ATTACK 25 E COMBI - stationary cast iron gas boiler 20-25 kW

Product detailed description

ATTACK COMBI - stationary cast-iron gas boiler

Stationary gas boilers with a cast-iron heat exchanger are among the most durable and time-tested heating solutions on the market. Cast iron as an exchanger material is highly resistant to corrosion and thermal shock, and it tolerates a less gentle operating regime (frequent switching, fluctuating return water temperature) better than thin-walled steel or aluminium exchangers. Thanks to this, cast-iron boilers have long enjoyed a reputation as robust and reliable appliances suitable not only for ordinary family houses but also for larger buildings, facilities, workshops, small businesses or multi-generational houses with higher heat demand.

Another advantage of the cast-iron body is easy maintenance and serviceability - a cast-iron exchanger usually consists of individual sections (segments), which means that if one of them is damaged, only the affected section needs to be replaced instead of the entire exchanger. In the long run, this can mean lower servicing costs compared to one-piece exchangers. The stationary design (a boiler standing on the floor, not wall-mounted) also simplifies installation at higher outputs and makes it easier to connect to a naturally draught chimney, which is commonly used in both older and newer boiler rooms.

The model you are currently viewing belongs to the ATTACK E COMBI line and is designed for a rated heat output of 20-25 kW. It is a stationary gas boiler in the COMBI version - in the context of this cast-iron ATTACK line, this means that the boiler itself does not have its own (built-in) domestic hot water (DHW) preparation. To heat domestic hot water, the boiler therefore needs to be complemented with an external indirectly heated DHW storage tank (e.g. from the ATTACK Z-series line - see the "How to connect" section below). This type of solution (boiler + separate storage tank) is common and proven especially where greater comfort of hot water draw-off at multiple points simultaneously is required, since storage tank water heaters can cover even occasionally higher DHW consumption better than instantaneous heating.

The ATTACK E COMBI line follows on from the KLQ COMBI and, within the ATTACK cast-iron range, represents an even higher equipment class. E COMBI boilers are intended for customers who, besides functional reliability, also require greater operating and handling comfort. The exact technical differences compared to the KLQ COMBI line (e.g. the specific burner type, control details) are not available in full detail in our source data - if you need to know exactly how the E COMBI differs from the KLQ COMBI for your specific output level, we are happy to verify this before purchase and help you choose based on the manufacturer's current technical documentation.

Why a cast-iron exchanger and not a steel or aluminium one?

When choosing a stationary gas boiler, customers often ask why they should opt for a cast-iron exchanger when cheaper steel or lighter aluminium solutions are also available on the market. The answer is mainly related to the long-term durability of the material. Cast iron has higher thermal inertia than thin-walled steel, which in practice means a calmer, less "nervous" combustion process and less material stress during sudden changes in return water temperature (for example, when a cold system starts up after a longer shutdown). At the same time, cast iron is naturally more resistant to condensation corrosion, which, under an unsuitable operating regime (long-term low return water temperature), can gradually damage thin-walled steel exchangers.

Another practical argument is the aforementioned sectional (segmented) design. A cast-iron exchanger does not consist of a single piece but of several sections stacked on top of each other and joined with sealing gaskets. If one section were to become damaged over the years of operation (which is unlikely even with cast iron, but theoretically possible, for example due to mechanical damage or extreme neglect of maintenance), a service technician can replace only the affected section without having to replace the entire boiler body. With one-piece steel or aluminium exchangers, this option generally does not exist, and exchanger damage means replacing the entire boiler or at least the whole exchanger block.

The disadvantage of cast iron is its higher weight (see the approximate figure in the technical parameters table below) and a somewhat slower thermal response when starting up from a cold state - however, for a stationary boiler intended for long-term, mostly continuous operation (not frequent switching on/off as with a cottage used only a few times a year), these are rather marginal disadvantages compared to the benefit of a long service life and easier repairability.

Who this boiler is suitable for

The ATTACK E COMBI stationary cast-iron boiler with an output of 20-25 kW is a suitable choice especially where a boiler room is available (it is not intended for wall mounting like common wall-hung combi boilers) and where a naturally draught chimney is available or planned. This typically includes:

• Family houses with their own boiler room or utility room, where long service life and easy serviceability of the appliance matter
• Larger buildings and multi-generational houses with higher heat demand, where the robustness of the cast-iron body is appreciated
• Smaller businesses, workshops, guesthouses or cottages with year-round or almost year-round operation, where the investment in a higher-quality body pays off through a longer service life
• Renovations where an older cast-iron boiler is being replaced and an existing chimney and gas piping sized for the given output are available

On the other hand, if you are looking for a compact wall-mounted solution for a flat or a smaller apartment core, or a boiler with its own fast domestic hot water preparation without having to deal with a separate storage tank, a cast-iron stationary COMBI boiler is probably not the most suitable choice - in that case, we recommend consulting us about a more suitable alternative from the ATTACK wall-hung condensing boilers.

