Condensing boiler with instantaneous DHW heating vs. tank-based DHW heating: which is better
Condensing Boiler with Instantaneous Hot Water Heating vs. Storage Tank Heating: Which Is Really Better?
When a customer comes into the store and says "I want a new gas boiler with hot water," the same question almost always follows: should the boiler have its own storage tank, or is instantaneous heating enough? This dilemma troubles households every time they replace their heat source, and the answer is not simple – it depends on family size, apartment or house layout, hot water usage habits, budget, and available space. In this article, I'll break down both solutions in depth, from the perspective of a technician who has seen hundreds of installations for customers with different needs and housing situations.
Before we get into the comparison, it's important to clarify the terminology. A condensing boiler with instantaneous DHW (domestic hot water) heating is a so-called combination boiler (combi) – it heats water directly the moment you turn on the tap. A storage system can take two forms: either a boiler with an integrated tank (a so-called boiler with storage), or a standalone gas or condensing boiler without instantaneous heating, to which an external tank (an indirectly heated cylinder) is connected as a separate element of the system. In this category of solutions, we'll focus mainly on comparing instantaneous DHW heating in a condensing boiler versus DHW storage tank heating.
How Instantaneous DHW Heating Works in a Condensing Boiler
The principle of instantaneous heating in a condensing boiler is relatively straightforward: the boiler has two heat exchanger circuits. The primary circuit serves heating (water circulates in radiators or underfloor heating), while the secondary circuit heats potable water. When you turn on the hot water tap, the boiler pauses heating, switches the three-way valve to DHW heating mode, and immediately starts heating the water passing through a plate or coil heat exchanger.
Modern condensing boilers such as the Vaillant VUW 236/5-3 ecoTEC pro or the Protherm Gepard Condens 18/25 MKV achieve a DHW heating output of 25 – 28 kW, which corresponds to a hot water flow rate of roughly 11 – 14 liters per minute at a temperature rise of 30 °C (i.e., from cold water to 45 – 50 °C). This is a value worth keeping in mind at all times – it represents the real capacity of the system in everyday use.
A key difference compared to a storage system is that water is not stored anywhere – it's heated instantly on demand. This has major advantages in terms of water hygiene (no stagnation, no risk of Legionella) and energy efficiency (no heat losses from a tank). On the other hand, capacity is limited by the boiler's output and the flow rate of water supplied from the mains.
How DHW Storage Tank Heating Works
A storage system works on a different principle: the boiler (or another heat source – heat pump, solar collectors) heats a predetermined amount of water in an insulated container. The tank can range in volume from 80 to 500 liters or more, with 150 – 200 liter tanks being most commonly used for an average family. The water is heated to a set temperature (usually 55 – 60 °C, for hygienic reasons) and kept in the tank ready for immediate use.
The tank can be vertical or horizontal, placed under the boiler, next to it, or in a completely different location. The boiler typically heats the tank once or twice a day (depending on settings and consumption), and throughout the day the stored thermal energy is gradually used up. When the tank cools below the set temperature, the boiler automatically reheats it.
A storage system can deliver a much larger hot water flow rate at peak demand (limited only by pipe diameter and mains pressure), because it draws from the accumulated energy reserve. The limitation, however, is the total tank volume – once the water is used up, you have to wait for the boiler to reheat the tank, which can take 30 – 60 minutes depending on the boiler's output and tank size.
Main Comparison: Instantaneous vs. Storage – 7 Key Criteria
1. Comfort When Drawing Hot Water
This is an area where storage systems have long dominated. A large tank ensures that even a family of five can shower and fill a bathtub in the morning without any drop in performance. Instantaneous heating is limited by the boiler's output – if the boiler delivers 25 kW of thermal output for DHW, and the difference between inlet and outlet water temperature is 30 °C, the flow rate will be roughly 11.9 l/min. A shower typically needs 7 – 9 l/min, so a single point of use is fine. The problem arises with simultaneous draw at multiple points: if someone is showering while someone else is washing dishes, the flow is split and the temperature may drop.
Modern boilers such as the Vaillant VUW 26CS/1-5 ecoTEC plus IoniDetect have a DHW output of up to 26 kW and a sophisticated control system that regulates output according to current demand. For an apartment or a smaller family house with one bathroom, instantaneous heating is fully sufficient. For a larger house with two bathrooms and a high likelihood of simultaneous use, a storage tank may be more advantageous.
2. Energy Efficiency and Operating Costs
A condensing boiler with instantaneous heating has one major advantage in terms of energy efficiency: no ongoing heat losses from a storage tank. A tank, no matter how well insulated, loses a certain amount of heat to its surroundings over 24 hours – for a typical 150-liter tank, this can be 1 – 3 kWh per day, which amounts to 365 – 1,095 kWh of wasted energy per year.
