Connecting a thermal storage tank to DHW for a heat pump or boiler – how to do it
Connecting a hot water storage tank to a heat pump or boiler – how to do it
Correctly connecting a hot water storage tank to domestic hot water (DHW) to a heat source is one of those technical details that determine whether the entire system operates efficiently and reliably, or becomes a source of ongoing problems. Customers often ask: "Is it simple?" – and the honest answer is: it depends on the heat source, the specific tank model, the installer, and many other circumstances. In this article, I will cover the topic from all important angles – from the basic principle through specific connection diagrams to typical errors from practice that I repeatedly encounter.
Why combine a storage tank with a heat pump or boiler at all?
A heat pump or boiler does not produce hot water "on demand" in seconds. Each heat source has a certain power and a certain heating time. If hot water were to flow directly from the source to the consumers without any storage capacity, the heat pump would have to start and stop dozens of times a day, dramatically shortening its lifespan and reducing efficiency. The boiler would have to operate in short, inefficient cycles. A storage tank elegantly solves this problem – it serves as a thermal buffer, into which the heat source gradually "charges" energy, and consumers then draw from it as needed.
This requirement is even more pronounced with a heat pump. A heat pump operates most efficiently in long, stable cycles. Each compressor start puts a strain on the system and reduces COP (coefficient of performance). A large storage tank allows the heat pump to run, for example, for one hour at a time and then shut off for several hours – instead of starting up every five minutes every time a tap is opened.
Basic connection scheme – what every system must include
Before we get into specific connection variants, it is important to understand the basic anatomy of the system. Every correct connection of a storage tank to DHW must address these circuits:
- Primary circuit – connects the heat source (heat pump or boiler) with the heat exchanger inside the tank
- Secondary circuit (DHW) – distribution of heated potable water from the tank to consumers
- Cold water – supply of cold potable water into the tank (inlet at the bottom)
- Safety components – expansion tank, safety valve, air vent
- Control – thermostat, control unit, circulation pump
The diagram shows the basic principle: the heat source heats the medium in the primary circuit (the pump drives water through the heat exchanger), the heat exchanger transfers heat to the potable water in the tank, which rises upward (hot water is lighter) and then flows to the consumers. Cold water enters the tank from the bottom. This principle applies to most commonly available DHW storage tanks.
Connecting to a heat pump – specifics and requirements
A heat pump presents a more demanding case for installers and customers themselves than a boiler. There are several reasons for this.
Temperature of the primary medium and achievable DHW temperature
Air-to-water heat pumps typically produce an output temperature of 45–55 °C under favorable conditions. In winter months, when the outside air is cold, the temperature drops and the achievable output may be only 40–48 °C. This is problematic from a DHW hygiene perspective – the recommended minimum DHW temperature is 55 °C to prevent the growth of Legionella bacteria. Manufacturers of DHW storage tanks respond to this by incorporating an electric resistance rod (so-called bimetal or stainless steel heating rod), which heats the water to the desired temperature when the heat pump is insufficient.
The so-called anti-Legionella function involves heating the entire tank volume to at least 60 °C once a week (or according to the setting), thereby eliminating any possible bacteria. With a heat pump, this function typically takes place through an electric heating rod, as the heat pump itself cannot reach this temperature. This fact must be considered when selecting a tank – not every model has a heating rod or a connection for it.
Size of the heat exchanger and volume of the primary circuit
Heat pumps operate with a higher flow rate and a lower temperature difference than gas boilers. Therefore, the heat exchanger in the tank must have sufficient surface area – for heat pumps, a minimum of 0.3–0.5 m² of heat exchanger surface per 1 kW of heat pump power is recommended. In other words, a 10 kW heat pump requires a heat exchanger with a minimum of 3–5 m² of active surface. Tanks designed primarily for boilers have smaller heat exchangers – when used with a heat pump, heat transfer would be inefficient and heating would take too long.
