the tank and the boiler heating runs constantly.
An approximate rule that works quite reliably in practice: 50 to 70 liters of tank capacity per person, with the lower figure applying to smaller apartments with a basic hygiene standard and the higher figure for houses with a bathtub and higher consumption. For a family of four, this results in 200 to 280 liters – which is why the 250-liter tank is so popular in this segment.
For 5 to 6 people, or for a household where heating is done with solid fuel or a heat pump and the tank also serves as an accumulator, customers often go for a 300-liter tank or even a 400-liter tank. A 400-liter tank is also suitable when you have a larger collector area on the roof – for example, 6 to 8 m² of flat collectors or 3 to 4 vacuum tubes.
More detailed analyses with calculations can be found in the article What volume of solar storage tank do I need for my house in this Knowledge Center.
Is there a difference between a tank with one and a tank with two heat exchangers?
Yes, and it is a difference that in practice determines how efficiently the entire system will function. A tank with one heat exchanger assumes that you have exactly one heat source – either a boiler or solar, but not both at the same time as separate circuits. If you want a boiler and solar collectors, you usually need two separate heat exchangers, one for each circuit.
Technically, they also differ in the placement of the heat exchangers. The lower (solar) heat exchanger is usually larger – an area of 1.5 to 2.5 m² is not unusual – because solar works with a lower medium inlet temperature (40 to 70 °C) and to transfer the same amount of heat, a larger area is needed. The upper (boiler) heat exchanger can be smaller, as the boiler delivers the medium at 70 to 90 °C and the heat flow per m² of the heat exchanger is significantly higher.
The recommended choice for most new builds and renovations is a solar storage tank with two heat exchangers including insulation – you get a tank ready for future integration of additional sources without the need to change the tank. If you are certain that you will always have only one heat source (for example, only a solar set without a boiler), one heat exchanger is sufficient.
The topic is further discussed in the article Storage tank with one or two heat exchangers: which is more suitable.
What medium is used in the solar circuit and can it leak into the TÚV tank?
In the solar primary circuit – that is, in the pipes between the collectors and the tank's heat exchanger – potable water never flows. The heat transfer medium is a mixture of water and propylene glycol or ethylene glycol with additives that prevent corrosion and sediment formation. Most commonly, it is an antifreeze mixture with a freezing point around −28 °C, which ensures protection even in the harsh winters of mountainous areas in Slovakia.
This medium flows only in the primary circuit – that is, from the collector down to the tank to the heat exchanger and back up. It must never mix with the water in the tank. The heat exchanger functions as a thermal wall – heat passes through the wall of the exchanger, but the media remain separated. This is a key safety and hygiene principle of every solar system.
In very exceptional cases – for example, in the event of mechanical damage to the heat exchanger or extreme overpressure – the medium could penetrate into the tank. That is why a safety valve and an expansion tank sized for the primary circuit are installed in solar systems. Such a failure is extremely rare in a properly installed and maintained system – in practice, I have seen it only once in many years, and it was caused by corrosion of a neglected heat exchanger.
Why is the tank not sufficient and the water is not warm, even though the collectors are working?
This is a classic problem that I get calls about every summer. There can be several causes, and it is good to systematically eliminate them:
- The tank is too small compared to the collector area – a small tank quickly heats up to the maximum temperature, the regulation stops the circulation pump, but the water in the tank is quickly consumed and further heating has to wait for the sun the next day.
- Air in the primary circuit – if the circulation pump is pushing air instead of the medium, heat is not transferred to the exchanger. The tank remains cold, even though the collectors are hot. Solution: air venting of the system.
- Malfunctioning regulation – the differential regulator compares the temperature of the collector and the tank. If the sensor on the tank is stuck, the regulator thinks the tank is sufficiently warm and will not start the pump.
- Clogged heat exchanger – in areas with hard water, limescale can deposit on the tank's heat exchanger. Heat transfer efficiency drops, and the tank heats up slowly or not at all.
- The tank is too large compared to the collector area – yes, a tank that is too large is also a problem. If you have 2 flat collectors (about 4 m²) and a 400-liter tank, in a typical September-October the tank will never be fully heated and the average temperature will be low.
Other faults and their diagnostics are described in detail in the article Common faults of solar storage tanks: overheating, corrosion and pressure loss.
What happens if the tank is not used in the summer?
