Project Documentation and Materials for Condensing Boiler Installation

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Why project documentation for a condensing boiler matters more than most people think

In my experience, most customers only start dealing with documentation when the inspection technician at final approval or the insurance company after a breakdown asks: "So where's the project documentation for this boiler?" That's exactly when it becomes clear what was neglected at the start. A condensing boiler isn't just an appliance you plug into an outlet – it's a gas or oil-fired appliance with a flue gas path, pressure system, electrical installations and control. All of this has legal requirements for documentation, design and commissioning.

This article explains in detail what documentation is required for installing a condensing boiler, who prepares it, what it must contain, and how the whole process from project to handover actually unfolds. This is a topic most guides skip, yet it's exactly what can delay final approvals, cause insurers to refuse to pay out, or lead an inspection technician to refuse to issue a report.

Overview of documentation by installation phase

The whole process can be divided into four phases: pre-project preparation, the design phase, execution and commissioning, and finally operational documentation. Each phase has its own mandatory and recommended documents.

1. Pre-project preparation – what needs to be resolved before contacting a designer

Before you even approach a designer, you need to have several input documents ready. Without them, the project can't even begin. In practice this sounds simple, but this is exactly where many jobs get stuck for weeks or months.

Technical documentation from the gas distributor (for a gas boiler): You need to know what gas pressure is available (usually 20 mbar for natural gas in a low-pressure network), what maximum input you can draw from the connection, and whether the connection is even sufficient for the new boiler. If you're renovating an older house with a boiler from the 1990s, where a 24 kW boiler with 80% efficiency was installed, and you now want to install a condensing boiler of the same output, gas consumption will be lower – however, the distributor must be informed about the change in boiler output. A new connection requires a separate procedure.

Building heat loss calculation: This is the foundation of every heating project. Without it, the designer cannot design the correct boiler output, pipe sizing, or radiator output. The article What condensing boiler output do I need – calculation based on area and insulation covers in detail how heat loss is calculated and what affects it. Here I'll just note that the designer should always work with the result of a standard calculation according to STN EN 12831, not rough estimates like "10 W/m²" or similar shortcuts.

Building documentation of the existing structure: Floor plans of all floors with marked rooms, their dimensions, wall and ceiling construction, insulation type and thickness, and window type. If such documentation doesn't exist (which is not unusual in older houses), the designer must rely on a site inspection and their own records.

Existing heating system documentation: If you're renovating the heating system, the designer needs to know what system already exists – whether the piping is steel or copper, what the design temperature is, and what type of radiators are used (cast iron radiators, steel panel radiators, underfloor heating). This affects compatibility with the condensing boiler and condensation conditions.

Project documentation – what it must contain

The project for installing a condensing boiler itself is not a single document but a set of several professional outputs. The scope depends on whether it's a flat, a family house, or a commercial building, and also on whether it's a replacement of an existing boiler or a new installation.

Central heating (CH) project

This is the key document. The central heating project must include:

Technical report – description of the system, proposed boiler output, boiler type, temperature parameters (e.g. 75/60 °C or 55/45 °C for a low-temperature system), heat source, control method, description of boiler connection with accessories (expansion vessel, safety valve, pump, mixing valve)
Heat loss calculation according to STN EN 12831, room by room
Heat output calculation for radiators – each radiator must have a designed output, type and size matching the room's heat loss
Hydraulic calculation – pipe sizing (pipe diameters, flow rates, pressure losses), circulation pump output design
Boiler connection diagram – functional diagram (P&ID) with boiler, expansion vessel, safety valve, manifold, circulation pumps, mixing valves
Pipe layout drawings – floor plan with pipe routes, anchoring points, vertical routing
Expansion vessel calculation and design – volume depends on the water content of the system and operating pressure
List of equipment and materials (bill of quantities)

Gas installation project (GI)

If the boiler is gas-fired, a gas installation project must be added to the CH project. It must meet the requirements of STN EN 1775 and related standards. The GI project includes:

Technical report describing the gas connection, type of gas (natural gas – methane G20), operating pressure, maximum input
Hydraulic calculation of the gas installation – sizing of gas pipes with regard to pressure losses so that the minimum pressure required by the manufacturer is ensured at the boiler inlet (usually 17–25 mbar)
Floor plan drawing with the gas pipe route, shut-off valves, pipe type and material
Location and type of gas meter, pressure regulator (if required)
Description of boiler room ventilation and emergency gas venting

The gas installation project must be signed by an authorized building engineer with the relevant specialization, or a technician authorized by the gas distributor.

Flue gas path project (flue gas discharge)

Condensing boilers have a sealed combustion system (type C) or an open one (type B, less common with condensing units). The flue gas discharge must be designed exactly according to the boiler manufacturer's requirements and applicable standards (STN EN 13384-1, STN EN 14471).

