Frequently Asked Questions about Low-Temperature Boilers with Storage Tank Connection
Frequently Asked Questions about Low-Temperature Boilers with the Option to Connect a Storage Tank
Low-temperature boilers with the option to connect a hot water storage tank are among the most common solutions in Slovak households, where central heating and hot water preparation are handled separately – that is, not via a flow heater built directly into the boiler. Although this is a relatively proven technology, customers still run into the same questions when choosing, installing, and operating these systems. Some are technical, others practical, and others concern settings or maintenance. This article aims to answer all of them – comprehensively, without unnecessary simplification, the way an experienced technician with decades of similar jobs under his belt would explain it to you.
What does "option to connect a storage tank" actually mean for a low-temperature boiler?
A low-temperature boiler with the option to connect a storage tank is a single-circuit boiler – it only heats the heating water. On its own, it does not prepare hot water for washing. Instead, it has hydraulic connections and control electronics that allow an external boiler (hot water storage tank) to be connected. The boiler then starts heating the storage tank according to a set program or on demand from the tank's thermostat – either via directly configured hydraulics or via an external pump on the storage tank circuit.
It's important to understand that "option to connect" is not just a mechanical matter (i.e., the presence of connections on the boiler body). It's also about whether the boiler has the appropriate controls – i.e., an input for a storage tank sensor (NTC temperature sensor), an output for controlling the storage tank pump, and heating priority logic. Without these controls, you would have to handle the entire wiring externally, which is more complicated and prone to errors.
Examples of boilers with this feature from the category discussed here include the Vaillant atmoTEC plus VU 200/3-5 or the Vaillant turboTEC plus VU 202/3-5 – both have a direct output for the storage tank pump and an input for the storage tank's NTC sensor.
What is the difference between a single-circuit boiler with a connected storage tank and a two-circuit (combi) boiler?
This is the number one question we still get. Simply put: a combi boiler has hot water heating built in – either via a flow-through heat exchanger or via a small built-in storage tank. A single-circuit boiler with a connected external storage tank has hot water heating handled externally, in a separate boiler tank.
In practical terms, the differences are as follows:
- Comfort: A combi boiler delivers hot water instantly, but at higher demand (multiple bathrooms, large family) it may struggle with output. A storage tank, on the other hand, must be heated up first, but then provides a large amount of hot water at once without a drop in output.
- Energy efficiency: A storage tank solution, when properly set up, can be more economical – the boiler heats the water in the tank in one go, rather than firing up every time a tap is opened. This is covered in more detail in the article Energy Savings When Combining a Low-Temperature Boiler with a Hot Water Storage Tank in our Knowledge Center.
- Price: A combi boiler is cheaper to install (a single unit), while a storage tank solution requires a larger investment in the tank and installation, but with the right choice can be more cost-effective in the long run.
- Space: A storage tank takes up room. In small apartments, this can be a decisive factor.
What outputs are available for low-temperature boilers with a storage tank connection, and what can you expect from them?
In this category you will find boilers with various rated outputs. For family homes and larger apartments, the most common range is 12 to 35 kW. However, it is important to distinguish what output means in the context of a low-temperature boiler:
- Heating output – what the boiler delivers to the heating system (radiators, underfloor heating).
- Storage tank heating output – when the storage tank has priority, the boiler directs its output to the storage tank circuit. The heating time depends on the boiler's output, the tank's volume, and the incoming water temperature.
For reference: a 20 kW boiler heats a 150-liter tank from 15 °C to 60 °C in approximately 30–35 minutes. A 28 kW boiler manages this in approximately 20–25 minutes. These figures are indicative and depend on the tank's heat losses and the heat exchanger's output.
Examples of specific models from our range: the Vaillant atmoTEC plus VU 240/3-5 with a rated output of 24 kW is suitable for medium-sized family homes, and can heat a 150–200 l storage tank in a reasonable time. For larger buildings, the Vaillant atmoTEC plus VU 280/3-5 with an output of 28 kW is available.
How does storage tank heating priority work on a low-temperature boiler?
Storage tank heating priority is a function that ensures that when the storage tank needs heating (its temperature has dropped below the set value), the boiler prioritizes heating the tank over space heating – meaning it turns off or limits heating for a certain time and directs full output to the storage tank circuit.