This is a scenario that regularly troubles owners of solar systems – a vacation, a cottage left empty for two weeks in July. The collectors are in the sun, but the tank has nowhere to transfer the heat and the temperature of the medium rises. This state is called stagnation.
During stagnation, the temperature in the primary circuit can reach 180 to 200 °C. The glycol mixture begins to degrade, steam forms in the pipes, and pressure rises sharply. The safety valve opens and releases part of the medium. The system does not burn out, but the degraded glycol solution must be checked and possibly replaced after the stagnation season.
Experience has shown that the simplest protection against absence is to heat the tank to the highest possible temperature (80 °C) before leaving, reduce the intensity of solar radiation by covering the collectors (a tarp, a special stagnation shield), or install a regulation that, upon reaching the maximum tank temperature, reflects the heat into a dummy load (for example, a cooling register in the boiler room). An investment in a high-quality expansion tank sized for stagnation conditions is indispensable for every solar system.
What is the difference between an enamelled and a stainless steel tank?
TÚV tanks – and thus also solar tanks – are manufactured in two basic material variants for the inner lining: enamelled steel and stainless steel (stainless steel).
Enamelled tank is the most common today. The steel tank has an inner layer of glass enamel, which resists corrosion and is hygienically safe. Enamel, however, has its limits – thermal shocks (sudden temperature changes), mechanical damage or poor enamelling can cause microcracks, through which the steel under the enamel begins to corrode. Therefore, enamelled tanks always contain a magnesium or zinc sacrificial anode, which slows down corrosion. The anode must be checked every 2 years and replaced if necessary – this is routine maintenance.
Stainless steel tank (typically made of AISI 316 or 304 steel) resists corrosion much better and a sacrificial anode is not necessary. The price is higher. In some areas with aggressive water (high chloride content), stainless steel may be preferred – chlorides are relatively benign for enamel, but for ordinary stainless steel they can be problematic if the material quality is low.
For most standard conditions in Slovakia (water hardness 15–25 °dH, without extreme aggressiveness), an enamelled tank with regular anode inspection is fully sufficient and cost-effective. The stainless steel variant makes sense with aggressive water or in projects where minimal maintenance is expected for many years.
How is the tank connected to the boiler and distributors?
Connecting the tank to an existing boiler room is the biggest question for many customers. The basic logic is simple: the tank has four main connections (sometimes more, but four are key) – inlet and outlet of the primary (solar) circuit on the lower heat exchanger, inlet and outlet of the secondary (boiler) circuit on the upper heat exchanger, plus the outlet of hot utility water at the top and the inlet of cold water at the bottom.
The boiler is connected to the top heat exchanger of the tank – either directly or via a three-way valve. If both the boiler and the tank have their own circulation pumps, the connection is controlled by hydraulic balancing to prevent the circuits from interfering with each other. This is where the distributor and collector come into play – a device that hydraulically separates the individual circuits and prevents them from influencing each other. For larger installations with multiple circuits (heating, floor heating, DHW), you can use, for example, an industrial stainless steel distributor/collector assembly with ball valves, which allows for a clear and safe connection of all branches.
A more detailed procedure for connecting the tank with the boiler and distributor is described in the article Connecting a Solar Storage Tank with a Boiler and Circuit Distributor. The overall installation – placement, piping routes, insulation – is described in the article Installation of a Solar Storage Tank: Procedure, Placement, and Installation Requirements.
What pressure can the tank withstand and what pressure do I have in the network?
The majority of solar storage tanks on the Slovak market are dimensioned for a maximum operating pressure of 6 bar on the DHW side (cold water / domestic hot water). This is more than enough for a standard distribution network, where water pressure usually does not exceed 4 bar.
On the primary solar circuit side, the pressures are lower – the system is filled and the charging pressure is set using the expansion tank's pre-charge pressure, typically to 1.5 to 2 bar. The safety valve of the primary circuit is usually set to 6 to 8 bar and protects the entire primary circuit from overpressure during stagnation.
If the pressure in the water supply network is higher than 4 bar, it is advisable to install a pressure-reducing valve before the tank – this protects not only the tank but also the piping and fixtures throughout the entire house. This is a fairly common situation in higher floors of buildings or in areas with strong water distribution.
How long will a solar storage tank last?