In practice, tanks with larger volumes (800–1500 l for a family home) and specially designed heat exchangers with larger surface areas are recommended for heat pumps. An example is the HPWB3000 storage tank, which is specifically designed for use with heat pumps and offers heat exchanger parameters appropriate for this type of source.
Hydraulic connection of a heat pump with a storage tank
Most modern air-to-water heat pumps have their own control unit that also controls the circulation pump of the primary circuit. When connected to a TUV storage tank, it needs to be determined whether the heat pump will supply only the TUV circuit, or also the heating circuit – and if both, how they will be switched.
A typical solution for a family house with a heat pump and a TUV storage tank looks like this:
- The heat pump is hydraulically connected via a three-way valve that switches between the heating circuit (floor heating or radiators) and the TUV tank
- The heat pump control unit monitors the temperature in the TUV tank (temperature sensor in the tank) and when the temperature drops below the set threshold, it switches the three-way valve to charge the tank
- Once the desired temperature is reached, the valve switches back to heating
- Prioritizing TUV over heating is common practice – it is logical, as TUV has an immediate need and the supply must be maintained
Connection to a gas or oil boiler – what is different
A boiler is a simpler partner in terms of connection to a TUV storage tank. It produces output temperatures of 60–80 °C routinely, so it can easily heat TUV to 55–65 °C and handle the anti-Legionella cycle without an auxiliary electric rod. On the other hand, condensing boilers operate most efficiently at lower return temperatures – here, attention must be paid to the hydraulics.
With a condensing boiler, it is advisable for the return from the primary circuit to the boiler to have as low a temperature as possible, because the boiler condenses the flue gases and recovers their heat precisely when the return temperature is below 57 °C. Therefore, it is advisable for the primary circuit circulation pump to operate with a relatively high flow rate (smaller temperature difference, i.e., lower Δt), which is somewhat at odds with the usual philosophy of heating systems. The installer must correctly set the pump and the thermostatic regulator.
Single-circuit vs. two-circuit boiler
The older approach was that the boiler had a built-in TUV tank (combination boiler). A modern solution is a single-circuit boiler (only for heating) combined with an external TUV storage tank. This solution has several advantages:
- The TUV volume is independent of the boiler – for a larger family, a larger tank is sufficient without replacing the boiler
- The boiler operates more efficiently, as TUV heating occurs separately, not at the expense of heating
- Easier boiler replacement without interfering with the entire TUV system
- Ability to combine multiple heat sources (e.g., boiler + solar collectors + TUV tank with two heat exchangers)
For larger families or properties with higher TUV consumption, larger tanks are suitable. ZGIB 850-liter storage tank covers the needs of a larger family (5–7 people), while for multi-generational homes, hotel facilities, or houses with a swimming pool, volumes such as ZGIB 1500 liters or even ZGIB 2200 liters are more appropriate.
Combination of two heat sources – solar collectors and boiler/heat pump
Many customers ask how to connect a storage tank when they have a heat pump or boiler as well as solar collectors. Here, tanks with two heat exchangers come into play – the lower heat exchanger is connected to the solar circuit, the upper one to the boiler or heat pump. Solar collectors heat the water in the lower part of the tank, and the boiler or heat pump heats the upper part to the desired temperature when the sun is not sufficient.
For the differences between tanks with one and two heat exchangers, read the article Storage tank for TUV with one or two heat exchangers – what is the difference. The topic of selecting the correct volume is covered in the article What storage tank volume for TUV do I need for my household.