On the other hand, a tank allows for night-rate electricity (for electric tanks) or off-peak charging from a cheaper tariff, or integration with a solar system, where the tank functions as an energy buffer. A condensing boiler without a tank does not allow this – it only heats water when you actually need it.
For a typical 4-person household in an apartment, the difference in annual DHW heating costs between a well-set-up condensing boiler with instantaneous heating and a storage system ranges from 50 – 200 EUR per year in favor of the instantaneous solution (for a purely gas-fired heat source).
3. Space Requirements and Installation
This is an area where an instantaneous condensing boiler clearly wins. A combi boiler is a compact device that usually fits on a wall in the kitchen or utility room, occupying an area of just 40 × 60 cm. A storage system requires additional space for the tank – a horizontal 150-liter tank measures roughly 55 × 110 cm, while a vertical 150-liter tank takes up a diameter of 50 – 55 cm and a height of 100 – 120 cm. In an apartment where every square meter counts, a tank can be a real problem.
In family houses with a utility room or boiler room, space constraints usually aren't a major issue. Here, the tank can be placed, for example, next to the boiler or under a staircase.
4. Hot Water Hygiene
This is an area where instantaneous heating has a significant advantage, yet it's rarely considered when buying a boiler. Storage heating carries the risk of Legionella pneumophila bacteria growth, which multiplies best at temperatures of 25 – 45 °C in stagnant water. That's why tanks must be periodically heated to at least 60 °C (so-called thermal disinfection), which requires either manual activation or an automatic function in the boiler.
Condensing boilers with instantaneous heating simply don't have this problem – water is not stored anywhere, it passes through the heat exchanger and goes directly to the tap. Within seconds, it's heated in the exchanger to 45 – 55 °C, an environment unsuitable for Legionella. From a hygiene standpoint, instantaneous heating is safer, which is why experts prefer it especially in apartment buildings.
5. Purchase Price and Installation Costs
A combination condensing boiler with instantaneous DHW heating typically costs between 700 – 1,500 EUR, depending on output and manufacturer. A storage system is more expensive because, in addition to the boiler (without DHW function), you need a tank (an extra 400 – 900 EUR), plus more installation work – the tank needs to be connected, a circulation pump installed, and piping extended. The total cost of a storage system, including labor, can be 800 – 1,500 EUR higher than an instantaneous solution.
On the other hand, a tank can be more economical in the long run if used in combination with solar collectors or charged during a night-time electricity tariff (in a hybrid system). In that case, the higher investment can pay off within 5 – 8 years.
6. Servicing, Lifespan, and Reliability
A combi condensing boiler has more moving parts within a single unit (three-way valve, DHW plate heat exchanger, DHW NTC temperature sensor), and the DHW plate heat exchanger is fairly prone to limescale buildup in hard water areas. This is actually one of the most common reasons for service calls – hard water (above 20 °F) can clog a plate heat exchanger within 3 – 5 years if there's no water softener or scale-reducing device installed.
A storage system has a different advantage: the boiler doesn't need to perform DHW heating in real time, so it's less stressed during peak demand. The tank itself is a simple stainless steel or enameled vessel with a sacrificial (magnesium) anode, which needs to be replaced every 2 – 3 years. With proper maintenance, a tank's lifespan is 15 – 25 years.
7. Flexibility and Combination with Solar Systems
A tank is practically essential when integrating solar collectors. The solar circuit heats the tank via a lower heat exchanger, and the boiler serves only as a backup source – topping up the tank when the sun isn't enough. The combination of a gas condensing boiler + solar collectors + tank is one of the most efficient solutions available today, saving 70 – 80% on DHW costs in summer.
An instantaneous condensing boiler doesn't cooperate directly with a solar system – without a tank, there's nowhere to store solar heat. For anyone interested in solar heating, a storage system is therefore the only sensible choice.
Who Is Instantaneous DHW Heating Ideal For?
From my experience, instantaneous heating is the right choice for:
- Apartments and smaller family houses up to 150 m² with one bathroom – space is limited, simultaneous water draw is minimal, and the comfort of instantaneous heating is fully sufficient
- Households of 1 – 4 people with typical hot water needs – shower, sink, kitchen faucet
- New-build apartments, where a tank simply won't fit due to layout constraints
- Replacement of old boilers in apartment buildings, where there was never a tank and space doesn't allow for one
- Customers who prioritize water hygiene and don't want to deal with periodic thermal disinfection of a tank
- Customers with a limited budget – an instantaneous boiler is cheaper to buy and install
A specific example from practice: a customer in a 3+1 panel-building apartment in Bratislava had an old Junkers gas boiler from 2001 with instantaneous heating. We replaced it with a Protherm Puma Condens 18/24 MKV – a compact condensing boiler with a 24 kW DHW output. The customer was surprised by how fast the boiler responds and how stable the hot water is. The boiler fit in the same spot as the old one, without any structural modifications. The annual savings compared to the old non-condensing boiler were about 180 EUR with the same level of comfort.