With proper installation, regular inspection of the sacrificial anode (once every 2 years), and adherence to operating parameters (temperature, pressure), you can expect a high-quality enamelled storage tank to operate without issues for 15 to 25 years. Stainless steel tanks can last at the upper limit or even exceed it.
In practice, the following factors shorten the lifespan:
- Neglecting the replacement of the sacrificial anode – the inner lining corrodes more quickly.
- Regular stagnation without replacing the degraded medium – the acidic medium attacks the heat exchangers and seals.
- Overheating beyond the maximum operating temperature for a prolonged period.
- Hard water without regular cleaning of the heat exchangers – limescale acts as a thermal insulator and also mechanically stresses the material during thermal cycles.
The replacement of the anode and inspection of the heat exchanger are described in the article Maintenance of a Solar Storage Tank: Cleaning, Anode and Heat Exchanger Inspection. These tasks are financially negligible compared to the cost of a new tank – neglect is always the more expensive option.
Do I need a storage tank with a solar set, or is an existing water heater sufficient?
Technically, you can connect the solar primary circuit via an external plate heat exchanger to an existing standard tank, but this solution has several limitations. An external heat exchanger adds hydraulic resistance, reduces heat transfer efficiency, and adds another component that can fail. In addition, you must have a place to install and hydraulically connect the external heat exchanger.
For long-term operation and maximum efficiency, it is always better to use a tank specifically designed for a solar system. The additional costs are relatively small compared to the overall investment in the solar system and will be offset by higher efficiency and lower failure rates. A tank with an integrated lower heat exchanger is hydraulically cleaner, more compact, and does not require additional components.
What is a storage tank with an embedded DHW tank (tank-in-tank)?
Tank-in-tank is a special construction where a smaller inner tank with drinking water is placed inside a larger tank (e.g., with water from a boiler or solar circuit). The drinking water is heated directly by contact with the walls of the inner tank – heat is transferred through the wall, but the media do not mix.
The advantage is a large heat transfer surface – the efficiency of heating is much higher than with a coil heat exchanger. The disadvantage is the price and the fact that the inner tank must be made of a non-corrosive material (usually stainless steel), which increases the overall cost. This type of tank makes sense mainly in combination with heat pumps or in larger installations where rapid DHW regeneration is a priority.
Details can be found in the article Solar Storage Tank with an Embedded DHW Tank: How It Works and When It Is Worthwhile.
What are typical dimensions and connections – what to check before ordering?
Before ordering a storage tank, it is essential to check several dimensional parameters to avoid unpleasant surprises during installation:
- Height and diameter of the tank – a 300-liter tank typically has a height of 175 to 195 cm and a diameter of 60 to 65 cm. It is necessary to check whether the tank will fit into the boiler room including insulation and whether it can be transported through the door (important in renovations with standard 80 cm doors).
- Thread sizes of the connections – solar connections are usually 3/4" or 1" internal thread, boiler connections 6/4" or 1". Compare with your existing installation to avoid unnecessary reductions.
- Height of the connections – different manufacturers place the nozzles at different heights. If you are replacing the tank with a new one, check whether the new tank has the connections at the same height, otherwise you will have to modify the piping routes.
- Position of the anode nozzle – the anode must be replaceable without dismantling the entire installation. The nozzle must be accessible.
More about dimensions and connections can be found in the article Dimensions and connections of solar storage tanks: what to check before purchasing, where there are also specific tables with measurements for common volumes.
Can the tank be placed horizontally?
Solar storage tanks are exclusively designed for vertical installation. The reason is physical: hot water is lighter and rises up, cold water sinks down – thermal stratification in the tank is key to the efficient operation of the system. The bottom (solar) heat exchanger heats the lower layer, hot water rises, the top heat exchanger (boiler) heats the upper layer. If you were to place the tank on its side, stratification would disappear, the entire volume would mix and efficiency would significantly drop.
Moreover, all connections and anode ports are designed for a vertical position – placing the tank horizontally would make maintenance more difficult, and in the case of a glazed tank, it could lead to uneven glazing during production, which shortens its lifespan.
Do I need any official permit or inspection for the tank?
A hot water storage tank as a pressure appliance is not subject to mandatory inspections according to Slovak regulations for standard household tanks up to 6 bar. Unlike gas-based hot water preparation, there is no legally prescribed regular professional inspection of the pressure tank itself.
