Integrating domestic hot water circulation with a heat pump – how to do it

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Integration of hot water circulation with a heat pump – how to do it

A heat pump as a heat source offers a wide range of options in a modern house – from heating through cooling to the preparation of hot tap water (HTW). One of the functions that proves to be very popular in practice and is technically not entirely trivial is hot water circulation. This article will guide you through the entire issue from the basic principle to specific solutions, products and typical errors from installation practice. If you have ever wondered why cold water runs for a few seconds before hot water comes out of the tap, or why your heat pump cannot handle circulation "on its own", this text is exactly for you.

What is hot water circulation and why it is important

Hot tap water circulation (shortened to hot water circulation or just circulation) is a system in which hot water continuously or at defined intervals circulates in a closed loop between the HTW tank and the outlets – i.e. taps, showers and bathtubs. The purpose is to ensure that hot water appears almost immediately when the tap is opened, without waiting for the "flushing out" of cold water from the pipe.

In practice, it looks like this: you have an HTW tank in the technical room, from which hot water leads to the bathroom on the second floor, to the kitchen and to another bathroom. Without circulation, the water in the pipe between the tank and the outlets cools down. When you open the tap, you have to wait until the cold (cooled) water runs out and the hot water from the tank arrives. In doing so, you waste water and energy. A circulation pump solves this problem by keeping the water in the pipe in motion, and thus hot.

In houses with a storage water heater connected to a heat pump, this requirement is even more relevant, because the tank is usually larger (200–500 liters), located far from the outlets, and the pipe runs are longer. Without circulation, waiting for hot water would take several tens of seconds and even a minute.

Why the heat pump itself does not control hot water circulation

This is where most customers get their first "shock" after installing a heat pump. They assume that a modern heat pump – with a touch screen, smart control, WiFi module – will automatically control HTW circulation as well. Reality is different: most heat pumps handle HTW circulation only as an optional function or not at all.

The reason is technical. A heat pump controls the primary circuit (coolant heating, heat exchanger, compressor), the secondary circuit (heating, possibly cooling) and the HTW tank (when to charge it). However, the HTW circulation pump is a device that operates in the potable water circuit – i.e. in a circuit that is legally and hygienically separated from the "heating" circuits. For this reason, the circulation pump is not directly connected to the heat pump's control electronics – at least not without additional accessories.

In addition, hot water circulation has its own control logic: it can run continuously (which is not energy efficient), according to a time schedule (e.g. morning 6:00–8:00, evening 18:00–22:00), or intelligently – based on demand (button at the tap, motion sensor). This logic is too specific for the heat pump to handle "out of the box" – this is exactly why specialized accessory kits exist.

Hot water circulation kit for heat pumps – what it contains and how it works

For customers using heat pumps of the ITEC-T, ATHENA-T, LEGEND, CALIBRA or ATLAS series, there is a directly adapted solution: KIT hot water circulation for these heat pumps. It is a compact kit that contains everything you need to integrate the HTW circulation function directly into the heat pump control – without the need for a separate external timer or additional programmable thermostat.

The kit typically includes a circulation pump (small, low-energy), the necessary valves, a check valve and a cable connection for the heat pump control unit. The circulation pump is connected to the hot water distribution after the tank, the return is connected back to the lower part of the tank (or to the tank's circulation input, if it has one). The control is done directly via the heat pump interface, where you set the time programs for circulation operation.

The advantage of the integrated solution over an external timer is precisely this direct contact with the regulation: the heat pump "knows" when the HTW tank is heating and can temporarily stop the circulation during tank charging, or coordinate it with the house's daily program. From an energy point of view, this represents a significant saving compared to circulation running on a timer without any intelligence.

Hydraulic layout of the circulation loop – where everything goes

Correct hydraulic layout of the circulation loop is key not only for functionality, but also for hygiene safety. Mistakes in this area can lead to the formation of Legionella, insufficient water heating, or mechanical problems. Let's go through it step by step.

Hot water outlet from the tank

Hot water leaves the tank from the top – where the water is hottest (stratification). From this outlet, the main TÚV branch runs through the house – to showers, sinks, kitchen. This branch contains the circulation pipe – either running parallel to each branch or shared at the end of the distribution.

Return pipe for circulation

The circulation pipe (return pipe) brings the cooled water from the distribution back to the tank. Important: the return pipe must be connected to the bottom part of the tank, not the top. If you connected it at the top, you would disrupt the thermal stratification of the tank and the whole system would be inefficient.

Circulation pump

The circulation pump is installed on the return pipe – that is, on the cold pipe leading back to the tank. Not on the hot water outlet. The reason is simple: the pump operates at a lower temperature, which prolongs its lifespan. In addition, it is hydraulically correct to place the pump so that it "sucks" from the distribution and "pushes" into the tank.

