Installing a Low-Temperature Boiler with a Built-in Tank: Procedure and What You Need to Know

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Installation of a Low-Temperature Boiler with a Built-in Tank: Complete Procedure and Everything You Need to Know Before Starting Work

A low-temperature boiler with a built-in domestic hot water (DHW) tank is a popular solution for family homes where the owner wants to avoid more complex installations with an external tank, while still needing reliable and sufficiently capacious hot water supply. Unlike a combi boiler with instantaneous DHW preparation, here you have a physical tank built directly into the boiler body – and this brings specific requirements for installation, boiler room dimensions, piping, hydraulics, and the pressure group. This article walks through the entire process from receiving the goods to the first commissioning of the boiler, with practical notes from real-world jobs.

If you're still considering whether a boiler with a built-in tank is right for you, first read the article How to Choose a Low-Temperature Boiler with a Built-in Tank: Volume, Output, and Type of Operation and What DHW Tank Volume Do I Need for a Family House or Apartment in our Knowledge Center. If you're wondering whether a condensing boiler might be a better choice, check out Condensing vs. Low-Temperature Boiler with Tank: Which Pays Off More.

What Is a Low-Temperature Boiler with a Built-in Tank and Why Does Installation Have Its Specifics

A low-temperature boiler operates with heat transfer fluid in the heating system at a temperature typically of 55–75 °C (unlike a condensing boiler, which is optimized for 30–55 °C). The built-in tank is an integrated DHW tank with a volume usually of 80–120 liters, which is indirectly heated via an internal heat exchanger from the central heating circuit. This solution saves space in the boiler room, eliminates external piping between the boiler and the tank, and simplifies the hydraulics – but it also brings several important differences compared to installing a simple instantaneous boiler:

Weight and dimensions: A boiler with a built-in tank is considerably heavier and taller than an instantaneous boiler of the same output. For example, the Thermona THERM PRO 14 TKX has a height of around 1,500 mm and a weight when the tank is full exceeding 150 kg. This must be taken into account during transport, carrying, and anchoring.
DHW pressure group: Tank heating requires a safety fitting on the cold water side with a safety valve, an expansion vessel for DHW, and a shut-off valve.
Anode: The tank has a magnesium anode, whose condition must be checked during installation and later monitored regularly (more in the article Rust, Limescale, and Tank Anode – How to Extend the Life of Your Boiler).
Condensate drainage from the DHW safety valve: The tank's safety valve occasionally drips during expansion – a drain to the sewer system is needed.
Space requirements: See the article Low-Temperature Boiler with Tank – Boiler Room Requirements, Dimensions, and Space.

Before Installation: What to Prepare and Check

Project Documentation and Inspections

Installation of a gas boiler (and most low-temperature boilers with a built-in tank are gas-fired) in Slovakia is subject to several regulations. Before starting work, you must have:

A heating system project prepared by an authorized person (a designer with the appropriate license), if it involves replacing a boiler over 10 kW in an existing building or a new installation in any building.
Installation must be carried out by a company or person with a valid certificate for the installation of gas appliances (in accordance with Decree No. 508/2009 Coll. and STN EN 1020/1).
After installation, a professional inspection and test of the gas appliance is required, without which the boiler cannot be legally put into operation.
The manufacturer's warranty (Thermona guarantees an extended warranty if conditions are met) is conditional on installation by an authorized service technician and registration of the boiler. This must be arranged before booking the installation company.

In practice, I have come across cases where the homeowner installs the boiler themselves and calls a service technician only for the first commissioning. This is not always a problem in terms of functionality, but it can be tricky in terms of warranty and legal operation. I always recommend having the installation carried out by an authorized company, even if you are technically skilled.

Receiving the Goods and Checking the Packaging

A boiler with a built-in tank is a large, heavy piece. When receiving it, always check:

The outer packaging – visible damage, dents, tears.
The contents of the package according to the delivery note – typically the DHW pressure group (expansion vessel, safety valve, air vent), installation kit, manual, and warranty card are included.
The condition of the tank – after unpacking, visually check the anode and the tank neck.
All flanges, gaskets, and screws.