Key features

• Stationary (floor-standing) design with a cast-iron heat exchanger - high resistance to corrosion and thermal shock
• Rated heat output 20-25 kW, natural gas fuel (LPG version available on request/according to the specific order specification)
• COMBI version = boiler without its own DHW preparation, designed to work with an external indirectly heated storage tank (e.g. ATTACK Z-series)
• Sectional (segmented) cast-iron exchanger - in case of damage, allows replacement of a single section instead of the entire body
• Connection to a naturally draught chimney (atmospheric combustion)
• Robust construction suitable also for larger buildings and facilities with higher heating output requirements
• Increased operating and handling comfort within the ATTACK cast-iron line
• The delivery includes the boiler body itself, including the burner and basic control - the external DHW storage tank, circulation pump, expansion vessel, chimney accessories and room thermostat are not included in the base price unless explicitly stated

Technical parameters

Note: the dimensions and weight are given as an approximate estimate based on the output category and the common construction of sectional cast-iron boilers of this class - we recommend verifying the exact value for a specific unit in the manufacturer's current technical data sheet or directly with us before ordering, especially if you are planning precise construction preparation of a room or recess for the boiler.

How to connect: boiler + external DHW storage tank

Since this is a COMBI version without its own domestic hot water preparation, this boiler needs to be connected together with a separate (external) indirectly heated DHW storage tank. In the ATTACK range, the so-called Z-series of storage tanks serves this purpose (e.g. Z100PLUS, Z125PLUS, Z100NTR, Z125NTRA, Z160NTRA, Z125OKC, Z160OKC, Z200OKC and other versions including horizontal wall-mounted OKCV variants) - we sell these storage tanks as a separate product; they are not included in the boiler's delivery or price.

For this output level (approx. 20-25 kW), we generally recommend a storage tank with a volume of 125-160 l, e.g. ATTACK Z 125 PLUS OKH NTR/HV, Z 125 OKC/NTR/HV or Z 160 OKC/NTR/HV (also available in the horizontal wall-mounted OKCV version, left/right) - a larger storage tank volume means more hot water "in reserve" for simultaneous draw-off in multiple bathrooms at once.

The connection principle is briefly as follows: the boiler heats the heating (heat-carrying) water, which circulates both to the heating system (radiators, underfloor heating) and through the exchanger (coil) in the DHW storage tank, where it transfers heat to the domestic water. Switching between the heating circuit and heating the storage tank is generally handled by a three-way valve (in the ATTACK range, e.g. models OT02 or OT44), controlled either by the boiler electronics or a separate room thermostat/controller. The exact wiring (piping route, placement of pumps, safety and mixing valves) depends on the specific boiler room layout and on whether it is a simple two-pipe system or a more complex system with multiple heating circuits - we recommend having the final connection designed and carried out by an authorised heating/gas professional who will also take into account locally applicable standards and regulations for connecting a gas appliance.

When ordering the boiler and storage tank, we are happy to help you choose a suitable combination of boiler output and storage tank volume according to the number of people in the household and the number of hot water draw-off points used simultaneously - do not hesitate to contact us before purchase if you are unsure about the right choice.

Maintenance and servicing in practice

As with any gas appliance, a regular service inspection (generally once a year, carried out by an authorised service technician) of the ATTACK stationary cast-iron boiler is not only a condition for maintaining the warranty but, above all, a prerequisite for safe and economical operation. For cast-iron boilers, servicing generally includes checking and, if necessary, cleaning the exchanger and burner of soot and dust, checking the tightness of the joints between individual sections, checking the chimney draught and flue gas path, and verifying correct combustion settings.

Compared to modern condensing boilers, maintenance of an atmospheric cast-iron boiler is generally simpler and cheaper - for example, there is no need to look after a condensate trap or an exchanger sensitive to low return water temperature. On the other hand, keep in mind that atmospheric cast-iron boilers of this construction do not achieve the efficiency of the most modern condensing solutions, which is a common and fair trade-off between higher initial robustness/simplicity and the highest possible energy efficiency. If the lowest possible gas consumption at any cost is a decisive criterion for you, we are happy to discuss alternatives from the ATTACK condensing range as well.