Check valve

A check valve must be installed on the circulation pipe – it prevents reverse water flow when the pump is not running. Without a check valve, there could be a thermosyphon effect (gravitational flow) or cold water intrusion into the hot water distribution. A check valve is a standard component in a hot water circulation set.

Pipe insulation

The circulation pipe must be thoroughly insulated – thermal insulation of at least 20–25 mm. An uninsulated circulation pipe is a pure energy loss: the tank has to be heated much more often, the heat pump works more, and electricity bills rise. In practice, we have seen cases where poor or missing insulation of the circulation pipe increased the heat pump consumption by 15–20 % compared to a properly designed system.

Setting and regulation of circulation in the heat pump control unit

Once the set is physically installed, it is time to set it up in the control unit (regulator) of the heat pump. Depending on the model and range, the parameters may be slightly different, but the principle is the same:

Circulation time program: You set the time windows during which the circulation pump runs. Recommended windows correspond to real household needs – in the morning (6:00–8:30), after lunch (11:00–13:00), in the evening (17:00–22:00).
Temperature condition: Some regulators allow you to set a minimum water temperature on the return pipe, at which it is not necessary to start the circulation. Typically 40–42 °C.
Storage tank priority: During charging of the TÚV tank by the heat pump, it is recommended to stop the circulation – this reduces thermal losses during the charging cycle and speeds up heating. Modern regulators handle this automatically.
Legionella protection: Thermal disinfection function (Legionella protect) – the tank is heated to 65–70 °C once a week. During this phase, the circulation should run to spread the disinfection to the piping distribution.

If your heat pump model does not natively support TÚV circulation control via its internal interface, the solution is an external timer switch or a programmable thermostat installed on the return pipe. This thermostat measures the return pipe temperature and starts the circulation pump whenever the temperature drops below the set value.

Relay set for heat pumps – when you need to transfer the signal

Some heat pumps – for example, from the Diplomat Optimum G3 range – do not send a classic binary (on/off) signal to the output for an external pump, but instead an analog control signal of 3–10 V. This signal is primarily intended for mixed circuit heating pumps, not for a simple TÚV circulation pump, which requires a classic on/off pulse.

In such cases, the Relay set – signal for the pump with Optimum 3–10 V function converts to on/off comes into play. This set contains a small relay that "monitors" the analog output signal from the heat pump regulator and converts it into a simple switching contact. When the regulator sends a signal (no matter how strong in the 3–10 V range), the relay closes and starts the circulation pump. When the signal disappears, the relay opens and the pump stops.

Installation of the relay set is not complicated, but it requires access to the electrical part of the heat pump, which should be done by a qualified electrician or service technician. Connecting it "blindly" just based on intuition is not a good idea – incorrect wiring can damage the control electronics of the heat pump.

Accessories for installation – other components to keep in mind

Complete integration of TÚV circulation with a heat pump requires several additional components that are easily overlooked during planning, and then remembered when the installer is on site.

Connection manifold for DUO models

Heat pumps from the CALIBRA DUO and ATLAS DUO ranges have specific requirements for hydraulic connection. For these models, a Connection manifold for TČ range CALIBRA DUO, ATLAS DUO is available, which ensures tight and properly dimensioned connection of the piping distribution to the heat pump. Using other (generic) manifolds can lead to leaks, vibrations, and in the worst case, damage to the heat pump inputs.

Self-regulating heating cable

Circulation pipe that runs through unheated areas – for example, an unheated basement, garage or technical corridor – can freeze on frosty days. The solution is Self-regulating heating cable for heat pumps ITEC – 2 m. This cable is wrapped around the pipe and automatically adjusts its output according to ambient temperature: it heats more at low temperatures and reduces output at higher ones. It is an elegant solution that does not require any additional regulation – the cable handles it on its own.

Expansion module

If your ITEC heat pump does not have enough outputs to control all devices (including the circulation pump), you can add an Expansion module for ITEC. This module adds additional control outputs and allows the heat pump to control TÚV circulation, electric water heater and other peripherals. For more information on when and why you need an expansion module, see the separate article Expansion module for heat pumps – when and why you need it in this Knowledge Center.

Energy balance and real savings – figures from practice

One of the most common questions is: "Is circulation economically justified?" The answer is not a simple yes/no – it depends on the specific situation. Let's take a closer look.

Energy consumption of the circulation pump: Modern low-energy circulation pumps of class A (for domestic TÚV distribution) consume 3–10 W of electrical energy. With 12 hours of daily operation, this is 36–120 Wh/day, i.e. 13–44 kWh/year. At an electricity price of 0.15–0.20 €/kWh, this is 2–9 € annually for the pump operation alone.

