ATTACK Z 125 OKC/NTR/HV Stationary Storage Tank

Product detailed description

ATTACK Stationary Cylinder Z 125 OKC/NTR/HV is an indirect-fired stationary domestic hot water (DHW) cylinder with a capacity of 125 litres, designed for free standing next to a stationary ATTACK gas boiler (e.g. EKO COMBI, PLQ COMBI, KLQ COMBI, E COMBI or EZ COMBI series). In its standard OKC/NTR/HV design, the cylinder does not generate heat on its own - the water inside it is heated by the heating water from the boiler, which flows through an internal steel heat exchanger (coil) placed in the tank. Thanks to this indirect heating principle, hot water is available quickly, hygienically, and without the risk of mixing with the heating circuit.

The Z 125 model is designed to complement an existing or newly installed boiler room with a separate DHW source with a sufficient supply of hot water for a household, apartment, or smaller business premises. The stationary design allows for easy placement on the floor of a boiler room, utility room, or pantry, without the need for wall mounting.

Key features

• Inner tank made of steel sheet with a high-quality enamelled (glazed) surface resistant to corrosion and limescale build-up
• Magnesium protective anode against electrochemical corrosion of the tank - extends the cylinder's service life and is easily replaceable during servicing
• Hardened PUR foam thermal insulation covered by an outer casing, minimising heat loss during hot water storage
• Internal steel heating coil (exchanger) with a heat exchange area of 1.0 m² for indirect heating of water from the boiler
• Suitable for connection to stationary ATTACK gas boilers (EKO COMBI, PLQ COMBI, KLQ COMBI, E COMBI, EZ COMBI series) as well as other heat sources with a properly sized heat exchanger
• Opening / pocket for a thermometer or thermostatic sensor for heating control
• Outlet for an optional electric heating element (purchased separately) for additional water heating outside the heating season, when the boiler is not running
• Drain valve for easy emptying and maintenance of the tank
• Robust construction with enamelling tested according to European standards for potable water tanks
• Manufactured under the ATTACK brand - a proven Slovak manufacturer of heating technology

Type and design

The designation OKC/NTR/HV in the name refers to the cylinder's compatibility with the whole range of stationary ATTACK gas boilers as well as other heat sources with an input for indirect heating (e.g. heat pumps with a suitable heat exchanger output). This is a proven standard design with an enamelled steel tank and a single heating coil.

The stationary design (unlike the wall-hung OKC or horizontal OKCV models) is placed directly on the floor next to the boiler. The advantage is easier installation (no need to anchor the full weight of a filled cylinder to the wall), stability, and the option to choose from a wider range of accessories (e.g. an additional electric heating element in the cylinder's pocket for additional heating outside the heating season).

Technical specification

Note: dimensions and weight are approximate estimates based on the cylinder's capacity and design type; exact values can be found in the manufacturer's data sheet or requested when ordering.

How indirect hot water heating works

The ATTACK Stationary Cylinder Z 125 OKC/NTR/HV is not a heat source - it has no burner or electric heating insert of its own (the latter is only an optional accessory). The heating principle is as follows: heating water heated in the ATTACK gas boiler flows via a circulation pump into an internal steel heat exchanger (coil) submerged in the cylinder's tank. Through the wall of the exchanger, it transfers heat to the domestic hot water filling the tank's internal volume of 125 litres. The domestic water and the heating water never physically meet - they are separated by the exchanger wall, so there is no mixing or contamination of the potable water by the heating medium (which may contain corrosion inhibitors and other additives).

Heating control is provided by the boiler's control unit in cooperation with a temperature sensor (surface-mounted or immersed in the cylinder's pocket) and usually a three-way diverter valve, which redirects the flow of heating water from the radiator circuit to the cylinder's exchanger upon a DHW demand. This gives priority to hot water preparation over space heating, and the household has hot water available practically immediately after the circulation pump starts.

The advantage of this solution over instantaneous heating (a combi boiler without a cylinder) is that the cylinder stores a supply of already heated water - it is therefore possible to draw a larger amount of hot water at once (for example, to fill a bathtub) without the output limitation that instantaneous heating in a boiler has.

