Instantaneous DHW Heating vs. Storage Tank: Pros and Cons for a Low-Temperature Boiler
Flow-through DHW Heating vs. Storage Tank Boiler: Which Is Better for a Low-Temperature Boiler?
When a customer decides between a combi boiler with flow-through domestic hot water (DHW) heating and a classic boiler with an external storage tank, it's rarely just a simple choice. In practice, people usually face this decision when confronted with a specific situation: a sealed basement with no room for a tank, a family house with four bathrooms and three people showering in the morning, or a panel-building apartment that had a tank before and now wants a replacement. Each of these cases has a different correct solution.
This article addresses exactly that question - what is more practical, economical, and comfortable in the context of low-temperature boilers. It examines both systems in depth, comparing them in terms of performance, energy efficiency, space requirements, and operating costs. And it shows where each solution has its place.
Basic Principle: How Both Systems Work
Flow-through DHW Heating in a Combi Boiler
A combi boiler - one that provides both heating and domestic hot water preparation in a single device - heats DHW on the principle of a flow-through heat exchanger. The moment you open a tap, the boiler "senses" a drop in pressure in the cold water circuit and automatically switches priority to DHW heating. Cold water from the mains passes through the heat exchanger, where it is heated to the required temperature, and flows directly to your tap. The boiler temporarily stops or limits heat supply to the heating circuit during this process.
This principle has one key technical characteristic: water is heated continuously, in real time, without accumulation. At the moment of water draw-off, the boiler must immediately deliver full output to the DHW exchanger. That's why combi boilers typically have a higher rated output than pure heating boilers - for example, the Vaillant atmoTEC pro VUW SK 200/3-3 has an output of 20 kW, while the heating demand of a typical family house is 8-14 kW. This "surplus" output is designed specifically for continuous DHW heating.
Storage Tank as a Hot Water Accumulator
A system with a storage tank works on the opposite principle: the boiler heats a supply of water in an insulated vessel (tank), typically with a volume of 80-300 liters. This heated water is available instantly when drawn, without waiting. The boiler heats the supply continuously - either according to a time program, or reacting to a drop in temperature in the tank (tank thermostat). With a correctly sized system, DHW draw-off itself occurs without direct involvement of the boiler - it is drawing energy from an accumulator.
A storage tank can be either directly integrated into the boiler (a so-called combi boiler with a tank, or "with a mini tank" of 8-15 l), or it can be a separate external tank connected to a standard heating boiler via a secondary circuit. The second configuration is typical for larger installations.
Comparison Table: Flow-through Heating vs. Storage Tank
| Criterion | Flow-through heating (combi boiler) | Storage tank |
|---|---|---|
| Space requirements | Minimal - just the boiler on the wall | Requires space for a tank (80-300 l) |
| Comfort with simultaneous draw-off | Limited output for 2+ draw-off points at once | High - depends on tank volume |
| Waiting for hot water | Practically none (once stabilized) | Minimal (depends on piping) |
| Standby losses | None (water is heated only on draw-off) | Constant heat losses (1-3 kWh/day depending on insulation) |
| Purchase price | Lower (one device) | Higher (boiler + tank + installation) |
| Legionella risk | Practically zero | Present - requires regular heating to 70 °C |
| Suitability for a low-energy house | Good | Very good (lower losses with a correctly sized tank) |
| Service demands | Lower - fewer system components | Higher - anode, tank, check valves |
Energy Efficiency: Where Flow-through Heating Excels, Where It Falls Short
In terms of pure energy efficiency of heating the water itself, flow-through heating is slightly more efficient than a storage tank system, because it eliminates so-called standby losses. A tank, even when well insulated, constantly loses heat to its surroundings. A typical 150-liter tank with standard thermal insulation loses roughly 1.5-2.5 kWh over 24 hours. At a gas price of €0.08-0.10/kWh, that amounts to €44-90 per year just in standby losses. For tanks with poorer insulation, or located in an unheated room, losses can be double that.
On the other hand, a combi boiler with flow-through DHW heating has its own specific disadvantage: every time the boiler switches on for DHW heating, so-called start-up thermal losses occur - the boiler heats up, combustion losses begin during the ramp-up to full output, and if the draw-off is short (e.g. quickly washing hands), the overall output is used inefficiently. This is relevant mainly for very frequent short draw-offs - typically in businesses or large families with children.
Hot Water Output: Where Flow-through Heating Hits Its Limits
This is an area where a storage tank system clearly dominates - with simultaneous draw-off from multiple points at once. A combi boiler with flow-through DHW heating can heat only a limited amount of water at any given moment, determined by its rated DHW output. For reference: a 20 kW boiler can heat water by about 30 °C at a flow rate of roughly 10 liters per minute. That's enough for one shower or one sink without any problems.