From the customer's point of view, this means that radiators may cool down slightly while the storage tank is being heated. If the tank is properly sized and the boiler has sufficient output, heating happens quickly (20–35 minutes), and the heating system practically doesn't have time to "cool down" enough for it to be noticeable in the rooms.
In practice, I encounter two problems with priority settings:
- Too low a storage tank hysteresis: The tank heats up too often (e.g., a drop of 2–3 °C triggers heating), causing frequent interruptions to space heating. I recommend setting the hysteresis to at least 5–8 °C.
- Too high a set storage tank temperature: A tank set to 70–75 °C takes a long time to heat, extending the priority period. 55–60 °C is sufficient for regular use; once a week it's advisable to run a thermal disinfection cycle at 65–70 °C to prevent Legionella.
You can find more on practical settings in the article Setting Storage Tank Heating Priority on a Low-Temperature Boiler in our Knowledge Center.
Which storage tank should you choose for a low-temperature boiler? Volume, material, heat exchanger
Choosing the right storage tank is just as important as choosing the boiler. The tank should be sized according to the household's actual hot water consumption, not by guesswork. As a rule of thumb: allow 50–60 liters per person, which for a family of 4 gives a tank of 200–240 liters.
The tank's heat exchanger should have sufficient surface area for fast heating. For low-temperature boilers (typically operating with a water temperature of 60–75 °C on the supply pipe to the tank), it's important that the heat exchanger is large enough – at low heat carrier temperatures, the exchanger's surface area is crucial. The minimum exchanger surface area for normal use should be:
- 150 l tank → heat exchanger of at least 1.2 m²
- 200 l tank → heat exchanger of at least 1.5 m²
- 300 l tank → heat exchanger of at least 2.0 m²
As for tank material: enameled steel is the most common and affordable option. Stainless steel is more expensive but more resistant to corrosion and doesn't require an anode. For locations with hard water, stainless steel is an advantage. You can find more on choosing a storage tank – its volume, material, and compatibility with a specific boiler – in the article What Hot Water Storage Tank Volume Do I Need for a Low-Temperature Boiler.
Atmospheric or turbo boiler – which fits a storage tank better?
Several factors play a role when choosing between an atmospheric and a turbo (fan-assisted) boiler with the option to connect a storage tank. In terms of the actual storage tank connection function, there is no fundamental difference between the two – both types have the hydraulics and controls needed for a storage tank. The difference lies in the way combustion and flue gas removal work.
Atmospheric boilers (e.g., the Vaillant atmoTEC plus VU 200/3-5) draw combustion air from the room and discharge flue gases via a chimney connection into a shared chimney. They require a functional chimney and an air supply to the room. They are suitable for homes with an existing chimney.
Turbo boilers (e.g., the Vaillant turboTEC plus VU 122/3-5) have a sealed combustion chamber, drawing air from outside via a coaxial pipe and also discharging flue gases through an external wall or roof. They do not require a chimney, which is an advantage in new builds and renovations without a chimney.
In terms of cooperation with a storage tank, both types are equally suitable. What matters is meeting the structural installation requirements. You can find a detailed comparison in the article Atmospheric vs. Turbo Low-Temperature Boiler – Which to Choose with a Storage Tank.
Can I connect a storage tank to an existing boiler that doesn't have one?
This is a question we get fairly often when a customer already has a boiler and wants to add a storage tank. The answer depends on the boiler model and its controls.
If the boiler doesn't have an input for the storage tank's NTC sensor and an output for controlling the storage tank pump, the wiring is more complicated. You would need to deal with an external storage tank controller, an external storage tank circuit pump, and a hydraulic connection (e.g., via a three-way valve or manifold). This solution is technically possible, but expensive and less elegant.
With the Vaillant atmoTEC plus and turboTEC plus boiler series sold in this category, storage tank connection is directly supported by the manufacturer – the boiler has pre-prepared inputs/outputs, and the connection procedure is documented in the installation manual. You can read more about the practical procedure in the article Connecting a Vaillant atmoTEC and turboTEC Boiler to a Hot Water Storage Tank.
What hydraulic connection is used for a low-temperature boiler with a storage tank?
In practice, we encounter two basic hydraulic schemes:
1. Connection via a storage tank pump (most common): The boiler has a dedicated output and input for the storage tank circuit. The storage tank pump is controlled directly by the boiler's controls. When the storage tank has priority for heating, the boiler starts the storage tank pump while simultaneously switching off (or limiting) the heating pump. This connection is simple, reliable, and standard for Vaillant boilers.