Who Is a DHW Storage Tank Better Suited For?
A storage system has its irreplaceable place in the following situations:
- Larger family houses with two or more bathrooms, where simultaneous water draw is a common occurrence
- Households of 5 or more people, where a real morning peak in demand occurs (everyone showers before work)
- Houses with a bathtub – filling a bathtub with 150 – 180 liters of hot water with an instantaneous boiler takes longer and may be inadequate at lower water pressure
- Properties with weak mains water pressure – an instantaneous boiler is sensitive to minimum pressure (usually 0.15 – 0.3 bar); a tank eliminates this issue
- Systems with hot water circulation – in larger houses with long DHW piping runs, circulation is installed (water in the pipe circulates continuously, so hot water flows immediately when the tap is opened). A tank is an ideal source for circulation
- Combination with solar collectors – as mentioned above, a tank is essential
- Cottages and recreational properties with irregular but intense demand – a weekend cottage where 8 people arrive on Friday evening and everyone wants a shower
An example from practice: a customer had a new-build 200 m² family house, husband + wife + 3 children. We chose a storage system – a condensing boiler for heating plus a 200-liter tank. In the morning, five people take turns in the bathroom, each for 10 – 15 minutes. A 200-liter tank is enough for four showers in this scenario without running out – the fifth person has to wait about 20 minutes for the boiler to catch up. With an instantaneous boiler, five consecutive showers (even taken in turns) would be no problem at all, since each person receives full output one after another. The customer ultimately chose the tank because they were planning to install solar collectors the following year.
Specific Models from Practice: What Has Proven Reliable
In the category of condensing boilers with instantaneous DHW heating, the Protherm Panther Condens 20/26 KKV has long been one of the most popular models. This boiler is especially popular among customers who need higher heating output (up to 20 kW) combined with a DHW heating output of 26 kW. Its advantage is a modulating burner that smoothly regulates output from 5 to 20 kW, significantly increasing condensing efficiency and saving gas. The Panther Condens is durable, has a proven hydraulic block, and shows good reliability in practice even after years of operation.
For customers looking for a more premium solution with advanced diagnostics and a long manufacturer's warranty, the Vaillant ecoTEC line is a suitable choice. Vaillant boilers have a long-standing excellent reputation for reliability and condensing efficiency.
If you're considering a specific model and can't decide, we also recommend reading our other articles in this section – such as Protherm vs. Vaillant Condensing Boilers with DHW: Comparison of Models and Parameters or How to Choose a Condensing Boiler with Instantaneous DHW Heating: What to Focus On, where you'll find a detailed comparison of specific model parameters.
Technical Parameters to Focus On When Choosing
Regardless of whether you choose instantaneous or storage heating, when choosing a condensing boiler with instantaneous DHW heating, pay attention to these key parameters:
- DHW thermal output [kW] – the higher, the greater the hot water flow rate. For a family house with one bathroom, 24 – 26 kW is sufficient; for a larger house or two bathrooms, 28 – 32 kW
- Specific flow rate D [l/min] – a standardized value according to EN 625, indicating the flow rate when heating water from 10 °C to 45 °C. Values of 10 – 14 l/min are common; boilers with a D value above 14 l/min are above average
- Minimum inlet water pressure [bar] – especially important in old apartment buildings with low water pressure. A typical value is 0.15 – 0.3 bar, though some boilers require at least 1 bar
- Type of DHW heat exchanger – a stainless steel plate heat exchanger is efficient but prone to limescale. A coil heat exchanger (spiral, copper, or stainless steel) is more durable but less efficient. In hard water areas above 18 °F, we recommend installing a scale reducer or water softener
- Burner modulation range – a boiler that can regulate output from 20% to 100% is much more efficient than one with fixed output. Most modern condensing boilers have modulation, but the range varies
- Efficiency class / seasonal efficiency (SCOP, ηs,h) – natural gas condensing boilers, when properly set up, achieve seasonal efficiencies of 90 – 98% (based on Hs, i.e., gross calorific value)
For proper boiler sizing, read our article What Output of Condensing Boiler with DHW Do I Need for My House, where you'll find calculation methods and practical tables.
