Installation of a Combined Low-Temperature Boiler with Instantaneous DHW Preparation

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Installing a combined low-temperature boiler with instantaneous DHW preparation: a complete expert guide

A combined low-temperature boiler with instantaneous domestic hot water (DHW) preparation is one of the most widespread central heating solutions in family houses and apartments in Slovakia. The entire system in one appliance – heating and hot water preparation – offers compact placement, a relatively low purchase price, and reliable operation when properly installed. However, that installation is exactly where most mistakes happen in practice. We have seen dozens of jobs where the boiler worked, but poorly or only temporarily – because some seemingly "unimportant" step was skipped during installation.

This article is intended for installation technicians, experienced DIYers with basic knowledge of heating technology, and ordinary customers who want to understand what actually happens during the installation of their boiler, what to check, and what to watch out for when taking over the work from a tradesman. We will cover preparation, the actual installation, setup, tightness testing, commissioning, and what is most often underestimated during reconstructions of old installations.

What is a combined low-temperature boiler with instantaneous DHW heating, and why does the type of installation matter

A low-temperature boiler operates with a boiler water temperature of up to 55–60 °C (unlike classic high-temperature boilers with 75–90 °C). This has consequences for the entire system – radiators must be sized for a lower temperature gradient, pipes must have the correct cross-section and be hydraulically balanced. Instantaneous DHW heating works by cold water passing through a heat exchanger directly in the boiler and being heated immediately upon draw-off – no storage tank, no waiting for the tank to heat up. This is convenient, but it places demands on the boiler's output as well as on the pressure and flow rate of cold water in the household network.

The key difference compared to a boiler without DHW preparation: when hot water is drawn, the boiler switches priority to DHW heating and heating is temporarily "paused". This mode has its own hydraulic requirements and is the reason why the entire installation must be designed and carried out differently than for a boiler intended solely for heating. If you connected a combined boiler the same way as a single-circuit boiler, the hot water would be cold or lukewarm, or the boiler would keep "cycling" (short start-up, shutdown, start-up again) – which shortens its lifespan.

Pre-installation preparation: what must be ready before the boiler arrives on site

The biggest savings in time and nerves during boiler installation come from thorough pre-installation preparation. From experience we know that missing wall sleeves, an incorrectly positioned wall bracket, or insufficient gas pressure are the reasons why installation takes twice as long as it should.

Structural readiness of the room

The room in which the boiler will be mounted must meet several conditions. For an atmospheric boiler (for example Vaillant atmoTEC pro VUW SK 200/3-3), a combustion air supply directly from the room and a functioning chimney for flue gas discharge are essential. The room volume must be at least 8 m³, preferably 10–12 m³. Coaxial turbo boilers (for example Vaillant turboTEC pro VUW SK 202/3-3) can also be installed in an enclosed room, since they draw in air and discharge flue gas through their own coaxial ducting through the outer wall.

The load-bearing wall must be able to support the weight of the boiler (typically 30–45 kg including the water in the system).
For a turbo boiler, prepare a sleeve through the perimeter wall (diameter according to system type – most often 100 mm for coaxial ducting or 2× 80 mm for separate ducting).
For an atmospheric boiler, verify the chimney's clearance and draught – a pressure of at least 3–5 Pa with the damper door closed, preferably 8–12 Pa.
A 230 V / 16 A grounded electrical outlet within reach of the boiler (not an extension cord).
A gas supply with a shut-off valve before the boiler – the gas shut-off valve must be accessible without tools.

Checking gas pressure and supply pipe cross-section

This is a step that many installers skip and then wonder why the boiler "drops" at full output or throws an error code. The static pressure of natural gas before the boiler should be 20 mbar (±2 mbar). The working pressure at full output should not fall below 17.5 mbar. Measurement is done with a manometer connected to the test nipple in front of the boiler's gas valve. If the pressure is low, the problem needs to be addressed with the gas distributor or by checking the supply pipe cross-section – for a larger boiler (20–24 kW) with a longer gas run, a DN 25 pipe instead of DN 20 may be required.

