ATTACK Z 125 OKC Wall-Mounted Storage Tank

Product detailed description

The ATTACK Wall-mounted Cylinder Z 125 OKC is an indirect-heated wall-mounted domestic hot water (DHW) cylinder with a capacity of 125 litres and a standard heat exchange area, designed to be hung on the wall next to a wall-mounted or floor-standing ATTACK gas boiler. The cylinder has no heat source of its own - the water inside it is heated indirectly, by means of heating water from the boiler circulating through an internal steel heat exchanger.

The wall-mounted OKC design saves floor space in the boiler room or utility room, as the cylinder hangs on the wall on the same bracket or next to the boiler. It is therefore a good choice especially for flats, smaller houses and spaces where there is not enough room for a floor-standing (freestanding) cylinder.

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 can be easily replaced during a service inspection
• Hardened PUR foam thermal insulation wrapped in an outer casing, which minimises heat loss while storing hot water
• Internal steel heating exchanger (coil) with a heat exchange area of 0.8 m² for indirect water heating from the boiler
• Suitable for connection to ATTACK floor-standing gas boilers (EKO COMBI, PLQ COMBI, KLQ COMBI, E COMBI, EZ COMBI series) as well as other heat sources with an appropriately sized heat exchanger
• Opening / pocket for a thermometer or thermostatic probe to control heating
• 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 draining 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 basic wall-mounted Z 125 OKC version is equipped with a standard single-circuit heat exchanger, sufficient for an average household of 2-4 people with an appropriately sized boiler. If the household has higher peak hot water demand, we recommend considering the sister model labelled "125/1m²" with a larger heat exchange area.

Structurally, this is a wall-mounted (hanging) cylinder - it is mounted on the wall using a supplied or separately purchased wall bracket, usually in close proximity to the boiler, so that connecting pipes are as short as possible and heat losses in the distribution are minimal.

Technical specification

Note: dimensions and weight are approximate estimates based on the cylinder's volume and design type; for exact values please refer to the manufacturer's technical data sheet or request them when ordering.

How indirect hot water heating works

The ATTACK Wall-mounted Cylinder Z 125 OKC is not a heat source - it has no burner or electric heating element 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 the internal steel heat exchanger (coil) immersed in the cylinder's tank. Through the wall of the exchanger, heat is transferred to the domestic water filling the tank's internal volume of 125 litres. The domestic and heating water never physically mix - 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 managed by the boiler's control system 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 when there is a demand for DHW. This means hot water preparation takes priority over space heating, and the household has hot water available practically immediately after the circulation pump starts.

The advantage of this solution compared to 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 the decision primarily on the number of people in the household and their hot water consumption habits:

• 100 litres - suitable for 1-3 people, a flat or 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, combination of shower and bathtub
• 160 litres - suitable for 4-5 or more people, households with higher demand for simultaneous consumption (e.g. more bathrooms)

This particular model, the ATTACK Wall-mounted Cylinder Z 125 OKC, has a capacity of 125 litres with a heat exchange area of 0.8 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. the OKC 125/160 series), also consider the output of your boiler: for boilers with higher output (25-30 kW), it is worth choosing the variant with a larger exchanger so that its output can actually be used for fast DHW heating.

The cylinder itself does not produce heat - it must be connected to a suitable source, typically an ATTACK cast-iron floor-standing 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 function of hot water storage. For combination with a heat pump or solar system, we recommend choosing specialised models from the HP or SOL series (other products in our range), which have a heat exchanger sized for lower heat transfer medium temperatures.

Installation and connection

The wall-mounted cylinder is fastened to a load-bearing wall using a supplied or separately purchased mounting bracket, usually in the immediate vicinity of the boiler. The wall must be able to bear the weight of the filled cylinder (empty weight approx. 31 kg + approx. 125 kg of water), so for plasterboard or otherwise non-load-bearing partitions, a suitable fastening system must be used, or the cylinder must be placed on a load-bearing masonry wall.

Cold and hot water connections and the heat exchanger circuit are connected according to the manufacturer's diagram, usually directly to the boiler or via a manifold. We recommend installing shut-off valves, a safety valve on the cold water inlet, and a check valve. Installation should be entrusted to a qualified plumber/heating technician, who will also connect the control system (temperature sensor, three-way valve) in cooperation with the boiler's control unit.

Maintenance and service

To ensure the ATTACK Wall-mounted Cylinder Z 125 OKC 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 inlet - regularly turn it to verify its function and the patency of the drip pipe
• Check the tightness of connections and the heat exchanger at every annual boiler service inspection
• In hard water areas, consider installing a water 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 Wall-mounted Cylinder Z 125 OKC is a pressure vessel that must be installed and operated in accordance with applicable technical standards and regulations, in particular EN 12897 (indirectly heated unvented water heaters) and related regulations for pressure equipment. Correct installation always includes a properly sized safety valve on the cold water inlet, which protects the tank against excessive overpressure during heating - its discharge/drip pipe must be routed freely into a 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 inlet that prevents water from flowing back into the water supply network during heating and thermal expansion). Without an expansion vessel, the safety valve may open repeatedly, resulting in unnecessary water loss.

Installation, connection to the boiler's heating circuit, and initial commissioning should always be entrusted to a qualified person authorised to install gas and heating equipment. Only professionally performed installation and a regular annual service inspection (together with a 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 the internal distribution systems of buildings, not for other technological purposes.

Position within the ATTACK range

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

We recommend combining this model primarily with ATTACK cast-iron floor-standing gas boilers from 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 include a DHW cylinder.

Frequently asked questions

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

The ATTACK Wall-mounted Cylinder Z 125 OKC is an indirect-heated type - it does not produce heat on its own. It must always be connected to a heat source, most commonly an ATTACK floor-standing gas boiler with provision for an external DHW cylinder (via a three-way valve), or another suitable source with a 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 models (for heat pumps, with a large-area exchanger for a low-temperature source) or SOL (with an additional solar exchanger), which are directly designed for these sources. This Z series model is primarily intended for a conventional gas boiler.

How long does the hot water in the cylinder last without heating?

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

Is it possible to add electric heating?

Yes, the cylinder has 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 out of operation.

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 conditions and regular maintenance (especially anode inspection and replacement) are observed.

How often does the magnesium anode need to be replaced?

We recommend checking the anode at least once a year during the boiler's 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 simultaneously), but also larger physical dimensions and weight. The choice depends on the number of people in the household and their water usage habits.

Can an ordinary wall bear the weight of the filled cylinder?

The filled cylinder weighs approximately 156 kg in total (empty weight + 125 l of water), so it must be mounted on a load-bearing masonry wall using suitable anchors, or a separate stand should be used. The wall-mounted cylinder must not be installed on thin, non-load-bearing partitions (e.g. plasterboard) without additional reinforcement.

Warranty and service

For ATTACK brand products, we provide a 5-year warranty on the insulated tank of the cylinder (provided installation conditions and regular maintenance are observed, including inspection and replacement of the magnesium anode) and 24 months on other parts and accessories from the date of dispatch from the factory. Service and spare parts (e.g. anodes, seals, accessories) are available in Slovakia. If you have any questions about selection, installation or warranty claims, please do not hesitate to contact us - we are also happy to help you choose a suitable boiler or control system for this cylinder.