How to choose the right cylinder for your boiler

When choosing the capacity of a Z series cylinder (100, 125 or 160 litres), it is recommended to base your choice primarily on the number of people in the household and hot water consumption habits:

• 100 litres - suitable for 1-3 people, an apartment or a smaller family house with standard consumption (showering, kitchen, bathroom without a bathtub or with occasional filling)
• 125 litres - suitable for 3-4 people, a typical family house, a combination of shower and bathtub
• 160 litres - suitable for 4-5 or more people, households with higher demand for simultaneous draw-off (e.g. multiple bathrooms)

This particular ATTACK Stationary Cylinder Z 125 OKC/NTR/HV model has a capacity of 125 litres with a heat exchange area of 1.0 m². The larger the heat exchange area, the faster the cylinder can "recharge" consumed hot water at the same boiler output - when choosing between a standard and an "enlarged" exchanger (where both are manufactured, e.g. OKC 125/160 series), also consider your boiler's output: for boilers with higher output (25-30 kW), it is worth choosing the variant with the larger exchanger so that its output can actually be utilised for fast DHW heating as well.

The cylinder itself does not generate heat - it must be connected to a suitable source, usually a stationary cast-iron ATTACK gas boiler (EKO COMBI, PLQ COMBI, KLQ COMBI, E COMBI or EZ COMBI series), which has provision for connecting an external DHW cylinder via a three-way valve. These boilers only prepare DHW instantaneously or not at all, and it is precisely the external Z series cylinder that adds the hot water storage function. When combining with a heat pump or solar system, we recommend choosing specialised HP or SOL series models (other products in our range), which have an exchanger sized for lower heat transfer medium temperatures.

Installation and connection

The stationary cylinder is placed on a flat, solid and level floor next to the boiler, with sufficient space for access to connections, inspection openings, and possible anode replacement. The recommended minimum ceiling height of the room should account for the cylinder's height of approx. 1100 mm plus handling space above it.

Connection of cold and hot water, circulation (if used) and the exchanger circuit to the heating boiler is carried out by an authorised plumber/heating engineer according to the manufacturer's installation instructions. Proper installation includes a safety valve on the cold water supply (sized according to tank capacity and mains pressure), shut-off fittings on all connections, and, if necessary, softening of the inlet water, which extends the tank's service life and reduces limescale build-up on the exchanger.

Maintenance and servicing

To ensure the ATTACK Stationary Cylinder Z 125 OKC/NTR/HV lasts as long as possible, we recommend following a regular service cycle:

• Check the magnesium anode at least once a year (more often with harder water) - a worn anode must be replaced in time, otherwise it loses its protective function and corrosion of the tank interior accelerates
• Desludging/draining the tank via the drain valve once every 1-2 years, especially in areas with harder water, where limescale and sludge settle at the bottom of the tank
• Check the safety valve on the cold water supply - regularly turn it to verify its function and the patency of the drain pipe
• Check the tightness of connections and the exchanger at every annual boiler service inspection
• With hard water, consider installing a softening or dosing device, which significantly extends the service life of both the exchanger and the tank

Regular maintenance is also a condition for warranty acceptance in the event of a fault caused by corrosion or limescale deposits.

Safety, standards and legislation

The ATTACK Stationary Cylinder Z 125 OKC/NTR/HV is a pressure vessel that must be installed and operated in accordance with applicable technical standards and regulations, primarily STN EN 12897 (indirectly heated water heaters) and related regulations for pressure equipment. Proper installation always includes a safety valve sized for the cold water supply, which protects the tank from excessive overpressure during heating - its discharge/drain pipe must be routed freely into the space with a slope and must not be closed off or blocked in any way.

We also recommend installing an expansion vessel on the cold water side (especially if there is a check valve or pressure reducing valve on the supply, which prevents water from flowing back into the water mains during heating and thermal expansion). Without an expansion vessel, the safety valve may open repeatedly, causing unnecessary water loss.

