Frequently Asked Questions About Condensing Boiler and Hot Water Tank Sets
Frequently Asked Questions about condensing boiler with hot water tank sets
Sets of condensing boilers with domestic hot water tanks are today one of the most frequently discussed topics when choosing a heating system for family houses, two-generation buildings, and smaller apartment buildings. Nevertheless, we repeatedly encounter the same questions from customers – sometimes technical, sometimes purely practical, sometimes related to installation or legislation. We compiled this article directly from practice: we summarized what customers ask before purchase, during selection, and even after installation. This is not a marketing overview – these are real questions from real orders, which we answer factually and technically accurately.
What actually is a boiler and tank set, and why address it as a whole?
A condensing boiler with hot water tank set is not just a "boiler plus tank". It is a hydraulically and control-wise interconnected system, where the type of hydraulic connection, tank size, boiler output, control method, and the order in which these components work all matter. When a customer buys the boiler and tank separately and lets the installer put it together "as they see fit," the result may work – but not necessarily optimally. It may happen that the tank heats slowly, the boiler is unnecessarily overloaded, or the control prioritizes heating the tank at a time when it's not actually needed.
A set as a whole means that the boiler and tank are designed and tested to work together. Manufacturers such as Vaillant or Protherm supply such sets including a hydraulic connection kit, pre-set controls, and in many cases even the intended assembled arrangement. This saves the installer time, reduces the risk of errors, and gives the customer confidence that the system works as it was designed to.
How does tank heating work in a system with a condensing boiler?
This is a question asked by every other customer – and most think the tank is simply "a container of water that the boiler heats." The reality is somewhat more complex.
A condensing boiler in a set with a tank operates in two modes: heating mode and hot water preparation mode. Most boilers in such a set do not have their own tank inside (i.e., it's not a combi storage boiler) – it's a flow-through boiler that heats the tank externally via a heat exchanger or its own connection. The tank has a coil heat exchanger (hose or tube type) inside, through which heating water from the boiler passes and transfers heat to the domestic water in the tank.
The boiler thus heats the heating water circuit, which passes through the exchanger in the tank. The controls (e.g., eRELAX, multiMATIC, or Thermolink P) control the three-way valve or tank pump to ensure the tank is preferentially heated to the set temperature (usually 55–65 °C). During tank heating, space heating is briefly interrupted – this usually lasts 15–30 minutes, and most modern boilers manage this without a noticeable drop in indoor temperature.
What boiler output do I need for a set with a tank?
This is one of the most important, yet often underestimated, questions. Customers sometimes assume that the larger the tank, the higher the boiler output needed. This isn't always true – it depends on several factors.
For a typical family house with a heating demand of around 10–15 kW and a 120–150 liter tank, a boiler with an output of 20–25 kW is sufficient. The output must cover both the heating demand and tank heating simultaneously. Since modern boilers operate in a modulated mode, they can temporarily work at a higher output (e.g., 24 kW) when heating the tank and heat it quickly, then return to lower output for space heating.
Practical example: A house with a heat loss of 9 kW, 4 people, 1 bathroom + shower. A 150-liter tank, 24 kW boiler – this is a typical, proven solution. When heating the tank (in the morning, before leaving for work), the boiler switches to full output, heats the tank from 20 °C to 60 °C in about 20–25 minutes, then continues maintaining the heating system temperature. Conversely, oversizing the boiler to 35 kW for the same system leads to so-called cycling – the boiler switches on and off too often, which reduces its lifespan and results in higher gas consumption than necessary.
Sets such as the Vaillant VU 246/5-3 ecoTEC pro + VIH R 150 B combine a 24 kW boiler with a 150-liter tank – one of the most common solutions for a 4-6 person household in a new build with a well-insulated envelope. If the house is an older solid-brick building, we usually recommend a higher output level and adjust the tank size to the number of bathrooms and occupants.
75, 120, or 150 liter tank – how to choose?
Tank volume depends on the number of people in the household, the number of bathrooms (i.e., draw-off points), usage pattern (shower vs. bath), and whether consumption is evenly spread throughout the day or concentrated into morning and evening peaks.
A practical rule of thumb: count 30–40 liters of hot water per person per day for showering, while a bath requires 80–120 liters per fill. The tank should have enough capacity to cover the household's morning draw-off without intermediate reheating – which is why for 4 people with one bath, we recommend a tank of at least 150 liters.