Report any transport damage immediately upon receipt, otherwise the complaint process becomes complicated. Photo documentation of the condition of the packaging and contents is always a good idea.

Space and Structural Requirements for the Boiler Room

A boiler with a built-in tank is considerably bulkier than an instantaneous boiler. For 14 kW versions (e.g. Thermona THERM PRO 14 KX), plan for a floor area of approximately 60 × 65 cm and a height of 150–160 cm. A service space of at least 600 mm in front and at least 200–300 mm on the sides must be maintained around the boiler. The floor must be level, solid, and able to bear the weight of the full appliance – an empty boiler can weigh 90–110 kg, and 170–200 kg when filled with water.

The boiler room must meet space volume requirements according to the boiler's input (STN EN 12828, STN 06 0310). For boilers up to 35 kW, a minimum volume of 7.5 m³ applies in most cases, but the specific conditions should always be verified with the designer. Permanent ventilation is also essential – supply of fresh air and extraction of air from the combustion chamber (for boilers with a sealed combustion chamber, this is handled by air supply to the burner via a plastic-coated pipe).

Step-by-Step Installation Procedure

Step 1: Positioning and Anchoring the Boiler

A boiler with a built-in tank is a floor-standing appliance – it is not mounted on brackets like most instantaneous boilers. Place it on a level, solid floor. Check the vertical position with a spirit level (the boiler must stand perpendicular, not tilted). Most manufacturers, including Thermona, supply leveling feet that can compensate for floor unevenness within a range of ±15 mm. It is not anchored to the wall like a wall-hung boiler, but if the floor is slippery or there is a risk of accidental shifting, some installers secure the boiler with a clamp or additional fixing.

Step 2: Connecting the Heating System

The boiler has connections on the rear or side wall for the supply and return pipes of the central heating (CH) system. The dimensioning and type of connection depends on the specific model – always follow the installation manual. These are usually threaded or flanged G 1" or G 1¼" connections. When connecting, observe the correct direction of the supply and return lines (marked in the manual and on the boiler body). It is advisable to fit ball valves in each branch so that the boiler can be disconnected without draining the entire system.

For low-temperature boilers that are not condensing, it is important to maintain a minimum return pipe temperature of approximately 45–55 °C (depending on the model) to prevent low-temperature corrosion of the heat exchanger. Some boilers have a built-in three-way valve (thermostatic mixer) to protect against a cold return line – check the documentation for the specific model, e.g. Thermona THERM 28 LXZ.A 5.

Step 3: Gas Connection

The gas supply must match the dimensions specified in the project and must not be undersized – if the pipe is too thin or the pressure drop too high, the burner will not reach its rated output. A gas shut-off valve (ball valve) with a quick-closing feature, purge, and test valve must be installed upstream of the boiler. Flexible hoses made of soft PVC must not be used on the pipeline – only steel or stainless steel pipes, or a certified flexible stainless steel hose (DN 20 or 25 depending on output).

After connecting the gas, perform a leak test on the entire gas piping (including new connections on the boiler) using a gas detector and soapy water. The natural gas pressure upstream of the boiler must be 1.8–2.5 kPa (depending on the manufacturer, see the installation manual). If the pressure is lower, contact the gas distributor.

Step 4: Connecting Hot and Cold Water (DHW Pressure Group)

The DHW tank is connected to the water circuit via a pressure group. A typical pressure group assembly (if not included with the boiler, it must be purchased separately):

Shut-off valve on the cold water (CW) line upstream of the tank
Check valve (prevents backflow during heating)
Expansion vessel for DHW (volume according to the tank, typically 8–12 l for an 80–120 l tank)
DHW safety valve (opening pressure 6 bar for most residential tanks)
Safety valve drain to the sewer system (heat-resistant plastic or copper pipe, the safety outlet must not be closed off)

The expansion vessel for DHW is different from the expansion vessel of the heating system – supply pressures and filling pressures differ. Do not confuse them, and always fill the DHW expansion vessel to the correct pre-charge pressure (usually 0.3–0.5 bar below the water supply pressure, typically 3–4 bar for a typical household).

