Energy Savings from Combining a Low-Temperature Boiler and a Hot Water Cylinder

We are still preparing products.

But you can look at other categories.

Combining a low-temperature boiler with a domestic hot water (DHW) storage tank is, in practice, one of the most efficient solutions for heating and hot water preparation in family houses and smaller residential buildings. Even though condensing boilers dominate the market today, low-temperature boilers with the option of connecting a storage tank still have a solid place – and with the correct configuration they can offer surprisingly low natural gas or propane consumption. In this article we'll go through the whole topic of energy savings, from the basic physics to specific settings and real examples from our installation practice.

What is a low-temperature boiler and why combine it with a storage tank?

A low-temperature boiler is a gas boiler that operates with a system flow temperature usually in the range of 40 – 75 °C, with the heating system designed so that the return temperature does not drop below the dew point of the flue gases (approx. 57 °C for natural gas). Unlike a condensing boiler, a low-temperature boiler does not actively use the latent heat of flue gas condensation, which makes it somewhat less efficient in terms of annual seasonal efficiency. Nevertheless, these boilers still have an efficiency of around 89 – 93%, are robust, relatively cheap to purchase, and easy to service.

The key question customers ask is: "Why not simply buy a combi boiler – i.e. a boiler with a built-in tank – instead?" The answer is simple: a combi boiler heats hot water on a flow-through basis, which means that with higher demand (e.g. a shower and washing dishes at the same time) the boiler's output is not sufficient or the water temperature fluctuates. A storage tank accumulates hot water, so you always have a large volume available at the required temperature regardless of the instantaneous load on the network or the boiler's output.

If you're interested in how to correctly choose a boiler for your storage tank, also check out our article How to choose a low-temperature boiler with the option of connecting a storage tank, where you'll find an overview of parameters and decision-making criteria.

Energy balance: where do the savings come from?

To understand exactly where savings arise when combining a boiler and a storage tank, we need to look at three key sources of energy loss in normal operation:

Frequent boiler starts and stops (cycling): Every boiler start requires energy to heat up the heat exchanger, burner, and flue system. The more starts, the higher the total consumption.
Heat losses from the storage tank: The storage tank radiates heat into its surroundings, continuously consuming energy to maintain temperature. Quality tank insulation minimizes this loss.
Non-optimal heating time: If the boiler heats the tank at any time rather than when it's energy-efficient to do so, more energy is consumed (e.g. during peak hours with higher gas prices, or when the boiler runs at an unfavourable operating point).

A properly set up combination of a low-temperature boiler and a storage tank addresses all three points: the boiler works in long, stable heating cycles for the tank, the tank accumulates heat and releases it on demand, and a timer or thermostat controls heating so the boiler runs at the optimal time rather than during the night without any need.

Hydraulic connection: how the boiler and storage tank work together

A low-temperature boiler is designed for direct hydraulic connection with a tank-based DHW heating system. In most cases, one of two basic connection schemes is used:

Direct circuit (boiler → tank → heating): The boiler sends hot water directly to the tank via a coil heat exchanger (the so-called boiler coil) and at the same time supplies the heating system. Priority is controlled by a solenoid valve or a three-way valve.
Parallel circuit with a three-way valve: The boiler circuit is split into a tank branch and a heating branch. The boiler's electronics or an external controller switches between the branches based on priority.

Most Vaillant atmoTEC and turboTEC low-temperature boilers have an integrated input for a storage tank signal and the corresponding 3-way valve. This is why these boilers are such a popular choice in combination with a storage tank – their connection is straightforward and doesn't require complex external control.

You'll find a detailed installation procedure, including pipe routing and valve settings, in the article Installing a low-temperature boiler with a storage tank connection step by step.

Storage tank heating priority: the key to savings

One of the most important functions of modern low-temperature boilers with a connected storage tank is the so-called DHW priority. It works on the following principle: when the tank temperature drops below the set value, the boiler interrupts space heating and focuses all its output on heating the tank. Only once the tank reaches the required temperature (typically 55–60 °C) does the boiler return to heating.

Why is this setting key for savings? For several reasons:

The boiler runs at maximum output: While heating the tank, the boiler doesn't need to "unnecessarily" throttle its output due to heating regulation. It works at a stable operating point, which is more energy efficient than constant modulation.
Shorter heating time = fewer starts: The faster the boiler heats the tank, the sooner it returns to efficient operation, and the fewer total start cycles it goes through per day.
More precise heating control: A heating system with a DHW storage tank as an accumulator is less sensitive to short interruptions, allowing more aggressive heating controller settings without a feeling of cold.

