Condensing vs. low-temperature boiler with storage tank: which one pays off more

We are still preparing products.

But you can look at other categories.

Condensing vs. low-temperature boiler with a tank: which one really pays off more?

This question comes up regularly – whether during the renovation of an old house or when choosing a boiler for a new build. Customers come to us with various situations: some have radiators sized for 80/60 °C, some want to save on gas, others need a domestic hot water (DHW) tank right in the boiler room without an extra vessel. The result? Confusion caused by a mass of parameters, model ranges and conflicting recommendations from various sources.

In this article, we'll break down both technologies in detail – not just on paper, but also from the perspective of real-world operation. We'll explain when a condensing boiler with a tank actually brings savings and when a low-temperature boiler with a built-in tank is the more sensible and economically advantageous choice. This includes specific figures, practical examples and technical details that will help you make an informed decision.

Technology basics: what each of them actually does differently

Before we can compare, we first need to understand how the two technologies differ on the level of physics, not just marketing.

How a condensing boiler works

A condensing boiler burns natural gas (or propane) just like a regular boiler – but it can additionally capture and reuse heat that would otherwise be lost with the hot flue gases through the chimney. Flue gases contain water vapour (produced by burning the hydrogen contained in natural gas). When the return pipe temperature is low enough – typically below 55–57 °C – this water vapour condenses in a special heat exchanger and releases its latent heat (condensation heat). This is the "extra" energy thanks to which a condensing boiler's efficiency can exceed 100 % (measured against the net calorific value of the fuel).

The key phrase: the return pipe temperature must be low enough. This is exactly where the fundamental practical problem lies. A condensing boiler operates efficiently only when the heating system allows it to. If you have old cast-iron radiators sized for 80/60 °C, you may run the boiler as a condensing unit, but you will only achieve condensation briefly during the season (in very mild weather). During cold days it operates in non-condensing mode – i.e. with the efficiency of a regular boiler, but at the price of a higher purchase cost.

How a low-temperature boiler works

A low-temperature boiler does not work with the condensation effect. Combustion takes place in a steel (or cast-iron) body, and heat is transferred directly to the central heating circuit. The flow temperature can be set to lower values – typically 40–75 °C – than with old atmospheric boilers (80–90 °C), and this is exactly what provides room for energy savings. A low-temperature boiler is therefore not an archaic technology; it is a boiler with modern control elements, quality insulation and optimised combustion, which however does not use the condensing effect.

Its main advantage? Compatibility and simplicity. It works well with classic radiators and existing chimneys (no need for a stainless-steel liner for acidic condensate), servicing is less demanding and the upfront investment is significantly lower.

Built-in DHW tank: why it changes the whole equation

Both types of boilers can have a built-in domestic hot water (DHW) tank. It is precisely the tank inside the boiler that is the condition which significantly affects the real efficiency of the whole system – for several reasons:

The DHW tank must be regularly heated to a higher temperature (at least 60 °C to protect against legionella), which for a condensing boiler means that while heating the water the boiler does not produce condensate and operates at lower efficiency.
For a low-temperature boiler with a tank, this "normal" operation at 60–70 °C is precisely its natural operating point – not a disadvantage.
The tank smooths out DHW draw-off peaks and reduces the frequency of burner start-ups – which saves gas and extends the boiler's service life regardless of the type of technology.

When a condensing boiler with a tank really brings savings

To be fair, a condensing boiler with a tank has clear advantages – but only under specific conditions. Here is an overview of situations when investing in condensing technology makes sense:

New build with underfloor heating or low-temperature panels

If you are building a heating system from scratch and choose underfloor heating (flow temperature 30–45 °C) or modern aluminium panel radiators sized for 55/45 °C, the condensing boiler will condense practically the whole season. In such a case, seasonal gas savings can reach 15–25 % compared to a classic low-temperature boiler. With consumption of, say, 2,000 m³ of natural gas per year and a price of €0.07/kWh, this can mean a saving of €180–300 per year. The payback period for the difference in boiler price (€1,000–2,000) could therefore be 5–10 years.

House with a pumped system and modern controls

Thermostatic heads, weather compensation control (the boiler adjusts the flow temperature to the outdoor temperature) and a circulation pump with a frequency inverter – these are the conditions under which a condensing boiler operates efficiently. The weather compensation curve ensures that in mild weather (outdoor +5 °C) the boiler does not unnecessarily heat the water to 75 °C, but only to 50–55 °C – and condensation occurs.