2. Connection via a three-way valve: Less common in typical family homes, but used in more complex systems. The three-way valve switches the water flow between the storage tank and heating circuits. The advantage is that the boiler can use a single pump for both circuits.
In any case, the storage tank circuit should have its own safety devices (safety valve, expansion vessel for the storage tank circuit, ball valves for servicing). The detailed installation procedure is described in the article Installing a Low-Temperature Boiler with a Storage Tank Connection Step by Step.
What are typical installation and operating mistakes?
After years in the field, I know that most problems don't stem from equipment failure, but from installation or setting mistakes. Here are the most common ones:
- Missing expansion vessel for the storage tank circuit: The hot water storage tank (potable side) requires its own expansion vessel for the sanitary circuit. This is not the same as the heating system's expansion vessel. Without it, the pressure in the tank will rise and the safety valve will regularly discharge water – the customer thinks there's a fault, when in fact there's simply a missing expansion volume.
- Storage tank circuit piping that is too thin: Good performance from the tank's heat exchanger requires sufficient flow. The storage tank circuit piping should be at least DN15, ideally DN20 for larger tanks.
- Storage tank connected without a magnesium anode or with a depleted anode: An enameled tank without a functional anode will corrode. The anode should be checked every 2 years.
- Storage tank temperature set too high: A temperature of 75–80 °C is unnecessary and increases the tank's heat losses. We recommend 55–60 °C for normal heating, with thermal disinfection set as a weekly program (65–70 °C for one hour).
- Missing thermostatic mixing valve at the storage tank outlet: If the tank is set to 60 °C, the outlet water is very hot. A thermostatic mixing valve blends the hot water with cold water to a safe temperature (45–48 °C) and prevents scalding.
You can find a detailed troubleshooting guide and diagnostics in the article Common Faults in Low-Temperature Boilers with a Connected Storage Tank and How to Solve Them.
What needs to be regularly checked and serviced?
The combination of a boiler and a storage tank requires regular maintenance of both components. During the annual inspection, a service technician should check:
- The boiler: combustion, seals, cleanliness of the heat exchanger, system pressure, function of the safety valve, function of the storage tank pump and NTC sensor.
- The storage tank: condition of the anode (for enameled tanks), limescale deposits in the heat exchanger (with hard water), tightness of the safety valve, function of the thermostatic mixing valve at the outlet, condition of the tank's thermal insulation.
- Storage tank circuit piping: any leaks, fitting corrosion, functionality of the ball valves.
A comprehensive service plan with frequencies and descriptions of tasks can be found in the article Maintenance and Servicing of a Low-Temperature Boiler with a Storage Tank – What and When to Check.
What should you watch out for when choosing a boiler with future expansion in mind (solar collectors, heat pump)?
If you plan to connect solar collectors or a heat pump for hot water heating to the system in the future, it's worth considering this already when choosing the storage tank. The tank should have two heat exchangers (a bivalent tank) – the lower one for solar or the heat pump, the upper one for the boiler. In this case, the boiler acts as a backup heat source.
The Vaillant atmoTEC plus and turboTEC plus boilers are compatible with this kind of setup – they can work with a bivalent storage tank, with the boiler only starting to heat when solar energy isn't sufficient. This solution is worthwhile from an energy standpoint, provided the house has suitable conditions for solar collectors (roof orientation, shading). You can find more on the system's energy efficiency in the article Energy Savings When Combining a Low-Temperature Boiler with a Hot Water Storage Tank.
What are the operating costs and payback period of the investment?
The total operating costs of a storage tank system compared to a combi boiler depend on several factors. In practice:
- The storage tank has heat losses – an insulated 150 l tank in an average environment (18 °C ambient) at a temperature of 60 °C loses approximately 1.5–2.5 kWh per 24 hours. At a gas price of €0.06–0.08/kWh, that's about €3–5 extra per month compared to a combi boiler without a storage tank.
- On the other hand, a storage tank solution allows for better use of cheap energy (off-peak electricity tariff, solar), which can more than offset the tank's heat losses.
- The higher purchase cost of a storage tank system (tank + installation) typically pays for itself within 3–6 years compared to a combi boiler, provided the tank is properly used and sized.
Frequently Asked Questions (FAQ)
Can I connect any storage tank to a low-temperature boiler, or does it have to be from the same manufacturer?
