Installing a Low-Temperature Boiler with a Cylinder Connection Step by Step

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Installation of a Low-Temperature Boiler with Cylinder Connection – Complete Step-by-Step Technical Guide

Installing a low-temperature boiler in combination with a domestic hot water cylinder is one of the most common solutions in Slovak households. Although this is a relatively standard job, in practice we still see the same recurring mistakes – incorrectly sized piping, a reverse-connected three-way valve, missing air venting, or insufficient pressure in the distribution system. The result? The boiler runs, but the cylinder doesn't heat up, or it heats slowly and inefficiently. This article will guide you through the entire installation from planning to commissioning, including specific dimensions, values, and technical details that no ordinary manual will tell you.

The whole topic of choosing a suitable boiler and cylinder is covered in the article How to Choose a Low-Temperature Boiler with Cylinder Connection Option, where you'll also find a comparison of parameters. Here we will focus exclusively on the installation procedure itself and the technical aspects directly related to it.

1. What You Need Before Starting the Installation – Plan and Materials

Every successful installation begins before the actual work, on paper (or in a CAD program). Before bringing the boiler to the site, you need clarity on several points:

Boiler output versus cylinder volume: For a 20 kW boiler (for example the Vaillant atmoTEC plus VU 200/3-5) we recommend a cylinder of at least 120–150 litres. For 24 kW (model Vaillant atmoTEC plus VU 240/3-5) we typically work with a 150–200 l cylinder. More on sizing can be found in the article What Cylinder Volume Do I Need for a Low-Temperature Boiler.
Placement of the boiler and cylinder: The cylinder should be as close to the boiler as possible – ideally within 1–1.5 metres. Every metre of piping between the boiler and the cylinder means heat losses and a longer heating time.
Boiler type – atmospheric or turbo: This affects the solution for flue gas discharge, combustion air intake, and the installation location. A detailed comparison is in the article Atmospheric vs. Turbo Low-Temperature Boiler – Which to Choose with a Cylinder.
Hydraulic diagram: Before purchasing materials, draw up a hydraulic diagram – where the three-way valve will be, where the pumps, where the expansion vessel, and where the air vents will go.

Basic list of materials and tools for a typical job

Low-temperature boiler with a cylinder outlet (e.g. Vaillant turboTEC plus VU 202/3-5)
DHW cylinder with heat exchanger – volume according to household needs
Three-way diverter valve (servo or motorised) 1" or 6/4"
Cylinder circuit circulation pump (if not integrated in the boiler)
Expansion vessel for the heating circuit (min. 8–12 l for a typical house)
3 bar safety valve for the heating circuit, 6 bar for the cylinder
Check valves on each circuit
Ball valves on each connection
Copper or multilayer piping DN 22 (3/4") for the main distribution pipe, DN 15 for secondary branches
Pipe thermal insulation – at least 13 mm for internal piping
Thermometer, pressure gauge, air vent
Wall sleeve for the flue pipe (for atmospheric boilers)
Electronic thermostat or controller with an input for the cylinder sensor

2. Preparing the Installation Site

The boiler is mounted on a load-bearing wall capable of carrying its weight. The Vaillant atmoTEC plus VU 280/3-5 (see the product here) weighs approximately 38 kg, while a cylinder filled with water in the 200 l version can exceed 240 kg – the cylinder must therefore always stand on the floor, never be wall-mounted. The floor under the cylinder must be level and able to bear this load.

For an atmospheric boiler (atmoTEC), you must ensure an air supply into the room of at least 2× 150 cm² of ventilation opening – one near the floor, one near the ceiling. Without a sufficient supply of combustion air, the boiler may show a fault or incomplete combustion. For a turbo boiler (turboTEC), you only need to deal with a 60/100 mm or 80/125 mm (coaxial) flue pipe, as the air supply is drawn from outside. More in the article Atmospheric vs. Turbo Low-Temperature Boiler – Which to Choose with a Cylinder.