Heat losses from circulation: This is where the bigger cost potential lies. Circulating hot water in pipes causes heat losses – the water cools down in the pipes and the tank must compensate for this loss. With well-insulated pipes (25 mm insulation, 15 m circulation loop length), the losses are about 30–60 W at an average water temperature of 55 °C and ambient temperature of 20 °C. Over a year of operation (12 h/day), this amounts to 130–260 kWh. At a heat pump COP of 3.5, this means a real electrical input of 37–74 kWh per year, i.e. 6–15 € annual costs to cover the heat losses of the circulation.

Water savings: On the other hand, water savings are significant – without circulation, we wait 20–60 seconds for hot water and waste 3–10 liters of cold water. With 6 people and 3 uses per day, this is 54–180 liters per day, i.e. 20–65 m³ per year. At a water price including sewerage of about 3–4 €/m³, this is 60–260 € per year, which circulation saves.

Conclusion from the figures: Although circulation has energy costs, with properly insulated pipes and intelligent control (not 24/7, but only as needed), it pays off in most households in terms of total costs (water + energy).

Legionella protection during TÚV circulation in a system with a heat pump

This is a topic that is underestimated in Slovakia, but is very important for health. The bacteria Legionella pneumophila multiplies optimally in drinking water at temperatures of 25–45 °C. Heat pumps, due to efficiency, usually heat the TÚV tank to 45–55 °C – which is a value at which Legionella can still occur in the pipe distribution, if the water stands in it for a long time or circulates at an insufficient temperature.

Properly set circulation combined with the tank's thermal disinfection function (Legionella program) solves this problem comprehensively:

The tank is heated to 65–70 °C once a week (by an electric coil or external source)
During this disinfection, the circulation pump runs intensively and distributes hot water throughout the entire distribution system
Disinfection lasts at least 30 minutes – the temperature in the return pipe must be above 60 °C throughout
After disinfection, the tank returns to normal operating temperature

Warning: at an operating temperature of 45–50 °C, there is a risk of Legionella especially in long, rarely used branches of the distribution system. Therefore, it is important to design the circulation pipe so that it serves all relevant branches, not just the main route.

Typical mistakes from practice and how to avoid them

After years of customer experience, we can compile a list of recurring mistakes when installing TÚV circulation in combination with a heat pump:

Connecting the return pipe to the top of the tank: Destroys stratification, reduces the efficiency of the entire system. The return pipe belongs at the bottom.
Missing check valve: The thermosiphon effect can pull cold water into the hot distribution system even when the pump is stationary.
Uninsulated circulation pipe: As mentioned, heat losses are huge. Every meter of uninsulated pipe costs several extra euros per year.
Circulation running 24/7: Although it works technically, it is economically disadvantageous. Circulation should run according to a time schedule or based on the return pipe temperature condition.
Too small a pump: The circulation pump must be dimensioned for the actual hydraulic conditions of the distribution system – pipe length, number of elbows, pipe diameter. A too small pump cannot maintain sufficient flow and the circulation does not fulfill its function.
Too large a pump: Generates unnecessary noise, vibrations and accelerates wear of fittings. Proper sizing is key.
Ignoring the thermal disinfection function: The health risk of Legionella cannot be underestimated. Check whether the Legionella program is activated in the heat pump controller.
Incorrect combination of accessories: For example, using a generic set instead of a set specifically designed for a particular heat pump model. The control signal may not be compatible, the pump may not start correctly or you may get error messages in the controller.

Step-by-step integration of TÚV circulation

For those planning to install or preparing for a meeting with a service technician, here is a brief overview of how the whole process works:

Design of the circulation pipe route: Mark the route from the tank to the most distant outlet and back on the floor plan. Determine the total length and number of elbows.
Selection of the circulation pump: Based on the length of the circuit and the pipe diameter, calculate the required hydraulic power (pressure head and flow rate). For a family house with a circuit up to 30 m, a small class A pump with a power of 3–8 W is usually sufficient.
Physical installation of the pipe: Ensure the pipe slope for air venting, install a check valve, ball valves for service disconnection, and an air vent.
Insulation of the entire pipe: Insulate the entire circulation pipe immediately, not just the visible sections.
Electrical connection of the pump: 230 V power supply via the output from the heat pump control unit (or via a relay set if signal conversion is needed).
Programming the controller: Set the circulation time schedule, return temperature condition, tank priority, and Legionella program.
Pressure test and venting: Before commissioning, the entire TÚV system is pressurized (minimum 6 bar for a drinking water system) and vented.
Function check: Verify that the circulation pump responds correctly to the control signal, and measure the return temperature after 30 minutes of operation (should be at least 45–50 °C).