The Vaillant VU 246/5-3 ecoTEC pro + VIH Q 75 B set with a 75-liter tank is a solution for a smaller household – typically an apartment or small house with 1–2 people where showering predominates. Conversely, the Vaillant VU 246/5-3 ecoTEC pro + VIH R 120 B with a 120-liter tank is suitable for a three-person household or a couple with a bath.
You can find a detailed guide to calculating tank volume in a separate article "What tank volume do I need: calculation based on number of people and bathrooms" in this Knowledge Center.
Can I connect any tank to an existing condensing boiler?
Technically yes – it is hydraulically possible to connect almost any tank with a heat exchanger to almost any boiler. In practice, however, it's not that simple, and we always recommend checking several parameters.
First, the tank's exchanger must have sufficient heat exchange area for the boiler's output. Cheap tanks often have a small exchanger, which slows down heating with a 24 kW boiler and forces the boiler to operate at a higher temperature, reducing condensing efficiency. Second, the tank should have the correct connection size for the hydraulic set used with the boiler. Third, controls – modern Vaillant or Protherm boilers only work with the controls properly when the tank has an eBUS/eRELAX/Thermolink compatible sensor or NTC probe in the right location.
A typical practical problem: a customer has an existing Protherm Gepard Condens boiler and wants to connect a "secondhand" tank from a different manufacturer. The tank has a different connection size, the exchanger is designed for 15 kW, and the boiler is 25 kW. The result: the tank heats up more slowly than it should, the boiler cycles, and the customer thinks the boiler is broken. The solution would be a set such as the Protherm Gepard Condens 25 MKO + FE 120 BM lower + Thermolink P, where everything is designed and tested to work together.
What is Thermolink P and why does Protherm use it in sets?
Thermolink P is a hydraulic connection set that serves as a manifold between the boiler and the tank. It is not just a "connector" – it includes a vent, shut-off valves, a check valve, and in many versions also connections for a weather compensation sensor and communication with the boiler's controls.
In the Protherm Gepard Condens 12 MKO + FE 120 BM lower + Thermolink P set, Thermolink P is precisely the key element that allows the boiler's controls to properly evaluate the tank temperature and prioritize its heating. Without the Thermolink, a separate tank control would be needed, which complicates the installation and increases costs. A detailed explanation of the Thermolink's function can be found in the article "Thermolink P and the hydraulic connection between boiler and tank: how it works" in this Knowledge Center.
Is a boiler + tank set better than a combi boiler?
This is a question that deserves a longer answer, because both systems have their merits – it depends on the specific case.
A combi boiler (or "combination boiler") has the tank and flow-through heating directly integrated into the boiler body. It's compact, cheaper to install, and doesn't take up much space. The disadvantage is that flow-through heating or a small internal tank (5–15 liters) has limited capacity for simultaneous multiple draw-offs. If two people turn on hot water at the same time, or someone runs a bath, the capacity may be insufficient.
A boiler + tank set (external, 75–200+ liters) has greater capacity, more stable water temperature during draw-off, and better efficiency – the tank serves as storage. The system is more resistant to consumption peaks, and the tank is heated during low-tariff periods or off-peak times. The disadvantage is a higher price, greater space requirements, and a more complicated installation.
For a family household of 4 or more people, with a bath or two bathrooms, a set with an external tank is almost always the better choice. For a studio flat or a small apartment with 1–2 people and a shower, a combi boiler may be entirely sufficient. A detailed comparison can be found in the article "Boiler with tank or flow-through water heater: which pays off more" in this Knowledge Center.
Where should I place the tank in the house – next to the boiler or elsewhere?
The tank should be as close to the boiler as possible. Every meter of pipe between the boiler and the tank means heat losses and slower heating. The ideal distance is up to 2–3 meters of pipe (not distance through air – pipe length). At greater distances, the pipework needs better insulation, and in some cases it's advisable to add a hot water circulation pipe.
An upright tank is usually placed next to the boiler or below it (for wall-mounted boilers). Larger tanks (150+ liters) are almost always upright, and their placement needs to be addressed during boiler room design. Wall-mounted tanks (e.g., VIH Q 75 B) are installed directly under the boiler, saving floor space – ideal for smaller boiler rooms or utility rooms in apartments.
The tank should not be placed in an unheated space (unheated garage, uninsulated basement) – it would unnecessarily lose heat, and in winter there would be a risk of the cold water freezing in cases of prolonged standing. If the tank is in a cold environment, expect higher energy consumption to keep it at temperature.
How long does hot water in the tank last without reheating?