Specific numbers and pipe sizing
One of the most common mistakes during installation is underestimating pipe dimensions. Too thin a pipe means high flow velocity, pressure loss, and noise in the system. For the primary circuit (heat source – tank heat exchanger), the following approximate recommendations apply:
- Power up to 10 kW: DN 20 (3/4") or DN 25 (1"), circuit length up to 15 m
- Power 10–20 kW: DN 25 (1") to DN 32 (5/4"), circuit length up to 25 m
- Power over 20 kW: DN 32 (5/4") or DN 40 (6/4"), larger installation
For the secondary TUV circuit (from the tank to the consumers), the total flow velocity in the pipe should not exceed 1.5 m/s in a standard residential installation. The pipe size depends on the simultaneous flow (number of open taps), not on the tank volume. Typically, DN 20 (3/4") or DN 25 (1") is sufficient for a family house with 2–3 bathrooms.
The expansion tank for the TUV circuit must be sized according to the total volume of TUV in the system. For a 1000-liter tank, an expansion tank with a volume of 25–50 liters is commonly used (depending on operating pressure and maximum temperature). The safety valve is set to 6 bar (for a TUV drinking water circuit in an apartment building it may be 10 bar, but in family houses the standard is 6 bar).
Practical installation procedure – step by step
Below I describe a typical procedure that has proven effective in practice for installing a TUV storage tank in a new family house with an air-to-water heat pump:
1. Preparation and tank placement
The tank must be placed so that all connections are accessible – inlet and outlet of the primary circuit (heat exchanger), cold water inlet from the bottom, TUV outlet from the top, thermostat connection, possibly an electric heating element connection and anode preparation. Leave at least 60–80 cm of space around the tank for operation and maintenance. More about space requirements can be found in the article Dimensions and space requirements of TUV storage tanks 500–3000 liters.
2. Connection of the primary circuit (heat exchanger)
The supply from the heat pump (or boiler) is connected to the inlet of the upper heat exchanger. The return goes from the outlet of the heat exchanger back to the heat source. The following components are mounted on this circuit: a ball valve (for possible isolation of the circuit), a dirt filter (Y-filter or mesh filter before the pump), a circulation pump (if the heat pump does not have one built-in), and a temperature sensor for regulation. The entire primary circuit must be purged of air before starting.
3. Connection of the drinking water circuit (cold/hot water)
The cold drinking water inlet is connected to the lower connection of the tank. The cold water supply pipe must have: a pressure-reducing valve (if the network pressure is above 4–5 bar), a check valve (to prevent TUV from flowing back into the cold water network), and a safety valve for TUV (set to 6 bar, with a discharge to the drain). The TUV outlet is connected to the upper connection of the tank and from there to the consumers.
4. TUV circulation pipe
In houses with a long TUV distribution (distance over 3–5 m from the tank to the last consumer), a TUV circulation pipe with a circulation pump is recommended. Circulation prevents long waiting times for hot water and unnecessary waste of cold water from the pipe. The TUV circulation pump typically operates on a timer or thermostat (if the temperature in the circulation pipe drops below the set value, the pump turns on).
5. Electrical connection (heating element, thermostat, control unit)
The electric heating element is connected to its own circuit breaker (typically 10–16 A for a 2 kW element). The thermostat sensor is placed in the designated position on the tank (usually in the middle of the tank height or according to the manufacturer's instructions). The heat pump control unit is connected to the tank thermostat – either directly via 0–10V or Modbus interface, or via a simple on/off contact.
6. Filling, purging, and pressure test
Before starting, the system is filled with drinking water, purged (including the heat exchanger of the primary circuit after it is filled with antifreeze mixture), and the tightness of all connections is checked. A pressure test at 1.5 times the operating pressure for 30 minutes is recommended. Details about the entire commissioning process can be found in the article Installation of a TUV storage tank – procedure, connection, and commissioning.
Common installation mistakes – what I have seen in practice
Over the years of contract work, I have encountered many installations where something was not working optimally. Here are the most common mistakes:
Mixing up connections (hot/cold)
Seemingly trivial mistake, but it happens. If the cold water supply is connected to the upper connection instead of the lower one, the stratification in the tank is disrupted – the cold water directly cools the upper storage and the TUV is cold or lukewarm. On the primary circuit heat exchanger, it is similar – mixing up the inlet and outlet of the heat exchanger reduces heat transfer efficiency (a counterflow heat exchanger works differently than a parallel flow one).
