Boiler with a Storage Tank or Instantaneous Water Heater: Which Pays Off More
Boiler with Tank or Instantaneous Water Heater: Which One Really Pays Off More?
This question is one that almost every customer faces when renovating a heating system or replacing an old boiler. And it's not a simple answer, because the right choice depends on a combination of factors – the size of the household, hot water consumption, the layout of the apartment or house, the existing installation, and, last but not least, financial possibilities. In practice, I've encountered both solutions in hundreds of projects, and I have to say: each has its own logic, but each also has its limits.
In this article, we'll break down both technologies in depth – how they work, where they excel, where they hit their limits, and what the economic comparison looks like in real conditions of Slovak households. If you're looking for a quick answer, there isn't one. But if you want to know why one or the other solution suits your situation, read on.
How an instantaneous water heater works – and what that means in practice
An instantaneous water heater – in practice most often in the form of a combi condensing boiler – heats water the moment someone in the household turns on the tap. Cold water from the mains flows through a plate heat exchanger or coil, and the boiler heats it to the required temperature. At first glance, a perfect solution: no storage losses, always fresh water, no waiting for it to heat up. But the reality is more complicated.
In order for the boiler to heat water continuously to 55–60 °C, it must have sufficient output for instantaneous heating. A standard combi boiler with an output of 24–26 kW can heat water at a rate of roughly 10–14 litres per minute – and only when it doesn't also need to heat the radiators at the same time. When does the problem occur? When there is simultaneous draw-off at two points at once – for example, a shower in the bathroom and a sink in the kitchen. The temperature drops, the flow isn't sufficient, and people in the shower experience uncomfortable temperature fluctuations.
Another practical problem: at very low flow rates (e.g. washing hands), a combi boiler may not fire up at all, or may do so with a delay. The result is that for the first 10–30 seconds, cold or lukewarm water flows out while the boiler starts up and the heat exchanger warms up. This isn't just discomfort – it's also water waste, which adds up to hundreds of litres a year.
How a boiler + tank set works – and where the difference lies
A condensing boiler with a storage tank set works on a completely different principle. The boiler – in this case, a heating-only unit designed exclusively for space heating – heats the hot water tank indirectly, via a built-in heat exchanger (coil) or an external plate heat exchanger. The tank stores ready-heated water, usually with a volume of 75 to 300 litres; draw-off from the tap gradually empties it, and the boiler automatically refills it.
The advantages are clear: hot water flows from the tap practically instantly (in terms of pipe distance, not waiting for heating), the flow rate is unlimited by the boiler's output – only by the pressure in the water mains, and simultaneous draw-off at multiple points is no problem as long as the tank isn't empty. Two showers at once? No problem, if you have a 150–200 litre tank and sufficient hourly boiler output for regeneration.
When a tank-based solution clearly wins
In practice, there are several types of households where a boiler + tank set has no real alternative:
- A family house with 4 or more people: Hot water consumption is so high and regular that a combi boiler simply can't ensure comfortable draw-off. A 150–200 litre tank handles the morning peak (when everyone is washing up before school and work) without a problem. A combi boiler would struggle with capacity at the same time.
- A house or apartment with two or more bathrooms: Simultaneous draw-off from multiple points is only a matter of time. A tank solves this elegantly – flow and pressure are determined by the water mains, not the boiler's output.
- Households with a bathtub: Filling a standard bathtub requires 120–160 litres of hot water. A combi boiler with an output of 24 kW can deliver about 15 litres per minute at 40 °C (mixed with cold water) at a flow rate of 10 l/min, so filling the tub takes 8–10 minutes – during which no one else can use another tap. A 150–200 litre tank solves this almost instantly and without restriction.
- Older houses with long pipe runs: In family houses where the boiler is far from the bathroom, a tank-based set allows the tank to be installed closer to the point of use, shortening the wait for hot water and minimizing pipe losses.
- Houses with solar collectors: A tank is a basic requirement for combining with solar. Connecting a combi boiler with solar is practically impossible – at least not without a separate tank.
When an instantaneous water heater (combi boiler) is a sensible choice
It would be unfair to claim that the instantaneous solution has no place. It does – but under specific conditions:
- Small apartment (1–2 people, one bathroom): Hot water consumption is low and predictable. Simultaneous draw-off at two points is rare. A combi boiler is more cost-effective and space-saving here.