We describe specific priority settings for Vaillant boilers in a separate article Setting storage tank heating priority on a low-temperature boiler, where you'll also find specific menu steps for different types of controllers.

Vaillant atmoTEC plus low-temperature boiler and storage tank: real-world figures

Let's look at a specific example. A family house with a floor area of 130 m², 4 people, in a location with an average winter (Trenčín, altitude 250 m). The house has an older radiator heating system designed for 75/65 °C, but after partial reconstruction of windows and roof, 65/55 °C is sufficient.

The customer originally had a 24 kW combi boiler that cycled – they recorded over 40 starts a day just for hot water preparation. After replacing it with a Vaillant atmoTEC plus VU 200/3-5 with a connected 150-litre storage tank, the number of daily boiler starts dropped to 6–8 (including heating), while annual natural gas consumption fell by 11–14%. At current gas prices, this represents a saving of €180–260 per year.

Why such a result? Because the atmoTEC plus VU 200/3-5 has a rated output of 20 kW, and the tank heats up (from 15 °C to 55 °C) at full output in about 28–35 minutes. During this time, the boiler runs steadily without cycling. The original combi boiler did the same job in 20–30 short cycles (every shower, every bath filling), and every start means loss of heat in the flue gases, heat exchanger loss, and mechanical wear.

For a larger house or a 5-person household with higher hot water demand, we recommend the Vaillant atmoTEC plus VU 240/3-5 with a 24 kW output, or possibly the Vaillant atmoTEC plus VU 280/3-5 with a 28 kW output. With higher output, the tank heats up faster, which is important in households where everyone showers within an hour in the morning.

Turbo vs. atmospheric boiler when combined with a storage tank – an energy perspective

The atmospheric vs. turbo (forced draught) question is relevant when combined with a storage tank mainly in terms of installation options and slightly different energy characteristics. From the perspective of savings when operating with a storage tank, the following applies:

Atmospheric boilers (atmoTEC): Require a chimney with natural draught. They are quieter, with no fans. Since there's no fan involved, there's no auxiliary electricity consumption for a fan (~40–80 W during operation). Combined with a storage tank, they are suitable where a chimney is available and wind pressure is lower (not high floors).
Turbo boilers (turboTEC): Have a sealed combustion chamber, are independent of room air, and can be placed almost anywhere (basement, bathroom, hallway). The fan consumes approximately 40–80 W of additional electricity, but with correct settings and tank operation this is a negligible item in the overall energy balance (approx. 8–20 kWh per year).

The Vaillant turboTEC plus VU 202/3-5 with a 20 kW output is a typical choice for new builds and renovations without a chimney, with a 150–200 litre tank. For smaller houses or lower demand, the Vaillant turboTEC plus VU 122/3-5 with a 12 kW output is sufficient, which combined with a 100–120 litre tank covers the needs of a 2–3 person household.

You'll find a detailed comparison of both types in the article Atmospheric vs. turbo low-temperature boiler – which to choose with a storage tank.

Storage tank insulation and heat losses: a practical view

A hot water storage tank is not a perfect thermos. Every tank loses heat to its surroundings – the rate of loss depends on the thickness and quality of the insulation, the water temperature in the tank, and the ambient temperature. That's why it's important to know:

Heat loss of common storage tanks: A quality tank with 100 mm of PU foam loses approximately 40–60 Wh per hour (at a water temperature of 60 °C and room temperature of 20 °C). Over 24 hours, that's 0.96–1.44 kWh of loss just to maintain temperature – without any draw-off at all.
Storage tank placement: A tank placed in a heated space (boiler room inside the house) has lower heat losses than a tank in an unheated garage. From an energy-saving perspective, it's ideal to place the tank as close as possible to the boiler and to the draw-off points.
Tank temperature: Every 5 °C increase in temperature raises heat losses by approximately 8–12%. Setting the tank temperature to 55 °C (instead of 65 °C) not only saves energy but also extends the tank's lifespan (less limescale buildup). However, for hygiene reasons (legionella), it's recommended to reheat the tank to 60–65 °C at least once a week.
Tank volume: A larger tank has a larger surface area, and thus greater absolute heat losses, but can be more efficient per litre of water. For a family house with 3–4 people, the optimal volume is 150–200 litres. More in the article What storage tank volume do I need for a low-temperature boiler.