Preparing the wall bracket and connection spacing

Every boiler has a fixed spacing of connections (gas, heating, cold water, and DHW). For example, for the Vaillant atmoTEC/turboTEC pro and plus range, the spacing is standardized and the manufacturer supplies an installation template. Anchor the bracket into the load-bearing wall with wall anchors – at least two anchoring points, for heavier boilers (over 35 kg) four points. The boiler must hang perfectly vertical (spirit level) – a tilted installation causes problems with venting and with the boiler's automatic air vent.

Actual boiler installation: step-by-step procedure

The installation itself follows a precisely defined order. Changing the order of seemingly minor steps can lead to problems that only appear after start-up – or after several months of operation.

1. Hanging the boiler and connecting the flue system

Hang the boiler on the bracket according to the installation template. Check the vertical position with a spirit level in two directions. Then connect the flue duct before fitting the water and gas connections – the reason is simple: with coaxial ducting you need access to the rear or top of the boiler, and later handling of a heavy boiler on the wall is uncomfortable and risky.

For turbo boilers with a coaxial outlet (80/125 mm or 60/100 mm), observe the maximum permitted duct lengths. For example, Vaillant turboTEC pro VUW SK 242/3-3 has a maximum equivalent length of about 9–10 m with 80/125 mm coaxial ducting (each 90° bend = 1.5 m equivalent). A longer duct increases resistance to air intake and flue gas discharge, which can lead to increased CO in the flue gas and fault-related boiler shutdown.

The slope of the flue pipe must be at least 3° (ideally 5°) sloping away from the boiler toward the outdoor terminal – to allow condensate drainage. Horizontal or tilted piping retains condensate, causing corrosion and noisy operation.

2. Connecting the gas pipe

Connect the gas pipe to the boiler via a ball valve (not a needle valve – it does not provide sufficient flow). The thread on the boiler is usually G 3/4" male. Use sealant suitable for gas media (Teflon tape for gas, or hemp fiber with Fermit paste, or a certified gas sealant). Rubber seals from plumbing technology are NOT suitable for gas.

After connection, a pressure test of the gas piping must be carried out according to STN EN 1775 – typically a test at operating pressure for 10 minutes, checked with a manometer and a foaming agent (gas detector or certified spray detector). The pressure test is a legal obligation and a condition of the boiler's warranty.

3. Connecting the heating circuit

The heating circuit is connected to the corresponding fittings (larger diameter than water). Before the boiler, install:

On the supply: a ball valve (the boiler must be disconnectable without draining the entire system).
On the return pipe: a ball valve and, before it, a filling/draining valve.
Safety valve (if not integrated in the boiler – in most modern boilers it is) must be set to a maximum working pressure of 3 bar; its discharge pipe must lead to a drain or a collection container, not onto the floor or into open space.
Expansion vessel – in modern boilers a small expansion vessel (8–10 liters) is integrated; for larger systems with a large water volume, an external expansion vessel is required.

The system volume and expansion vessel size are critical parameters. If the system is large (for example an old house with old cast-iron radiators), the integrated vessel is not sufficient and the pressure rises above 3 bar when the system heats up – the safety valve opens and water leaks out. Solution: install an additional external expansion vessel on the return pipe as close as possible to the boiler inlet. Volume calculation: system water volume × 0.08 (coefficient for the 10–80 °C temperature range) / (1 – expansion vessel pre-charge pressure / maximum working pressure). In practice, for an average 150 m² family house with gas radiators, this comes out to an additional 18–25 liters of external vessel.

4. Connecting the cold water and DHW piping

The cold water supply to the instantaneous DHW heat exchanger must be fitted with:

A ball valve immediately before the boiler.
A strainer filter (100–150 μm) – impurities in the water clog the DHW heat exchanger and lead to reduced output and faults. This is one of the most common reasons for service calls on older installations.
A check valve – prevents back-contamination of the water supply with hot water from the boiler.
A water pressure reducing valve, if the network pressure is above 3.5 bar (common in new buildings or when supplied from a water tower) – the instantaneous heat exchanger is not a pressurized storage tank, and pressures exceeding 6 bar can damage the exchanger.