The flue gas path project is handled either as part of the CH project or as a separate document. It must include:

Calculation of chimney flue capacity – verification that the diameter, height and thermal resistance match the boiler's parameters (the flue gas temperature of a condensing boiler is very low, typically 50–80 °C, which is why plastic or stainless steel ducting in the 60/100 mm or 80/125 mm system is commonly used)
Method of routing the flue system – coaxial (air/flue gas in one pipe) or separate (dual)
Solution for condensate drainage from the flue gas path
Drawing of the flue pipe route with dimensions, heights, number of bends, and connection to the facade or chimney
Assessment of fire safety distances from combustible structures

The flue gas path is in fact the most commonly underestimated part of the documentation. I know of cases from practice where a customer bought a condensing boiler and wanted to connect it to an existing brick chimney. Without a project and a calculation of the chimney system, the inspection technician refused to issue a report. The reason: the brick chimney did not have a certified liner resistant to condensate.

Electrical installation project

A condensing boiler is an electrical appliance. It requires a 230 V / 50 Hz supply with a dedicated circuit breaker (usually 10 A or 16 A depending on output), grounding, and in many cases also a mains filter or backup power supply (UPS) for sensitive electronics. The electrical installation project for the boiler room includes:

Power supply diagram for the boiler, controls and pumps
Bonding requirements (equipotential connection) of metal parts
Location of the boiler's main switch
Wiring diagram for external controllers, thermostats, CO/CH4 detectors (mandatory for some types of installations)

This project doesn't always need to be separate. If the boiler is installed in a flat and the electrical supply is not being changed, an inspection report of the electrical installation is sufficient. If electrical installation work is being carried out, a project prepared by an authorized person is required.

Who may prepare the project and who may carry it out

This is an area where customers often ask questions and where there are many misunderstandings. Act No. 138/1992 Coll. on Authorized Architects and Building Engineers sets out who is entitled to prepare project documentation for buildings.

For the central heating and heating system project, an authorized building engineer specializing in technical, technological and energy equipment of buildings is required. For the gas installation project, it is either the same engineer or a technician with special authorization under the relevant decree.

Execution (the actual installation) may be carried out by:

A company or self-employed tradesperson with professional competence under the relevant decree (for gas installation – a person with a certificate for gas appliances)
Manufacturers recommend a service technician trained by the manufacturer for boiler installation – otherwise the warranty may be voided
Inspection of the electrical installation is carried out by an authorized electrical inspection technician under STN 33 1500
Inspection of flue gas paths (chimneys) is carried out by a certified chimney sweep company

Important: an installer ≠ a designer. Even if the installer is experienced, they may not prepare project documentation unless they hold the relevant authorization. This rule also applies to a seemingly simple boiler-for-boiler replacement.

Boiler replacement without a project – when it's legal and when it isn't

This is a very practical question that comes up constantly. The answer depends on the scope of the changes and whether it concerns a flat or a house.

According to the Building Act and its amendments:

Replacing a boiler with one of the same type and output (e.g. an atmospheric boiler replaced with a condensing one, both 24 kW, in the same location, without changing the piping or flue gas path): In most cases, no building permit or notification is required. Nevertheless, you must have a report of a professional inspection and test of the gas appliance from an authorized technician!
Boiler replacement with modification of the flue gas path (e.g. a new chimney liner, a new pipe on the facade): This is a notification of minor construction or building modification. It depends on the local building authority.
New boiler installation (where there was no heating before): Always requires a project, a building permit or notification, and final approval.
Installation in an apartment building (change to shared flue or gas connection): Requires the administrator's consent, a project, and in many cases also the consent of other owners.

From experience: the most common scenario in family house renovations is replacing an old boiler with a condensing one along with a new flue gas path. The building authority usually accepts a notification with project documentation, without requiring a full building permit. Nevertheless, I recommend checking in advance with the local building authority – some municipalities have different practices.

Technical documentation from the boiler manufacturer – what to look for and where

Every condensing boiler manufacturer provides a set of technical documents. These are essential for the designer and also important for the installer. You'll usually find them in the Manuals, Documents, Price Lists section on the seller's website or directly on the manufacturer's website.

