Thermolink P and hydraulic connection of the boiler with a cylinder: how it works

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Thermolink P and hydraulic connection of the boiler with the storage tank: how it works

When I meet with customers ordering a condensing boiler set with a separate domestic hot water storage tank in practice, most of them primarily focus on the boiler's output, the tank's volume, and the price. The hydraulic connection between the boiler and the tank – i.e., what physically links these two devices and ensures their mutual communication – usually doesn't cross their mind. Yet it is precisely this connection that determines whether the whole set works reliably, efficiently, and safely, or becomes a source of problems from the very first year of operation.

This article focuses specifically on the Thermolink P system from Protherm, used in sets with Gepard Condens boilers and FE series storage tanks. I will explain what Thermolink P actually is, why manufacturers include it in these sets, how the hydraulics of the whole system work, and what to watch out for during installation and operation. If you're interested in a broader perspective – for example, a comparison of Protherm vs. Vaillant sets or choosing the right tank volume – you'll find answers in other articles in our Knowledge Center.

What is Thermolink P and what is it for

Thermolink P is a hydraulic connection assembly – essentially a compact hydraulic module that physically and functionally connects the Protherm Gepard Condens wall-mounted condensing boiler with external domestic hot water tanks, specifically from the FE (Flat Energy) series. It's not just a piece of pipe with valves. It's a well-designed component that integrates several functions at once:

Hydraulic connection – supplies and returns water between the boiler's primary circuit and the tank's secondary circuit
Three- or four-way diverter valve – switches the boiler between heating the heating system and preparing domestic hot water (DHW)
Circulation pump for the DHW circuit – ensures active water circulation between the boiler and the tank during heating
Electrical connection – links the boiler with the tank and ensures communication between the boiler's control unit and the tank's temperature sensor
Compact placement – the whole assembly is mounted directly under the boiler, making installation clear and space-saving

The key word here is integration. An alternative to Thermolink P would be to address each of these functions separately – a four-way valve on its own, a pump on its own, a connection on its own. That would work too, but it would be significantly more labor-intensive to install, more prone to errors, and more complicated to service.

Hydraulic principle: how switching between DHW and heating works

A condensing boiler in a set with a storage tank always has two basic tasks to handle: heating the building and heating domestic hot water. The problem is that these two tasks are mutually exclusive – the boiler cannot fully heat both the tank and the heating system at the same time. It has to switch between them.

In systems without a storage tank (a boiler with an instantaneous DHW head), this isn't an issue – hot water is heated immediately, directly in the boiler. But in a set with a separate tank, we need a mechanism that determines when the boiler works for the tank and when for the radiators.

Thermolink P provides this mechanism via a diverter valve. The principle works like this:

The tank contains a temperature sensor (NTC sensor). When the water temperature drops below the set value (typically 45–55 °C, depending on the setting), the sensor sends a signal to the boiler's control unit.
The boiler's control unit activates the diverter valve in Thermolink P – the valve redirects the water flow from the heating circuit to the tank circuit.
At the same time, the circulation pump of the tank circuit (also part of Thermolink P) starts – the pump circulates water between the boiler and the tank's heat exchanger.
The boiler starts heating water for the tank, typically at an outlet temperature of 60–70 °C.
Once the tank reaches the required temperature, the sensor detects it, the control unit closes the tank circuit, and switches the boiler back to heating.

The entire heating cycle of the FE 120 BM tank with the Gepard Condens 25 MKO boiler takes about 25–35 minutes with cold water (15 °C) and a target temperature of 55 °C. With hot standby water (tank maintained at 45 °C, heated up to 55 °C), it's only 8–15 minutes.

Why the FE tank (bottom connection) and not another type

Protherm FE (Flat Energy) series storage tanks are designed specifically to work with Gepard and Panther Condens boilers. Their design has several characteristics important for understanding the whole hydraulic system:

Flat shape – compared to classic cylindrical tanks, FE tanks have a flatter profile, which allows them to be placed in a narrower space (e.g., in a utility cabinet or built-in furniture). For example, the FE 120 BM model is 1,230 mm high and only 590 mm wide.

Bottom connection – the variant labeled "bottom" has its hydraulic connections routed at the bottom, allowing direct connection to Thermolink P without long pipe runs. Both the primary circuit (boiler – heat exchanger) and the secondary circuit (tank – DHW draw-off) are accessible from the lower part of the tank.

Built-in heat exchanger – the FE tank has an internal coiled heat exchanger (tube-type or plate-type, depending on the model), through which the boiler heats the stored water without direct mixing. Primary water (from the boiler) flows through the exchanger, while secondary water (potable) is heated indirectly. This is important both from a hygiene standpoint and for corrosion protection of the boiler.

