Product detailed description

ATTACK Z125.1 OKC ENTR HV 2.2 - DHW storage tank with built-in 2.2 kW electric heating

The Z125.1 OKC ENTR HV 2.2 model (order code Z125OKCENTRHV22) extends the standard indirectly heated OKC storage tank with a built-in electric heating element with an output of 2.2 kW and its own thermostat. It is therefore a hybrid solution: primary heating is provided by the heating coil connected to a gas, solid fuel, or electric boiler, while secondary (backup or supplementary) heating is provided by the built-in electric element powered directly from the 230 V mains.

How combined heating works

The Z125.1 OKC ENTR HV 2.2 storage tank has one heating coil inside the tank for connection to an external heat source, and at the same time a flange fitted with a 2.2 kW electric heating element with its own operating thermostat. During normal operation in the heating season, DHW is heated conventionally from the boiler via the coil - quickly and with low operating costs. When the boiler is out of operation (summer shutdown, planned maintenance, malfunction, cottage without permanent heating), the electric element takes over heating automatically or by manual switching, and at an output of 2.2 kW it heats 125 litres of water in approximately 3-5 hours, depending on the inlet temperature.

The electric element has an integrated adjustable thermostat that maintains the set DHW temperature (typically 55-60 °C) and protects against overheating; the element is also equipped with a thermal fuse against dry running and overheating of the heating coil.

Key features

• Volume of 125 litres, vertical design for hanging/standing
• One heating coil for connection to the boiler
• Built-in electric heating element 2.2 kW / 230 V with thermostat
• Independent DHW heating even without a running boiler (summer operation, cottage, boiler failure)
• Enamelled steel tank with magnesium anode
• Rigid PU insulation with low heat losses
• Thermal fuse against dry running and overheating
• Compact dimensions suitable even for smaller utility rooms

Technical parameters

Installation

The storage tank is installed in the same way as the classic Z 125 OKC model - by hanging it under the boiler or on a separate bracket, with the coil connected to the boiler's heating circuit and the DHW distribution with a safety valve (max. 0.6 MPa). In addition, the electric element must be connected to a separate protected 230 V circuit (we recommend a residual current device and a separate circuit breaker of at least 16 A) in accordance with valid electrical installation standards - this part of the installation must be carried out by a qualified electrician.

Switching between boiler heating and electric heating can be handled manually (with the electric element's switch) or automatically using an external timer switch or a smart plug, which activates the electric element only when the boiler is not in operation.

Maintenance and service life

In addition to the standard annual inspection of the magnesium anode, we also recommend checking the functionality of the thermostat and thermal fuse of the electric element on this model, ideally during a service inspection before the start of the heating season. In case of a fault, the electric heating element can be replaced separately without having to drain the entire storage tank, if it is fitted with a ball valve under the flange.

Step-by-step commissioning procedure

1. Hanging the storage tank under the boiler or on a separate bracket
2. Connecting the coil to the boiler's heating circuit
3. Connecting the electric heating element to a separate protected 230 V circuit by an electrician
4. Connecting the cold and hot water distribution with a DHW safety valve (max. 0.6 MPa)
5. Filling and venting the storage tank and the heating circuit
6. Setting the thermostat of the electric element and the temperature on the boiler (recommended 55-60 °C)
7. Functional test of both heat sources separately and in combination
8. Pressure test and leak check of all connections
9. Handover of documentation and instructing the user on switching between heat sources

Safety warnings

The electric heating element must be powered from a separately protected circuit with a residual current device; the electrical part must be installed exclusively by a qualified electrician. The DHW safety valve (0.6 MPa) must always remain functional and must not be closed. When operating the boiler and the electric element simultaneously, make sure that the maximum water temperature does not exceed 95 °C and the pressure 0.6 MPa - we recommend setting both sources to a reasonable operating temperature of 55-60 °C.