But if a shower and a washbasin run at the same time, or a bathroom and a kitchen, the total flow rate increases and the temperature drops. The boiler simply cannot heat a larger volume of water to the same temperature. This is a concrete disadvantage that customers with larger families feel every day. For example, in a house with four people where two bathrooms are active simultaneously in the morning, a flow-through system can really be stretched thin.
A storage tank system doesn't have this problem: if the tank is filled and pre-heated, you can draw water from the shower and the kitchen at the same time - the tank handles it without a temperature drop, until it starts emptying below a certain level. For such households, a 150-200 liter tank is the more comfortable solution.
Practical Example: A Family House with 4 People
From practice: a customer in Trenčín had a three-story family house, 4 people, two bathrooms. Originally they had a 24 kW combi boiler - in the morning when the husband went to the downstairs bathroom and the daughter to the upstairs one, the water in both places was lukewarm. After switching to a system with an external 200-liter tank, the problem was solved - the tank was charged overnight and in the morning handled both bathrooms without any limitation. Such cases are fairly common in larger households and are important to consider when choosing a boiler.
Low-Temperature Boiler and Flow-through DHW Heating: Technical Context
Low-temperature boilers work with a return heating water temperature typically below 60 °C (in most operating conditions even 40-55 °C). This is their characteristic feature - they work efficiently at lower heating medium temperatures, which is advantageous for underfloor heating and modern radiators with a larger surface area.
With flow-through DHW heating, however, this brings one important technical challenge: domestic hot water should have an outlet temperature of at least 45-50 °C (ideally 55-60 °C) for hygienic reasons and comfort. To reach this temperature during flow-through heating, the boiler needs to briefly increase its output and operating temperature - typically to 60-70 °C. This momentary temperature increase is designed directly into the firmware of combi boilers and happens automatically with every DHW draw-off.
That's why modern low-temperature combi boilers, such as the Vaillant turboTEC pro VUW SK 202/3-3 or the Vaillant atmoTEC pro VUW SK 240/3-3, feature a dual-circuit design with DHW priority - the boiler's electronics intelligently control when and to what extent the temperature is raised for DHW heating, while minimizing energy losses and protecting the heat exchanger from thermal shocks.
Space Requirements and Installation Demands
A combi boiler with flow-through DHW heating is substantially simpler to install. The entire unit hangs on a single wall, takes up minimal space, and requires no additional equipment. This is one of the main reasons why combi boilers are so popular in panel-building apartments and smaller houses or apartments - there's simply nowhere to put a tank. And when a bathroom or kitchen is being renovated in an apartment, customers usually don't want to lose another square meter to a tank.
A storage tank system, on the other hand, requires more space. An external 150-liter tank has dimensions of roughly 500 × 1100 mm (diameter × height). A 200-liter tank is typically 550 × 1250 mm. These dimensions have to be placed somewhere - usually in a utility room, boiler room, or basement. If the house doesn't have such a room, or it's small, this can be a problem.
Moreover, the tank must be connected via a primary circuit with the boiler, meaning additional piping, a tank circuit pump, a three-way valve, and controls. All of this increases both the installation cost and later servicing demands.
Hygienic Aspect: Legionella Risk and Sanitary Minimum
This is a topic that must not be overlooked with storage tank systems. The bacterium Legionella pneumophila multiplies in stored water at temperatures of 25-45 °C. With correctly set tank operation (constant temperature above 55 °C, or a weekly thermal shock to 70 °C), the risk is minimal, but if the tank operates at lower temperatures or is in standby mode without draw-off for a long time, the risk is real.
With flow-through heating in a combi boiler, this problem is practically zero - water doesn't accumulate anywhere, it passes through the exchanger immediately and is hygienically safer. For family houses this is a marginal factor, but for buildings with an intermittent operating cycle (cottages, cabins, recreational properties), a flow-through system is significantly safer from a hygienic standpoint.
Real-World Scenarios: When We Recommend What
Scenario 1: Apartment in a Panel Building, 2-3 People, One Bathroom
This is a classic case for a combi boiler with flow-through DHW heating. Space is limited, the family is small, and DHW draw-off occurs gradually (not simultaneously). A 20-24 kW combi boiler handles this without any problems. Customers usually get used to setting the desired DHW temperature on the boiler (typically 45-50 °C), and the system works for years without issues. For example, the Vaillant atmoTEC pro VUW SK 200/3-3 is ideal for this application.