A more detailed guide on the installation procedure of accessories can be found in the article Connecting and installing heat pump accessories step by step in this Knowledge Center. If you are interested in the overall installation of the heat pump, we also recommend the article Heat pump installation – procedure, requirements, and common mistakes.

Special situations and advanced scenarios

Circulation in a house with mixed heating (heat pump + stove/boiler)

In houses where the heat pump works together with a stove with an exchanger or a condensing boiler as a backup, the control of TÚV circulation is a bit more complicated. The TÚV tank can be supplied from multiple sources, and when switching between them, the circulation control must adapt. Most modern heat pump controllers account for this situation – it is important to correctly configure the source priorities and signal the active source.

Circulation in a larger house or apartment building

In larger buildings with longer pipe runs (over 50 m total length of the circulation loop), a single pump may be insufficient. The solution is either a larger pump or a hydraulically balanced system with multiple branches. In apartment buildings with central TÚV preparation, circulation is legally required (STN EN 806-4), if the distance from the tank to the outlet is greater than a certain value.

Integration with a smart home system

Some customers want to control TÚV circulation via a smart home system (KNX, Loxone, Home Assistant). In such a case, the circulation pump can be connected via a smart relay (Shelly, Sonoff, etc.), with control running in parallel – the smart system takes over control outside of the set time programs in the heat pump controller. It is important that the smart system does not interfere with the safety functions (Legionella program, tank protection temperatures).

Most frequently asked questions (FAQ)

Do I need hot water circulation if I use a heat pump?

No, circulation is not legally required in family houses if the distance from the tank to the outlet is not extremely long. However, it is highly recommended from the perspective of comfort and water saving, especially in houses with longer TÚV pipe runs or with more than 3–4 people. The heat pump itself cannot solve circulation without an additional set of accessories.

Can I install hot water circulation later, after the heat pump is already running?

Yes, and this is a fairly common situation. Customers first install the heat pump and the TÚV tank, and only later – when they feel discomfort from waiting for hot water – proceed with the installation of circulation. Technically, the additional installation is possible without problems, but you should expect that the technician will have to interfere with the existing TÚV piping and the electrical part of the heat pump when connecting the control line.

What should be the water temperature in the circulation loop?

The minimum temperature of hot water at the outlets should be 55 °C (according to hygiene standards for Legionella prevention). The return temperature of the circulation loop should not drop below 50 °C. If the return temperature drops below this value, either the pipe insulation is insufficient or the TÚV tank is not heated to a sufficient temperature. The heat pump should have the TÚV tank set to at least 55–60 °C.

How long does the return on investment for a circulation set take?

For a typical household (4 people, 15–25 m piping) and a complete set with installation (including labor) in the range of 400–700 €, the return on investment from water savings is about 80–150 €/year, roughly 3–6 years. If you consider only the economics without the subjective value of comfort (instant hot water), these are realistic figures. If you take comfort value into account, the return on investment is practically immediate.

What happens if I set the circulation to 24/7 operation?

Technically, it works, but economically it is inefficient. Circulation at night (e.g., from 00:00 to 06:00), when no one uses hot water, only unnecessarily cools the pipes and forces the TÚV tank to heat more often. We recommend setting a time schedule according to the household's real needs – in the morning before waking up and in the evening before sleeping. The result is the same comfort, but significantly lower heat losses.

Can circulation damage the TÚV tank or the heat pump?

No, if the hydraulic connection is correct. Circulation in the drinking water circuit is completely separate from the heat pump's heating circuits. The TÚV tank usually has a dedicated inlet for circulation (at the lower third of the tank). If the tank does not have this inlet, the return is connected directly to the cold inlet of the tank via a three-way valve or hydraulic distributor – always clarify this detail with a designer or service technician.

Conclusion

Integrating hot water circulation with a heat pump is not rocket science, but it requires a properly planned procedure, suitable accessories, and a responsible approach to hygiene safety (Legionella). If you have a heat pump from the ITEC, ATHENA, LEGEND, CALIBRA, or ATLAS series, we recommend solving the circulation directly through accessories designed for these models – for example, the hot water circulation set, or optionally add a expansion module if the controller does not have the necessary outputs.

Properly set up TÚV circulation will bring you immediate comfort, water savings, and – if the piping is well insulated and the control is intelligent – an acceptable energy balance. In combination with a modern heat pump that centrally controls the entire system, you get a solution that really works as it should. If you are deciding on the selection of a heat pump and its power, also check out other articles in this Knowledge Center – for example, How to choose a heat pump – what to focus on before purchase or What heat pump power do I need for my house.

Do you have a question on this topic?

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