Operating economics

Combined heating allows you to flexibly choose the cheaper available energy source - during the heating season, heating from the boiler is generally more economical (especially with solid fuel), while outside the season or in the case of surplus photovoltaic production, electric heating via the built-in 2.2 kW element may be more advantageous. This flexibility makes the Z125.1 OKC ENTR HV 2.2 model particularly interesting for households with a photovoltaic power plant that want to maximise the use of their own generated electricity instead of selling it to the grid at a low purchase price.

Who the Z125.1 OKC ENTR HV 2.2 is suitable for

You will particularly appreciate this model if: you run a cottage or cabin without year-round heating, want to be sure of hot water even during a boiler shutdown or failure, or want to switch off the boiler completely in summer and heat DHW more cheaply from your own photovoltaics via the electric element. 125 litres is a suitable volume for a household of up to 4 people.

How to navigate the ATTACK Z / PZO range of storage tanks

The ATTACK brand offers several types of hot water storage tanks that differ in their heating method and purpose. Below is a brief overview to help you find your way when choosing a related or replacement product:

• Z .. OKC / OKCV - Basic indirectly heated storage tank with one coil, connected to the boiler. The simplest and most affordable DHW preparation solution if you already have or plan to have a boiler.
• Z .. OKC ENTR HV 2.2 ▸ this is the product you are currently viewing - Same principle as OKC, plus a built-in 2.2 kW electric element - a choice for cottages, summer operation without a boiler, or as a safeguard in case of boiler failure.
• Z .. SOL - Two coils - boiler and large-area solar. A choice for households with solar collectors who want to make use of free solar heat as well.
• Z .. HP - One large-area coil optimised for low outlet temperatures from a heat pump. A choice for households with a heat pump as the main heat source.
• PZO .. PLUS - Directly heated, own gas burner, does not need a boiler. A choice for cottages, workshops and premises without central heating.

For all indirectly heated variants (OKC, ENTR, SOL, HP), the heating output can be supplemented or backed up with an ATTACK TH electric heating element (outputs 2/2.5/4.5/6 kW) via a 6/4" flange, which is part of our offer as a separate product.

Common mistakes when choosing and installing

A frequent misunderstanding is expecting the 2.2 kW electric element to fully replace the boiler's output during simultaneous high demand - in reality, it is a backup/supplementary source, not a full replacement for fast heating from the boiler. Another common mistake is forgetting to install separate protection for the electric element's circuit, or undersizing it, as well as neglecting the annual inspection of the magnesium anode, just as with the standard OKC model. When choosing between the Z125.1 model and a standard OKC without an electric element, consider whether you will actually use the independence from the boiler, or whether a cheaper basic model would be sufficient.

Frequently asked questions

How do I switch between heating from the boiler and electric heating?

The basic version allows simple manual switching on/off of the electric element using a switch; for automatic control, we recommend adding an external timer switch or a smart plug.

Can both heat sources (boiler and electricity) be switched on at the same time?

Yes, technically this is not a problem, but in practice it does not make sense - normally one source is used according to current need and energy cost.

How long does it take the 2.2 kW element to heat the entire volume?

Approximately 3-5 hours from 15 °C to 55-60 °C, depending on the 125 l volume and the inlet water temperature.

Is the electric element protected against dry running?

Yes, the element has a built-in thermal fuse that disconnects it from the power supply in case of overheating (e.g. when running without water).

Can the electric element be replaced separately in case of a fault?

Yes, the element is fitted in a separate flange, and in case of a fault, only it can be replaced, without the need to replace the entire storage tank.

Do I need a separate electric circuit?

Yes, we recommend a separately protected 230 V circuit with a residual current device; installation must be carried out by a qualified electrician.

Does the storage tank work even without connection to a boiler, using electricity only?

Yes, the storage tank can operate exclusively on electric heating if you do not yet have a boiler connected or it is out of operation - this is a standard method of use, e.g. in new buildings before the boiler room is completed.

