How to Read a Condensing Boiler Data Sheet and Price List – What to Watch Out For

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How to read the technical data sheet and price list of a condensing boiler – what to pay attention to

When a customer sits down for the first time to read the technical data sheet of a condensing boiler, it is usually greeted by a dozen tabular columns full of numbers, abbreviations in English, German or Polish, and footnotes that they would rather not even look at. The result? They choose based on price and brand, because "that must be enough". From practice, I know that this approach leads to situations where the boiler is running, but never achieves the promised efficiency, where it is not possible to connect a tank that the customer already has at home, or when the installer arrives on site and finds out that the flue gas route does not lead in the right direction. A technical data sheet is not bureaucracy. It is a plan according to which the whole system is built – and those who read it carefully will save themselves from several unnecessary visits, complaints and nerves.

This article will take you through each important section of the technical data sheet and price list of a condensing boiler, explain what each value actually means in practice, and point out the most common mistakes in their interpretation. If you are still considering what power you need, read the article What power of a condensing boiler do I need – calculation based on area and insulation, where the methods of calculating heat loss are discussed in detail.

What is a technical data sheet and why it exists

A technical data sheet (in English technical data sheet, in German Technische Daten, abbreviated as TDS or TD) is a document that the manufacturer is obliged to issue for each boiler model. Within the framework of European legislation, a boiler sold on the EU market must comply with the ErP (Energy-related Products) directive, regulation No. 813/2013 and at the same time have a calculated and published seasonal energy efficiency (ηs or η_s,h). A technical data sheet is a formalized collection of all parameters according to which the boiler is designed, compared and installed.

The price list is a different document – it speaks about commercial conditions, prices, accessories and warranty conditions. In practice, these two documents are often distributed together, which can be confusing if we do not know where one ends and the other begins. Below we will look at each group of parameters separately.

Power parameters – what is actually hidden behind the numbers

Rated and modulated power

The first row of the table that everyone is interested in is the power in kilowatts. Here you need to be careful, because modern condensing boilers are almost always modulated – that is, they can operate in a range from minimum to maximum power. This range is written in the technical data sheet, for example, as 2.5 – 24 kW or 4.0 – 28 kW. The minimum is just as important as the maximum.

Why? Because most of the year the boiler operates at partial load – not at full power. If the heat loss of a family house is, for example, 9 kW and the boiler has a minimum of 4 kW, it will modulate correctly and achieve the condensing mode. If, however, for the same house we choose a boiler with a minimum of 9 kW, it will always run at the edge and constantly switch on and off – so-called cycling, which reduces the lifespan and efficiency. Comparing the modulation ratio is therefore key when choosing the right model, which I discuss in more detail in the article How to choose a condensing boiler – criteria for selection for a family house and an apartment.

Efficiency – η and Pn, Ppart, p4

The efficiency of a condensing boiler is given in several values and each has a different calculation basis:

η at Pn (100 % load, 80/60 °C): a classic test condition, high-temperature mode. Here condensation practically does not occur, so the values are usually around 97–99 %. This number is the least interesting for condensing boilers.
η at Ppart (30 % load, 50/30 °C): condensing mode, return temperature below the dew point (~57 °C for natural gas). Here the real advantage of condensation is shown – values 106–109 %.
η_s (seasonal efficiency): a calculated value according to EU regulation, taking into account the entire heating year including standby losses. The minimum standard currently valid for condensing boilers is 92 % η_s. High-quality models achieve 93–98 %.

Be careful with manufacturers who in their advertising materials only mention a high value at low-temperature testing and do not mention seasonal efficiency. The only comparable value across the entire market is precisely η_s, because it is measured by the same methodology.

NOx Emission Classes

Standard EN 483 / EN 15502 defines NOx emission classes from 1 to 6 – the higher the class, the lower the nitrogen oxide emissions. Most modern condensing boilers meet class 5 (NOx emission limit ≤ 70 mg/kWh), while some premium models reach class 6 (≤ 56 mg/kWh). This is particularly important in regions with stricter local legislation and also when applying for certain types of subsidies. The NOx class is always stated in the certificates and should also be included in the technical data sheet.