Combination with a solar system for DHW preheating

If you have solar collectors, the DHW tank is preheated to 40–50 °C for most of the year and the boiler only "tops it up" to 60 °C. In such a case, a condensing boiler works with warmer water for a shorter time – which paradoxically reduces its condensing advantage when preparing DHW. But the overall system is energy efficient. You can read more on this topic in the article How to connect solar collectors to Thermona low-temperature boilers with a tank.

When a low-temperature boiler with a tank is the better choice

And now the other side of the coin – which is, in practice, far more common than one might think.

Older buildings with conventional radiators

This is by far the most common scenario from real-world project experience. A family house from the 1980s or 1990s, cast-iron or steel panel radiators sized for 75/60 °C or even 80/60 °C. The owner is replacing an old atmospheric boiler with something more modern. The salesperson offers them a condensing boiler as a "money-saving solution".

Reality? Condensation occurs only when the return pipe temperature is below 57 °C. But with such radiators and an outdoor temperature of −10 °C, you need at least 70–75 °C at the flow – and the return pipe runs at 55–60 °C. Condensation is minimal or non-existent. The boiler operates as a standard non-condensing unit, only it was more expensive. The customer pays a premium for technology they cannot use.

In such a scenario, a low-temperature boiler with a tank is a much more sensible investment. It has a lower purchase price, simpler servicing, longer service life (less aggressive environment without condensate) and, in reality, consumes only slightly more gas – a negligible difference compared to a condensing boiler operating without condensation.

Apartment buildings and smaller spaces with limited room

Compact low-temperature boilers with a built-in tank are simpler to install and do not require stainless-steel chimney liners (condensing boilers produce acidic condensate that corrodes ordinary masonry chimneys). If the house has a functional classic chimney and lining it is not possible or economically justified, a low-temperature boiler is de facto the only sensible choice.

Lower installation and servicing costs

A condensing boiler needs a condensate drain (carbonic acid, pH 3.5–5.5), a neutralisation box, a stainless-steel or plastic flue and more careful regular maintenance of the heat exchanger. A low-temperature boiler has a classic chimney connector, a simpler flue, and servicing requirements are lower. Over 15 years of operation, the difference in servicing costs can reach €800–1,500, which should be factored into the overall economic balance. You can read more on this topic in the article Maintenance and servicing of a low-temperature boiler with a tank: what and how often to check.

Specific models from practice: Thermona THERM PRO and THERM LXZ

Let's look at real products available in the category of low-temperature boilers with a built-in tank and explain who each one is intended for.

Thermona THERM PRO 14 KX and 14 TKX – a reliable base for a typical family house

The Thermona THERM PRO 14 KX and Thermona THERM PRO 14 TKX series are classic low-temperature representatives with a built-in DHW tank, designed for family houses with a standard heating system. The KX model has a built-in DHW tank, while the TKX model is a variant with a larger tank and solid-fuel connectivity – meaning it allows a bivalent connection with a solar collector or a solid-fuel boiler as an additional heat source.

In practice, these boilers are encountered most often when replacing old gas or electric boilers in houses from the 1990s. The customer gets a modern controllable boiler with an integrated tank, saves space in the boiler room (no need for a separate DHW vessel), simplifies the piping and reduces heat losses compared to the old boiler. You can find out more about how to choose the correct tank volume in the article What DHW tank volume do I need for a family house or apartment.

It is important to note that the same model exists in two variants distinguished by URL – the alternative variant of THERM PRO 14 KX and the alternative variant of THERM PRO 14 TKX – which may correspond to a different design (for example, a different type of circulation or accessories). Always check the technical data sheets to know what accessories are included in the package and what needs to be purchased separately.

Thermona THERM 28 LXZ.A 5 – a more powerful model for demanding applications

The more powerful Thermona THERM 28 LXZ.A 5 belongs to the LXZ range – low-temperature boilers with higher heat output (28 kW) and modernised control. It is suitable for larger family houses, or for houses with higher DHW consumption (larger family, shower cubicles, bathtub). The tank is integrated, which saves boiler room space, and the tank capacity is set to cover peak demand without the need to oversize the boiler.

This model is also interesting for a possible future connection of solar DHW preheating – the tank has a secondary heat exchanger prepared. If you are considering such a solution, read the article How to connect solar collectors to Thermona low-temperature boilers with a tank, where you will find a detailed procedure as well as the limitations of such a connection.

Economic analysis: where the real savings are and where it's just marketing

Let's break down a specific calculation example that we use when advising customers.

House built in 1992, 160 m² floor area, cast-iron radiators sized for 80/60 °C, family of 4.

Natural gas consumption with the old atmospheric boiler: 2,800 m³/year. Estimated consumption after replacement with a low-temperature boiler (better control, lower water temperature): 2,300–2,400 m³/year. Estimated consumption after replacement with a condensing boiler (with the same radiators, only partial condensation): 2,100–2,200 m³/year.