Frequently asked questions

1. Is domestic hot water (DHW) heating included in the price of this boiler?
No. This is a COMBI version without its own DHW preparation - the boiler only heats the heating water. An external indirectly heated storage tank (e.g. from the ATTACK Z-series) needs to be added for domestic hot water heating, which is ordered and paid for separately.

2. Which DHW storage tank is suitable for a boiler with an output of 20-25 kW?
For this output level (approx. 20-25 kW), we generally recommend a storage tank with a volume of 125-160 l, e.g. ATTACK Z 125 PLUS OKH NTR/HV, Z 125 OKC/NTR/HV or Z 160 OKC/NTR/HV (also available in the horizontal wall-mounted OKCV version, left/right) - a larger storage tank volume means more hot water "in reserve" for simultaneous draw-off in multiple bathrooms at once.

3. How does the E COMBI line differ from other lines of the ATTACK COMBI cast-iron range (EKO, PLQ, KLQ, E, EZ)?
In short: within the ATTACK range, the E COMBI line is positioned above the KLQ COMBI, with increased operating comfort. We are happy to verify and clarify the exact technical differences compared to KLQ COMBI at this output on request.

4. What type of fuel is the boiler designed for, and can it be converted to another fuel?
In the basic version, the boiler is designed for natural gas. An LPG (propane-butane) version is available on request/according to the specific order specification - we recommend stating this requirement when ordering or inquiring with our sales team in advance.

5. What is the warranty on this boiler?
ATTACK stationary gas boilers are covered by a 30-month warranty from commissioning, but no more than 36 months from the date of dispatch from the manufacturer's warehouse. An extended 5-year warranty applies to the cast-iron exchanger body. The exact warranty conditions are governed by the warranty certificate enclosed with the specific unit, and the prescribed commissioning conditions must be observed (generally by a professional service technician).

6. Can the boiler be connected to an existing naturally draught chimney?
Yes, this is an atmospheric boiler designed to be connected to a naturally draught chimney. The suitability of a specific chimney (diameter, height, material) needs to be assessed individually, ideally during a site visit or in cooperation with a chimney sweep/inspection technician, especially if the boiler is being connected to an older masonry chimney.

7. Can the boiler be connected to a system with multiple heating circuits (e.g. radiators + underfloor heating)?
Yes, combining multiple circuits with different heating water temperatures is common - however, this requires adding a mixing circuit with its own pump and control (in the ATTACK range, e.g. sets DPP25007/DPP25008 or ESBE versions DPP25020-DPP25022). We recommend discussing the connection design with a heating engineer or directly with our team.

8. Can the boiler be ordered without a DHW storage tank, just the body itself?
Yes - this product represents the boiler body itself. The DHW storage tank, circulation pump, chimney accessories, three-way valve and room thermostat/controller are ordered as separate items according to the needs of the specific installation - we are happy to prepare a complete list of the accessories needed, tailored to your project.

9. What is the difference between this atmospheric cast-iron boiler and a modern condensing boiler?
An atmospheric cast-iron boiler operates with a higher flue gas temperature and does not utilise the condensation of water vapour from the flue gases for additional heat recovery, so its efficiency does not reach the levels of modern condensing boilers. In exchange, it offers a simpler construction, lower sensitivity to return heating water temperature, and generally simpler, cheaper maintenance without the need to deal with condensate drainage. The choice between a cast-iron atmospheric and a condensing solution is always a trade-off between initial robustness/simplicity and the highest achievable efficiency - we are happy to help you weigh both options according to your priorities.

10. Where can I order the boiler, and how does delivery work?
You can order the boiler directly through the atria.sk online shop. When ordering, we also recommend considering ordering a suitable DHW storage tank from the Z-series and any additional accessories (chimney parts, three-way valve, circulation pump, room thermostat) at the same time, so you have a complete solution ready for installation. If you have any doubts when choosing the combination of boiler + storage tank + accessories, do not hesitate to contact us before ordering - we are happy to advise so that you don't order something you won't ultimately need, or, conversely, so that you don't miss anything essential.

Warranty and service

ATTACK E COMBI stationary cast-iron gas boilers are covered by a standard warranty of 30 months from commissioning of the boiler, but no more than 36 months from the date of dispatch from the manufacturer's warehouse (whichever period occurs first). The manufacturer provides an extended 5-year warranty on the cast-iron exchanger body itself. To claim the full warranty period, the commissioning conditions set by the manufacturer must be observed (generally by a professional service technician authorised for this activity), and a proper service book/warranty certificate must be kept. If you have any questions about the warranty, servicing, or choosing suitable accessories (DHW storage tank, circulation pump, chimney parts, controls), do not hesitate to contact us - we are happy to help you choose and order a complete tailored solution.