Control and time programming: where the greatest potential for savings lies

Modern Vaillant atmoTEC and turboTEC low-temperature boilers, combined with an external controller, allow you to set a time program for tank heating. In practice, this is one of the most effective methods of energy saving, because:

Tank heating is triggered only when you really need it – for example, in the morning between 5:30 and 7:30 (before a shower) and in the evening between 17:00 and 19:00 (before an evening bath).
At night, the tank isn't heated unnecessarily – overnight losses are minimal, and when the boiler reheats it in the morning, it's more energy-efficient than maintaining the temperature throughout the whole night.
If a household has a dual-tariff electricity meter (with a heat pump or electric backup heating), or a lower gas tariff at night, heating can be programmed for the cheaper time period.

It's important to know that the time program must not be set too "aggressively" – if the tank remains without heating for a long time (e.g. over a weekend when you're away), a hygienic heating cycle to 65 °C must be triggered manually or automatically. Most Vaillant controllers handle this automatically with the "thermal disinfection" function.

We describe connecting the boiler to a controller and detailed settings in the article Connecting a Vaillant atmoTEC and turboTEC boiler with a hot water storage tank.

Effect of boiler circuit temperature on low-temperature boiler efficiency

Low-temperature boilers are designed to operate efficiently at lower heating circuit temperatures. The lower the boiler outlet temperature (while maintaining thermal comfort), the higher the combustion efficiency. This is a physical law – a lower flue gas temperature means that more energy was extracted from the fuel.

In practice, this means: if you have an older house with radiators designed for 75/65 °C, but after renovating the building envelope 60/50 °C is sufficient, you will significantly increase the boiler's efficiency. However, when combined with a storage tank, a complication arises: the DHW tank needs a minimum of 55 °C, ideally 60 °C. So the boiler temperature cannot drop below this threshold, otherwise the tank won't be hot enough.

The solution is hydraulic separation of circuits: the tank is connected to the "hot" boiler circuit (60–65 °C), while heating is reduced to 50–55 °C by a mixing valve. This solution allows the boiler to work efficiently when heating the tank (higher temperature = faster heating = fewer starts) while maintaining a low heating temperature (higher boiler efficiency during normal operation).

Specific savings: comparison of scenarios in numbers

Let's look at three typical installation scenarios we've handled:

Scenario A: Older house, 3 people, atmoTEC + 150 l storage tank
Original state: 24 kW combi boiler, consumption of 1,850 m³ of gas per year. After replacing it with a Vaillant atmoTEC plus VU 200/3-5 with a 150-litre storage tank: consumption of 1,640 m³ of gas per year. Savings: 210 m³/year, at a price of €0.72/m³ = approximately €151/year. Payback period on the investment in the tank and installation (approx. €900): 6 years.

Scenario B: New build, 4 people, turboTEC + 200 l storage tank
A well-insulated new build of 140 m². Boiler Vaillant turboTEC plus VU 202/3-5, 200-litre storage tank. Gas consumption: 1,250 m³/year (heating and DHW combined). Compared to the original design with a combi boiler (1,410 m³/year): savings of 160 m³/year = approximately €115/year. The customer also appreciates the comfort – plenty of hot water even when two people shower at the same time in the morning.

Scenario C: Renovation of a 4th-floor apartment, 2 people, 12 kW turboTEC + 100 l storage tank
Vaillant turboTEC plus VU 122/3-5, 100-litre storage tank. 65 m² apartment. Gas consumption: 780 m³/year. Without a storage tank, combi boiler cycling would mean approximately 920 m³/year. Savings: 140 m³/year = approximately €100/year. The customer also noted that the hot water temperature is now constant – with the combi boiler, he and his wife weren't satisfied with the temperature fluctuating during longer showers.

Maintenance as a factor in energy efficiency

Energy isn't only lost due to poor settings or outdated technology – we also lose it through neglected maintenance. Low-temperature boilers with a connected storage tank require regular maintenance, which directly affects consumption:

Descaling the storage tank: Limescale deposits on the tank's heat exchanger act as a thermal insulator. A 1 mm layer of limescale reduces heat transfer by 10–15%, a 3 mm layer by up to 30%. The boiler has to run longer to heat the tank to the required temperature.
Cleaning the boiler's burner and heat exchanger: A dirty burner burns less efficiently. Regular servicing (once a year) keeps the boiler at its nominal efficiency.
Checking the safety valve and expansion vessel: Faults in the pressure system can cause heat loss through leaks or unnecessary boiler shutdowns.
Checking the storage tank's anode protection: The magnesium anode protects the tank from corrosion. Its wear should be checked every 2 years, otherwise there's a risk of tank damage and heat loss through corroded joints.

You'll find a complete inspection schedule in the article Maintenance and servicing of a low-temperature boiler with a storage tank – what and when to check.