Difference between the condensing and low-temperature boiler: about 150–200 m³/year, which at a price of €0.065/kWh (calorific value 10.55 kWh/m³) amounts to a saving of €103–138 per year. Difference in boiler prices: approximately €1,200–1,800. Difference in installation and chimney lining costs: another €500–1,200. Total difference in upfront costs: €1,700–3,000. Payback period: 12–25 years – which is longer than the average service life of a boiler.

This is not an argument against the condensing boiler as such – it is an argument for a realistic assessment of your specific situation. In a new build with underfloor heating, the figure would be completely different (payback period of 5–8 years).

Practical scenarios from project experience

Scenario 1: Renovation of an older house with radiators retained

A family replaced a 25-year-old atmospheric boiler without a tank. They had a separate electric DHW heater (300 L, old and inefficient). They were considering a condensing combi boiler or a low-temperature boiler with a tank. They decided on a low-temperature boiler with a built-in tank from the Thermona THERM PRO series – they saved on the boiler price as well as on installation (no need for chimney lining, no need to replace the chimney connector). The 80 L tank fully covers the daily DHW consumption for a family of 3. Monthly heating + DHW costs dropped by 38 % compared to the original state (combination of the old boiler and the electric water heater).

Scenario 2: New build with underfloor heating

An investor building a family house was deciding between a condensing boiler and a heat pump. In the end, they chose a condensing boiler with a tank (new build, underfloor heating 35/30 °C, weather compensation control). The boiler condenses practically the whole season, and consumption is significantly lower than with a regular boiler. In this case, condensing technology is fully justified and the payback period for the extra investment is realistic.

Scenario 3: Apartment building, shared boiler room

The administrator of an apartment building from the 1980s is dealing with the replacement of the central boiler. The piping and radiators are old, sized for high temperatures. The chimneys are masonry, without liners. Replacing them with a condensing boiler would require comprehensive lining of the entire chimney structure (a costly undertaking). Solution: a more powerful low-temperature boiler without condensation, compatible with the current chimney. Savings from more modern control and better combustion efficiency are real and immediate, without enormous additional investments.

Technical parameters to compare when choosing

When choosing a specific boiler – regardless of type – pay attention to these parameters:

Heat output (kW): must match the house's heat losses. Oversizing is just as bad as undersizing – unnecessary cycling shortens the boiler's service life. The heat loss calculation should be done by a designer, not estimated by a salesperson.
DHW tank volume (L): 80–100 L is common for 2–3 people, 120–150 L for 4 people, and for higher consumption or bivalent connection with solar, 200 L+. A detailed calculation can be found in the article What DHW tank volume do I need for a family house or apartment.
ErP energy efficiency class: at least A for heating, A for DHW preparation. Condensing boilers achieve A+ or A++, low-temperature boilers standardly A.
Tank type: enamelled (longer service life, more resistant to corrosion, but more sensitive to thermal shocks), stainless steel (no enamel layer, but more expensive) or plastic liner (cheapest, but shorter service life). Don't forget to check the anode – more in the article Rust, limescale and the tank anode – how to extend the boiler's service life.
Compatibility with a solar system: if you plan to add solar in the future, choose a boiler with two heat exchangers in the tank or with a solar connection. Thermona's TKX range models allow this.
Dimensions and boiler room space: low-temperature boilers with a tank are larger than standalone flow boilers. Measure the available space. More about the requirements in the article Low-temperature boiler with a tank – boiler room requirements, dimensions and space.

The DHW tank as a real decisive factor

Many customers, when choosing a boiler, underestimate the DHW tank and focus only on the type of heating technology (condensing vs. low-temperature). Yet a tank integrated directly into the boiler body brings a whole range of benefits that are independent of the heating technology:

It eliminates the need for a separate DHW vessel and saves boiler room space.
It shortens the cold and hot water piping, thereby reducing heat losses.
It allows precise setting of the DHW heating time program (night setback, legionella heating once a week to 65 °C).
It reduces the frequency of burner start-ups compared to flow-through systems – heat accumulation over time means fewer starts = a longer service life for the combustion system.

From a daily comfort perspective, an 80–120 L tank is suitable for most standard family houses in most cases. The tank is continuously replenished from the boiler, so a short-term peak (two people showering at the same time) does not necessarily empty the tank completely – it depends on the tank temperature setting and the boiler's output.