- Apartments in a block of flats with central heating: If the building has its own heat source only for hot water, a small instantaneous heater makes sense.
- Limited space: If there simply isn't room for a tank in the bathroom core or utility room, a combi boiler with continuous heating is the only option.
- A temporary solution during renovation: Sometimes the cheapest start is a new compact boiler without a tank, with the tank added in a second phase.
Economic comparison: what costs more and what saves more
Let's look at specific numbers. We'll compare a typical set for a family house with 4 people.
The purchase price of a combi condensing boiler ranges from €700–1,200, plus installation of approx. €250–400. In total, an investment of €950–1,600. A condensing boiler with a tank set (for example Vaillant VU 246/5-3 ecoTEC pro + VIH R 120 B) costs €1,400–2,200 including installation, depending on the tank size and the specific model.
At first glance, a combi boiler looks more advantageous – and it really is cheaper at the start. But let's look at operation. A condensing boiler with a tank has lower heat losses thanks to better heating regulation and the possibility of using a night tariff for hot water preparation (heating the tank during a cheaper tariff). Vaillant VIH R series tanks have excellent thermal insulation (loss of only 1.5–2.0 kWh/24 hours), so the tank retains its temperature for a long time even without reheating. For a standard four-member household, the difference in annual operating costs is €30–70 in favour of the tank solution. With an investment difference of around €600–700, this means a payback period of 8–15 years – which seems long, but the overall lifespan of the boiler is 15–20 years, and the tank lasts 15–25 years.
Standby losses – myth or reality?
One of the most common arguments against a tank is the so-called standby loss – heat that the tank radiates into its surroundings even when no one is drawing water. It's a real phenomenon, but its magnitude depends on the quality of the insulation. Modern tanks with PU foam (polyurethane insulation) have a loss of 1.5–2.5 kWh per 24 hours at a maintained temperature of 60 °C. Annually, that's 550–900 kWh, which at a gas price of approx. €0.08–0.10/kWh represents €44–90 extra per year compared to instantaneous heating.
However, this figure needs to be corrected for an important factor: a tank in a heated utility room (typically in a basement or boiler room) actually releases this lost heat into the space – which can be desirable in winter. A tank placed in an unheated space has higher losses, and these heat losses are genuinely lost.
Another factor: a tank allows better use of condensation. A heating-only boiler that heats the tank works in long cycles with a lower firing frequency – meaning fewer starts and more condensation. A combi boiler with continuous heating starts up briefly and more frequently, which reduces efficiency (not every start-up reaches full condensation).
Real-world sets: what it looks like in practice
Let's look at a few real-life scenarios I've encountered in practice:
Scenario 1: Family house, 4 people, 2 bathrooms
The customer had an old electric storage water heater (200 l) and a separate gas boiler for heating. When replacing the boiler, we proposed the Vaillant VU 246/5-3 ecoTEC pro + VIH R 150 B set with Multimatic 700 control. The 150-litre tank covers the morning needs of all 4 people (2 showers, 1 wash) with a margin. The boiler is heating-only – its entire capacity goes purely into heating, or into quick tank regeneration. Result: satisfied customer, annual savings compared to the electric water heater of approx. €350–400, higher comfort than before.
Scenario 2: Two-room apartment, 2 people, 1 bathroom
Here we recommended a combi boiler. Water consumption is low, a tank would be unnecessary and would take up expensive space in the bathroom core. A combi boiler works without issues in this situation – simultaneous draw-off at two points practically never occurs, and there's no bathtub available (only a shower). Lower investment, acceptable operation.
Scenario 3: Older family house, 2 adults + 3 children
A classic case where a combi boiler failed. The morning peak (children before school, both adults before work – all within 45 minutes) exhausted the capacity of the instantaneous heater. The solution was the Protherm Gepard Condens 25 MKO + FE 120 BM lower + Thermolink P set with a 120-litre tank. Thermolink P provides the hydraulic connection between the boiler and tank without the need for an external hot water circulation pump – a space-saving and reliable solution. A year after installation, the customer reported zero problems with water temperature even during peak times.
Scenario 4: Low-energy house with solar collectors
Here, a tank was a necessity. Solar collectors require storage volume – usually 200–300 litres. The boiler serves only as a backup heat source in winter. A combi boiler wouldn't work at all in this set.