The DHW outlet (hot water) leads directly to the hot water distributors throughout the house. Remember that for instantaneous boilers without a storage tank, hot water circulation is neither possible nor desirable – unlike storage tank systems. If the customer insists on circulation (long pipe runs), the solution is a small 30–50 liter storage tank after the boiler, but this changes the nature of the system. We discuss this choice in more detail in the article Instantaneous DHW heating vs. storage tank: advantages and disadvantages with a low-temperature boiler.

5. Electrical connection and thermostat

Connect the boiler to the electrical network with a 3× 1.5 mm² cable (phase, neutral, protective conductor). Circuit breakers: 10 A (usually sufficient for most boilers up to 28 kW; the boiler's electrical input is typically 150–200 W). The boiler must NEVER be connected via a socket adapter without grounding – grounding is a safety requirement and a condition of the warranty.

A room thermostat or weather-compensated controller is connected to the boiler's terminal block (terminals TA or similar, according to the diagram in the manual). Most modern boilers accept a potential-free contact (a standard thermostat) or the OpenTherm protocol (for smart thermostats with output modulation). OpenTherm control is significantly more economical – the boiler modulates its output continuously according to the thermostat's demand, instead of the classic on/off cycle. Investment in an OpenTherm thermostat usually pays back within 1–2 heating seasons.

Hydraulic balancing and setting the heating system pressure

After physically connecting all pipes and fittings comes a step that laypeople, and sometimes even cheap installers, skip: hydraulic balancing. Incorrect balancing causes some radiators to remain cold while others overheat, and the pump becomes overloaded or, conversely, fails to deliver sufficient flow. For low-temperature boilers this is especially important, because they operate with a smaller temperature gradient and a small flow error has a greater impact on heat output.

Basic procedure:

Fill the system with water and vent it – start from the lowest point, working upward. Filling pressure: 1.0–1.5 bar when cold (the exact value depends on the building's height: every 10 m of height = +1 bar of required pre-charge).
Set the flow on each radiator's thermostatic valve preliminarily according to the design calculation (hydraulic calculation). If none is available, use the proportional method based on radiator output.
Start the pump and boiler, set the boiler water outlet temperature to 60 °C (or as per the design).
Measure the temperature at the inlet and outlet of each radiator – the temperature drop should be the same across all of them (typically 10–15 K for low-temperature systems). If the drop is larger, the radiator is getting too little water – open the valve. If the drop is smaller, it is getting too much – throttle the valve.

In practice, hydraulic balancing is done in two rounds: rough setting while hot and fine tuning after the system stabilizes (usually 30–60 minutes of operation).

Boiler settings: output, temperatures, gas pressure

Correct boiler setup is just as important as the physical installation. The manufacturer supplies the boiler set to maximum output and for natural gas (G20). If your locality uses propane-butane or a mixed gas, conversion is necessary – replacing the nozzles and reprogramming the control unit. This conversion must only be carried out by a certified technician.

Setting maximum heating output

Modern condensing or low-temperature boilers should not operate continuously at maximum output – that would mean they are undersized. A correctly sized boiler should operate at 80–90% output at an outdoor temperature of -12 °C (the design temperature for most of Slovakia). Boilers such as the Vaillant atmoTEC plus VUW CZ/SK 240/3-5 have adjustable output, allowing adaptation to a specific system.

If the boiler is oversized (common during reconstructions, when an old large boiler is replaced with a new modern one of the same rated output), set the maximum heating output lower – the boiler will run longer and more economically, instead of short cycles at high output.

Setting the weather compensation curve

The weather compensation curve defines what the boiler water temperature should be at a given outdoor temperature. A steeper curve = higher temperature at the same outdoor temperature. For underfloor heating, typically a flat curve (slope 0.5–0.8); for low-temperature radiators, medium (0.8–1.2); for old cast-iron radiators, steeper (1.2–1.8). Correct curve setting reduces gas consumption by up to 10–15% compared to an incorrectly set curve.

