ATTACK AK 400 Storage Tank (insulation included in price)

Product detailed description

Why a buffer tank determines heating efficiency

The ATTACK Buffer Tank AK 400 (insulation included) is a steel accumulation (buffer) tank with a volume of 400 litres, designed for storing hot heating water from any heat source - a wood gasification boiler, heat pump, electric boiler, gas boiler or solar collectors. This is a tank without a built-in heat exchanger (sometimes also referred to as a "hydraulic separator" or "buffer tank"), which serves purely to store heating water - heating of domestic hot water (DHW) with this type of tank is handled externally, for example via a storage water heater or flow module.

The greatest benefit of a buffer tank is seen when combined with a solid fuel gasification boiler, such as the ATTACK DPX or ATTACK SLX. These boilers achieve the highest efficiency and cleanest combustion precisely when burning at full, rated output - i.e. when they burn a full load of wood "in one go" without throttling the air supply. Without a buffer tank, the boiler would often have to operate in a choked mode (to avoid overheating the system), leading to worse combustion, higher tar formation and shorter boiler and chimney lifespan. With the ATTACK Buffer Tank AK 400 (insulation included), the boiler burns the fuel at full output at once, and the surplus heat is stored in the water reserve in the tank - from where the heating system gradually draws it according to the household's current needs, even several hours after the boiler has burned out.

The same principle applies to an electric boiler or heat pump: a 400 l tank allows the heat source to work in longer, less frequent cycles instead of constantly switching on and off for short intervals, which extends the lifespan of the compressor/heating elements and reduces energy consumption. This is a typical volume for a standard family house heated by an ATTACK DPX or SLX wood gasification boiler - a tank of this size can store the heat from one or more boiler loads and gradually release it into the heating system over the following days, significantly reducing the number of boiler starts and stops.

Key features of the AK400K tank

• Volume of 400 litres - designed to provide a sufficient heat reserve between heating cycles at a recommended heat source output of 16 - 20 kW.
• Soft, removable polyester insulation 100 mm thick - significantly reduces heat loss to the surroundings during long-term storage of hot water, while being easy to unfasten for inspection or servicing of the tank and connections.
• Outer insulation cover made of leatherette (PVC fabric) in silver-grey - resistant to abrasion and easy to maintain, visually blending in as part of the boiler room interior rather than a "technical" element.
• 5 connection ports distributed around the shell and on the top/bottom of the tank, allowing connection of the heat source, heating circuits, an additional electric heating element (backup heating), venting, draining and temperature sensors.
• Steel shell with an internal corrosion-resistant surface treatment, rated for a max. operating pressure of 3 bar and max. temperature of 95 °C - a safe margin over typical heating water operating temperatures (60-80 °C).
• Suitable for upright installation thanks to feet/a stand, which at this volume facilitates precise temperature layering (stratification) - warmer water naturally stays on top, cooler water at the bottom, increasing the usability of the accumulated heat.
• Option of additional installation of an electric heating element (e.g. ATTACK TH100, TH102, TH103 or TH107) directly into one of the tank's ports - suitable as a backup or supplementary heat source outside the heating season or in case of a main source failure.
• Without a built-in heat exchanger - the tank is designed purely for accumulating heating water; if you also need DHW preparation via a built-in heat exchanger, check our range of AS/HR/HRS/TUV/TUVS tanks with a heat exchanger.

Technical parameters

Note: the dimensions and weight of the empty tank are approximate estimates corresponding to the given volume; we recommend verifying the exact dimensions with the seller before ordering, prior to installation (e.g. in case of limited boiler room space or narrow doors/staircase).

How to correctly choose the volume of a buffer tank

The most common recommended rule when choosing a buffer tank for a solid fuel gasification boiler is: allow approximately 20 to 25 litres of accumulation volume for each 1 kW of rated boiler output. This rule is based on the fact that a boiler burns one load of fuel at full output in approximately 1.5 to 3 hours, and the heat from this load needs to be stored somewhere until the heating system gradually consumes it. For a tank with a volume of 400 litres, this means it is suitable for a heat source with an output of approximately 16 to 20 kW.