Installation: What to Prepare
Installation of a condensing boiler with instantaneous DHW heating must always be carried out by an authorized person – a qualified installer with valid certification for gas appliance installation. Before installation, it's necessary to verify:
- The diameter and condition of the existing gas line and gas pressure (the boiler requires a minimum supply pressure of 20 mbar for natural gas G20)
- The flue gas discharge method – condensing boilers require a coaxial or twin-pipe flue system, and condensate must be discharged into the sewage system via a neutralization tank
- Inlet water pressure and hardness – it's recommended to measure the pressure with the tap open and check the water hardness with the local water utility
- Existing heating pipework – old steel radiators combined with a condensing boiler work fine, but it's advisable to set a lower temperature gradient (e.g., 55/45 °C instead of 75/60 °C) to achieve condensing mode
You can read more about the entire installation process in the article Installing a Condensing Boiler with Instantaneous DHW Heating: Procedure and Requirements.
Frequently Asked Questions (FAQ)
Is instantaneous DHW heating in a condensing boiler really instant, or do you have to wait?
It's not completely instant – it usually takes 10 – 30 seconds between opening the tap and hot water arriving. This isn't a boiler fault, but rather the length of the pipe between the boiler and the outlet (the water in the pipe is cold and has to be pushed out by the hot water coming from the boiler). In larger houses with long DHW pipe runs, this is solved by installing a DHW circulation pump, which keeps the water in the pipes warm. The instantaneous boiler itself reacts practically immediately – the burner ignites within 2 – 5 seconds of opening the tap. You can read more on this topic in the article Instantaneous DHW Heating in a Condensing Boiler: How It Works and What Affects Comfort.
Can I add a tank to an existing condensing boiler with instantaneous heating later?
It depends on the boiler model. Some condensing boilers with instantaneous heating have an input for an external tank (a so-called priority input) – in that case, adding a tank is easy. Other boilers are designed exclusively for instantaneous heating, and a tank cannot be connected. Always check the technical documentation before purchase to see whether a specific model allows connection of an external tank.
What is the gas consumption for instantaneous heating vs. a storage system?
For the same amount of hot water used, gas consumption is comparable; however, a storage system has additional heat losses from the tank – approximately 1 – 3 kWh/day, which over a 365-day season amounts to an extra 365 – 1,095 kWh/year. At a gas price of 0.07 EUR/kWh (approximate 2024 price), this represents an extra 25 – 77 EUR per year to the disadvantage of the storage system. In practice, this also depends on the quality of the tank's insulation (energy efficiency class A – G for tanks). A well-insulated Class A tank loses significantly less than a cheaper Class C or D tank.
Is instantaneous heating suitable for a large bathtub?
For a small bathtub with a volume of 80 – 100 liters, instantaneous heating is fully sufficient – filling takes about 8 – 10 minutes. For a large freestanding bathtub with a volume of 180 – 250 liters, filling can take 15 – 25 minutes, which may be inconvenient for some. It also depends on water pressure – at low water pressure, the flow into the tub is slower. If you're planning a larger bathtub and fast filling matters to you, a 200-liter tank is the safer choice.
What is the lifespan of a DHW plate heat exchanger in a combi boiler?
With soft water (up to 10 °F) and regular servicing, a DHW plate heat exchanger can function without problems for 10 – 15 years. With hard water (above 20 °F) and no water treatment, limescale buildup can occur within as little as 3 – 5 years. Prevention is simple: installing a magnetic filter, a scale reducer, or a full water softener. Replacing a plate heat exchanger typically costs 150 – 300 EUR for the part plus labor. Regular annual boiler servicing can catch early signs of scaling in time.
Which manufacturer is better: Protherm or Vaillant?
Both manufacturers are part of the same Vaillant Group, so they share some technologies. In practice, both are reliable, and service is readily available in Slovakia. Protherm is a traditional brand with a longer history on the Slovak market, while Vaillant is perceived as a premium brand with more advanced diagnostics and a broader network of service partners. You'll find a more detailed comparison in the article Protherm vs. Vaillant Condensing Boilers with DHW: Comparison of Models and Parameters.
Conclusion: Which Solution to Choose?
After reading this article, it should be clear that there's no universally "better" solution – there's a solution that's better for a specific situation. For most apartments and smaller family houses with one bathroom and up to 4 people, instantaneous DHW heating in a condensing boiler is the optimal choice: it's cheaper to buy and install, more energy-efficient, more hygienic, and space-saving. For larger houses, households with multiple bathrooms, planned solar integration, or intensive simultaneous water use, a storage system is a sensible investment that pays off in the long run.
If you're still not sure what's right for your specific situation, browse the entire category of condensing boilers with instantaneous DHW heating and compare the available models – or read other articles in our Knowledge Center, where you'll find answers to most technical questions before and after purchase, including Setting Temperature and Pressure in a Condensing Boiler with DHW for Optimal Savings and Maintenance and Servicing of a Condensing Boiler with DHW: What to Check and How Often.
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.