Scenario 2: Family House, 4-5 People, Two Bathrooms
Here we would only recommend a combi boiler with flow-through heating with a margin - i.e. a boiler with higher DHW output (for example 28-32 kW). But even then, the question of simultaneous draw-off remains. Customers with a larger family and two bathrooms are happier with a 150-200 liter storage tank system, or a boiler connected to a tank - a less powerful (and therefore cheaper to run) boiler is sufficient for DHW heating there, and the tank covers morning peaks.
Scenario 3: Renovation of an Older House with a Boiler Room
A house from the 1980s, a boiler room with space, an original 120-liter tank, transitioning to gas. Here the customer usually wants to keep the comfort of a tank but modernize the boiler. The solution is a modern low-temperature boiler for heating plus an external tank. A combi boiler is not ideal here - if the house is larger (for example a heated area of 180 m²), a 150-200 liter tank and a correctly sized boiler give a better overall result.
Scenario 4: New Extension or New Build with Underfloor Heating
A low-energy house, underfloor heating, low temperature demand. Here flow-through heating in a combi boiler is very suitable - heating runs at 35-45 °C, the system is compact, and a combi boiler is an ideal choice. A tank would add unnecessary complications and losses here.
Impact on Boiler Choice: Combi vs. Pure Heating
If you choose flow-through DHW heating, you're automatically heading toward a so-called combi boiler (VUW - Vaillant Unite Wärme, i.e. combined). These boilers have a built-in secondary DHW exchanger, a three-way valve, a flow sensor, and corresponding control software. The DHW output of such a boiler is typically 18-28 kW, while the heating output is somewhat lower (modulating, typically 8-20 kW).
If you choose a storage tank system, you can use a pure heating boiler (without a built-in DHW exchanger) plus an external tank - or a combi boiler with a tank-charging function. Vaillant offers a separate line of heating boilers and tanks for this case (e.g. VUW boilers with a connected tank). A combi boiler with an output such as the Vaillant turboTEC pro VUW SK 242/3-3 (24 kW) can also be connected with an external tank as a priority system - where DHW is heated using the boiler's exchanger and the tank serves as an accumulator.
You can find more about how to choose the right boiler output in the article What Boiler Output with DHW Heating Do I Need for My House, which also includes specific calculation formulas. And if you're deciding between an atmospheric and a turbo type, we also recommend the article Atmospheric vs. Turbo Boiler with DHW Preparation: Which Type Suits You Better.
Long-Term Operating Costs
Let's compare a specific situation for a 4-member family, DHW consumption of about 150 liters at 50 °C daily, inlet water temperature 10 °C:
Energy needed to heat DHW per day: Q = m × c × ΔT = 150 × 4186 × 40 = about 6.96 kWh/day → ~2,540 kWh/year. With a boiler efficiency of 88-92% (low-temperature gas boiler) and a gas price of €0.09/kWh, this works out to about €248-260/year just for DHW heating.
To this, for a storage tank system, standby losses must be added: for a well-insulated tank, about 1.8 kWh/day = 657 kWh/year = an additional ~€66/year. That's a real difference which, over 10 years of operation, represents €660. For an old tank with poorer insulation, it could be as much as €1,200 over 10 years.
On the other hand, the purchase price of a tank (80-200 liter tank + installation) ranges from €600 to €1,800. So the payback isn't always clear-cut and depends on the specific conditions.
Service and Maintenance: What to Watch for with Each System
A combi boiler with flow-through DHW preparation has one potentially problematic additional element: the flow-through DHW exchanger, which is in direct contact with domestic water and can become clogged with limescale. In areas with hard water (hardness above 14 °dH), the exchanger clogs faster, DHW draw-off pressure drops, and in extreme cases a fault can occur. We therefore recommend regular inspection (every 2 years) in hard water areas and cleaning the exchanger if necessary.
A storage tank system requires different care: the sacrificial anode (magnesium or titanium) protects the tank's inner shell from corrosion and must be regularly checked and replaced - typically every 2-3 years. The tank's safety valve, thermometer, and any sediment at the bottom should also be checked. You can find more details on maintenance in the article Maintenance and Servicing of Combi Low-Temperature Boilers: What and How Often.
Special Topic: Vaillant atmoTEC pro vs. turboTEC pro for Flow-through DHW Heating
In the category of low-temperature combi boilers with flow-through DHW heating, the Vaillant atmoTEC and turboTEC series are among the most popular devices on the Slovak market. Their main difference lies in the type of flue gas discharge and combustion air supply.
atmoTEC pro is an atmospheric boiler - it takes combustion air from the room, and flue gases are discharged through a chimney or shaft. This means the boiler must be installed in a room with sufficient ventilation and must have access to a chimney or shaft. The advantage is simpler and cheaper installation, the disadvantage being dependence on the condition of the chimney and the need for ventilation openings. A typical representative is the Vaillant atmoTEC plus VUW CZ/SK 240/3-5, which belongs to a higher line with extended controls.
turboTEC pro is a pressurized (turbo) boiler - it draws air from outside via a coaxial pipe and discharges flue gases along the same route, but in the opposite direction. It doesn't depend on a chimney and can be installed anywhere with access to a facade or roof. For apartment buildings without chimneys and for new builds, turboTEC is the preferred choice.