Minimum space requirements

Side clearance of the boiler from the wall: min. 150 mm (service access)
Bottom clearance of the boiler from the floor: min. 200 mm (for atmospheric type)
Space in front of the boiler for servicing: min. 700 mm of clear area
Distance of the cylinder from the boiler: ideally up to 1,500 mm, maximum 3,000 mm (otherwise excessive heat losses)
Height of the boiler from the floor (centre of mounting brackets): typically 1,400–1,600 mm – check the technical documentation of the specific model

3. Mounting the Boiler and Installing the Cylinder

The boiler is mounted according to the manufacturer's instructions – use only the supplied mounting frame or brackets. For Vaillant models, the boiler mounting frame is held against the wall, the drilling points are marked, 10 mm diameter holes are drilled (10×60 mm wall plugs, M8 screws), the frame is fixed, and then the boiler is hung on the frame's hook. Important: use a spirit level to check that the boiler hangs exactly vertically and horizontally – otherwise there is a risk of the safety valve and expansion vessel level functioning incorrectly.

The cylinder stands on the floor. Before installation, check the anode protection (magnesium anode) – most cylinders have it directly at the top cover. Set the cylinder upright and secure it so it cannot tip over (for larger 200+ l models it is advisable to strap it to the wall). Check the inlet and outlet ports of the heat exchanger – usually marked with an arrow or R/S (return/supply) labelling.

Preparing the cylinder connections – procedure

The cylinder usually has the following connections (from top): DHW outlet (hot water to appliances), cold water inlet (from the mains), and on the side or bottom, the heat exchanger connections (boiler circuit). On the side of the cylinder there is a port for the immersion temperature sensor of the cylinder – this sensor is absolutely essential for correct heating priority control. If the cylinder is not equipped with a sensor port, you must use a clamp-on sensor fixed directly to the cylinder body with a thermally insulated cover.

Install the safety valve directly on the cold inlet of the cylinder – recommended opening pressure 6 bar, route the safety valve discharge to a drain or catch container
After the safety valve (on the cylinder side) don't forget the check valve – it prevents hot water from flowing back into the cold water supply during expansion
Hot water outlet – a direct branch to the appliance distribution, heat meter (if required)
Heat exchanger connections – supply (from the boiler, higher temperature) and return (to the boiler, lower temperature). Correct wiring/piping will become apparent at the first test run – the cylinder must heat up

4. Hydraulic Connection – Installing the Three-Way Valve and Pump

This is technically the most critical part of the whole installation. A low-temperature boiler with cylinder connection uses a three-way diverter valve to switch between two circuits: cylinder heating (priority) and space heating. In practice we encounter two basic schemes:

Boiler with built-in control and three-way valve – some Vaillant models have cylinder control directly integrated (e.g. via system control). The three-way valve is connected to the cylinder hydraulic kit directly behind the boiler.
Boiler without integrated cylinder control – the three-way valve is external, controlled by the boiler's control unit or an external controller. The cylinder circuit pump may be either integrated in the boiler or external.

In a typical job with the Vaillant turboTEC plus VU 122/3-5 or Vaillant turboTEC plus VU 202/3-5 models, the cylinder circuit pump is integrated directly in the boiler, and the three-way valve is fitted externally – usually into the cylinder hydraulic kit supplied by the manufacturer or a third party. The heating circuit pump is also in the boiler. An external pump for the cylinder is only added for longer pipe runs or for cylinders with a higher hydraulic resistance of the heat exchanger (≥ 1,500 mm of piping, large cylinders over 300 l).

How to correctly install the three-way valve – step by step

Install the valve in the boiler's flow pipe – usually directly behind the boiler elbow, before the branch splits into heating and cylinder.
Check the flow direction according to the arrow on the valve body – flow must pass through the valve, not bypass it.
Fit the servo or motorised actuator onto the valve body exactly as per the instructions (usually a locking screw or a click-fit).
Route the actuator's electrical wiring to the boiler control unit or external controller – use a 2-core cable of at least 0.75 mm², up to 10 m in length, 1.5 mm² for longer runs.
Test the manual control of the valve before filling the system – most valves have a manual switching button on the actuator.