Missing safety valve or incorrect setting
Legislation requires a safety valve on the DHW side. If it is missing or set too high, dangerous pressure increases may occur in the event of tank overheating. On the other hand, a safety valve set too low (e.g., at 3 bar) opens unnecessarily often, resulting in water loss.
Missing expansion tank
Water expands when heated. Without an expansion tank, this volume has "nowhere to go," and system pressure rises. The safety valve will relieve the pressure, but with every heating cycle, the system loses water, which is not correct. The expansion tank for DHW must be properly sized – a tank that is too small will not fulfill its function.
Insufficient insulation of pipes and tank
If the primary circuit or DHW piping is not properly insulated, heat losses are enormous. In an uninsulated technical room, a tank can lose 3–5 °C overnight. Insulating the tank and piping pays for itself quickly. More on this topic in the article Insulation of DHW Accumulation Tank – Why It Is Important and What to Consider When Choosing.
Incorrect position of temperature sensor
The temperature sensor in the tank determines when the heat source turns on or off. If the sensor is placed too high (in the hot zone), the tank is only partially charged, and the storage is insufficient to meet demand during high consumption. If it is too low, the heat source runs unnecessarily long. Manufacturers usually indicate the correct sensor position in the documentation.
Large industrial and commercial installations
For larger buildings – guesthouses, small hotels, apartment buildings, wellness centers – the requirements for DHW increase dramatically, and one heat pump or boiler is not enough. In such cases, tanks with a volume of 2000 liters or more are commonly installed, with multiple heat sources (boiler cascade, multiple heat pumps, heat recovery from cooling, etc.).
For such applications, for example, the ZGIB 1000-liter accumulation tank can be used as one module of a larger system, or directly the ZGIB 2200-liter tank for a central storage unit of a larger building. With such large tanks, it is also necessary to consider floor statics (a full 2200-liter tank weighs over 2.3 tons including its own weight), access to the technical room, and other logistical issues.
Control and automation – intelligent system management
Modern installations do not use only a simple bimetal thermostat. Heat pump control units, smart home systems, and external controllers allow:
- Time-controlled tank charging – DHW heating takes place during low electricity tariff periods (night hours, periods of photovoltaic surplus)
- Predictive control – the control unit knows that there will be high consumption at 7:00 AM and starts charging the tank earlier
- Integration with photovoltaics – if there is currently a surplus of FV production, the system heats DHW to a higher temperature (stores the surplus as heat)
- Remote monitoring – via an app, it is possible to monitor the tank temperature, daily heating, energy consumption, etc.
These are features that make a DHW accumulation tank not only a passive storage but an active element of an intelligent home energy system. The condition is that the heat source (heat pump or boiler) has a control unit with the appropriate inputs and outputs, and that the installer is able to correctly set up these functions.
Safety requirements and legislation
A DHW tank in Slovakia must meet the relevant standards (STN EN 12897 for pressurized hot water, or EN 806 for internal plumbing). For installation, the following applies:
- A pressurized DHW tank is a pressure vessel and must be registered (at pressure above 0.5 MPa and volume above 10 liters, it falls under the SÚBP decree)
- The safety valve must be certified and set according to the tank parameters
- Installation of a gas boiler must be performed only by a person with the appropriate certificate (gas technician)
- Inspection and control of the pressure vessel must be carried out according to current legislation (regularly for larger tanks)
- The water supply connection must meet the requirements of STN EN 806 and related standards, and backflow prevention is mandatory
A typical customer usually does not encounter legislative issues – if the installation is performed by a qualified installer, they should ensure these obligations themselves. However, it is good to know what to expect from the installer. Common signs of problems, such as pressure loss, leakage from the safety valve, or unexplained temperature drops, are discussed in the articles Common faults of DHW accumulation tanks and how to recognize them and Maintenance and service of DHW accumulation tanks – what and how often to check.