Hydraulic Parameters – Where Installation Problems Hide

Maximum Operating Pressure and Minimum System Pressure

The technical data sheet lists the maximum operating pressure (PMS – maximum allowable operating pressure), usually 3 bar for most domestic condensing boilers. However, be careful – it also specifies the minimum pressure required for the boiler to start, typically 0.8–1.0 bar. If the system pressure drops below this minimum (which can happen due to insufficient filling or losses via the expansion vessel), the boiler will lock and display an error code.

From practice: a customer once called me saying his boiler was throwing error E5 (low pressure) every morning. The problem was simple – his system was only filled to 0.7 bar instead of the recommended 1.2–1.5 bar. It was enough to add water and check the expansion vessel. The technical data sheet clearly stated this value – no one had just read it before installation.

Maximum Flow and Pressure Loss in the Heat Exchanger

For the sizing of the pump and the entire hydraulic system, the pressure loss of the boiler itself at nominal flow is crucial. It is given in kPa or mbar. For example, a 24 kW boiler may have a pressure loss of 18 kPa at a flow rate of 960 l/h – this is an input value for calculating the characteristic curve of the entire system's pump. If the boiler is installed with a too weak pump or with an overly long pipe layout without properly set circulation pumps, the heat exchanger overheats and the boiler faults.

Expansion Vessel Volume and Gas Pressure

Some technical data sheets also list the volume of the built-in expansion vessel (e.g., 8 l or 10 l). This value is important for assessing whether the built-in expansion vessel is sufficient for the volume of the heating system. As a rule of thumb: a system with a water volume exceeding 100 liters requires an expansion vessel larger than what is typically mounted on the boiler – in that case, an external one must be added. This is a detail that is easily overlooked.

Gas pressure: the technical data sheet specifies the minimum and maximum input pressure of natural gas (for G20) or propane (G31). Typical values for natural gas are 17–25 mbar. If your supply line provides a different pressure, the installer must adjust the regulator. Without this basic check, improper combustion may occur, with consequences ranging from reduced efficiency to dangerous conditions.

Flue Gas Parameters and Requirements for Flue Gas Ventilation

Type of Flue Gas Ventilation According to EN 483

This is perhaps the most frequently overlooked parameter in the technical data sheet. The boiler can be classified into several categories based on flue gas ventilation:

B23 / B33: combustion air drawn from the room, flue gases vented outside via a separate pipe. Requires a ventilated room, now limited.
C13 / C33 / C43 / C53 / C63: closed system – air and flue gases routed through one wall (coaxial) or separately to the façade, roof, or chimney.
C63: boiler without an integrated fan, connected to an external flue and air system – typically for shared flue shafts.

Each type requires a different range of accessories (coaxial elbow, adapter, extension) and different building conditions. Sometimes a customer buys a boiler and only at the site it turns out that the required length of the coaxial line is longer than allowed by the technical data sheet (e.g., maximum equivalent length of 10 m). In that case, it is necessary to either move the boiler or choose another model. More about these requirements can be found in the article Installation of a Condensing Boiler – Procedure, Space Requirements, and Flue Gas Ventilation.

Flue Gas Temperature and Condensate

Flue gas temperature at maximum output (test condition 80/60 °C) used to be 200–250 °C for older boilers. For a condensing boiler, it is 55–75 °C under the same conditions, and drops below 40 °C in low-temperature mode (40/30 °C). This value determines which material of the flue pipe you can use – condensing boilers always require plastic or stainless steel double-walled coaxial piping. An old masonry chimney without a lining is unsuitable for a condensing boiler without renovation.

The amount of condensate (in l/h or kg/h) is also listed in the technical data sheet. Typically 1.5–3 l/h at full condensing mode for a 24 kW boiler. Condensate has a pH of 3.5–5 (acidic!) – its removal must be handled with resistant piping, and in many municipalities, a neutralization chamber is required before discharging into the sewer. If the technician forgets this during installation, damage to the sewer and fines from the network operator are possible.