Checking combustion and flue gas analysis

This is the step that distinguishes a professional from a "fixer". Flue gas analysis using an analyzer (Testo, Wöhler, and similar) verifies whether the boiler is combusting correctly. Monitored parameters:

CO₂ content in flue gas: for natural gas, 8.0–9.5% (optimum).
CO content in flue gas: below 100 ppm (regulatory limit), in practice it should be below 30 ppm.
Flue gas temperature: for a low-temperature boiler, 120–160 °C (depends on output and boiler water temperature).
Combustion efficiency: at least 90%, a quality boiler with correct settings achieves 92–94%.

If CO₂ is low (below 8%), the mixture is lean – too much air, higher consumption. If CO₂ is high (above 10%) and CO is rising, the mixture is rich – incomplete combustion, risk of CO poisoning. Adjustment is done by regulating the air supply (damper) and gas pressure at the boiler's gas valve.

Pressure test and installation protocol

Before commissioning the boiler, a pressure test of the hydraulic part is mandatory. Procedure: fill the system with water, vent it, and raise the pressure to 1.5 times the operating pressure (typically 4–4.5 bar) using a filling pump. Monitor the pressure for at least 30 minutes (most often 60 minutes without a drop is required). Visually check all joints – threads, flange connections, fittings. Any leak must be fixed before the boiler is put into operation.

The gas piping pressure test is carried out according to STN EN 1775, usually with air at 1.1 times the operating pressure (22–25 mbar for natural gas at medium pressure) for 10 minutes, or according to the distributor's regulations. The result is recorded in a protocol.

The installation protocol (installation and first start-up report) includes: date, address, boiler type and serial number, measurements of gas and water pressures, flue gas analysis results, boiler parameter settings, and the signature of the responsible installer. Without this protocol, no warranty claim arises with the manufacturer.

Specifics of reconstruction jobs: old house, new boiler

Reconstruction jobs are technically the most demanding. Typical scenario: the customer has a 30-year-old steel natural gas boiler in the house, cast-iron radiators, galvanized steel pipes, and an old plumbing installation. They want to replace the boiler with a modern combined low-temperature one with instantaneous DHW heating.

Here are the key things to verify before ordering the boiler:

Condition and output of existing radiators: cast-iron radiators have a larger volume and longer thermal inertia – that is fine for a low-temperature boiler. But if their output parameters were tested at 90/70 °C, their output at a low-temperature 65/50 °C may be 30–40% lower. You must recalculate whether they are sufficient to cover the heat loss.
Condition of the pipes: galvanized steel after 20–30 years has a narrowed cross-section (deposits). Measure the flow and hydraulic resistance before deciding whether to keep the piping. A new combined boiler with an instantaneous DHW exchanger is sensitive to dirt – without thorough flushing of the pipework, rust and scale will enter the exchanger.
Flushing the system: before connecting the new boiler, perform a thorough chemical flush of the system (citric acid solution or a special cleaning agent) and neutralize afterward. The system must be clean.
Corrosion inhibitor: after flushing and filling the system, add a corrosion inhibitor (e.g. Fernox F1, Sentinel X100, or equivalent). This investment of a few dozen euros will extend the life of the pump and heat exchanger by years.
Water hardness: in areas with hard water (above 300 mg/l CaCO₃, for example around Trnava, Nitra, parts of Záhorie), a water softener is required when filling the system. Hard water forms limescale in the DHW and heating exchangers – after 2–3 years of operation this significantly reduces output and increases consumption. This is particularly critical for instantaneous boilers, where the exchanger has a small surface area and rapid water flow accelerates scaling.

Another common scenario: the customer is switching from electric heating or solid fuel to a combined gas boiler. In this case there is no existing heating system (if any at all), so the designer plans the entire piping from scratch. This is an opportunity to size everything correctly from the ground up and choose a suitable boiler based on the building's actual heat loss. We discuss output selection in the article What output boiler with DHW heating do I need for my house, where you will also find concrete calculation methods.