The most common mistakes when choosing and what to avoid

Based on experience from dozens of projects – here are the mistakes that keep recurring:

Buying a condensing boiler for an old house with an unsuitable system – a classic. The customer pays a premium, condensation doesn't occur, and savings are minimal.
An undersized DHW tank – a family of 4, a 60 L tank. Result: cold water after 10 minutes of showering. You need to calculate the daily DHW requirement realistically.
Forgetting about chimney requirements – a condensing boiler requires a resistant chimney (stainless steel, plastic) due to the acidic condensate. A masonry chimney without a liner would collapse over the years.
Ignoring servicing costs – choosing the cheapest boiler without available servicing and spare parts is a path to problems. Thermona has a service network in Slovakia.
Not reading the project documentation and measuring the boiler room space – a boiler with a 120 L tank can be 1.6–1.8 m tall and over 80 cm deep. It won't fit into a low boiler room with narrow doors. More on this in the article Installing a low-temperature boiler with a built-in tank: procedure and what you need to know.

FAQ: Frequently asked questions

Is a condensing boiler always more economical than a low-temperature one?

No. A condensing boiler is only more economical when it actually condenses – that is, when the return pipe temperature drops below the dew point (approx. 57 °C for natural gas). With old radiators sized for 75–80 °C, condensation occurs only sporadically (in mild weather). In such a case, the difference in gas consumption between a condensing and a low-temperature boiler is only 3–8 %, which, given the higher purchase price of the condensing boiler, leads to a very long payback period.

Can I connect solar collectors to a low-temperature boiler with a tank?

Yes, but the tank must have two heat exchangers (a bivalent tank) – one for the solar circuit and one for the boiler. Models such as the Thermona THERM PRO 14 TKX have this variant available. A simpler tank with a single heat exchanger does not allow a solar connection without an external vessel. A detailed procedure can be found in the article How to connect solar collectors to Thermona low-temperature boilers with a tank.

How often does the tank anode need to be replaced?

Magnesium anodes are recommended to be checked every 2 years and replaced according to their condition – typically every 2–4 years, depending on water quality and tank volume. Hard water (high calcium content) wears out the anode faster. Neglecting anode replacement leads to internal rusting of the tank and a costly repair or replacement of the entire boiler. More in the article Rust, limescale and the tank anode – how to extend the boiler's service life.

What is the difference between the KX and TKX models of the Thermona THERM PRO?

The KX model is the basic variant with a simple DHW tank and a single heat exchanger – suitable as a standalone boiler without a solar connection. The TKX model has a bivalent tank with two heat exchangers, which allows the connection of solar collectors or another primary heat source (for example, a fireplace insert with a water heat exchanger) as a supporting system for DHW heating.

Can a low-temperature boiler with a tank handle very cold days (−15 °C and below)?

Yes, a low-temperature boiler has no problem with low outdoor temperatures – unlike heat pumps. It operates at full output without limitation. It is important that the boiler is correctly sized for the house's heat losses at the design outdoor temperature (depends on the region, for most of Slovakia −12 to −15 °C). The DHW tank operates normally on such days, since its heating is independent of the outdoor temperature.

Is installing a low-temperature boiler with a tank simpler than a condensing one?

In most cases, yes. A low-temperature boiler does not require a condensate drain, a neutralisation box, or special chimney lining (it works with a classic masonry chimney). This simplifies installation and reduces the installation cost by €300–1,200 depending on the specific situation. On the other hand, a boiler with a tank is physically larger and heavier – you need to plan for transport logistics and sufficiently large boiler room doors.

Conclusion: which boiler really pays off

The answer to the headline question is not clear-cut – and anyone who tells you otherwise without having seen your house, your chimney and your radiators is simply lying to you. The truth is that:

A condensing boiler with a tank pays off if you have a new build with a low-temperature heating system (underfloor heating, modern panel radiators) or if you plan a complete overhaul of the heating system. In that case, the savings are real and the payback period is reasonable.
A low-temperature boiler with a built-in tank is the right choice for most renovations of older houses with existing radiators, for houses with masonry chimneys without liners, for customers looking for a lower upfront investment with sensible operation, and for those where the condensing gain would be minimal due to the system's temperature.

Low-temperature boilers with a built-in tank are not an obsolete technology – they are proven, reliable machines with simpler servicing, a lower upfront price and full compatibility with existing installations. In Slovak conditions, where housing stock from the 1980s and 1990s with classic radiators predominates, they are the economically more advantageous and practical choice for most customers.

If you're not sure which model is right for you, also read the article How to choose a low-temperature boiler with a built-in tank: volume, output and type of operation – there you'll find a step-by-step process for arriving at the right choice based on your specific house's parameters.

Do you have a question about this topic?

Not sure what to decide, or dealing with a specific situation in your household? Write to us - we'll be happy to advise.