ATTACK 35 SLX PROFI - wood gasification boiler

Product detailed description

ATTACK 35 SLX PROFI – wood gasification boiler with 35 kW output

ATTACK 35 SLX PROFI is a modern wood gasification (pyrolysis) boiler with a nominal output of 35 kW, designed for heating family houses, cottages, businesses and smaller industrial buildings where quality firewood is available. The SLX Profi series combines the proven cast iron and steel construction of the ATTACK brand with controlled two-stage combustion technology, thanks to which it achieves significantly higher efficiency and lower emissions than classic wood boilers with bottom or top burning. The ATTACK SLX Profi series is designed with an emphasis on a larger fuel hopper volume and a longer interval between individual fuel loads, which is especially appreciated when heating larger buildings or if you prefer less frequent refuelling. The larger combustion chamber also allows longer logs to be loaded, simplifying fuel preparation.

The 35 SLX PROFI boiler belongs to energy class A+ and meets the current emission requirements (ecodesign) imposed on solid fuel boilers in the European Union. Due to the gasification principle, the firewood in the boiler is not burned all at once, but gradually – first it is gasified (pyrolysed) in the upper hopper chamber under insufficient air supply, and the resulting combustible gas (a mixture of CO, H2 and hydrocarbons) is then, together with supplied secondary air, perfectly burned in the ceramic combustion nozzle and lower combustion chamber at temperatures exceeding 900 °C. The result is a longer burning time per load of wood, higher efficiency (typically above 88 %) and significantly lower wood consumption per kilowatt-hour of heat produced compared to a classic boiler.

How wood gasification works and why it is more efficient

A common wood boiler with bottom burning burns fuel all at once with insufficiently controlled air supply, producing a large amount of unburned combustion products (tar, soot, CO) that escape through the chimney and clog the flue. The ATTACK 35 SLX PROFI gasification boiler, on the other hand, divides combustion into two separate phases taking place in two chambers, one above the other. In the upper hopper chamber, the wood gasifies at high temperature and with a lack of oxygen (there is no direct combustion), while the resulting combustible wood gas is pushed by the combustion air fan through a ceramic (firebrick) nozzle into the lower combustion chamber. There, the gas mixes with secondary air and burns completely at a very high temperature. This process brings three main benefits: significantly higher combustion efficiency, minimal tar and soot formation (less cleaning, longer lifespan of the flue and chimney) and the possibility of smooth output regulation according to current heat demand, since the fan regulates the intensity of combustion instead of manual chimney damper adjustment.

Key features of the ATTACK 35 SLX PROFI boiler

• Wood gasification combustion with two-stage combustion and high efficiency above 88 %, energy class A+.
• Large hopper space with a volume of approximately 130 l, allowing a larger amount of wood to be loaded and a longer interval between refuelling.
• Turbulators in the heat exchanger – rotating turbulators increase the turbulence of flue gases in the heat exchange area, improve heat transfer to the heating water and reduce the temperature of flue gases leaving through the chimney, increasing boiler efficiency.
• Mechanical heat exchanger cleaning using an external lever/handle – the operator easily removes ash and soot deposits from the heat exchange surfaces with a few movements of the lever, without having to disassemble the boiler.
• Robust cast iron/steel construction of the boiler body with increased wall thickness resistant to operational wear and thermal stress.
• Ceramic combustion nozzle and combustion chamber lining made of quality firebrick resistant to the high temperatures generated when burning wood gas.
• Combustion air fan with output control ensuring an optimal air-to-fuel ratio at various boiler loads.
• Electronic control with display and option of weather-compensated control
• Large service door for convenient access to the combustion chamber, nozzle and heat exchanger for cleaning and maintenance.
• Safety heat exchanger / option to connect a cooling circuit against boiler overheating (we recommend supplementing with a thermal safety valve in accordance with applicable Slovak legislation).
• Ready for conversion to automatic operation using the SL20968 kit for burning pellets (COMBI Pellet), giving the boiler fully automatic refuelling capability.
• Low emissions and compliance with ecodesign requirements for solid fuel boilers valid in the EU.

