Vaillant ecoTEC plus VU 35CS/1-5 IoniDetect - condensing boiler 4.3 - 37.7 kW

Product detailed description

Vaillant ecoTEC plus VU 35CS/1-5 IoniDetect – powerful condensing boiler 4.3 – 37.7 kW

Vaillant VU 35CS/1-5 ecoTEC plus IoniDetect is the most powerful model of the current (fifth) generation of the popular ecoTEC plus range of wall-mounted condensing boilers, which Vaillant offers in three power levels (10, 25 and 35). With a modulated thermal output of 4.3 – 37.7 kW (at a temperature gradient of 50/30 °C), or 4.0 – 34.8 kW at a gradient of 80/60 °C, it is designed for larger family houses, buildings with higher heat loss, older non-insulated buildings, or cases where it is necessary to cover both heating of a larger area and faster preparation of a larger volume of hot water in combination with a storage water heater. The high modulation ratio of 1:9 (i.e. an almost ninefold range between minimum and maximum output) also allows even this most powerful model of the range to smoothly cover periods with lower heat loss (spring, autumn) without unnecessary cycling, when only a fraction of its maximum output is required.

Just like the 10CS/1-5 model, this version is also designed as a "VU" – i.e. as a heat source intended primarily for heating, while domestic hot water preparation is handled in combination with an external Vaillant storage water heater (for example uniSTOR VIH R or actoSTOR). It is therefore not a combined instantaneous boiler with immediate water heating without a storage tank (the so-called "VUW" variant) – this concept with a storage tank is deliberately more suitable for larger households with a higher and more simultaneous demand for hot water (for example a multi-member household with several bathrooms), where the capacity of purely instantaneous heating might not be sufficient with simultaneous draw-off at several points.

IoniDetect technology and combustion with a wide power range

Even this most powerful model of the range is equipped with the IoniDetect system – automatic detection of the quality and composition of the supplied natural gas via an ionization electrode, which continuously fine-tunes the ratio of gas to combustion air in real time. For a model with such a wide modulation range (4.3 to 37.7 kW), this technology has particularly great practical significance – the quality of combustion may not behave linearly when the burner power level changes, and it is precisely this continuous automatic correction that ensures combustion remains optimal across virtually the entire power range, not just at the nominal maximum. The system also allows safe operation with an admixture of hydrogen into natural gas of up to 20% by volume, making this boiler a sensible investment with a view to the gradual change in the composition of distributed natural gas in the coming years.

Efficiency, emissions and energy class

The standardized heat utilization rate for this model reaches up to around 109% (relative to the calorific value of the fuel), which is a typical high value for condensing technology of this range. The boiler achieves energy class A for heating (according to the ErP regulation), and when supplemented with a suitable controller (for example sensoCOMFORT or the VRT range), it is possible to achieve up to the highest class A+++ on the energy label of the entire heating package. The NOx emission class reaches value 6 (the strictest commonly stated class for gas appliances), with measured NOx emissions of around 30.1 mg/kWh – i.e. significantly below common limit values, which is particularly important in locations with stricter air quality requirements or for buildings undergoing environmental certification assessment.

Dimensions, weight and design

Due to its higher output, this model has a slightly greater construction depth than the smaller power variants of the range – the external dimensions are 720 × 440 × 382 mm (height × width × depth) with a weight of approximately 40.5 kg. Despite the higher output, the boiler thus remains a compact wall-mounted device that can be installed on the wall of a standard utility room without requiring a dedicated boiler room space, which a comparably powerful older-type stationary (cast iron or steel) boiler would require. The hydraulic connection of the heating circuit is via a 3/4" thread, gas connection via a 1/2" thread. The maximum operating pressure of the heating water is 3 bar, the outlet temperature is adjustable in the range of 15 – 80 °C (maximum 85 °C). The declared sound pressure level during operation is approximately 38 dB(A), meaning the boiler is among the quiet heat sources suitable even for installation near living areas.

Flue gas discharge and connection options

The basic design of the boiler uses a concentric (coaxial) system for flue gas discharge and combustion air supply with a diameter of Ø 60/100 mm – this solution is easy to install and suitable for most common installations with routing through the facade or roof. For buildings where, for layout or structural reasons, it is more advantageous to route the combustion air supply and flue gas discharge separately (for example when connecting to an existing masonry chimney, or for longer flue routes), a variant with a split connection of Ø 80/125 mm is also available – this exact function is served by a separate connection kit from our accessories range. The boiler can also be connected to a common (collective) chimney system with a 60/100 mm discharge using a certified connection kit designed for this type of installation in multi-storey buildings.

Electronics, control and communication interfaces

The boiler is powered by a standard mains connection of 230 V / 50 Hz, and the electronic equipment corresponds to protection level IPX4D. The boiler communicates via the eBUS bus (connection of room and weather-compensated controllers) and LINbus (peripheral components), which simplifies diagnostics and any future expansion of the system with additional control elements. The built-in touch LCD display offers intuitive control directly on the boiler, including display of current output, temperature and operating messages. In combination with an internet communication module (for example sensoNET or VR 921) and a smart controller, the entire system can be controlled remotely via the myVAILLANT application – including setting a weekly heating program, monitoring consumption and remote diagnostics in the event of a fault, which can help a service technician speed up problem identification even before a physical visit.