However, the entire solar system – including the tank – must be put into operation in accordance with the design documentation and technical standards (STN, EN 12975, etc.) for solar thermal systems. If you are claiming a subsidy (for example, from the "Green Households" program), the installation must be carried out by a qualified person and meet the conditions of the relevant assistance scheme.
The electrical part (circulation pump, control system) is subject to an electrical installation inspection, which must be carried out by a qualified technician. This is a standard part of any custom installation.
Most frequently asked questions (FAQ)
Can I connect the tank directly without an expansion vessel?
No, an expansion vessel is an essential part of every closed pressure system – that is, also of a solar storage tank connected to the water supply or primary circuit. Without it, the water would have nowhere to expand during heating, pressure would rise uncontrollably, and the safety valve would open with every heating cycle. The sizing of the expansion vessel depends on the tank volume and network pressure – for a 250-liter tank, an expansion vessel of 18 to 25 liters is commonly used on the TÚV side, while the primary solar circuit is sized separately according to the volume of the primary circuit and stagnation conditions.
What temperature should be set in the tank?
From a hygiene perspective, a minimum of 60 °C is recommended – at this temperature, the bacterium Legionella pneumophila dies, which is a health risk in stagnant water at lower temperatures. Some controls allow so-called thermization – once every 24 hours, the tank is heated to 70 °C regardless of the normally set operating temperature. From an energy-saving perspective, the normal operating temperature is kept at 55 to 60 °C, which is a reasonable compromise between hygiene and losses of stored energy.
What happens if the tank stands idle for a long time without being used?
A solar storage tank that has been inactive for a long time (e.g., disconnected for several months and filled with water) can be problematic from a hygiene standpoint – stagnant warm water in the range of 25 to 50 °C is an ideal nutrient medium for Legionella bacteria. Before putting it back into operation, it is advisable to completely flush the tank, sterilize it by heating it above 70 °C, and check the sacrificial anode. If the tank has been standing empty for a long time, it is better to drain it and close the inlets.
How long does it take to heat the tank from 15 to 55 °C using solar energy?
It depends on the intensity of solar radiation, the area of the collectors, and the tank volume. Approximately: on a good sunny day (July, 800–900 W/m²) with 2 flat collectors (4 m²), the system's energy output is around 2.5 to 3 kW. To heat 250 liters of water by 40 °C (from 15 to 55 °C), you need about 11.6 kWh. At a power of 2.5 kW, this takes about 4.5 hours of continuous sunlight. In reality – with morning fog, variable cloud cover, and nighttime cooling – it can take 6 to 8 hours of total daylight.
Should I buy a tank with or without insulation?
Always with insulation. A tank without insulation loses about 3 to 5 kWh per day at room temperature in the boiler room (20 °C) and water temperature of 55 °C – these are unnecessary losses that reduce the return on your solar investment. A high-quality insulation made of cold-cured polyurethane foam with a thickness of 50 to 80 mm reduces daily losses to less than 1 kWh. Tanks available on the market, such as a solar storage tank with two heat exchangers including insulation, are delivered complete, so solving these losses is not your additional concern.
What is the difference between a bivalent and a monovalent tank?
A monovalent tank has one heat exchanger and operates with one heat source. A bivalent tank has two heat exchangers and can accept heat from two sources at the same time – typically solar collectors and a boiler. The term "bivalent" thus practically corresponds to a tank with two heat exchangers, and is used more frequently in German and Austrian technical literature, from which it has entered our terminology. For a combined solar-boiler system, a bivalent tank is the correct choice.
Conclusion
A solar storage tank is the heart of every solar system for the preparation of hot water. Most of the questions customers ask relate to three main areas: the correct choice of volume and type, the correct connection and integration with other heat sources, and long-term reliability including maintenance. For each of these areas, there are separate detailed articles in this Knowledge Center – for example, How to choose a solar storage tank: volume, heat exchangers and system type for a comprehensive selection, or Maintenance of a solar storage tank for long-term operation.
If you are planning a specific purchase decision, have the number of people in your household, the area and type of collectors you plan to or already have, the type of secondary heat source (boiler, heat pump, electricity), and the boiler room dimensions at hand. With this information, you can choose the tank precisely and without unnecessary compromises.
Do you have a question about this topic?
Having trouble deciding or dealing with a specific situation in your household? Write to us – we are happy to help.