Impact on comfort: temperature, pressure, flow
Comfort when drawing hot water is one of the criteria that is underestimated when buying and overestimated in everyday use. A tank-based solution has clear advantages here:
- Stable temperature: The tank maintains temperature throughout its volume (if properly set to heat to 60 °C), so water flows out of the tap consistently. A combi boiler can experience fluctuations with variations in inlet water pressure, changes in flow, or a clogged heat exchanger.
- Flow rate: A tank is limited only by the pressure and pipe diameter in the house, not by the boiler's output. At 3 bar and 3/4" piping, the flow from a hot tap can easily be 15–18 l/min – a combi boiler at the same flow rate would barely reach 40 °C in the hot water.
- Speed of availability: A properly positioned tank (close to the bathroom) delivers hot water within 5–15 seconds. A combi boiler must first detect flow, fire up the burners and heat the exchanger – this takes 20–40 seconds, during which cold water flows.
In this context, the Protherm Gepard Condens 12 MKO + FE 120 BM lower + Thermolink P set is also interesting, designed for smaller houses or apartments with higher comfort demands. A 12 kW boiler combined with a 120-litre tank handles a 3-member household without problems and is very reasonably priced. You can find more on how to choose the right tank size in the article What tank volume do I need: calculation based on number of people and bathrooms.
Technical lifespan and reliability of both solutions
In terms of lifespan, the situation is as follows. A condensing boiler (whether heating-only or combi) has a lifespan of 15–20 years with regular servicing. A hot water tank's lifespan depends on water quality – in areas with hard water (hardness above 20 °N), limescale can build up on the heat exchanger surface in the tank, reducing heating efficiency and shortening the exchanger's lifespan. The solution is regular inspection, or possibly installing a water softener.
A combi boiler has an internal plate heat exchanger for hot water preparation – and this is its Achilles' heel. It's sensitive to hard water, impurities in the mains, and if maintenance is neglected, it can become clogged or crack. Replacing a plate heat exchanger in a combi boiler costs €150–350 for the part plus labour. A tank with a clogged coil can be treated with chemical cleaning or anode replacement, which is a cheaper fix.
From the perspective of overall reliability of the set, splitting functions across two devices (boiler + tank) is an advantage – if one component fails, the other still works. If the tank breaks down, the boiler still heats. If the boiler has a fault, the tank still holds hot water for several hours. With a combi boiler, any fault is a problem for both functions at once.
Floor-standing or wall-mounted tank – how it affects the choice of sets
With sets, we encounter two basic types of tanks. Floor-standing tanks (such as the FE 120 BM in Protherm sets) are placed on the ground, have a larger volume and usually a larger heat exchanger – meaning faster regeneration. Wall-mounted tanks (such as the VIH Q75 B in the Vaillant VU 246/5-3 ecoTEC pro + VIH Q75 B set) are smaller, more space-saving, but the volume is limited – typically 75–100 litres. For a 3-member household with one bathroom, this may be borderline.
You can find more about the advantages and disadvantages of both types in the article Floor-standing vs. wall-mounted tank with a condensing boiler: differences and advantages.
Hygiene aspect: Legionella and hot water safety
A rarely discussed but important topic: a hot water tank must be operated correctly from a hygiene perspective. The bacterium Legionella pneumophila multiplies in stagnant hot water at temperatures of 25–50 °C. That's why the tank must be set to a minimum of 60 °C and regularly (at least once a week) heated to 70 °C – the so-called hygienic flush. Modern controllers (e.g. Multimatic 700 in the Vaillant set or Thermolink P in Protherm sets) do this automatically.
A combi boiler is theoretically safer in terms of Legionella – the water is heated continuously and doesn't stand in a tank. But in practice, the entire distribution system matters – stagnant water in long pipe branches is also a risk.
Control and smart management: an advantage of tank-based sets
Modern condensing boiler with tank sets allow for sophisticated control that isn't possible with a simple combi boiler. For example:
- Night-time tank heating: The tank is heated at night during a cheaper electricity tariff (if an electric heating element is available as a backup source) or during off-peak gas tariff hours.
- Hot water priority: The boiler temporarily pauses heating and focuses its entire output on heating the tank – quick regeneration without compromises.
- Smart control via app: Systems such as Vaillant sensoAPP or myVaillant connect allow you to set heating schedules, monitor remotely and receive notifications.