If, for example, you have a wood gasification boiler with an output in this range (from the ATTACK range, these include some models of the DPX or SLX series - both Standard and Profi versions), the ATTACK Buffer Tank AK 400 (insulation included) is appropriately sized for your system. For a higher boiler output, or if you plan to combine multiple heat sources (e.g. a wood boiler + heat pump, or a boiler + solar collectors), we recommend choosing a larger volume, or connecting multiple tanks in a cascade - so that the accumulation volume corresponds to the sum of the outputs of the connected sources.

With a volume of 400 l, it is also true that the larger the tank, the longer it can supply heat even after the boiler has completely burned out - with a well-designed and sufficiently large system, it is common for a family house to be heated for 12 to 24 hours from a single boiler load and one buffer tank, depending on the outdoor temperature, the heat loss of the house and the temperature gradient of the heating system (lower temperatures, e.g. with underfloor heating, allow more of the stored heat to be used than high-temperature radiator systems).

It is also worth remembering that a buffer tank pays off even in combination with automatic heat sources such as an electric boiler or a heat pump - although these sources can modulate output more smoothly than a solid fuel boiler, they still benefit from the tank by reducing the number of switching cycles, which saves the lifespan of the compressor, circulation pumps and electronics.

Connection, installation and backup heating options

The ATTACK Buffer Tank AK 400 (insulation included) is typically connected as a hydraulically separated buffer element between the heat source and the heating system (radiators, underfloor heating, fan-coil units, etc.) - the heat source charges the tank via one pair of ports, while the heating system draws heat via the other pair, hydraulically separating both circuits and allowing the system to run more stably even with fluctuating flow on the source side. The remaining ports (5 in total on this tank) can be used, for example, for a temperature sensor, an air vent valve, a safety valve, or for connecting an additional electric heating element.

The option to install an electric heating element is a practical solution for periods when you don't want to heat with solid fuel (e.g. outside the main heating season), or as a backup in case of a main source failure. Our range includes the electric heating elements ATTACK TH100 (2.5 kW), TH102 (4.5 kW), TH103 (6 kW) and TH107 (2 kW), all with their own thermostat and G6/4" thread - compatible with the ports of this buffer tank. We recommend matching the element's output to the tank volume and the required heating speed.

When installing the ATTACK Buffer Tank AK 400 (insulation included), allow for sufficient manoeuvring space in the boiler room - given a diameter with insulation of approximately 850 mm and a height of approximately 1450 mm (both values approximate, see above), it is necessary to verify in advance the clearance of doors, staircases and the room itself. We also recommend leaving a free space of at least 300-500 mm around the tank on the side with the connection ports, to ensure convenient access during servicing, gasket replacement, or sensor inspection/refilling.

We recommend filling the tank only with treated heating water in accordance with applicable standards (e.g. EN 14868 or the boiler manufacturer's recommendation) - hard water with a high mineral content accelerates limescale formation on the inner walls and shortens the lifespan of the entire system, including the boiler and circulation pumps.

Connection with other heating system components

The ATTACK Buffer Tank AK 400 (insulation included) is only one link in the heating chain. Customers most often combine it with the ATTACK DPX or ATTACK SLX wood gasification boiler (both Standard and Profi versions, output range approximately 15-35 kW), whereby the boiler output should correspond to the recommended range of 16-20 kW mentioned above. When combined with a heat pump or electric boiler, the tank serves as a hydraulic separator and heat store, reducing the number of switching cycles of the source. If you also need to prepare domestic hot water, since this particular tank is a version without a heat exchanger, we recommend adding a separate DHW storage water heater to the system (e.g. from the ATTACK Z range - Z100, Z125, Z160, Z200 and others, available in our range), or choosing one of the AS/HR/HRS/TUV/TUVS series models instead of this tank, which have the heat exchanger built directly into the tank.