Step 5: Connecting Flue Gas Discharge and Combustion Air Supply

Low-temperature boilers with a sealed combustion chamber (turbo) are connected via coaxial or twin-pipe plastic-coated (plastic or stainless steel) piping. The diameter and maximum length of the piping system are strictly specified by the manufacturer – for example, for DN 60/100 mm, a typical maximum equivalent length is 4–6 m horizontally plus 2–3 m for elbows (each 90° elbow = approx. 1 m of equivalent length). These values vary depending on the boiler model.

For pressurized boilers (sealed chamber), it is possible to route the flue gases through the perimeter wall to the outside (horizontal outlet), into a shaft, or through the roof. If routing the flue gases horizontally, the outlet must be at least 1.5 m from a window, door, or ventilation opening and must not be directed towards a public road at a height below 2 m. These requirements are set by STN EN 13384 and local regulations. Always check with a chimney sweep company or installer.

For boilers with an open combustion chamber (atmospheric boilers), a chimney with sufficient draught and sufficient air supply to the boiler room is required.

Step 6: Electrical Connection and Controls

The boiler requires 230 V / 50 Hz, PE grounding (the boiler must be grounded), and 10–16 A protection (depending on the model). The socket or fixed outlet must be located within reach of the service space. The following are typically connected to the boiler's terminal block:

Thermostat or room controller (OT/eBus or potential-free contact)
Outdoor temperature sensor (if the appliance is equipped with weather-compensated control)
Possibly a solar controller (for boilers with solar preparation – see the article How to Connect Solar Collectors to Thermona Low-Temperature Boilers with Tank)

The Thermona THERM PRO 14 TKX boiler has integrated weather-compensated control with an outdoor sensor – in this case, connecting the outdoor sensor is essential for correct operation.

Step 7: Filling the System, Venting, and Pressure Check

Before the first commissioning, the heating (CH) system must be filled with water. The filling water should have a hardness of up to 10 °dH (up to 178 mg/l CaCO₃) – with harder water, faster limescale build-up in the heat exchanger is a risk. If the local water is hard (above 15 °dH), it is advisable to use water treatment (softener, magnetic filter, or inhibitor dosing). The working pressure of the cold system should be 1.2–1.5 bar (for a two-story family house); after heating to 70 °C, the pressure must not exceed 3 bar.

Venting takes place via the automatic air vent (AAV) on the boiler as well as manually via the venting valves on radiators and underfloor heating manifolds. After the initial filling, it is common to need to vent the system repeatedly even after several days of operation – air is gradually released from the pipes and from the heated water.

Fill the DHW tank by opening the cold water supply and opening a hot water outlet at one of the taps until a steady stream of water without air bubbles flows.

Step 8: First Commissioning (Putting into Operation)

The first commissioning must be carried out by an authorized service technician (manufacturer's authorized service). For Thermona, this is a condition for the extended warranty to remain valid. During the first commissioning, the service technician:

Checks all connections and the tightness of the gas piping.
Sets the burner output to the nominal value (measuring CO₂ in the flue gases, checking flue gas temperature).
Sets the DHW tank temperature (usually 55–65 °C – higher to eliminate legionella, lower to save energy).
Sets the control parameters: heating curve (weather-compensated control), frost protection, tank heating times.
Performs a test of the safety features (function of the safety valve, high-limit thermostat).
Issues a report on the professional inspection and test.