What is the difference compared to the standard Z OKC model without an electric element?

Functionally, the storage tank is identical; it only adds a built-in 2.2 kW electric element with a thermostat, which the standard OKC model does not have and would require additional installation of a separate ATTACK TH element.

Can I use the electric element for heating from photovoltaics?

Yes, this is a common and increasingly popular solution - a surplus controller from a photovoltaic power plant directs surplus generated energy to water heating instead of selling it to the distribution grid.

What is the electricity consumption when heating the entire volume?

Approximately, at an output of 2.2 kW and a heating time of 3-5 hours, consumption is around 6.6-11 kWh per heating cycle for the entire 125-litre volume; the exact value depends on the inlet and target temperature.

Can the 2.2 kW element be subsequently replaced with a more powerful one?

Yes, when replacing it, an ATTACK TH element with a higher output (e.g. 4.5 or 6 kW) can be fitted, provided this is allowed by the building's electrical connection and protection.

Is the electric element included in the price of the storage tank, or is it an extra charge?

The 2.2 kW element is already included in this model from the factory and is included in the price; there is no need to buy it separately.

Does the element protect the storage tank against freezing in an unheated cottage?

Not directly; the element is used for heating DHW according to the thermostat's set temperature, not as frost protection. If there is a risk of the entire system freezing in cold weather, we recommend draining the storage tank and pipework for winter or ensuring permanent heating of the building.

How long does the electric heating coil last with regular use?

With correct installation, compliance with the rated voltage, and regular inspection (annual check of the magnesium anode and thermostat), the service life of the coil is comparable to that of standalone ATTACK TH elements, i.e. generally many years of trouble-free operation.

Is the model with an electric element also suitable for a flat with central heating?

Yes, even in a flat with central heat supply it can be useful as a supplementary source, for example during the summer shutdown of the central heat supply for maintenance, when hot water is otherwise not available from the central source either.

When is the extra cost for the built-in electric element worth it

When deciding between the Z125.1 OKC ENTR HV 2.2 model and a standard OKC storage tank without an electric element, consider how often you would actually use independence from the boiler. If you have a cottage with summer-only operation without heating, are planning photovoltaics, or simply want a safeguard in case of boiler failure, the extra cost for the built-in 2.2 kW element will quickly pay for itself in the comfort and certainty of hot water at any time of the year. If, on the other hand, your boiler runs year-round and you do not need independence, a cheaper basic OKC model without an electric element will suffice.

Comparison of the two volumes in the ENTR range

We offer the ENTR range in two volumes - 100 and 125 litres. The 100-litre model is more compact and suitable especially for smaller flats, cottages, or households of 2-3 people, where compactness is a priority rather than space. The 125-litre model provides somewhat higher comfort with simultaneous consumption from multiple points and is more suitable for a 3-4 person household. The price difference between the volumes is relatively small compared to the difference in comfort, so if in doubt, we recommend considering the larger volume, provided the utility room space allows it.

Summary

The Z125.1 OKC ENTR HV 2.2 model is an ideal choice wherever you want to be sure of hot water regardless of the boiler's status - thanks to the built-in 2.2 kW electric element, it is a practical combination of the reliability of boiler heating and the flexibility of electric backup heating in one compact storage tank. It is a solution particularly popular with owners of cottages and recreational properties, where the heating boiler is not used year-round, but the need for hot water persists regardless of the season.

Warranty and service

The manufacturer ATTACK provides a 5-year warranty on the storage tank body from the date of sale, subject to the conditions stated in the warranty certificate (regular anode inspection, compliance with maximum operating pressure and temperature, installation of safety equipment in accordance with applicable legislation). A standard warranty period of 24 months applies to other components of the storage tank (insulation, casing, electrical components, sensors, accessories). Service and spare parts for the entire ATTACK range are provided by a network of authorised service partners in Slovakia and the Czech Republic; our team will be happy to advise you on choosing the right part or contacting the nearest service centre.