This depends on the quality of the tank's insulation, ambient temperature, and the set tank temperature. Modern tanks with 50–80 mm insulation typically lose 1–3 °C per hour at an ambient temperature of 20 °C. This means that a 150-liter tank heated to 60 °C will cool to about 45–50 °C over 10 hours (overnight) – still a sufficient temperature for comfortable use.
In practice, most controls set up so-called maintenance reheating – the boiler switches on once every 4–6 hours (or when the temperature drops below the set lower limit) and reheats the tank. This happens automatically, and the customer usually doesn't even notice it. Energy consumption for maintaining tank temperature can be reduced by setting a night setback – the controls allow the tank to drop to a lower temperature at night and schedule a morning reheat before the typical morning draw-off.
Does the tank need to be disinfected, and why?
Yes, and while this question is asked less often, it's important from a hygiene standpoint. Hot water standing in the tank at a temperature of 35–45 °C is an ideal environment for the growth of Legionella pneumophila bacteria – the cause of legionellosis. The risk is real, not hypothetical.
Prevention is simple: the tank should be regularly (recommended once a week, at least once every two weeks) heated to 60–65 °C for at least one hour. Most modern controls have a "thermal disinfection" or "anti-Legionella heating" function – automatically overheating the tank once per set interval. This function needs to be activated in the control settings, which the installer usually does during commissioning.
Important: stainless steel tanks are somewhat more prone to biofilm buildup than enamelled tanks, but the same recommendation for thermal disinfection applies in both cases. If the tank is out of use for a longer period (holidays, holiday home), always overheat the water before using it again.
What if the tank doesn't heat water fast enough?
Slow tank heating is one of the most common complaints after installation – and in most cases, neither the tank nor the boiler is at fault, but rather the control settings or hydraulic connection.
The most common causes: the three-way valve is not set correctly or is clogged, the tank circuit pump runs at low speed, the tank has an unsuitable exchanger for the boiler's output, the boiler temperature is set too low (e.g., 55 °C, while the tank needs at least 60 °C at the exchanger inlet to reach 55 °C in the tank), or there is air in the circuit. These problems are addressed by an experienced service technician when setting up the system.
Watch out for one specific case: with weather-compensated control, the boiler may operate at a low outlet temperature during transitional seasons (spring, autumn) – e.g., 40 °C – which is not enough for efficient tank heating. In that case, the minimum boiler temperature for tank heating must be set to at least 60 °C, independent of the weather compensation curve. This is a parameter in the control settings, usually set by the installer during installation. More on typical problems can be found in the article "Common faults in condensing boiler and tank sets and their solutions".
What are the operating costs of a boiler + tank set compared to a combi boiler?
This is a question customers ask increasingly often – and the answer isn't clear-cut. Several factors affect operating costs.
A condensing boiler in a set with a tank operates more efficiently when heating the tank, because it can utilize the full condensing effect (the return temperature from the tank exchanger is typically 30–40 °C, which is ideal for condensation). With a properly designed system, operating costs are comparable to a combi boiler, sometimes 5–10% lower.
On the other hand, the tank has heat losses – it loses energy even when not in use. With a poorly insulated tank or one in a cold environment, these losses can be significant. A well-insulated tank (e.g., VIH R 120 B or FE 120 BM lower) has heat losses of up to 1–2 kWh per day at an ambient temperature of 20 °C – which is about 30–60 kWh per month, i.e., a few euros extra per month compared to a flow-through system. For the comfort and convenience of draw-off, this is an acceptable cost for most households.
Do I need any permits for installing the set?
Installing a gas condensing boiler with a tank does not require a building permit in the classic sense – however, it is a regulated activity. The boiler must be installed by a person with a valid authorization for installing gas appliances (certificate in accordance with Decree No. 508/2009 Coll.). The installation must comply with technical standards (STN EN 15420, STN 06 1008, STN 73 4301, and others).
After installation, a professional tightness test, combustion setting, and gas appliance inspection must be carried out. The result is an inspection report, which the customer receives from the installer and keeps for the insurance company or building administrator, if needed.
Important: boiler service inspections are a condition for maintaining the warranty with most manufacturers (Vaillant, Protherm) and are required once a year or once every two years, depending on the manufacturer. Detailed information can be found in the article "Maintenance and servicing of a boiler and tank set: what and how often to check".
Can I install the set in an apartment in a prefab building?
Yes, but with certain limitations. A condensing boiler in an apartment building requires a solution for flue gas discharge – either an existing shared flue (in prefab buildings usually type C, coaxial or twin-pipe flue), or individual flue routing through the perimeter wall (if the building and administrator allow it). The tank must be sized to fit the apartment's spatial possibilities – in most apartments, a realistic tank size is up to 120 liters; a 150-liter tank is hard to fit in most bathrooms or utility rooms in prefab buildings.