Maintenance and inspection of the tank during operation

The ATTACK Buffer Tank AK 400 (insulation included) requires minimal but regular maintenance. We recommend checking once a year (for example before the start of the heating season) the tightness of all 5 connection ports, the condition of the air vent valve and the safety valve of the heating circuit, as well as the overall condition of the outer insulation cover - the silver-grey leatherette cover is resistant to normal abrasion, but in case of mechanical damage (e.g. during moving or handling in the boiler room), we recommend repairing the damaged area promptly to avoid unnecessary heat loss.

Since this is a tank without a built-in heat exchanger, the inside of the tank comes into contact only with treated heating water, not drinking water - the risk of limescale buildup is thus significantly lower than with DHW storage tanks, but we still recommend visually inspecting the inside of the tank through an inspection opening (if the model is equipped with one) once every few years (e.g. during a system shutdown), or at least monitoring the quality and colour of the heating water during system desludging. Cloudy or rusty water may indicate a need for water treatment or a check for corrosion in the system.

During a long-term shutdown (e.g. outside the heating season), there is no need to drain the tank - on the contrary, leaving water in the system protects the internal surface from corrosion caused by air exposure. If you plan to take the tank completely out of operation for a longer period, consult a professional installer about the appropriate preservation procedure.

Frequently Asked Questions

Do I also need to buy a heat exchanger for this tank?

No, this AK400K model cannot be additionally fitted with a built-in heat exchanger - it serves purely for accumulating heating water. If you need both heating water storage and domestic hot water preparation via a heat exchanger in one tank, choose from our AS, HR, HRS, TUV or TUVS series.

How large a boiler can I connect to this tank?

For a volume of 400 litres, we recommend a heat source with an output of approximately 16 to 20 kW, according to the rule of 20-25 litres of accumulation per 1 kW of output. For a more powerful source, it is more suitable to choose a larger volume or connect several tanks in a cascade.

Can the tank also be used with a heat pump or electric boiler?

Yes, the AK400K buffer tank works equally well with a heat pump, electric boiler, gas boiler or solar collectors - in each case it reduces the number of switching cycles of the source and balances the differences between heat production and consumption.

Is insulation included in the delivery?

Yes, the tank is supplied including soft, removable polyester insulation 100 mm thick with an outer leatherette cover in silver-grey.

Can I fit the tank with an electric heating element for backup heating?

Yes, the tank has 5 connection ports, one of which can be used to install an electric heating element, for example ATTACK TH100, TH102, TH103 or TH107 with a G6/4" thread.

What is the maximum operating pressure and temperature of the tank?

The tank is rated for a maximum operating pressure of 3 bar and a maximum operating temperature of 95 °C, providing a safe margin over typical heating water operating temperatures of 60-80 °C.

What are the exact dimensions and weight of the tank?

Approximately, the diameter with insulation is around 850 mm, the height is around 1450 mm, and the weight of the empty tank is around 75 kg. These are estimated values corresponding to a volume of 400 l - we recommend verifying the exact dimensions with the seller before installation in confined spaces.

What is the warranty period for the buffer tank?

We provide a 24-month warranty on ATTACK buffer tanks from commissioning, but no more than 30 months from the date of shipment from the factory.

Warranty and final recommendation

The manufacturer ATTACK s.r.o. provides a standard warranty of 24 months from commissioning on the ATTACK Buffer Tank AK 400 (insulation included), but no more than 30 months from the date of shipment from the manufacturing plant. To maintain the warranty, we recommend connecting and commissioning the tank in accordance with the installation manual and applicable heating technology standards, using treated heating water and regularly checking the condition of the insulation and the tightness of the connection ports. We are happy to advise on choosing the right tank volume by phone or e-mail - just let us know the type and output of your heat source, and we will recommend the most suitable model from the ATTACK range.