5. Installing the Expansion Vessel, Safety Valve and Air Venting

The expansion vessel must be sized for the total volume of the heating system. For a typical family house with a system water content of 80–120 litres and a pressure difference of 1–2.5 bar, a 12–18 l expansion vessel is sufficient. The pre-charge pressure of the expansion vessel must correspond to the static height of the system – add 1 bar for every 10 metres of system height. Typically, the pre-charge is 0.5–1 bar for a single-storey house, 1–1.5 bar for a two-storey house.

Install the heating circuit safety valve (set to 3 bar) as close to the boiler as possible, on the flow pipe, with no shut-off element between the boiler and the safety valve. The discharge of the safety valve must be visible – into a siphon, a container, or outside, and certainly not "blindly" into a wall or insulation.

Install the air vent (automatic or manual) at the highest point of each hydraulic branch. Automatic air vents are practical but tend to leak at higher pressure – therefore always combine them with a ball valve underneath (open during normal operation).

6. Filling the System, Pressure Testing and Venting

Before filling the system with water, visually check all joints – threaded connections must be properly tightened, fittings correctly crimped or soldered. Fill the system slowly, through the filling valve, with open mains water (a mains water pressure of 2.5–3 bar is sufficient to fill most systems).

During filling, gradually open the air vents from the lowest point upwards – air escaping with the first water is normal. After filling, check the pressure on the gauge – it should be 1–1.5 bar (with cold water). After the system heats up to operating temperature, the pressure will rise to 1.5–2.5 bar, which is normal.

Pressure test: Close the filling valve and leave the system under full pressure for 30 minutes. A pressure drop of more than 0.1 bar indicates a leak – check all joints. If the system is tight, you can proceed with the electrical connection and commissioning.

7. Electrical Connection and Control Setup

The electrical connection of the boiler with the cylinder is just as important as the hydraulic connection. Low-temperature Vaillant boilers from the atmoTEC plus and turboTEC plus series have dedicated terminals on the control board for the cylinder temperature sensor, the three-way valve (or cylinder pump), and an external room thermostat.

Electrical connection procedure:

Cylinder temperature sensor (NTC): Insert the sensor into the cylinder port or clamp it onto the cylinder body. Route the sensor cable (usually a white 2-core cable) to the boiler and connect it to the terminals marked "SP" or "Ths" (depending on the model). The sensor resistance at 25 °C is typically 10 kΩ – you can verify this with a multimeter before connecting it to the boiler.
Three-way valve (servo actuator): Connect the cylinder valve actuator to the boiler control terminals according to the wiring diagram in the manual – these are usually 230 V AC terminals for the actuator (live, neutral, possibly a feedback signal). For a motorised actuator with a potentiometric feedback contact, make sure the polarity is correct.
External thermostat / controller: Connect the room thermostat to the eBUS terminals or a potential-free contact (depending on the control system). Digital Vaillant controllers (e.g. VRT 50, VRT 350) use the eBUS – a 2-core 0.75 mm² wire, without polarity.
Earthing: The boiler must be earthed – a PE conductor brought from the distribution board; check the presence of earthing at the socket near the boiler or via a direct connection.

Setting cylinder parameters in the boiler control

After wiring and before the first start-up, set the following parameters in the boiler control (using Vaillant atmoTEC plus / turboTEC plus as an example):

Desired cylinder temperature: Typical setting 55–60 °C. Below 50 °C there is a risk of Legionella (Legionella pneumophila) growth, above 65 °C there are unnecessary energy losses and a risk of scalding. For households with small children, set a maximum of 55 °C and add a thermostatic mixing valve at the tap.
Cylinder hysteresis: The temperature difference between switching heating on and off. Recommended setting 5–8 K (degrees). When the cylinder temperature drops 5–8 K below the setpoint, the boiler starts priority cylinder heating.
Maximum cylinder heating time: Some controls allow you to set a maximum time for priority cylinder heating (e.g. 30 minutes) – protection against the cylinder blocking space heating in case of a sensor fault.
Anti-bacterial function (Legionella Protection): Once a week (usually at night), the cylinder is heated to 70 °C for 30 minutes. Activate this in the control menu – this function is explicitly recommended by hygiene standards.