Most common mistakes when installing a combined boiler with instantaneous DHW heating

From dozens of jobs and service calls, we have compiled the mistakes that occur most often:

Forgotten filter on the cold water supply to the DHW system: within a month, the exchanger clogs, hot water becomes cold on draw-off, a service call and acid cleaning of the exchanger follow – unnecessary costs.
Incorrect expansion vessel or missing external one: the safety valve keeps opening, water leaks, and the system needs constant refilling – the customer thinks the boiler is "leaking".
Missing check valve on the cold water supply: contamination risk; in some areas required by the water utility.
Forgotten hydraulic balancing: the customer complains that some rooms are warm and others cold. The boiler is working correctly; the problem is an uneven flow distribution.
Excessively long flue duct on a turbo boiler: higher temperature in the combustion chamber, longer start-up time, lower output during DHW draw-off, increased CO.
Incorrect thermostat placement: a thermostat placed on a wall near a heat source (radiator, kitchen stove) – the boiler switches off prematurely, the house is cold. The thermostat belongs on a north-facing or neutral wall at a height of 1.5 m, in a "representative" room (living room or hallway).
Inadequate frost protection: a boiler in an unheated boiler room or utility room – during frosts below -5 °C, the cold water supply in front of the boiler can freeze (if uninsulated). Solution: thermal insulation of the supply pipe and a functional anti-freeze protection in the boiler (automatic activation at low temperature).

Handing over the boiler to the customer: what they need to know

A technically flawless installation becomes problematic if the customer does not know how to use the boiler correctly. Handover should include:

Explaining how to operate the controller/thermostat (program setting, temperature, holiday mode).
Showing where the water pressure gauge is and how to top up water (filling valve) – the cold pressure should be 1.0–1.5 bar. If it drops below 0.8 bar, the boiler shuts down with a fault code.
Location of the main gas shut-off valve and the boiler's main electrical protection.
Information about annual maintenance (cleaning the exchanger, checking ignition electrodes, flushing the water strainer, checking pressure and inhibitor condition). More on this in the article Maintenance and servicing of combined low-temperature boilers: what and how often.
Explaining what to do in case of a fault (the most common error codes, reset, when to call service). A list of the most frequent faults can be found in the article Common faults of Vaillant atmoTEC and turboTEC boilers and their solutions.

The customer should physically receive: an operating manual in Slovak, a warranty card with the installation date and installer's signature filled in, the first start-up protocol, and the contact details of a service technician.

Special situations: installation in an apartment versus a family house

Installation in an apartment building has additional specifics. First, written consent from the building administrator is required, usually along with an inspection report from an authorized person. Second, discharge of a turbo boiler's flue gas through the façade of an apartment building is subject to the building's rules and must not disturb neighbors – the terminal must not be below the windows or balcony of a neighboring apartment. Third, boiler noise level is a more important parameter – the boiler in an apartment must be placed with consideration for neighbors (nighttime operation, thin partition walls).

Vaillant atmoTEC and turboTEC are well rated in terms of noise level (30–38 dB(A)), but we still recommend anti-vibration pads on the bracket and flexible hose connections instead of rigid pipe joints directly on the boiler – this transmits fewer vibrations into the building structure.

In a family house the situation is more flexible, but a new requirement arises: long water piping runs. With instantaneous DHW heating and long piping between the boiler and the draw-off point (a bathroom upstairs, far from the boiler room), the customer waits 30–60 seconds for hot water while cold water runs out. This is not a boiler fault – it is a characteristic of instantaneous systems. The solution is either a circulation pump with a storage tank (which changes the nature of the system) or accepting this fact. More on this in the article What hot water flow rate can a boiler with instantaneous DHW heating provide.

Choosing between an atmospheric and a turbo boiler for reconstruction or new construction

This decision significantly affects the installation method and costs. An atmospheric boiler (Vaillant atmoTEC pro VUW SK 240/3-3) requires a functioning chimney – this is a disadvantage in new buildings without a chimney and an advantage in old houses where a chimney exists and is in good condition. A turbo boiler (

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