Typical Installation Mistakes and How to Avoid Them

From experience, I know that a large proportion of the faults customers report shortly after installation are not caused by a fault in the boiler itself, but by an installation error. Here are the most common ones:

Missing or undersized DHW expansion vessel: Manifests as regular dripping from the DHW safety valve. The tank's safety valve dripping is not a fault – it is a safety feature that reacts to insufficient space for thermal expansion. Solution: correctly size and fill the DHW expansion vessel.
Air in the system: The boiler is noisy, radiators are cold at the top. Solution: thorough venting, or checking the automatic air vent.
Incorrect direction of the CH supply and return connection: The boiler does not heat or has low efficiency. Always check the flow direction according to the diagram in the manual.
Flue gas discharge too short or too long: The boiler enters a fault state (ignition fault, high flue gas temperature). Follow the maximum equivalent length according to the technical documentation.
Missing drain for the DHW safety valve to the sewer: During thermal expansion, water drips onto the boiler room floor. Always connect the drain.
Incorrectly set heating curve: The house is overheated or underheated. With weather-compensated control, ongoing fine-tuning is needed according to the actual conditions of the house (insulation, orientation, window area).
Water too hard without treatment: Limescale in the heat exchanger after 2–3 seasons reduces output and shortens lifespan. Solution: softener or inhibitor. More in the article Rust, Limescale, and Tank Anode – How to Extend the Life of Your Boiler.

Specifics of Installing the Thermona PRO and LXZ Models

The manufacturer Thermona is one of the established European heating technology manufacturers with a long tradition and an extensive service network in Slovakia. For the category of low-temperature boilers with a built-in tank, it offers several model lines that differ in output, tank volume, and functional equipment.

The Thermona THERM PRO 14 KX is a compact floor-standing boiler with a built-in DHW tank, intended for smaller family houses or larger apartments. A maximum heat output of 14 kW will cover a house with a heat loss of up to approximately 11–12 kW (a well-insulated family house of 120–150 m²). The tank has a volume of 80 liters, which, for a standard family of 3–4 people, covers the morning demand when the preparation time is set correctly.

The Thermona THERM PRO 14 TKX is a version with integrated weather-compensated control and an outdoor sensor. Weather-compensated control adjusts the heating water temperature to the outdoor temperature and ensures more comfortable and energy-efficient operation. When installing this version, the outdoor temperature sensor must be mounted on the north or northwest wall of the house (not in direct sunlight, not above the flue gas outlet). The sensor is connected via a 2-wire cable to the boiler's terminal block.

The Thermona THERM 28 LXZ.A 5 is a more powerful model with 28 kW and a larger built-in tank. This boiler is suitable for larger family houses or houses with higher DHW consumption (5 or more people, a whirlpool bath, etc.). The connection and installation are essentially the same as for the 14 kW versions, but a larger gas supply diameter (at least DN 20), a larger CH and DHW expansion vessel, and a more robust pressure group must be planned for.

After Installation: What to Check in the First Weeks of Operation

In the first 2–4 weeks after installation, it is common for the system to "settle in" and for some minor phenomena to appear that should be monitored:

Drop in system pressure: A normal occurrence during venting – air is released and the system "loses" a bit of pressure. If the pressure drops below 0.8 bar, top up water via the filling valve. If the pressure drops quickly (more than 0.3 bar per day) even after venting, look for a leak.
Noises in pipes and radiators: Gurgling, clinking – usually air. Vent the radiators manually.
Dripping from the DHW safety valve: If it drips after every tank heating cycle, the DHW expansion vessel has low pre-charge pressure or is too small. Check the pre-charge pressure (with a bicycle pump via the Schrader valve) and compare it with the supply pressure value.
DHW temperature: Check the actual temperature at the outlet (thermometer at the tap). With the boiler set to 60 °C, you should get at least 50–55 °C at the outlet (losses through the piping). If the difference is greater, check the length of the circulation circuit or the insulation of the DHW piping.

Regular maintenance (annual servicing, anode inspection, filter cleaning) is key to the long service life of the entire appliance. More in the article Maintenance and Servicing of a Low-Temperature Boiler with Tank: What and How Often to Check.