Detailed settings for cylinder priority and control scenarios are discussed in a separate article, Setting Cylinder Heating Priority on a Low-Temperature Boiler.

8. First Start-Up and Function Verification

The first start-up of the boiler should be carried out in the presence of an authorised service technician – for Vaillant boilers, the first start-up is tied to the warranty conditions and must be performed by a registered service partner. Nevertheless, it is good to know what is checked during commissioning.

Checking system pressure: Before switching on the boiler, the pressure must be in the range of 1.0–1.5 bar (cold water). If the pressure is lower, top up water through the filling valve.
Checking air venting: After the system's first heat-up, check the air vents again – air released from the water when heated collects in the upper parts of the system.
Verifying three-way valve function: When the control starts priority cylinder heating, the three-way valve switches to the cylinder position (verify by the current draw of the actuator or by direct observation of the actuator). The cylinder should start heating up – the temperature at the cylinder sensor must rise.
Verifying temperatures: Measured boiler flow temperature (80–85 °C in boiler mode during cylinder heating) and cylinder return temperature (40–50 °C at the start of heating, rising to 55–60 °C at the end of the cycle).
Checking the gas path: Manually check for gas leaks at the boiler's gas connections – use a soap solution or a gas detector. There must be no leaks whatsoever.
Recording in the service book: Installation date, all measured values, information about the cylinder and components used. This forms the basis for future servicing and any warranty claims.

9. Common Installation Mistakes and How to Avoid Them

In practice we repeatedly see the same mistakes in similar jobs, leading to operational problems:

Reversed cylinder heat exchanger connections: The supply and return branches are swapped – the cylinder heats extremely slowly or not at all. Solution: check the flow direction and swap the branches if necessary.
Missing cylinder sensor or faulty sensor contact: The boiler cannot detect the cylinder temperature and either heats the cylinder continuously or not at all. Always check the sensor resistance and the terminal block contacts.
Undersized expansion vessel: The system regularly discharges through the safety valve. A large cylinder with a large heat exchanger volume significantly increases the total system volume – don't forget to include the cylinder heat exchanger volume in the expansion vessel calculation.
Insufficient flow in the cylinder circuit: Piping that is too long or undersized (e.g. DN 15 instead of DN 22) causes slow heating. For larger cylinders (over 150 l) always use DN 22 (3/4").
Atmospheric boiler disadvantaged by lack of air supply: The boiler shuts down or burns irregularly – the room is not sufficiently ventilated. Check the vents.
Cylinder without anode protection or with a worn-out anode: Corrosion of the cylinder within a few years, a characteristic "rotten egg" smell (hydrogen sulphide). The anode must be checked every 2 years.

Typical faults and their systematic diagnosis are discussed in detail in the article Common Faults of Low-Temperature Boilers with a Connected Cylinder and Their Solutions.

10. Pipe Thermal Insulation and Energy Efficiency

Thermal insulation of the piping between the boiler and the cylinder is an investment that pays for itself quickly. Uninsulated DN 22 piping at 70 °C in a 20 °C room loses approximately 15–20 W per metre. With a boiler operating 8 hours a day over a 200-day heating season, that amounts to 24–32 kWh/m per year – at a price of €0.20/kWh, this means €5–6 per year for every metre of uninsulated pipe. Insulation costing €3–5 per metre therefore pays for itself within one year.

Use pre-insulated flexible multilayer pipes or elastomeric foam insulation (Armaflex, Kaiflex) with a minimum thickness of 13 mm for pipes up to DN 22, and 19 mm for DN 28 and above. For piping routed through an external wall or in an unheated space, double the insulation thickness.

More tips on the energy efficiency of the boiler-cylinder combination can be found in the article Energy Savings with a Combination of a Low-Temperature Boiler and a DHW Cylinder.