What the designer and installer should have available:

Boiler technical data sheet – rated output (min/max), input, efficiency η, NOx class, weight, dimensions, connections (diameter and spacing), water operating pressure (min/max), minimum return temperature for condensation, electrical input, IP protection rating
Installation manual – requirements for minimum distances from walls and ceiling, boiler room requirements (ventilation, minimum dimensions, temperature), manufacturer's flue system specifications (which are approved, maximum equivalent length, number of bends)
Wiring/piping diagrams – for various system configurations: direct circuit, mixed circuit, DHW tank, solar system, cascade connection of multiple boilers
Operating manual – handed over to the customer at commissioning
Service manual – for technicians, contains diagnostic codes, parameter settings, service procedures
Declaration of Conformity (DoC) – CE certificate, a legal requirement for placing the product on the EU market
Energy label – mandatory for boilers since 2015 (Ecodesign), states seasonal energy efficiency ηs and class (e.g. A+++ for condensing boilers)

If you're wondering how to read a technical data sheet and what all the values mean, this is covered in more detail in another article in our Knowledge Center: How to read a condensing boiler's technical data sheet and price list – what to watch out for.

Professional inspection and test report

Before a boiler is first put into operation, an authorized person must carry out a professional inspection and test of the gas appliance. The result is a report of the professional inspection and test, without which the boiler must not be commissioned. This is not a mere formality – it is a legal requirement (the relevant decree and standard STN EN 1775).

The inspection report includes:

Identification of the equipment (type, serial number, output, year of manufacture)
Gas installation tightness check (pressure test)
Functional check of safety features (safety valve, flow limiter, high-limit thermostat)
Functional check of flue gas discharge and combustion air supply
Measurement of flue gas values (CO, CO2, O2, temperature, lambda)
Electrical part check (grounding, protective conductors)
Operator instruction (confirmed by signature)
Record of boiler setup and adjustment

The inspection report is carried out by a technician holding a certificate for professional equipment inspections. This certificate is not automatically included in an installer's trade license – verify it in advance.

In addition to the inspection report, a new installation also involves:

Pressure test of the heating system (water part) – the system is pressurized to 1.5 times the operating pressure (usually 3–4 bar) and the pressure drop is monitored for at least 30 minutes
Flue gas path tightness check – visual inspection, possibly a pressure test of the chimney liner
Electrical installation inspection – issued by an authorized electrical inspection technician, valid for 5 years (for a flat) or 3 years (for a boiler room as technical equipment)

Boiler commissioning – what the customer must receive

After completing the installation and carrying out the inspections, the installer/service technician hands over the complete documentation to the customer. This is a legal obligation and also the basis for making a warranty claim. The customer should receive:

Boiler operating manual (in the local language)
Warranty card (service booklet) with the first service inspection filled in
Report of the professional inspection and test
Report of the heating system pressure test
Electrical installation inspection report
Boiler Declaration of Conformity (CE)
Boiler energy label
Project documentation (if it exists)
Boiler setup protocol (temperatures, pressures, control parameters)

From experience, the warranty card/service booklet is the document customers most often come to me with a problem about: "The technician didn't fill anything in here." If the service technician doesn't complete the service record at the initial installation, the manufacturer may claim, in the event of a warranty claim, that the boiler was not properly commissioned. Always insist on a signed service record with a date and the company's stamp.

Documentation when combined with other systems

Today, a condensing boiler is very often combined with other sources or systems. Each combination brings additional documentation requirements.

Condensing boiler + solar system

When combining a condensing boiler with solar DHW heating (or solar heating support), a solar installation project is required, including calculation of collector area, tank volume, and the hydraulic diagram with the storage boiler or combination tank. You can learn more about combination options in the article Condensing boiler and solar heating – combination options and savings.

Condensing boiler + heat pump (bivalent installation)

In a bivalent installation (heat pump as the primary source + condensing boiler as the peak/backup source), the project is more complex. It must address the hydraulic connection of both sources, the control logic (when the boiler starts, when the heat pump runs), and an economic feasibility calculation.

Cascade connection of multiple boilers

In larger buildings (apartment blocks, commercial buildings), boilers are connected in a cascade. The project for a cascade system must address the hydraulic separator, cascade control, boiler rotation, monitoring, and the distance of media. This is exclusively a matter for an authorized designer.

How to properly archive documentation

The boiler documentation should be stored directly at the boiler (in the boiler room, in the technical room) or in an accessible place where the service technician can find it during a regular inspection. We recommend:

A plastic folder or binder kept directly at the boiler
A digital copy (scan/photo) stored in the cloud or an email account
When transferring the property, hand over the documentation to the new owner – this is a legal obligation when selling real estate

Inspection reports have limited validity (usually 3 years for gas equipment, 3–5 years for electrical). After the validity period expires, a new inspection must be carried out, otherwise the equipment is technically in a non-compliant state. You can learn more about what and how often to check in the article Maintenance and servicing of a condensing boiler – what and how often to check.

