ATTACK Z 125 PLUS OKH NTR/HV Stationary Storage Tank

Product detailed description

ATTACK Stationary Storage Tank Z 125 PLUS OKH NTR/HV is an indirectly heated stationary domestic hot water (DHW) storage tank with a volume of 125 litres, designed for free-standing installation next to a stationary ATTACK gas boiler (e.g. EKO COMBI, PLQ COMBI, KLQ COMBI, E COMBI or EZ COMBI series). The tank, with improved thicker insulation for lower heat losses (PLUS version), does not generate heat itself - the water inside it is heated by heating water from the boiler, which flows through an internal steel heat exchanger (coil) located 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 PLUS model is designed to complement an existing or newly installed boiler room with a standalone DHW source with sufficient hot water reserve for a household, apartment or smaller business. The stationary design allows for easy placement on the floor of the 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 tank's lifespan and is easily replaceable during a service inspection
• Hardened PUR foam thermal insulation covered by an outer jacket, which minimises heat losses during hot water storage
• Internal steel heating exchanger (coil) with a heat exchange area of 1.2 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 to other heat sources with a correctly sized heat exchanger
• Opening / pocket for a thermometer or thermostatic sensor to control heating
• Outlet for an optional electric heating element (purchased separately) for additional 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 drinking water tanks
• Manufactured under the ATTACK brand - a proven Slovak manufacturer of heating technology

Type and design

The designation PLUS in the name means an improved version with a thicker layer of thermal insulation made of hardened PUR foam, which reduces the standby heat losses of the tank - the water stays hot longer and operating costs for maintaining its temperature are reduced. It is especially worthwhile where the boiler does not run continuously and the tank needs to maintain its temperature for longer between individual heating cycles.

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

Technical specification

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

How indirect hot water heating works

The ATTACK Stationary Storage Tank Z 125 PLUS OKH NTR/HV is not a heat source - it does not have its own burner or electric heating element (the latter being only an optional accessory). The heating principle is as follows: heating water heated in the ATTACK gas boiler is pumped by a circulation pump into an internal steel heat exchanger (coil) submerged in the tank. Through the wall of the exchanger, it transfers heat to the domestic water filling the internal volume of the 125-litre tank. The domestic water and the heating water never physically meet - they are separated by the wall of the exchanger, so there is no mixing or contamination of drinking water by the heating medium (which may contain corrosion inhibitors and other additives).

Heating control is provided by the boiler's controller in cooperation with a temperature sensor (surface-mounted or immersed in the tank's pocket) and usually a three-way switching valve, which redirects the flow of heating water from the radiator circuit to the tank's exchanger when DHW is required. This means that 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 over instantaneous heating (combi boiler without a storage tank) is that the tank stores a reserve 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 tank for your boiler

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

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

This particular ATTACK Stationary Storage Tank Z 125 PLUS OKH NTR/HV model has a volume of 125 litres with a heat exchange area of 1.2 m². The larger the heat exchange area, the faster the tank can "recharge" the consumed hot water at the same boiler output - when choosing between a standard and an "enlarged" heat exchanger (where both are produced, e.g. the OKC 125/160 series), also consider your boiler's output: for boilers with higher output (25-30 kW), it is worthwhile to opt for the version with a larger heat exchanger, so that its output can actually be used for fast DHW heating.

The tank 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 tank via a three-way valve. These boilers only prepare DHW instantaneously or not at all, and it is precisely the external Z series tank that adds the hot water storage function to them. 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 a heat exchanger dimensioned for lower heat carrier temperatures.

Installation and connection

The stationary tank is placed on a flat, solid and horizontal 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 takes into account the tank height of approximately 1150 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 technician according to the manufacturer's installation instructions. Proper installation includes a safety valve on the cold water inlet (sized according to the tank volume and network pressure), shut-off valves on all connections, and, if necessary, softening of the inlet water, which extends the tank's lifespan and reduces limescale build-up on the exchanger.

Maintenance and service

To ensure the ATTACK Stationary Storage Tank Z 125 PLUS OKH NTR/HV lasts as long as possible, we recommend following a regular service cycle:

• Checking 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 from the inside 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
• Checking the safety valve on the cold water inlet - regularly turning it to verify its function and the patency of the drip pipe
• Checking 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 lifespan 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 Storage Tank Z 125 PLUS OKH 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 dimensioned for the cold water inlet, which protects the tank from excessive overpressure during heating - its discharge/drip pipe must be led freely into a space with a slope and must not be closed 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 backflow of water into the water supply network during heating and thermal expansion). Without an expansion vessel, the safety valve may open repeatedly, causing unnecessary water loss.

Always entrust the 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 a boiler inspection) is also a prerequisite for warranty acceptance in the event of a claim. The 125-litre tank is intended exclusively for heating drinking (domestic) water in the internal distribution systems of buildings, not for other technological purposes.

Position in the ATTACK range

The ATTACK Stationary Storage Tank Z 125 PLUS OKH NTR/HV belongs to the Z series - indirectly heated DHW storage tanks by ATTACK, which, in addition to this model, also includes other volumes (100, 125, 160, 200 litres), designs (stationary 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 tank 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 tank tailored exactly to the boiler room layout and household 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 tank, since the boiler itself does not include a DHW tank.

Frequently asked questions

Do I need a boiler in addition to the 125 l tank, or does it work standalone?

The ATTACK Stationary Storage Tank Z 125 PLUS OKH NTR/HV is an indirectly heated type - it does not generate heat on its own. It must always be connected to a heat source, most often a stationary ATTACK gas boiler with provision for an external DHW tank (via a three-way valve), or to another suitable source with sufficiently sized output.

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

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

How long does hot water stay hot in the tank without heating?

Thanks to the 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 tank keeps usable water temperature for several hours up to overnight; for a longer standby period without the boiler running, it is advisable to add the optional electric heating element.

Is it possible to purchase electric additional heating?

Yes, the tank has provision (a pocket/outlet) for an electric heating element with 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 switched off.

What is the warranty on the tank?

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 shipment, provided installation conditions and regular maintenance (especially anode inspection and replacement) are followed.

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 inspection and an earlier replacement date may be necessary - a worn anode loses its anti-corrosion function.

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

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 water consumption habits.

Is a stationary tank better than a wall-mounted 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 load the wall, while the wall-mounted 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 (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 shipment from production. Service and spare parts (e.g. anodes, seals, accessories) are available in Slovakia. If you have any questions about selection, installation or warranty claims, do not hesitate to contact us - we will be happy to help you choose the right boiler or control system for this tank as well.