11. Handover of the Work and Customer Training

A correct installation doesn't end with switching the boiler on. The customer must be properly instructed – not everyone knows what to do when the boiler displays a fault code, or why the cylinder heats up to 70 °C once a week, even overnight. Provide the customer with:

Technical and operating documentation for the boiler and cylinder
Warranty certificates for both appliances
A record of the first start-up and the measured values
Contact details for the service technician
Instructions for basic tasks: topping up pressure, manually venting a radiator, resetting the boiler after a fault
Recommended date for the first service (usually 12 months after installation)

What to check and when during regular maintenance is discussed in detail in the article Maintenance and Servicing of a Low-Temperature Boiler with a Cylinder – What and When to Check.

Frequently Asked Questions (FAQ)

Do I always need to install an external cylinder circuit pump with a low-temperature boiler with cylinder connection?

Not necessarily. Models such as the Vaillant turboTEC plus VU 202/3-5 or VU 122/3-5 have an integrated cylinder circuit pump directly in the boiler. An external pump is only needed if the distance between the boiler and the cylinder is greater than 1.5–2 m, or if the cylinder has a heat exchanger with high hydraulic resistance. For most typical installations with a shorter connection, the internal pump is fully sufficient.

What temperature should the DHW cylinder be set to?

The recommended operating temperature of the cylinder is 55–60 °C. This temperature provides sufficient comfort while maintaining a safe hygienic level. Below 50 °C, conditions favourable to Legionella bacteria growth are created. Most modern controls also allow an automatic weekly heat-up to 70 °C (anti-bacterial function) – always activate this function. At the taps, we recommend adding a thermostatic mixer that reduces the temperature of the outflowing water to a safe 45 °C.

The cylinder is heating up too slowly – what could be the cause?

The most common causes are: reversed cylinder heat exchanger connections (supply and return branches swapped), undersized piping (DN 15 instead of DN 22), incorrect three-way valve function (not opening fully to the cylinder position), or a malfunctioning integrated cylinder circuit pump in the boiler. Check each point in turn. If the boiler flow temperature is rising but the cylinder is not heating, the problem is almost always in the hydraulics or the position of the three-way valve.

Is a safety valve needed on both the cylinder and the heating circuit?

Yes, each circuit needs its own safety valve set to the appropriate pressure. Heating circuit: 3 bar. Cylinder (sanitary side – drinking water): 6 bar. Both valves are mandatory according to STN EN standards and are a condition of the manufacturer's warranty. The cylinder's safety valve is installed on the cold inlet of the cylinder and must have a discharge leading to a visible location (siphon, drain).

Can I connect a cylinder to an existing low-temperature boiler that was not originally designed for it?

It depends on the specific boiler model. Some older low-temperature boilers do not have integrated cylinder control and lack terminals for the cylinder sensor and an output for the three-way valve actuator. In this case, it is possible to add external cylinder control (e.g. a digital controller with cylinder priority), which controls the three-way valve and cylinder circuit pump independently of the boiler control. This solution works but is more complicated. If you plan to replace the boiler, choose a model with native cylinder support – for example from the category low-temperature boilers with cylinder connection option.

How long does a typical boiler-with-cylinder installation take?

For an experienced technician with all materials ready, a direct replacement of an old boiler with a new one (with existing piping kept) realistically takes 1 day – approximately 6–8 hours of work. For a completely new installation, including all piping, chimney work, and electrical installation, allow 2–3 working days, depending on the extent of the construction work involved. The first start-up and final inspection require an additional 1–2 hours.

Conclusion – What to Remember for Every Job

Installing a low-temperature boiler with cylinder connection is technically and organisationally more demanding than a simple boiler replacement. It requires precise hydraulic planning, correct sizing of all components, and careful control setup. Boilers such as the Vaillant atmoTEC plus VU 200/3-5 or the Vaillant turboTEC plus VU 202/3-5 have integrated cylinder support, which significantly simplifies the installation process – but even with these models, the technician's correct procedure matters.

Key principles to remember: hydraulics before electronics, insulate every pipe, pressure test before start-up, replace the cylinder anode every 2 years, and train the customer on the basic tasks. These are the things that distinguish quality installation work from the kind a customer will be complaining about a year later.

Do you have a question on this topic?

Can't decide, or are you dealing with a specific situation in your household? Write to us - we'll be happy to advise.