Subsidies and their documentation requirements

If you plan to use a state subsidy for a boiler replacement (e.g. the Green Households II program or other support schemes), the documentation requirements are even stricter. Usually required:

An expert assessment or energy audit confirming the condition before replacement
Photo documentation of the condition before and after the work
Invoice from an authorized contractor (a company registered in the relevant official list)
Report of the professional inspection and test with a date
Technical data sheet of the new boiler with the energy label
Declaration of Conformity

Without complete documentation you won't receive the subsidy, or it may have to be repaid. From experience, most customers lose the subsidy because they had the boiler installed before submitting the application, or used an unauthorized installer.

The most common project documentation mistakes from practice

Over the years I have repeatedly seen the following problems:

Undersized boiler output – the designer didn't calculate heat losses, just estimated based on area. Result: the boiler can't keep up during frosts, runs at full output without condensing, and uses more gas.
Unresolved return temperature – an old radiator system with high temperatures (75/60 °C) doesn't allow condensation. The designer should have proposed a mixing valve or radiator replacement, but only specified the boiler. Result: the boiler condenses minimally, efficiency is like that of a non-condensing unit.
Missing expansion vessel calculation – the installer used the same vessel as the old boiler. In a larger system this isn't enough, and the safety valve occasionally releases.
Incorrect flue gas route – too long a route, too many bends. The boiler struggles with flue gas discharge and shuts down on a fault.
Missing condensate drainage from the flue gas path – condensate from the flue leaks into the wall or floor. The project didn't address the siphon and sewage connection.
Project on paper, different in practice – the installer changed the piping routes, radiator type, or boiler location, but the project was never updated. At final approval or an insurance claim, the actual condition differs from the documentation.

Frequently Asked Questions (FAQ)

Do I need a project even when replacing an old boiler with a new condensing one, if nothing else changes?

If the boiler location, gas connection, heating pipes and flue gas path are not changing, a CH or GI project is generally not legally required. However, a report of the professional inspection and test from an authorized technician is mandatory before commissioning. Nevertheless, I recommend having at least a simple boiler setup protocol prepared – it will protect you in the event of warranty claims and any insurance claim. If the flue gas path is changing (e.g. a new liner, a new pipe), a construction notification is usually needed.

Who can tell me what project documentation is required in my case?

Primarily the local building authority (or municipal office) – they decide what requires a notification or permit. Also the gas company (distributor) for the gas part, and an authorized person for the inspection of gas appliances. A designer experienced in building services can assess the situation during a site visit and tell you exactly what you need.

What happens if I don't have project documentation and something breaks or damage occurs?

The insurance company may refuse or reduce payment for a claim (fire, explosion, flooding) if it proves that the installation was not carried out according to regulations and without the required documentation. Legal liability for damage passes to the property owner or installer. In the case of a fatal accident or serious damage, criminal prosecution may also follow. This is not scaremongering – such cases exist.

How long is the report of the professional inspection and test of the gas appliance valid?

According to the relevant decree and standard STN EN 1775, a regular professional inspection and test of gas equipment is mandatory once every 3 years for technical equipment (boiler rooms). For residential equipment (gas appliances in flats), the period is also 3 years, while an annual service inspection carried out by an authorized boiler service technician is recommended by manufacturers and is a condition for maintaining the warranty.

Can I change the boiler settings myself without having to report or document it?

Normal operating settings (desired temperature, time schedule, comfort DHW temperature) can be freely changed by the user according to the operating manual. Service settings (maximum output, heating curves, gas pressure setting) may only be changed by an authorized service technician, and changes should be recorded in the service log. Unauthorized interference with service parameters leads to loss of warranty and may compromise operational safety.

Where can I find technical data sheets and wiring diagrams for specific boilers?

Technical data sheets, installation manuals, wiring diagrams and other documents can be found in the Manuals, Documents, Price Lists section on atria.sk. We provide the available manufacturer documents for each boiler. If you're looking for documentation for a specific model and haven't found it, contact us – we can usually obtain documents that the manufacturer doesn't have directly online as well.

Conclusion – documentation is not bureaucracy, it's protection

Project documentation and supporting documents for installing a condensing boiler are not just pointless paperwork. They guarantee that the system will work correctly and safely, that you'll be protected in the event of an insurance claim, that no one can dispute your warranty, and that you'll have a clean slate if you ever sell the property. Properly documented installation also increases the value of the property.

If you're planning to install a condensing boiler, I recommend starting with the documentation – get advice from a building services designer in advance, find out what your local building authority requires, and only then choose the boiler and installer. The order matters: first the project, then the purchase, then the execution. Not the other way around. You can find more about choosing the right model in the article How to choose a condensing boiler – selection criteria for a family house and a flat, and about common operational issues in the article Common condensing boiler faults and how to fix them.

Do you have a question on this topic?

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