Environmental aspect: lower consumption = lower emissions

Energy savings aren't just about money. Lower natural gas consumption directly means fewer CO₂ emissions. Natural gas produces approximately 2.0 kg of CO₂ per m³ when burned. If the customer in Scenario A saves 210 m³ of gas per year, that's 420 kg of CO₂ per year that won't be released into the atmosphere. Over the 15-year lifespan of a boiler room, that's more than 6 tonnes of CO₂ – and that's from just one household.

Low-temperature boilers with a connected storage tank are therefore not only an economically advantageous solution, but also a responsible choice from the perspective of a household's long-term carbon footprint. In the context of current EU energy policy and rising emission allowance prices (ETS), this aspect is becoming increasingly relevant for ordinary customers too.

Frequently Asked Questions (FAQ)

What are the real energy savings when switching from a combi boiler to a low-temperature boiler with a storage tank?

In practice, savings depend on several factors – household size, number of people, original consumption, and the condition of the house's insulation. From our installation experience, we observe savings of 8–18% of annual natural gas consumption. The highest savings are achieved by households with higher hot water demand (4–5 people), where the combi boiler cycled intensively. On average, this amounts to €100–250 per year at current gas prices.

Can I connect a storage tank to an existing low-temperature boiler, or do I have to replace the boiler?

It depends on the specific boiler. Some older boilers have an integrated tank output (output D) – in that case, a tank and a three-way valve are enough. Other boilers don't have this and would require an external controller and hydraulic modifications, which can be costly. We always recommend consulting a professional installer who will inspect the boiler and assess the options. With Vaillant atmoTEC plus and turboTEC plus boilers, connecting a storage tank is supported as standard.

What temperature should I set the hot water storage tank to?

From a hygiene perspective, a minimum of 55 °C is recommended, ideally 60 °C. At lower temperatures, there's a risk of Legionella pneumophila bacteria multiplying, which thrive in hot water at temperatures of 25–50 °C. Once a week, the controller should automatically trigger a hygienic heating cycle to 65 °C (so-called thermal disinfection). From an energy perspective, 60 °C is a compromise between safety and consumption.

What is storage tank priority, and should it be turned on or off?

Storage tank priority is a function in which the boiler temporarily interrupts space heating and focuses its entire output on heating the tank. We recommend keeping it on – the tank heats up faster, the boiler cycles less, and the system is more energy-efficient. The only exception is extreme winter weather, when a long interruption of heating would cause a significant drop in room temperature (e.g. in old houses without sufficient thermal inertia) – in that case, some installers choose to turn off the priority and let both circuits run in parallel.

What storage tank volume is optimal for a 4-person household?

For a 4-person household with typical demand (showering, dishwashing, possibly bathing), a storage tank of 150–200 litres is generally recommended. A 150 l tank covers morning showers for 4 people within a short interval, while 200 l is suitable when tank heating is scheduled for only a short time block. Learn more about choosing the volume in the article What storage tank volume do I need for a low-temperature boiler.

Does combining a low-temperature boiler with a storage tank pay off economically compared to a condensing combi boiler?

A condensing combi boiler has higher efficiency (98–109% vs. 89–93%), so purely in terms of gas consumption it comes out better than a low-temperature boiler. However, a low-temperature boiler with a storage tank has advantages in comfort (a constant volume of hot water) and long service life (fewer cycles, simpler servicing). Economically, the decision depends on the specific purchase price, service costs, and gas prices. In many projects – especially renovations where an old chimney already exists and the customer doesn't want to invest in a new flue system – a low-temperature boiler with a storage tank remains a very competitive choice.

Conclusion: Why combining a low-temperature boiler and a storage tank still pays off

A low-temperature boiler combined with a hot water storage tank is not an outdated technology – it's a proven, reliable, and, with the right settings, energy-efficient solution. The key to savings isn't just choosing the right boiler, but a whole set of measures: correct hydraulics, tank priority, a heating time program, quality tank insulation, and regular maintenance of the entire system.

For family houses with four or more people, where hot water demand is high, a storage tank connected to a low-temperature boiler can save €150–250 per year on the gas bill while also significantly improving comfort – no more temperature fluctuations in the shower, no more waiting for hot water when drawing it at multiple points at once. The Vaillant atmoTEC plus and turboTEC plus boiler ranges are designed from the ground up for exactly this kind of combination, and their connection to a storage tank is straightforward and reliable.

If you're deciding on a specific model or sizing the boiler's output for your storage tank, we also recommend reading our article Frequently asked questions about low-temperature boilers with the option of connecting a storage tank, where you'll find answers to dozens of other practical questions straight from installers and customers.

Do you have a question about this topic?

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