Most frequently asked questions (FAQ)
Do I need to replace the accumulation tank when switching from a gas boiler to a heat pump?
Not always, but it depends on the specific tank model. If your existing tank has a sufficiently large heat exchanger (at least 0.3–0.4 m² per kW of heat pump power), you can keep it. However, most older tanks installed for gas boilers have too small a heat exchanger and low volume, which leads to inefficient operation with a heat pump. I recommend consulting with an installer and the heat pump manufacturer.
Where should the temperature sensor be placed – at the top, in the middle, or at the bottom?
It depends on the function of the sensor. A sensor for tank charging control (when the heat source is activated) is typically placed in the lower third of the tank – when the temperature in the lower part drops below the set value, it means the supply is running low and it is time for a new heating cycle. A sensor for anti-Legionella control is placed in the middle or upper part. Always follow the manufacturer's instructions for the tank and heat source.
How long does it take to heat a full DHW accumulation tank from cold water?
It depends on the tank volume, heat source power, and the desired output temperature. A rough calculation: the energy needed to heat 1000 liters from 15 °C to 55 °C is approximately 46.5 kWh. A heat pump with 10 kW power (COP 3 means 30 kW thermal power, but in DHW mode it usually operates with a lower COP) will heat this tank in 4–6 hours. A 24 kW boiler in about 2 hours. In practice, the tank is never completely cold – only part of the volume is drawn off and cold water is added, so the actual charging time is shorter.
Is corrosion protection of the tank interior necessary?
Yes, this is an important topic. DHW accumulation tanks for potable water are either enamelled (interior vitrified) or made of stainless steel (stainless steel). Enamelled tanks have anodic protection – magnesium or titanium anodes that must be regularly checked and replaced (typically every 2–3 years). If the anode burns out and is not replaced, it is impossible to prevent corrosion of the enamelled tank surface, and the tank will be damaged. Stainless steel tanks do not require anodes, but they are more expensive. More in the article Maintenance and service of DHW accumulation tanks.
What if I want to connect photovoltaics as an additional heat source via a resistance heater?
This is an increasingly common situation. It is solved by installing an electric heating element (resistance heater) into the tank, connected to a surplus FV energy regulator. When the FV installation produces more energy than the household consumes, the regulator redirects the surplus to the heating element and heats the DHW. Such a system can cover a large part of the annual DHW heating for free. The condition is that the tank has a prepared input for the heating element (most modern models do).
What is the recommended temperature in a DHW accumulation tank during normal operation?
From a hygiene perspective, at least 55 °C throughout the entire tank volume. In practice, the thermostat is set to 60 °C so that after drawing off cooled water (during high consumption), there is still a reserve above 55 °C. The anti-Legionella cycle heats the entire volume to 60–65 °C at least once a week. On the other hand, excessively high temperatures (above 70 °C) unnecessarily burden the system, increase heat losses, and accelerate limescale buildup.
Conclusion – correct installation pays off
Connecting a thermal storage tank to TUV (domestic hot water) to a heat pump or boiler is not technically impossible or overly complicated – but it requires careful planning, the right choice of components and quality execution. Shortcuts, low-quality materials or ignoring safety elements will eventually catch up with you, usually at the worst possible moment. An investment in proper hydraulics, good regulation and a quality tank (such as the ZGIB series for standard home applications or the HPWB3000 series specifically designed for heat pumps) will pay off in the form of reliable and efficient operation for many decades.
If you are considering the choice of a specific tank, volume or other technical criteria, I recommend reading the article How to choose a thermal storage tank for TUV – volume, heat exchanger and other criteria, which deals with this topic in more detail. And if you feel like you are still missing an answer to some question in this area, check out Common questions about thermal storage tanks for TUV – you will find more practical answers there.
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