Technical parameters of the ATTACK 35 SLX PROFI

Differences between the Standard and Profi series

The 35 SLX PROFI boiler is equipped with the Profi series, which, compared to the basic Standard version, brings an electronic control unit with a clear display, more precise combustion air control via a motorised damper, and the option to connect a room thermostat or weather-compensated control based on outdoor temperature. This allows the boiler to automatically adjust its output to the current heat demand of the house, saving wood and extending the interval between refuelling. The Profi version also has thicker insulation of the boiler body, which reduces heat loss into the boiler room and increases operating comfort, especially during year-round operation.

Installation and commissioning

ATTACK SLX gasification boilers operate with high flue gas temperature in the combustion chamber and are therefore sensitive to so-called low-temperature corrosion, which occurs if water that is too cold (below approx. 60 °C) returns to the boiler. During long-term operation with a low return temperature, acidic combustion products condense on the walls of the heat exchanger, significantly shortening the lifespan of the boiler body. Therefore, we always recommend installing a mixing (three-way) circuit with a thermostatic head – for example ATTACK / Oventrop or ESBE LTC361 – in the heating system, which maintains the temperature of the water returning to the boiler at least at 60 °C until the boiler reaches operating temperature. This mixing circuit is also a condition for the extended warranty on the boiler body to be recognised (see the Warranty section below).

Due to the nature of gasification combustion (the boiler works best at output close to nominal), we strongly recommend supplementing the heating system with a buffer tank (e.g. the ATTACK AK or AS series with a suitable volume according to the boiler output and the heated area). The buffer tank allows the loaded batch of wood to be burned at once at optimal output, and the heat is "stored" for gradual use in the heating circuit throughout the day. Without a buffer tank, the boiler's output is frequently throttled, which increases tar formation, reduces efficiency and shortens the lifespan of both the boiler and the flue. Installation, connection to the flue, connection to the electrical network and initial commissioning should always be carried out by an authorised heating engineer/service technician in accordance with the project documentation, applicable legislation (especially STN standards and boiler room regulations) and the manufacturer's installation manual. Initial commissioning should be carried out by an authorised ATTACK service technician, which is also one of the conditions for warranty claims.

Fuel and its preparation

The recommended fuel for the ATTACK 35 SLX PROFI boiler is dry firewood (preferably hard deciduous wood – beech, oak, hornbeam) with moisture up to 20 %. The moisture content of wood significantly affects both boiler output and lifespan – wet or freshly cut wood (moisture 40 % or more) significantly reduces the calorific value, causes incomplete gasification, increased tar and soot formation, and can even lead to combustion extinguishing. Therefore, store wood for at least 1–2 heating seasons under a well-ventilated shelter. The length of the logs should correspond to the dimensions of the boiler's hopper chamber (usually up to approx. 50 cm), and log thickness should be approx. 8–15 cm for even and smooth gasification.

Option of conversion to automatic operation – COMBI Pellet

If you are considering a more convenient, automatic operation in the future without the need for daily wood loading, the ATTACK 35 SLX PROFI boiler can be additionally converted to burn wood pellets using the SL20968 kit with a 30 kW pellet burner. This conversion kit is designed for ATTACK 20-35 SLX Profi boilers and includes a burner, pellet hopper and the necessary control electronics, including connection to the existing boiler control system. When ordering the conversion kit, you need to provide the serial number of the specific boiler to which the kit will be fitted, so that the service technician can verify compatibility and prepare the correct components. After conversion, the boiler functions as a fully automatic pellet heat source, while the original wood-burning function can, depending on the version, be retained as a backup (combined wood/pellet operation).

Maintenance and cleaning

Regular maintenance is key to maintaining the high efficiency and long lifespan of the ATTACK 35 SLX PROFI boiler. We recommend emptying the ash pan and the space under the grate depending on the intensity of heating, usually several times a week. Mechanical cleaning of the heat exchanger using the external lever should be carried out according to the operating instructions, ideally once a day during intensive seasonal operation – this extends the interval between major service cleaning and maintains the declared boiler efficiency. We recommend having a comprehensive service inspection of the boiler carried out at least once a year (before the start of the heating season), including checking the door seal, the condition of the ceramic nozzle, the combustion chamber lining and the functionality of the fan and controls.