Electrical Parameters and Boiler Control

Electric Energy Consumption

Condensing boilers are dependent on electricity – they need it for the control electronics, combustion fan, circulation pump, and possibly control modules. Typical consumption at full output is 80–150 W, and in standby mode 5–20 W. Annual electricity consumption for a boiler with 2,000 hours of annual use is roughly 160–300 kWh – this is a significant item and is considered in some projects (e.g., with photovoltaic panels).

Important is also the information about phasing: most domestic boilers are single-phase (230 V / 50 Hz), while larger boilers over 50 kW are usually three-phase (400 V). Swapping this would damage the boiler. The IP rating (e.g., IP44) indicates resistance to water and dust – for installation in a boiler room with possible splashing water, a minimum of IP44 is required.

Communication protocols and intelligent control

Modern condensing boilers support various communication protocols for connection to a thermostat or smart home system:

OpenTherm: two-wire digital protocol, allows regulation of heating water temperature according to actual load. The boiler and thermostat "talk" to each other – this is the most efficient way to control a modulating boiler.
0–10 V analog input: older, but still used method for external control (e.g. from MaR systems).
On/Off (eBus terminals, potential-free contacts): basic two-position control, the boiler runs at a fixed temperature. A cheap solution, but it loses the benefit of modulation.
EMS / EMS+ / Bosch EMS2, Vaillant eBus, Viessmann LON/KNX: proprietary protocols of individual manufacturers – compatibility with the thermostat must be verified before purchase!

This is a point that customers often overlook. They buy a smart thermostat for the tank and then find out that their boiler supports only OpenTherm, not the proprietary protocol of the selected thermostat. Result: unnecessary purchase and another service technician visit.

Dimensions, weight and space requirements

The technical specification always includes a plan and front view drawing of the boiler with the position of all connecting nozzles (heating, return, cold water, hot water, gas, flue gases, condensate). These dimensions are in millimeters and are binding – not approximate.

What to pay attention to:

Service space: in addition to the physical dimensions of the boiler, the technical specification lists the minimum service zone – free space that is necessary in front of the boiler and on the sides for the removal of the burner, heat exchanger or siphon. It was usually 50–60 cm in front of the boiler. If you install the boiler in a niche, you must measure it in advance.
Weight: a 24 kW boiler typically weighs 30–45 kg. Some combined boilers with built-in storage tanks reach 80–100 kg. This affects the type of mounting and wall load capacity.
Minimum room height for certain types of flue gas exhaust (e.g. vertical coaxial flue) or for the possibility of folding the burner during service.

If you plan to install a condensing boiler in an apartment or in a limited space, I recommend reading Project documentation and installation materials for a condensing boiler, where you will find an overview of all the materials you need to have prepared before the building permit.

How to read the price list of a condensing boiler

Base price vs. price of a complete installation

The manufacturer's or distributor's price list usually lists the price of the boiler itself – without accessories. It is important to understand this right away, because the actual price of a complete installation can be 30–60 % higher. What is typically not included in the base price:

Flue gas / coaxial set (90° elbow, extensions, roof or façade end piece)
Expansion vessel (if the built-in capacity is insufficient)
Service valve for gas, check valves, ball valves for heating
Neutralization shaft for condensate
External thermostat or smart controller
Hot water tank (if the boiler is not combined)
Installation and commissioning

When a customer compares boiler A for 900 € and boiler B for 1 100 €, while boiler A has more expensive accessories (e.g. proprietary flue system for 350 €) and boiler B has compatible standard accessories for 150 €, the result is the opposite of what appears at first glance.