You can find more on comparing models in the article Vaillant atmoTEC pro vs. atmoTEC plus: What's the Difference and Which to Choose.
Frequently Asked Questions (FAQ)
Can a combi boiler with flow-through DHW preparation supply two bathrooms simultaneously?
It depends on the boiler output and the draw-off requirements. A boiler with 24 kW of DHW output can heat water at a total flow rate of about 11-12 l/min by 30 °C. A shower consumes roughly 6-8 l/min, a washbasin 4-5 l/min. With simultaneous draw-off from a shower and a washbasin, you're at the limit of the output. The water temperature may be 5-10 °C lower than when drawing from a single point. For comfortably supplying two full bathrooms at once, a storage tank system is more suitable.
Is flow-through heating in a combi boiler hygienically safe?
Yes, flow-through heating is hygienically safer than a storage tank system. Water doesn't accumulate in a tank, where Legionella bacteria could multiply. Water is heated directly upon draw-off, passes through the exchanger, and goes to the tap. This system is also recommended for recreational properties with intermittent operation.
How long does it take for hot water to flow from a combi boiler?
The boiler activates within 2-5 seconds of opening the tap (the flow sensor detects the draw-off). Heating the exchanger itself takes another 2-8 seconds, depending on the setting and the distance of the draw-off point from the boiler. Overall, you typically wait 10-20 seconds for hot water, with most of that time taken up by cold piping between the boiler and the tap - this is not a problem with the boiler, but with pipe length. A DHW circulation pump reduces this time to a minimum.
Is it worth connecting an external tank to a combi boiler, or is it unnecessary?
Vaillant atmoTEC and turboTEC combi boilers can technically be connected with an external tank (via a secondary circuit), where the tank is heated by the boiler on a priority basis. This solution makes sense with higher DHW consumption (4+ people, 2+ bathrooms). You lose the advantage of compactness, but gain the comfort of a tank. However, you should consider that the combi boiler has a built-in DHW exchanger that isn't used in this configuration - so you're paying for a function you don't use. For a tank system from the start, a pure heating boiler is usually more suitable.
How large a tank is needed for a family of 4?
General rule of thumb: 35-50 liters of tank capacity per person. For a 4-member family, that means 150-200 liters. If two bathrooms are active simultaneously in the morning, we recommend 200 liters. For households with flexible scheduling (people don't bathe all at once), 150 liters is sufficient. A tank smaller than 100 liters for 4 people is on the edge and relies on the boiler actively reheating the tank during draw-off - which leads to a temperature drop during long showers.
What happens to heating when a combi boiler is heating DHW?
A combi boiler has built-in DHW priority - during domestic hot water heating, it limits or completely interrupts heat supply to the heating circuit. This interruption usually lasts a few minutes (the length of a shower or dishwashing). The heating system has accumulated thermal capacity (water in radiators, underfloor heating), so a brief interruption of a few minutes has no effect at all on room temperatures. Problems only arise with very frequent and long DHW draw-off - then heating can indeed be somewhat compromised. In practice, however, this happens only rarely.
Conclusion: Flow-through DHW Heating in a Low-Temperature Combi Boiler - Who It's For and Who It Isn't
Flow-through DHW heating in a low-temperature combi boiler is a simple, compact, and energy-efficient solution for households with one bathroom and up to 3-4 people, where DHW draw-off doesn't occur simultaneously at multiple points. It's ideal for apartments, smaller houses, new builds with underfloor heating, and recreational properties with intermittent operation. It requires no additional space, has a lower purchase price, and carries no hygienic risk from water accumulation.
A storage tank system is better for larger family houses with 4+ people and two or more bathrooms, where hot water is drawn simultaneously from multiple points. It offers higher comfort, greater DHW capacity, and better resistance to peak draw-offs - at the cost of higher purchase costs, space requirements, and higher standby operating losses.
If you're deciding between specific combi boiler models, take a look at the complete category of boilers with flow-through DHW heating on Atria.sk. You can find further details on choosing correctly in the article How to Choose a Low-Temperature Boiler with Flow-through DHW Preparation: What to Watch Out For and in the overview Frequently Asked Questions about Low-Temperature Boilers with Flow-through DHW Heating.
Have a Question on This Topic?
Not sure what to decide, or dealing with a specific situation in your household? Write to us - we'll be happy to advise.