Technically, you are not required to buy a storage tank from the same manufacturer as the boiler. What matters is that the tank has a heat exchanger with sufficient surface area (see the values above), correctly positioned connections (heat exchanger inlet/outlet, cold water inlet, hot water outlet, NTC sensor connection), and a matching volume. Vaillant offers its own uniSTOR series of storage tanks, optimized for atmoTEC and turboTEC boilers, but tanks from other brands with suitable parameters will also work without issues. Always check that the storage tank's NTC sensor input is compatible with the boiler's controls (connector type, sensor type).
What is the minimum temperature at which the boiler starts heating the storage tank?
The boiler starts heating the storage tank when the tank's temperature (measured by the NTC sensor) drops below the set minimum value. The adjustable target storage tank temperature is usually in the range of 40–70 °C (depending on the model and controls). The hysteresis – i.e., the difference between the temperature at which heating switches on and off – is typically adjustable within a range of 3–10 °C. The boiler heats the tank to the target temperature plus the hysteresis, which prevents unnecessarily frequent cycling. Recommended configuration: target temperature 60 °C, hysteresis 8 °C (the boiler switches on at 52 °C, off at 60 °C).
What happens if the storage tank runs out (hot water is used up) during a long shower?
If the entire contents of the tank cool down or are used up, the boiler starts priority heating and the tank begins heating up again. During this time (20–40 minutes depending on the boiler's output and the tank's volume), the hot water will not be hot enough. The solution is a larger tank, a higher tank temperature (with a properly set thermostatic mixing valve), or simply not wasting water during long showers. For a family of 4 with normal habits, a 200-liter tank at a temperature of 58–60 °C is sufficient.
Does the storage tank need to be filled or prepared in any way before first start-up?
Yes, the storage tank must always be filled with water before the first heating cycle. You must never start heating an empty tank – you will damage the heat exchanger and possibly also the anode. Before first start-up, it's advisable to flush the tank with clean water, check that there are no impurities inside from manufacturing or installation, and only then fill it and start heating. We recommend performing the first heating cycle at the maximum temperature (thermal disinfection) to eliminate any possible bacterial contamination from installation.
Can I operate the storage tank without a connected boiler (for example, running only on solar collectors in summer)?
Yes, if you have a tank with two heat exchangers (bivalent) and a solar system with its own controller, you can happily run on the solar circuit alone in summer with the boiler shut down. The boiler will only switch on automatically when solar energy isn't sufficient and the tank's temperature drops below the set value. This hybrid mode is the most energy-efficient, and under suitable conditions can cover hot water heating in summer completely without gas.
How do I know if the storage tank pump or the storage tank's NTC sensor isn't working properly?
The symptoms are fairly clear: the storage tank doesn't heat up (temperature remains permanently low despite the boiler running), or the boiler heats up but the water in the tank stays cold. On the boiler display, most modern controllers will show an error code – e.g., F75 for a pressure sensor failure, or another code indicating a problem with the storage tank circuit. For accurate diagnostics, we recommend calling a certified service technician, who will measure and verify the function of the pump (flow, pressure) and the sensor (NTC sensor resistance at a given temperature). A list of common faults and their solutions can be found in the article Common Faults in Low-Temperature Boilers with a Connected Storage Tank and How to Solve Them.
Conclusion: Why a storage tank solution with a low-temperature boiler is worthwhile
Combining a low-temperature boiler with an external hot water storage tank is not a compromise or an outdated approach. It's a well-thought-out, flexible, and, with proper installation, very reliable solution that can cover both heating and hot water needs for family homes as well as larger residential spaces. Compared to combi boilers, it offers greater comfort when drawing water, better adaptability for integrating renewable sources, and, with the right configuration, savings in operating costs.
The key to satisfaction is choosing the right boiler output (for example, the Vaillant turboTEC plus VU 202/3-5 for a medium-sized house), a matching storage tank volume and type, professional installation with complete hydraulic accessories, and correct configuration of the controls including storage tank priority. All of these steps require expert assessment – but the result is a system that reliably works for decades without major issues.
If you're still not sure about your choice, we recommend reading the article How to Choose a Low-Temperature Boiler with the Option to Connect a Storage Tank, where you'll find a systematic selection process from output to a specific model. You can find all products from this category clearly listed on the low-temperature boilers with the option to connect a storage tank category page.
Do you have a question about this topic?
Not sure how to decide, or dealing with a specific situation in your household? Write to us - we'll be happy to help.