Warranty conditions in the price list

Warranty conditions vary from manufacturer to manufacturer and are always part of the price list / commercial conditions. The basic legal warranty in Slovakia is 24 months. Most condensing boiler manufacturers offer an extended warranty (3–5 years) under the following conditions:

Installation by an authorized installer (documented by a commissioning report)
Annual service by an authorized service technician
Product registration on the manufacturer's website within a specified period (e.g. within 30 days)
Use of original spare parts during service interventions

These conditions are always in the price list or warranty card. If the customer does not meet them, the extended warranty is void – and this is a common source of disputes in claims. At the same time, some manufacturers exclude from warranty damage caused by unsuitable water quality (hardness above 3.5 mmol/l), which is a real problem in some Slovak regions.

Comparison of models within one series

Manufacturers typically sell boilers in series with different outputs (e.g. 12, 18, 24, 28, 35 kW) and they also differ in equipment (basic model, model with integrated storage tank, model with solar controller). In the price list it is important to track:

Price difference between adjacent outputs – sometimes the difference is only 50–80 €, so it is worth going one step higher for more reserve when expanding the house or during extreme cold.
Availability of the model in stock – some outputs are available immediately, others are ordered. A price list with a date of issue is relevant only if it is current (not older than 6 months).
End of production and availability of spare parts – older models may be cheaper, but if they are at the end of their life cycle, availability of spare parts may be problematic in 5–10 years.

Certificates and standards in the technical specification

At the end of the technical specification or in a separate appendix you will find a list of certificates. For the Slovak market, the following are relevant:

CE marking: mandatory for placing on the EU market, confirms conformity with directives (PED, GAD, EMC, LVD).
ErP class: energy label similar to appliances – from A to A+++. Condensing boilers must have at least A, premium ones reach A+.
EN 15502 / EN 677: standards for performance and safety of gas condensing boilers.
DVGW (Germany) / KIWA (Netherlands) / CE-GAZ (France): national certificates that manufacturers add voluntarily and are a sign of higher quality testing.
PED 2014/68/EU directive: pressure directive, relevant for boilers with power above 10 kW.

The presence of DVGW or KIWA certificates indicates that the manufacturer has undergone a more rigorous external audit than required by the basic CE certification. For a comparison of boilers in terms of quality and reliability, I recommend the article Condensing Boilers – Comparison of Brands and Models by Efficiency and Price.

Typical mistakes when reading technical data sheets – from practice

Over the years of working in this segment, I have seen these recurring misunderstandings:

Confusing heating power with hot water heating power: a combination boiler may have 24 kW for heating and 28 kW for hot water. These are different values, and if we make a mistake, we underestimate comfort or overdimension the system.
Overlooking the minimum burner load: choosing a boiler that is too large for a small house, where even the minimum power is not sufficient for long modulation.
Not checking the date of the technical data sheet: an older data sheet may contain different values than current production (e.g., after a burner change in 2022). Always request a document with the date of issue or revision.
Ignoring footnotes: they contain the conditions under which the stated values apply – e.g., "an efficiency of 108.5 % applies at an incoming gas temperature of 15 °C and a return temperature of 30 °C".
Not noticing the boiler's compatibility with biogas mixtures: some boilers are certified for natural gas G20 and biogas (biomethane) up to a certain percentage. With the increasing share of biomethane in the grid, this is becoming practically relevant.

Practical step-by-step guide to reading a technical data sheet

I recommend the following approach when you sit down with a boiler's technical data sheet:

First, check the date of the document – if it is older than 2 years, request an updated one.
Find the modulation range of power (min–max kW) and compare it with the calculated heat loss of the building.
Check the seasonal efficiency η_s – this tells the truth about actual operating costs.
Verify the type of flue gas exhaust and check if it matches your building conditions.
Check the hydraulic ports and their position – do they match the existing layout?
Read the electrical requirements – phase, power consumption, communication protocol.
Go through the list of certificates – this confirms compliance with legislation and any possible subsidy requirements.
Look at the warranty conditions in the price list – what conditions must be met for an extended warranty?

If you are unsure about any parameter, do not hesitate to consult a specialized sales representative or heating system designer – it is better to have a question answered before installation than to deal with problems after it. More practical customer scenarios can also be found in the article Common Questions about Condensing Boilers – Subsidies, Legislation, Operating Costs.

