Product detailed description

ATTACK Z 300 HP - DHW storage tank for heat pumps

The Z 300 HP model belongs to a range of storage tanks designed specifically to work with air-to-water or ground-to-water heat pumps. A volume of 300 litres combined with a large-area heat exchanger makes this tank a reliable solution for domestic hot water preparation, even in low-energy houses where the main heat source is a heat pump.

Why a heat pump needs a different storage tank than a gas boiler

A gas or electric boiler can easily supply heating water at a temperature of 70-80 °C, so a standard boiler-fed storage tank only needs a smaller heat exchanger - the temperature difference between the heating water and the water in the tank is large, and heat transfer is fast even through a smaller surface area. A heat pump, on the other hand, works best with a low heating water output temperature (usually 35-55 °C) for efficiency reasons - the higher the temperature the heat pump has to produce, the lower its coefficient of performance (COP) and the higher its electricity consumption.

If you connected a heat pump to a standard boiler-fed tank with a small heat exchanger, the pump would have to produce water at a higher temperature so that enough heat could pass through the small surface area within a reasonable time - this would significantly reduce its efficiency and increase operating costs. The Z 300 HP tank therefore uses a heat exchanger with a considerably larger heat transfer area, enabling efficient heat transfer even at a low temperature difference between the heat pump (e.g. 45 °C) and the water in the tank (e.g. from 40 °C to 50 °C).

Key features

• Volume of 300 litres, grey insulation, vertical design
• Large-area heat exchanger optimised for low heat pump output temperatures
• Maintains a high heat pump COP even during DHW preparation
• Enamelled steel vessel with magnesium anode
• Rigid PU insulation with low heat losses
• Built-in sleeve for the heat pump control temperature sensor
• Prepared for the addition of a 6/4" electric heating element as a backup/bivalent source
• Also compatible with a conventional boiler, should you later change your heat source

Technical parameters

Assembly and installation

The Z 300 HP tank is placed on the floor of the technical room as close as possible to the heat pump's indoor unit, in order to minimise heat losses in the piping between the pump and the tank. Connection of the heat exchanger to the heat pump circuit is carried out by the heat pump installation company according to its manual - it is important to set the priority DHW heating in the heat pump control system and to correctly place the temperature sensor into the tank's sleeve.

We also recommend adding an electric heating element (ATTACK TH) via the free 6/4" flange as a bivalent source for extremely cold days, when the heat pump's output drops, or for regular thermal disinfection (anti-legionella programme) at a temperature above 60 °C, which the heat pump alone usually cannot reach efficiently.

Maintenance and service life

Checking the magnesium anode once a year also applies to this model. We also recommend regularly (according to the heat pump manufacturer's instructions) performing thermal disinfection of the tank at a temperature above 60 °C, if not activated automatically, as a prevention against the growth of legionella bacteria, which can multiply especially at the lower operating temperatures typical of heat pumps.

Step-by-step commissioning procedure

1. Placing the tank on the floor as close as possible to the heat pump's indoor unit
2. Connecting the large-area heat exchanger to the heat pump circuit according to its manual
3. Fitting the heat pump control temperature sensor into the tank's sleeve
4. Connecting the cold and hot water piping with a DHW safety valve (max. 0.6 MPa)
5. If applicable, adding and connecting an electric heating element as a bivalent source
6. Filling and venting the tank and the heat pump circuit
7. Setting priority DHW heating and temperature in the heat pump control system
8. Setting and testing any thermal disinfection (anti-legionella programme)
9. Pressure test, leak check and handover of documentation to the user

Safety warnings

The DHW safety valve (0.6 MPa) must always remain functional, and its discharge must freely release any overpressure. Electrical connection of the heat pump and any additional electric heating element must be carried out by a qualified electrician. When preparing DHW with a heat pump, we recommend setting regular thermal disinfection above 60 °C (e.g. once a week) as a prevention against the growth of legionella bacteria, since the typical operating temperature with a heat pump (45-55 °C) is more favourable for their growth than the higher temperatures typical of boilers.

Operating economics

The main economic advantage of the Z 300 HP tank is maintaining a high heat pump coefficient of performance (COP) even during DHW preparation, thanks to the large-area heat exchanger. With a standard boiler-fed tank with a small heat exchanger, the heat pump would have to operate at a higher output temperature, which for air-to-water types can reduce the COP from around 3.5-4 to 2.5-3 or lower, significantly increasing electricity consumption for preparing the same amount of hot water. A properly sized tank for a heat pump therefore pays off in the long run, despite a higher purchase price compared to a standard boiler-fed tank.

Who is the Z 300 HP suitable for

This tank is intended primarily for households with an air-to-water or ground-to-water heat pump as the main source of heating and DHW preparation. A volume of 300 litres is suitable for a larger household or a household with higher demands for hot water comfort (multiple bathrooms, bathtub). If you plan to add a heat pump in the future, we recommend choosing this model directly instead of a standard boiler-fed tank.