Which buildings the 35CS/1-5 model is most suitable for

The choice of the correct power level of the boiler should always be based on the heat loss of the specific building calculated according to STN EN 12831, not just on an approximate rule based on area. The 35CS/1-5 model, with its range up to 37.7 kW, is particularly suitable for larger multi-storey family houses, older buildings with a lower level of insulation, buildings with a higher number of heated rooms, or for combining heating with a larger storage hot water heating for a multi-member household. The wide modulation range (ratio 1:9) also ensures that the boiler can operate efficiently even in transitional periods of the year, when the actual heat demand represents only a small fraction of its maximum output – this prevents excessive cycling, which would otherwise reduce the lifespan of the burner and heat exchanger.

If you have any doubts about the suitable power level for your specific building, we recommend contacting our technical advisor – we will be happy to help with an approximate estimate of the required output based on the age, area and insulation condition of the building, or recommend preparing a precise heat loss calculation.

Installation, commissioning and legislative requirements

Installation, connection to the gas supply, electrical network and flue gas discharge, as well as the commissioning of the boiler itself, may only be carried out by a professionally qualified person with the appropriate authorization for the installation and servicing of gas appliances, in accordance with applicable legislation. Proper commissioning includes checking the tightness of the gas supply, checking the correct draft and tightness of the flue gas discharge, setting combustion parameters, filling and venting the heating system, as well as setting the controls to match the character of the specific system (radiator, underfloor or mixed heating). Proper commissioning by an authorized technician and its entry into the warranty certificate is a condition for maintaining the full manufacturer's warranty.

For this more powerful model, we also recommend paying increased attention to the sizing of the heating system's expansion vessel (for larger buildings and a larger volume of water in the system, it may be necessary to add an external expansion vessel in addition to the one built into the boiler) and to the quality of the filling heating water – especially for larger systems with older piping, it is advisable to consider filtration or water chemistry treatment to prevent deposits on the boiler's plate heat exchanger.

Compatibility and recommended accessories

The 35CS/1-5 model is fully compatible with the entire current ecosystem of Vaillant fifth-generation ecoTEC plus accessories – hot water storage heaters of the uniSTOR range (VIH R 120, VIH R 150 and larger) and actoSTOR, room and weather-compensated controllers (sensoCOMFORT, sensoTOUCH, VRT 350f, VRT 51f, VR 940f), an internet communication module for control via myVAILLANT, as well as installation accessories – hydraulic connection between boiler and storage tank with a safety group and siphon, insulating aesthetic covers, a kit for separate combustion air supply Ø 80/125 mm, a kit for connection to common chimneys 60/100 mm, or flue gas plugs for service shutdown. Due to the higher burner output, we recommend always verifying, when choosing a conversion or service kit, that the specific part is designed for the power category up to 35 kW – some conversion kits differ between the power levels of the range.

Maintenance and regular servicing

In accordance with applicable legislation on the operation of gas appliances, a regular, usually annual, service inspection carried out by an authorized service technician must be ensured – this includes checking the burner and heat exchanger, measuring flue gases, checking the tightness of the gas circuit and the functionality of safety elements. For a more powerful boiler, which is typically deployed in larger systems with a higher volume of heating water, we recommend paying increased attention during service inspections to the condition of the expansion vessel and the pressure in the system, since a larger volume of water also means greater sensitivity of the system to a potentially insufficient expansion volume during larger temperature fluctuations.

Why choose this particular Vaillant ecoTEC plus model

The ecoTEC plus range has long been among the best-selling condensing boilers on the Slovak market thanks to proven German manufacturing quality, a wide service network and long-term availability of spare parts. The 35CS/1-5 model, as the most powerful representative of the current fifth generation, brings, along with high output, the same modern equipment as the smaller models of the range – IoniDetect technology, a touch display, readiness for smart control and low operating noise. This is a solution suitable precisely where a higher output is needed due to a larger area or higher hot water demand, without having to resort to a more expensive stationary boiler or a cascade of several smaller units.

Frequently asked questions

Broader context – investing in a more powerful heat source

When choosing a more powerful boiler such as the 35CS/1-5 model, the decisive factor is a correct assessment of the actual heat loss of the building – an oversized boiler brings not only a higher initial investment but also less efficient operation during transitional periods of the year. This is precisely why Vaillant places emphasis on a wide modulation range even for the most powerful model of the range, so that even when choosing a higher output (for example with a reserve for a possible future expansion of the heated area or an extension), operation remains efficient even outside peak winter days.

For customers considering between several power levels of the ecoTEC plus range, we recommend not buying a boiler based only on an approximate estimate according to the house's area, but basing the decision on an actual heat loss calculation, or consulting our technical team – we will be happy to assess your specific situation, including recommended accessories (storage tank, controls, flue gas discharge solution), and prepare a tailored price offer.

It is also worth mentioning that in a cascade solution (connecting several boilers into one system for very large buildings or apartment blocks), it may in some cases be more advantageous to use several smaller units instead of one maximum-output boiler – such a solution brings redundancy (if one boiler fails, the others remain functional) and, in many operating modes, also higher overall efficiency thanks to better use of the modulation range. For a typical family house, however, a single 35CS/1-5 model boiler is usually a fully sufficient and economically most advantageous solution – we recommend consulting a cascade connection with our technical department only for buildings with significantly above-standard heat loss or specific operating requirements, for example when combined with large-capacity storage water heating for multiple residential units.