Always entrust installation, connection to the boiler's heating circuit, and initial commissioning to a qualified person authorised to install gas and heating equipment. Only professionally performed installation and a regular annual service inspection (together with the boiler inspection) are also a prerequisite for warranty acceptance in the event of a claim. The 125-litre cylinder is intended exclusively for heating potable (domestic) water in indoor building plumbing systems, not for other technological purposes.

Position in the ATTACK range

The ATTACK Stationary Cylinder Z 125 OKC/NTR/HV belongs to the Z series - indirect-fired DHW cylinders by ATTACK, which, in addition to this model, also includes other capacities (100, 125, 160, 200 litres), designs (stationary OKH NTR/HV, wall-hung OKC, horizontal wall-hung OKCV) and special variants (e.g. with an additional 2.2 kW electric heating element built into the cylinder body, a solar SOL version with two exchangers, or an HP version designed for heat pumps). Thanks to the wide range of variants, it is possible to find a cylinder tailored exactly to the boiler room layout and the household's needs.

We recommend combining this model primarily with stationary cast-iron ATTACK gas boilers of the EKO COMBI, PLQ COMBI, KLQ COMBI, E COMBI and EZ COMBI series, which have provision for connecting an external DHW cylinder, since the boiler itself does not contain a DHW cylinder.

Frequently asked questions

Do I need a boiler as well as the 125 l cylinder, or does it work on its own?

The ATTACK Stationary Cylinder Z 125 OKC/NTR/HV is an indirect-fired type - it does not generate heat on its own. It always needs to be connected to a heat source, most often a stationary ATTACK gas boiler with provision for an external DHW cylinder (via a three-way valve), or another suitable source with sufficiently sized output.

Can the cylinder also be connected to a heat pump or solar collectors?

Yes, in principle it can, however we recommend the specialised HP series (for heat pumps, with a large-area exchanger for low-temperature sources) or SOL series (with an additional solar exchanger), which are directly sized for these sources. This Z series model is primarily intended for a conventional gas boiler.

How long does hot water stay warm in the cylinder without heating?

Thanks to quality PUR insulation, standby losses are low, but the exact time depends on the room temperature and the difference from the water temperature. Generally, under normal conditions, the cylinder maintains usable water temperature for several hours up to overnight; for a longer idle period without the boiler running, it is advisable to add the optional electric heating element.

Is it possible to purchase electric heating separately?

Yes, the cylinder has a provision (pocket/outlet) for an electric heating element with a thermostat, which is purchased separately (e.g. output of 2-6 kW depending on the model). This allows DHW heating even outside the heating season, when the boiler is shut down.

What is the warranty on the cylinder?

We provide a 5-year warranty on the insulated tank, and 24 months on other parts (e.g. anode, seals, accessories) from the date of dispatch, provided installation and regular maintenance conditions are met (especially anode inspection and replacement).

How often does the magnesium anode need to be replaced?

We recommend checking the anode at least once a year during the boiler service inspection. With more aggressive or harder water, more frequent checks and earlier replacement may be needed - a worn anode loses its anti-corrosion function.

What is the difference between the 100, 125 and 160 litre capacities?

The difference is mainly in capacity - a larger volume means more accumulated hot water available at once (e.g. when filling a bathtub while showering at the same time), but also larger physical dimensions and weight. The choice depends on the number of people in the household and water consumption habits.

Is a stationary cylinder better than a wall-hung one?

Neither type is universally "better" - the choice depends on the boiler room layout. The stationary (free-standing) design is easier to install and does not put weight on the wall, while the wall-hung design saves floor space where it is limited.

Warranty and service

For ATTACK brand products, we provide a 5-year warranty on the insulated tank of the cylinder (subject to compliance with installation conditions and regular maintenance, including inspection and replacement of the magnesium anode) and 24 months on other parts and accessories from the date of dispatch from production. Service and spare parts (e.g. anodes, seals, accessories) are available in Slovakia. If you have any questions about selection, installation or claims, please do not hesitate to contact us - we will be happy to advise you on choosing a suitable boiler or controls for this cylinder as well.