It is precisely for the FE 120 BM tanks combined with the Gepard Condens 12 MKO and 25 MKO boilers that the Thermolink P system was designed. Sets like Protherm Gepard Condens 12 MKO + FE 120 BM bottom + Thermolink P or Protherm Gepard Condens 25 MKO + FE 120 BM bottom + Thermolink P are designed as functional units – all three components work together as one system.

Hydraulic diagram: what flows where and why

To properly understand how Thermolink P functions, it's useful to trace the flow of water through the whole system. We distinguish three hydraulic circuits:

Boiler's primary circuit (heating water)

This is a closed circuit with treated (inhibited) water that flows only inside the boiler and the heating system. The boiler heats the water to the required temperature (e.g., 50–75 °C for heating, or 60–70 °C for tank heating), and the pump drives it into the system. When heating the tank, Thermolink P redirects part of this flow into the tank's heat exchanger.

Tank's secondary circuit (water in the heat exchanger)

This circuit is physically connected to the primary circuit via Thermolink P – hydraulically speaking, it's an extension of the primary circuit. Water from the boiler enters the tank's heat exchanger, transfers heat to the tank water, and returns cooled back to the boiler. This circuit remains closed; it never mixes with potable water.

Potable water circuit (tank water for consumption)

This is the cold water circuit from the water main, which enters the tank from the bottom, is heated indirectly through the heat exchanger, and rises as hot water for taps. This circuit is not closed – it is under mains pressure (typically 2–5 bar) and water leaves it as it is drawn off.

Thermal priority of the tank: what it means and when it shows

In the control logic of the Gepard Condens boiler, the tank is set with what's called thermal priority. This means that when the tank signals a need for heating, the boiler prioritizes it over space heating. In practice, this looks like this:

Imagine a winter night, -10 °C outside, the boiler continuously heating. At 6:30 in the morning, someone in the household takes a shower – the tank empties and cools down. The sensor in the tank detects a temperature drop below 45 °C and immediately sends a heating request. Regardless of the current heating demand, the boiler switches its full output to the tank.

This principle has the advantage that the tank heats up quickly and at the boiler's full output. The downside? During tank heating (which takes 10–35 minutes), the radiators receive no heat. In a normally insulated house with a properly sized heating system, this is negligible for a short interruption – the thermal inertia of the building and radiators covers this gap without any noticeable drop in room temperature.

However, if the tank were heated too often (for example, due to an incorrectly set hysteresis, excessive tank heat losses, or an unnecessarily high DHW target temperature), unnecessary switching could occur, reducing heating efficiency. In such a case, it's advisable to check the tank's insulation and the boiler's hysteresis setting (standard 5–10 K).

Installation of Thermolink P: practical experience from installations

From dozens of installations we have carried out with customers, I can say that Thermolink P is one of the less problematic components – but only if the installation is properly prepared. Here are the issues that come up in practice:

Position and space under the boiler

Thermolink P is mounted directly under the Gepard Condens boiler – this is its natural position according to Protherm's design documentation. This assumes there is enough space under the boiler: at least 30–35 cm from the bottom edge of the boiler to the first obstacle (shelf, floor, other equipment). If the boiler is installed too low (which happens during renovations when the boiler is mounted on existing brackets), installing Thermolink P may be problematic or it may need to be offset using a side connection kit.

Distance from the tank

The FE tank is usually placed next to the boiler – i.e., at a distance of 0.5–1.5 m. Thermolink P's connections are designed for short pipe runs (DN 22 or DN 28, copper or press-fit). If the tank is further away (for example, in a different room), you need to account for greater heat losses in the connecting pipework and proper pipe insulation.

In one project, we dealt with a situation where the tank had to be in a bathroom 4 m away from the boiler in the utility room. We insulated the pipework with 25 mm thick Armaflex, which was fully sufficient. Without insulation, such a distance would result in unnecessary heat losses and longer heating times.

Electrical connection

Thermolink P includes an electrical connector that plugs into the boiler (usually a BUS connector or direct terminals, depending on the boiler model). It also includes a cable for the tank's temperature sensor. This sensor is placed in the sensor pocket on the FE tank (located in the lower third of the tank, corresponding to the position of the cold inlet zone). Incorrect sensor placement (for example, too high) would cause the tank to overheat, because the sensor reads the temperature of the upper hot layer and the boiler assumes the tank is full of hot water even though the lower part is cold.

Filling and venting

After installation, the primary circuit (boiler + Thermolink P + tank heat exchanger) needs to be properly vented. Thermolink P includes an automatic air vent, but during the first filling, it is advisable to manually go through all the venting valves in the system. Air in the tank circuit would cause pump noise and reduce heat transfer efficiency.