Before installation, you need to resolve: consent from the building administrator or owners' association, assessment of the flue's condition, possible flue replacement, and the gas supply capacity in the building's riser.
Frequently Asked Questions (FAQ)
Can I control the boiler + tank set via phone?
Yes, most modern Vaillant and Protherm sets support internet control. The multiMATIC or eRELAX (Vaillant) controls, or Thermolink P combined with an internet module, allow remote setting of heating temperature, tank temperature, heating time schedule, and monitoring boiler status via a mobile app. Some sets (e.g., with multiMATIC 450) have internet connectivity included directly in the controller, others require an additional WiFi module purchased separately.
How long does installation of a boiler and tank set take?
When replacing an old boiler and tank with a new set in an existing boiler room, expect 1–2 working days. For a new build or complete renovation without existing pipework, installation may take 2–4 days. It depends on whether new gas, water pipework, a change to the flue, or electrical wiring for the controls needs to be addressed. After installation, the service technician spends another hour setting up the boiler, controls, and tank. The whole process from first contact to handover to the customer for a standard order takes 1–3 weeks (including material delivery time and installer's schedule).
Is the tank included in the boiler's warranty?
The tank and boiler each have their own separate warranty, even though we sell and supply them as a set. Vaillant ecoTEC pro boilers have a standard 2-year warranty; with manufacturer registration and regular service inspections, the warranty can be extended up to 5 years. Tanks usually have a 2-year warranty on functionality and 5–25 years on the tightness of the vessel (depending on the manufacturer and material). Enamelled tanks generally have a longer warranty on the vessel than stainless steel tanks under normal use.
What happens when the tank corrodes?
Most tanks have a built-in anode (a magnesium rod or a titanium active anode) that protects the inner surface of the tank from corrosion. The magnesium anode needs to be checked and replaced every 2–4 years (depending on local water hardness). If the anode is neglected and the tank corrodes, repair is not possible – the tank must be replaced. Therefore, regular anode inspection is part of every annual service check. In areas with very hard water (over 30 °dH), consider softening the incoming water, which will extend the life of both the tank and the boiler's exchanger.
Can I combine the tank with solar panels?
Yes, and this is one of the most efficient combinations for households with a suitable roof orientation. A solar tank usually has two exchangers – a lower one for the solar circuit, an upper one for the boiler. The boiler only reheats the tank when the sun isn't enough. With a well-designed system, a solar source can cover 50–70% of annual energy consumption for hot water preparation. However, this combination requires a tank with a solar exchanger (a so-called bivalent tank) and appropriate controls. Standard tanks such as VIH R 120 B or FE 120 BM lower are not designed for solar combination – a bivalent tank is required. Both manufacturers have a separate product line for this.
What to do if the tank doesn't heat water even after an hour of boiler operation?
First step: check whether the boiler is actually heating the tank or just heating the space (the three-way valve must switch to the tank position). If the boiler is running but the tank isn't heating, the most common causes are: a stuck or faulty three-way valve, air in the tank circuit (venting needed), a faulty tank circuit pump, or incorrectly set controls (tank priority not activated). Another group of causes can be a clogged tank exchanger (limescale) – this happens with hard water after several years without regular maintenance. The solution is a service inspection and possibly chemical descaling of the exchanger.
Conclusion: A condensing boiler with tank set as a system solution
A condensing boiler with hot water tank set is currently the optimal choice for most families in terms of comfort, energy efficiency, and operating costs. The key to satisfaction is not just choosing the right boiler and tank, but their properly designed combination, hydraulic connection, and control settings. That's why it makes sense to buy proven sets from reputable manufacturers – such as the Vaillant VU 246/5-3 ecoTEC pro + VIH R 150 B with multiMATIC 700 controls or the Protherm Gepard Condens 25 MKO + FE 120 BM lower + Thermolink P – where the manufacturer guarantees compatibility and the installer works with proven parameters.
If you need help choosing the right set for your specific case, we recommend starting with the articles "How to choose a condensing boiler with hot water tank set" and "Upright vs. wall-mounted tank with a condensing boiler: differences and advantages" in this Knowledge Center. And before placing your order, be sure to check out the article "Installing a boiler and tank set: process, requirements, and what to prepare in advance", which will help you get ready for installation without surprises.
Have a question on this topic?
Can't decide or dealing with a specific situation in your household? Write to us - we're happy to help.