Frequently asked questions

Is a buffer tank needed for the ATTACK 35 SLX PROFI boiler?
It is not a legal requirement, but it is strongly recommended. A gasification boiler works most efficiently at output close to the nominal 35 kW, so the buffer tank allows a batch of wood to be burned at once under optimal conditions and the heat to be stored for later use. Without it, output throttling occurs, along with increased tar formation and reduced boiler lifespan.

What is the recommended wood moisture content?
The recommended moisture content of firewood is up to 20 %, which usually corresponds to wood dried for at least 1–2 years under shelter. Wetter wood reduces efficiency and increases the risk of fouling the heat exchanger and chimney.

What is low-temperature corrosion and how can it be avoided?
It is damage to the boiler body caused by condensation of acidic combustion products at too low a return water temperature. It is solved by installing a mixing (three-way) circuit that maintains the temperature of the water returning to the boiler at a minimum of 60 °C.

Can the 35 SLX PROFI boiler be converted to pellets later?
Yes, using the SL20968 conversion kit designed for 20-35 SLX Profi boilers, which equips the boiler with a pellet burner and the necessary control electronics for fully automatic operation.

What is the difference between the Standard and Profi series?
Compared to the Standard version, the Profi series brings more precise electronic control with a display, the option of weather-compensated control based on outdoor temperature, and usually thicker insulation of the boiler body, resulting in higher operating comfort and slightly lower fuel consumption during long-term operation.

Who is allowed to carry out the initial commissioning of the boiler?
Initial commissioning, connection to the flue and electrical network should always be carried out by an authorised heating engineer or an authorised ATTACK service technician, in accordance with the installation manual and applicable legislation. This is also a condition for warranty validity.

What flue diameter does the boiler require?
The ATTACK 35 SLX PROFI boiler is standardly equipped with a 200 mm chimney connection; however, the exact chimney draft and diameter must be assessed individually according to the height and type of chimney (we recommend consulting a chimney sweep or designer).

Why choose the ATTACK 35 SLX PROFI

ATTACK is a Slovak boiler manufacturer with a long tradition and an extensive service network throughout Slovakia, which is a great advantage when heating with solid fuel – spare parts (ceramic nozzles, seals, fans, control units) are readily available and service is not dependent on imports from abroad. The 35 SLX PROFI boiler is designed to last for decades of operation if the recommended conditions are followed (protection against low-temperature corrosion, quality dry wood, regular maintenance). The combination of cast iron and steel components, ceramic combustion chamber lining and well-thought-out mechanical heat exchanger cleaning is among the proven solutions tested over the years in practice, appreciated by both installation companies and end customers in everyday operation.

When choosing the output of the 35 SLX PROFI boiler, be guided primarily by the heat loss of the heated building calculated according to the applicable standard (STN EN 12831), not just approximately according to the floor area of the house. An undersized boiler will not be able to cover the heat loss on the coldest days, while a significantly oversized boiler works more often at partial output, which, for solid fuel boilers, reduces efficiency and increases tar formation. If in doubt about the correct choice of output, we recommend consulting a heating designer or directly an ATTACK sales representative.

Emissions and environmentally friendly operation

Thanks to two-stage controlled combustion and high operating temperature in the combustion chamber, the ATTACK 35 SLX PROFI boiler achieves significantly lower emissions of particulate matter, CO and organic compounds compared to classic wood boilers with bottom burning. The boiler meets the requirements of the ecodesign regulation (Directive 2009/125/EC and related implementing regulations) imposed on solid fuel boilers placed on the market in the EU. Lower emissions also mean fewer deposits in the chimney and flue, reducing the risk of soot fire in the chimney and extending the interval needed between chimney sweep cleanings.

Warranty

The manufacturer provides a standard warranty of 36 months from the date of boiler commissioning for ATTACK SLX wood gasification boilers, but no more than 42 months from the date of boiler dispatch from the factory. If protection against low-temperature corrosion is installed (a mixing circuit maintaining a return temperature of at least 60 °C), an extended 7-year warranty on the boiler body can be claimed. For the warranty to be valid, the initial commissioning must be carried out by an authorised ATTACK service technician and the prescribed method of operation must be observed (especially fuel moisture and protection against low-temperature corrosion). Exact warranty conditions can be found in the warranty certificate supplied with the boiler.