- Solar integration: The controller automatically prefers solar heating and the boiler only tops up when there's a shortfall.
Frequently Asked Questions (FAQ)
Can I add a tank to a combi boiler later without replacing the boiler?
Yes, but with reservations. If your combi boiler is prepared for connection to an external tank (most modern models allow this via an additional control module), you can add a tank later. However, a combi boiler isn't optimized for heating a tank – it doesn't have hydraulic hot water priority, and control will be less efficient than with a heating-only boiler. If you're planning to add a tank in the future, it's better to opt for a heating-only boiler as the core of the set from the start. You can find more about sets in the article How to choose a condensing boiler with hot water tank set.
How many litres of tank are enough for a 4-member family?
The recommended volume for 4 people with one bathroom is 120–150 litres. With two bathrooms or a bathtub, I recommend at least 150–200 litres. You can find the exact calculation based on habits and the number of bathrooms in the article What tank volume do I need: calculation based on number of people and bathrooms. As a rule of thumb: 40–50 litres of tank volume per person is a comfortable standard at a tank temperature of 60 °C.
Is a tank always less energy-efficient due to heat losses?
Not necessarily. Modern tanks with PU insulation (such as Vaillant VIH R or Protherm FE BM) have heat losses of 1.5–2.5 kWh/24h, representing 550–900 kWh annually. This is offset by the higher efficiency of a condensing boiler operating in long cycles, better use of low temperature levels, and the ability to optimally time the heating. The overall energy balance of a tank-based solution is comparable to or better than instantaneous heating for larger households.
What is Thermolink P and why is it used in Protherm sets?
Thermolink P is a hydraulic connection module that ensures the correct connection of a Protherm Gepard Condens boiler (MKO type) to a hot water tank without the need for an external circulation pump for the hot water circuit. It manages the priority of tank heating, switches between heating mode and hot water preparation, and hydraulically separates the circuits. It's an elegant and compact solution that simplifies installation. You can find a more detailed description of its function in the article Thermolink P and hydraulic connection of the boiler with the tank: how it works.
Is a tank-based solution worthwhile for a low-energy house with low consumption?
In a low-energy house, heating consumption is low, but hot water consumption remains the same as in a regular house – it depends on the number of occupants, not the building's thermal insulation standard. If there are more than 2 people in the house or two bathrooms, a tank-based solution makes sense even in a passive house. Moreover, low-energy houses often have solar collectors installed – and these can't be used without a tank.
How long does installation of a boiler + tank set take and what needs to be prepared in advance?
Standard installation of a set (replacing the old boiler, installing the new boiler and tank, connecting, venting, setting up the controls) takes 1–2 working days. In advance, you need to ensure: a prepared space with sufficient dimensions for the tank, gas supply and condensate drain, flue gas discharge (chimney inspection or preparation for a turbo flue), cold water near the tank, and electrical power for the boiler. You can find a detailed procedure in the article Installing a boiler and tank set: procedure, requirements and what to prepare in advance.
Conclusion: so which one really pays off more?
After everything we've discussed, the answer is clearer, though still context-dependent. If you're a household with more than 3 people, two bathrooms or a bathtub, or you're planning solar heating – a condensing boiler with tank set is the clear choice. The higher initial investment pays off through better comfort, lower operating costs and a longer system lifespan.
If you're a small apartment with one person or a couple, one bathroom and a shower – a combi boiler is cheaper, more space-saving and functionally sufficient. But even here, there are exceptions to the rule.
In any case, I recommend consulting your situation with an expert before deciding, who will take into account the layout of the space, water hardness, number of draw-off points and your habits. Sets such as Vaillant VU 246/5-3 ecoTEC pro + VIH R 120 B or Protherm Gepard Condens 25 MKO + FE 120 BM lower + Thermolink P are proven solutions for most family houses in Slovakia – but only when properly sized and correctly installed do they bring real benefits.
You can find more information on the various aspects of choosing and operating in other articles in the Knowledge Centre, for example Vaillant ecoTEC pro vs. Protherm Gepard and Panther Condens: comparison of sets, Common faults of a condensing boiler and tank set and their solutions, or Maintenance and servicing of a boiler with tank set: what and how often to check.
Do you have a question on this topic?
Can't decide, or are you dealing with a specific situation in your household? Write to us - we'll be happy to advise.