Comparison with the Vaillant solution: different philosophy, same goal

Customers often ask us why Vaillant ecoTEC pro sets don't have an equivalent to Thermolink P. The answer is simple – Vaillant handles it differently. Vaillant ecoTEC pro boilers have a built-in three-way diverter valve directly in the boiler body. This means the boiler itself can switch between heating and tank heating without needing an external hydraulic module.

Sets like Vaillant VU 246/5-3 ecoTEC pro + VIH R 120 B or Vaillant VU 246/5-3 ecoTEC pro + VIH R 150 B are therefore hydraulically simpler – the tank connects directly to the boiler's outlets without an intermediate component. The tank circuit pump is also integrated into the boiler.

In contrast, the Protherm + Thermolink P solution is more modular – the Gepard Condens in its basic version is a boiler for heating only (or with an instantaneous DHW head), and Thermolink P is an add-on that extends it with full cooperation with a separate tank. This offers greater flexibility in tank selection and easier replacement of individual components in case of failure.

You'll find a more detailed comparison of both systems in the article Vaillant ecoTEC pro vs. Protherm Gepard and Panther Condens: comparison of sets in our Knowledge Center.

Settings and control: what the user can influence

Most customers think that after installation all settings are fixed. In fact, the user (or the service technician during commissioning) can adjust several parameters according to the specific needs of the household:

Required tank temperature

Typically set to 55–60 °C. This temperature is hygienically safe (eliminating Legionella bacteria requires at least 60 °C at least once a week) and energy efficient. A higher temperature (65–70 °C) extends the time without needing to reheat, but increases the tank's heat losses. A lower temperature (45–50 °C) saves energy but may pose a hygienic risk during long standby periods. Most Gepard Condens boilers allow setting within the range of 40–65 °C directly from the control panel.

Hysteresis (switching range)

Hysteresis determines how many degrees the tank temperature must drop below the set value before heating starts. With a setting of 55 °C and a hysteresis of 5 K, heating starts when the temperature drops to 50 °C. A larger hysteresis (8–10 K) means less frequent but longer heating cycles. A smaller hysteresis (2–3 K) means frequent, short cycles. A value of 5 K is recommended as a compromise.

Tank priority

Some boiler models allow you to set whether the tank has absolute or parallel priority. With absolute priority, the boiler completely interrupts heating while heating the tank. With parallel priority, the output is split – the tank receives, for example, 70% of the boiler's output, with the rest going to heating. This setting is more for a service technician than for a regular user.

Legionella function

The Gepard Condens boiler with Thermolink P has a built-in automatic thermal disinfection cycle (the so-called Legionella heating function). Once a week (usually at night) the tank is heated to 70 °C and held at that temperature for 30 minutes. The function is activated in the boiler's settings – for any tank larger than 100 liters, its activation is recommended.

Typical problems and their causes: what happens in practice

Over years of working with customers, a fairly consistent list of problems has emerged in sets with Thermolink P. Most of them have a simple solution if diagnosed correctly.

The tank doesn't heat up, or heats up slowly: The most common cause is a stuck or jammed diverter valve in Thermolink P. Another possibility is a faulty or disconnected tank temperature sensor. Less commonly: a clogged heat exchanger (mainly with hard water and no inhibitor).

The boiler switches between the tank and heating too often: Either a small hysteresis, or poor tank insulation (the tank cools down quickly). Sometimes the cause is also unsuitable sensor placement.

Noise when the tank pump runs: Air in the tank circuit. The entire primary circuit, including the tank's heat exchanger, needs to be vented.

Water leak from Thermolink P: Most often from the diverter valve seals after intensive or long-term operation. Thermolink P has seals that can be replaced by a service technician without needing to replace the entire module.

You'll find more on troubleshooting and diagnostics in the article Common faults of a condensing boiler and tank set and how to solve them.

Regular maintenance: what applies to Thermolink P as well

As a hydraulic component, Thermolink P requires minimal but regular attention during the boiler's annual service:

Leak-tightness check – visual inspection of all joints and seals, testing for any moisture traces
Diverter valve function check – the service technician verifies through a manual test in the boiler's diagnostic mode whether the valve switches correctly
Inspection and cleaning of the tank circuit pump – the circulation pump can tend to seize after a summer break (when the tank hasn't been heated for several months); some versions of Thermolink P have a pump with automatic anti-seizing start-up
Primary circuit pressure check – same as for the boiler, the recommended static pressure is 1.2–1.5 bar
Inhibitor check in the primary circuit – chemical test of the system water quality (pH, hardness, inhibitor content)

Detailed procedures and frequency of these tasks are described in the article Maintenance and servicing of a boiler and tank set: what and how often to check.