Frequently asked questions (FAQ)

What does it mean that a boiler has an efficiency of 109 %? Isn't that physically impossible?

It is not a mistake or marketing. An efficiency above 100 % is possible because the reference value (100 %) corresponds to the lower heating value of the fuel (Hu), which does not include the latent heat of water vapor in the flue gases. A condensing boiler can recover this latent heat through the condensation of water vapor – thus its actual energy gain relative to Hu can exceed 100 %. If we calculated relative to the higher heating value (Hs), the efficiency would be, for example, 98.5 %. Both methods are correct, it is just important to know which one is used in the given technical data sheet.

Why do the values in the technical data sheet differ from those in the manufacturer's catalog?

The catalog is a marketing document – it presents values under the most favorable test conditions, rounds up, and selects attractive numbers for the customer. The technical data sheet is a binding technical document with more precise values including test conditions. In case of a dispute (e.g., a complaint), the technical data sheet is binding, not the catalog. Always work primarily with the technical data sheet.

Can I connect a 24 kW boiler to a house where the calculation came out to 11 kW of heat loss?

It depends on the minimum power of the boiler. If the boiler has a minimum of 2.5 kW, it will modulate between 2.5 and 11 kW most of the year in real operation – this is acceptable. If the minimum is, for example, 9 kW, the boiler will not be able to work for a long time under the required load, it will cycle and shorten its lifespan. Oversizing in a ratio of 2:1 (24 kW for a house with a need of 11 kW) is on the upper limit of acceptable – a 18 kW model with modulation from 2–3 kW would be better.

What is ErP class and why is it different from the A+++ class on refrigerators?

ErP (Energy-related Products) is a European energy labeling system for heating equipment. Unlike refrigerators and washing machines (where the scale goes from A to A+++), the scale for boilers is defined differently: the minimum standard for condensing boilers is class A, and premium boilers achieve A+. Classes B to G are reserved for older and less efficient boiler types (non-condensing). The class values are directly derived from the seasonal efficiency η_s.

What to do if the technical data sheet contains values only in German or English?

Most European manufacturers issue technical data sheets in the language of the target market – for Slovakia, this should be a Slovak or Czech document. If the distributor provides only a German or English sheet, it is legally acceptable (European standards do not require technical data sheets in every national language, only installation manuals and user instructions), but practically inconvenient. In such a case, request the distributor to provide a translation of key parameters or assistance from a designer. Abbreviations such as Pn, Ppart, η, ΔT, PMS, NOx are international and their understanding is independent of the document's language.

Where can I find the technical data sheet and price list if the seller did not provide them?

The technical data sheet is mandatory for the manufacturer to make available – it is usually found in the "Downloads" or "Documents" section directly on the manufacturer's website for a specific model. If you cannot find it, you can contact the distributor's technical department or the manufacturer directly. The price list is a commercial document that distributors and sellers usually issue only to registered partners, but approximate retail prices are available on e-shops such as atria.sk. Current price lists and technical data for condensing boilers can also be found in the section Tutorials, Documents, Price Lists on this page.

Conclusion – the technical data sheet is your best ally

The technical data sheet of a condensing boiler is not just a dry collection of numbers intended only for designers and installers. It is a document that – if you know how to read it – will answer all the key questions before you spend even a euro. Performance, efficiency, flue gases, hydraulics, dimensions, electricity, certificates, and warranty conditions form a complete picture of what you are buying and what it entails in practice. When combined with a properly interpreted price list, you have a complete basis for comparing boilers, selecting installation accessories, and making an informed decision about the entire heating system.

If you have made it this far, you are likely working on a specific project. In that case, I recommend also reading the article Maintenance and servicing of a condensing boiler – what and how often to check, where you will learn how proper handling of the boiler after installation prolongs its lifespan and protects warranty conditions – a topic that directly relates to what you have just learned about warranty conditions in the price list.

Do you have a question about this topic?

Can't decide or are you dealing with a specific situation in your household? Write to us – we are happy to help.