How to navigate the ATTACK Z / PZO range of tanks

The ATTACK brand offers several types of hot water storage tanks, differing 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 tank with a single heat exchanger, connected to a boiler. The simplest and most affordable solution for DHW preparation if you already have or are planning to install a boiler.
• Z .. OKC ENTR HV 2,2 - 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 backup in case of boiler failure.
• Z .. SOL - Two heat exchangers - 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 ▸ this is the exact product you are viewing - One large-area heat exchanger optimised for low heat pump output temperatures. A choice for households with a heat pump as the main heat source.
• PZO .. PLUS - Directly heated, with its 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 available in our range as a separate product.

Common mistakes in selection and installation

The most common and costly mistake is using a standard boiler-fed tank with a small heat exchanger in combination with a heat pump - such a solution works, but in the long run it increases electricity consumption and reduces the pump's service life due to the need for a higher output temperature. Other mistakes include the absence of regular thermal disinfection (legionella risk at low operating temperatures) and incorrectly set DHW heating priority in the heat pump control system. For the Z 300 HP model, we recommend having the sizing of the tank volume and heat pump output assessed by a professional installation company already at the design stage.

Frequently asked questions

Can I use this tank with a gas boiler as well?

Yes, the large-area heat exchanger works without problems with a boiler too, only its advantage (efficient heat transfer at low temperature) is fully utilised specifically with a heat pump.

Why should I pay more for a heat pump storage tank?

The higher price pays off through lower heat pump electricity consumption - with a small heat exchanger, the pump would have to operate at a higher output temperature and lower COP throughout its operation, which in the long run increases operating costs much more than the price difference of the tank.

Can the heat pump reach even 60°C in this tank to prevent legionella?

Most heat pumps can reach a higher temperature, but with lower efficiency. For reliable thermal disinfection, we recommend adding an electric heating element, which reaches 60 °C and higher quickly and reliably.

Is a special temperature sensor required?

Yes, the heat pump control system requires its own temperature sensor placed in the tank's sleeve, which is supplied with the heat pump, not with the tank itself.

How large a tank do I need for my heat pump?

The recommended volume depends on the heat pump output and the number of people in the household; 300 litres is a common choice for a larger family house, and we recommend consulting the exact sizing with the heat pump supplier.

Can I also add an electric element as a backup source?

Yes, the tank has a free 6/4" flange ready for fitting an ATTACK TH electric element as a bivalent source for cold days or thermal disinfection.

How often should the anode be checked when operating with a heat pump?

The recommended check is the same as for other tanks - once a year, or more often with harder water.

Is the tank also suitable for ground-to-water heat pumps, not just air-to-water?

Yes, the large-area heat exchanger is suitable for any type of heat pump with a low heating water output temperature, i.e. both air-to-water and ground-to-water (brine-to-water).

By how much does the COP decrease when using a standard boiler-fed tank instead of the HP model?

The exact value depends on the specific heat pump, but as a rough estimate, the COP drop during DHW preparation can reach 20-35%, which translates into higher electricity consumption for the same hot water comfort.

How often should thermal disinfection against legionella be performed?

The recommended frequency is usually once a week at a temperature above 60 °C; the exact recommendation depends on the heat pump manufacturer's instructions and local hygiene regulations.

Can I later connect the tank to a different heat source?

Yes, the large-area heat exchanger is universal and also works with a boiler, so a change of heating source does not require replacing the tank.

Is this model compatible with all heat pump brands?

Yes, connection is made via a standard water pipe interface on the heat exchanger, which is compatible with heat pumps of any brand; we recommend verifying the exact connection parameters with the heat pump installation company.

How much larger is the heat exchanger area compared to a standard boiler-fed tank?

A large-area heat exchanger usually has tens of percent more heat transfer area than a standard boiler heat exchanger of the same volume; the exact ratio depends on the specific design.

Effect of heating water temperature on DHW heating output

The actual DHW heating output of the Z 300 HP tank depends on the temperature difference between the heating water from the heat pump and the water temperature in the tank. At low outdoor temperatures, the heat pump's output and output temperature decrease, which can extend the time needed to heat the entire volume. This is exactly why a large-area heat exchanger is so important - it maintains sufficient heat transfer even at a lower temperature difference, whereas a standard boiler heat exchanger would result in noticeably slower heating or the need to raise the heat pump's output temperature at the expense of its efficiency.

Comparison of Z 200 HP and Z 300 HP volumes

We offer the HP range in volumes of 200 and 300 litres. The smaller 200 l volume is suitable for a 2-4 member household with average consumption, while the larger 300 l volume is recommended for households with more people or higher comfort demands (multiple bathrooms, simultaneous showering and bathtub filling). Since a heat pump heats water more slowly than a gas boiler, it is worth choosing a larger tank volume for higher expected consumption, so that the system has a sufficient supply of hot water even during peak demand without waiting for reheating.

Summary

ATTACK Z 300 HP is the right choice of storage tank for any household with a heat pump - the large-area heat exchanger ensures that the investment in the heat pump fully realises its potential even during hot water preparation, without an unnecessary drop in efficiency caused by an undersized heat exchanger. The combination of a heat pump with a properly sized Z 300 HP tank is one of the most efficient and environmentally friendly ways of preparing hot water available on the market.

Warranty and service

The manufacturer ATTACK provides a 5-year warranty on the tank vessel from the date of sale, subject to compliance with 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 the other components of the tank (insulation, jacket, 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 help you choose the right part or find the nearest service centre if needed.