When a set with Thermolink P makes sense

To conclude this section, let's summarize who the Gepard Condens + FE 120 BM + Thermolink P set is the right choice for:

It's a suitable solution for households with 1–4 people, where a 120-liter tank volume is sufficient (if you're not sure, see the article What tank volume do I need: calculation based on number of people and bathrooms). The Gepard Condens 12 MKO boiler with an output of 3.4–12 kW is ideal for apartments and smaller family houses with a heat loss of up to 10–11 kW. The 25 MKO model with an output of 6.5–25 kW will cover larger houses up to 180–200 m².

The Thermolink P system makes sense wherever you want a modular solution – where the boiler and tank can be physically separated (different rooms, tank in another location), where you want the option of replacing the tank with a larger one in the future without replacing the whole boiler, or where you're loyal to the Protherm brand and want to be sure of full component compatibility.

Frequently Asked Questions (FAQ)

Can I connect a tank from a different brand (not FE from Protherm) via Thermolink P?

Technically yes, it's hydraulically possible – Thermolink P is a standard hydraulic module with common DN 22/28 connections. However, electrical compatibility of the temperature sensor could be a problem: the Gepard Condens boiler is designed for NTC sensors with a 10kΩ characteristic at 25 °C (Protherm/Bosch standard). A tank from another brand may have a different sensor or a different sensor pocket location, which would require modification. We recommend combining only components designed for the given set – for reasons of warranty and correct functioning of the control logic.

What happens if Thermolink P fails? Will heating still work?

Yes – if Thermolink P fails in the "tank" position, the boiler cannot heat the building. If it fails in the "heating" position, the tank won't heat up. Most Thermolink P failures are either electrical (sensor, connector) or mechanical (stuck valve). Both conditions will show up as an error code on the boiler's display. Heating can usually be maintained by manually setting the valve to an emergency position (directly bypassing the tank) – this procedure should be part of the installer's service documentation.

Is the FE 120 BM tank sufficient for a 4-member family with two bathrooms?

For a 4-member household with two bathrooms, 120 liters is at the borderline of sufficiency – it depends on consumption habits. If the family showers in both bathrooms simultaneously and also uses a bathtub, we recommend considering a 150-liter tank. If everyone showers one after another (which is the more common case), 120 liters is fully sufficient – the boiler reheats the tank to full capacity in 15–25 minutes. Detailed calculations can be found in the article What tank volume do I need: calculation based on number of people and bathrooms.

Why is the "bottom" version of the FE tank specified in the set?

The FE 120 BM "bottom" tank denotes the variant with hydraulic connections located at the bottom of the tank. This is necessary for cooperation with Thermolink P, which is mounted under the boiler and has a short connection to the tank. There is also a variant with side connections – that would require different pipe routing and is not primarily intended for this set.

Can I also connect a solar collector to the Gepard Condens + Thermolink P set?

The FE 120 BM tank is also available in versions with two heat exchangers – the lower exchanger for solar collectors, the upper one for the boiler. In this case, Thermolink P and the boiler would work the same way – supplementing the heating whenever solar power isn't enough. This solution is energy-efficient but requires solar control and proper tank sizing (for solar connection, a volume of at least 200 liters is recommended). The standard FE 120 BM + Thermolink P set without a solar heat exchanger is not intended for solar use.

How long does the installation of the boiler + Thermolink P + tank set take?

For an experienced installer, the complete installation (removing old equipment, installing the boiler, tank, and Thermolink P, hydraulic connection, electrical wiring, initial start-up and setup) usually takes 6–8 hours under simple conditions. If it's a renovation with complex piping or requiring masonry work, it can take 1–2 days. You'll find more about preparation and the installation process in the article Installing a boiler and tank set: procedure, requirements, and what to prepare in advance.

Conclusion: Thermolink P as a well-designed system member

Thermolink P is not just "an extra piece of pipe." It's a well-designed hydraulic module that allows the Gepard Condens boiler to work efficiently and safely in a set with a separate domestic hot water tank. It integrates a diverter valve, a tank circuit circulation pump, and an electrical connection – all compactly, reliably, and with minimal space requirements under the boiler.

If you're considering a set with the Gepard Condens 12 MKO boiler, take a look at Protherm Gepard Condens 12 MKO + FE 120 BM bottom + Thermolink P – a complete set including Thermolink P for smaller houses and apartments. For larger houses, the Protherm Gepard Condens 25 MKO + FE 120 BM bottom + Thermolink P with a higher output of 25 kW is more suitable.

If you prefer the Vaillant solution with an internal three-way valve in the boiler, check out, for example, the Vaillant VU 246/5-3 ecoTEC pro + VIH Q75 B set for a smaller tank, or other variants within the category. Each approach has its advantages – what matters is that the whole set is designed as a functional unit, properly sized for the specific household, and professionally installed.

Do you have a question on this topic?

Can't decide, or dealing with a specific situation in your household? Write to us – we'll be happy to advise.