Frequently Asked Questions (FAQ)

Can I install a boiler with a built-in tank myself?

Technically, this depends on your skills and the scope of the work. The mechanical part – installing water and CH piping, securing the unit, filling the system – can be done by a technically skilled homeowner. However, the gas connection and first commissioning must be carried out by a person with a valid authorization to install and operate gas appliances (according to Decree No. 508/2009 Coll.). Without a professional inspection and test, the appliance is not legally operated and the manufacturer's warranty is not valid. In addition, insurance companies may refuse to pay out in the event of damage caused by an improperly installed appliance.

What size DHW expansion vessel do I need for an 80-liter tank?

For an 80-liter tank and a standard supply pressure of 3–4 bar, we recommend a DHW expansion vessel with a volume of 8–10 liters. The exact calculation depends on the water supply pressure, the maximum tank temperature, and the vessel's pre-charge pressure. General rule: DHW expansion vessel = approx. 10–12% of the tank volume. If the supply pressure is higher (above 4 bar), it is advisable to install a pressure reducing valve upstream of the pressure group.

What temperature should I set the DHW tank to?

The recommended tank temperature is 55–65 °C. At 55 °C, limescale formation and gas consumption for DHW heating are significantly reduced, but regular hygienic heating cycles to 70 °C (the so-called anti-legionella cycle) must be ensured at least once a week for 30–60 minutes. Today's boilers, including Thermona models, have an anti-legionella cycle adjustable in the controller menu. Never set the tank below 50 °C as a permanent operating temperature – legionella multiplies rapidly in the 25–50 °C range.

The boiler goes into fault mode after switching on – what is the most common cause with a new installation?

With a new installation, the most common causes of faults are: (1) low system pressure (below 0.8 bar) – top up the water; (2) air in the system – vent it; (3) incorrect connection of the flue gas piping – check the orientation and maximum length; (4) low gas pressure – check that the gas valve is open and the pressure upstream of the boiler; (5) incorrect control configuration – check the settings according to the manual. Most fault codes are explained in the operating manual. If you are unsure, call an authorized service center.

Do I need to replace the chimney/flue pipe when replacing an old boiler?

Not necessarily, but very likely yes, if you are switching from a boiler with an atmospheric burner and traditional chimney outlet to a turbo burner with a sealed combustion chamber and plastic-coated piping. New low-temperature boilers with a sealed chamber require DN 60/100 or 80/125 plastic-coated piping (coaxial), or a twin-pipe system. They cannot be connected to a traditional brick or ceramic chimney without professional renovation and lining. This must be addressed before ordering the boiler – have the chimney certified by a chimney sweep company.

Where should the outdoor temperature sensor be mounted for weather-compensated control?

Mount the sensor on the north or northwest facade of the house at a height of approximately 2–2.5 m above ground level. It must not be in direct sunlight (which would distort the measurement), must not be above the flue gas outlet, nor under a roof that shields it from the actual outdoor temperature. The ideal location is where the sensor measures the "real" outdoor temperature without being affected by the building's thermal bridges or direct sunlight. Route the sensor cable (2 × 0.5 mm² CYKY or compatible) away from power cables, or in a separate conduit.

Conclusion: It Pays to Get the Installation Right the First Time

Installing a low-temperature boiler with a built-in tank is no more complicated than other types of gas boilers, but it has its specific requirements – especially regarding the DHW pressure group, the boiler room's spatial layout, correct expansion vessel setup, and first commissioning by an authorized person. Investing in quality installation and correct initial setup pays off many times over in lower operating costs, a long service life of the appliance, and the absence of unnecessary faults.

If you're not sure which model to choose, take a look at the products in the category of boilers with built-in tank at atria.sk, or read more topics in our Knowledge Center – Common Faults of Low-Temperature Boilers with Tank and How to Recognize Them or Frequently Asked Questions About Low-Temperature Boilers with Built-in Tank – where you'll find answers to common operational questions after commissioning your boiler.

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