Condensing Boiler with Instantaneous Water Heating vs. Storage Tank: Difference and Selection
Condensing Boiler with Instantaneous Water Heating vs. with a Storage Tank: Difference and Choice
When a customer is deciding on a new condensing boiler, the first question is usually about output. The second question – often more important in practice – is: "Do I need a boiler with its own storage tank, or is instantaneous domestic hot water heating enough?" This exact choice determines whether your morning shower will be comfortable or stressful, whether the boiler room takes up half the basement or just a corner in the hallway, and also whether your monthly gas bill will be slightly lower or higher. In this article, we'll examine both principles in technical depth, compare them for various types of households and homes, and finally give you concrete guidance on how to choose.
How Instantaneous Domestic Hot Water Heating Works in a Condensing Boiler
A condensing boiler with instantaneous domestic hot water (DHW) heating (in short, a "combi" or dual-function boiler) heats domestic hot water directly at the moment you turn on the tap. Inside the boiler is a plate or tube heat exchanger. Cold water from the plumbing flows through it, and the burner briefly increases output to heat the water to the required temperature – usually 40 to 60 °C.
The whole process works like this: you open the hot tap, a flow sensor detects the movement of water, the control unit switches the boiler to DHW heating mode, combustion output increases (usually to the boiler's maximum), and within 5 to 15 seconds hot water is flowing. During this time, the heating circuit temporarily "waits" – the boiler can only serve one function at a time.
The consequence is direct: the boiler's output must be sufficient to heat the water flow instantly. A standard shower consumes 8 to 12 liters per minute, a kitchen sink another 4 to 6 liters. The boiler must therefore be able to raise the water temperature by 25 to 35 °C at a flow rate of 10 to 14 l/min. This requires a thermal output of 15 to 28 kW just for DHW. This is exactly why combi boilers commonly have outputs of 20 to 35 kW even in small apartments – not because of heating needs, but because of hot water.
The advantage of the instantaneous principle is compactness – the entire boiler, including electronics, pump, and heat exchangers, fits into a cabinet approximately 40 × 70 × 35 cm in size. A hot water storage tank is not needed, so there's no heat loss from a tank (so-called standby losses). An instantaneous boiler heats water only when you actually need it.
How Storage Tank Domestic Hot Water Heating Works
Boilers marked with the letter "P" (or "S" according to German terminology, or simply "heat-only boiler") are single-function – they only handle heating. Domestic hot water is heated by an external storage tank connected to this boiler. The tank typically has a volume of 80 to 500 liters and contains an indirect heat exchanger – a coiled tube through which hot water from the boiler circulates. This transfers heat to the reserve of cold water in the tank.
The principle is thus indirect heating: the boiler heats water in the heating circuit, which flows through the exchanger in the tank and heats the DHW reserve. The boiler requires additional heating time for the tank, usually in the morning and evening, or according to the temperature control settings.
An important detail: the tank heats water with priority, meaning that when the tank starts heating, the boiler temporarily interrupts space heating and directs all energy to the tank. Modern control units sophisticate this priority – for example, in summer mode the boiler only heats the tank, while in winter it alternates between the tank and radiators.
Main Differences: Table Comparison
| Parameter | Instantaneous Heating (Combi) | Storage Tank Heating |
|---|---|---|
| DHW storage tank | not needed | essential (80–500 l) |
| Installation space | small – just a wall-mounted boiler | larger – boiler + tank |
| DHW heating | instant, no reserve | reserve capacity anytime |
| Simultaneous draw-off | limited (1–2 draw-off points) | larger tank = more draw-off points |
| DHW efficiency | very high – heating only on demand | slightly lower – tank heat losses |
| Investment costs | lower – boiler only | higher – boiler + tank + installation |
| Boiler output requirements | higher (20–35 kW even for a small house) | lower – output sized for heating only |
| Legionella risk | practically zero – water is not stored | requires a regular thermal disinfection cycle |
| Combination with solar or heat pump | more difficult, not always possible | designed directly for combination |
What Is the Real Difference in Hot Water Comfort?
This is where theory and practice sometimes diverge. Technically speaking, a boiler's instantaneous heating can supply hot water to one shower and a kitchen sink without any problem – provided the boiler has adequate output. The problem arises when two showers are opened at the same time, or when someone is filling a bathtub while water is also running in the kitchen.
Let's take a concrete example from practice: a family house with five people, two bathrooms, a spacious kitchen. In the morning, between 6:30 and 8:00, demand for hot water is enormous – three people shower in quick succession. A combi boiler can handle this – but only if they shower one at a time. When two try to shower simultaneously, the flow is divided, the boiler can't keep up with heating, and the water temperature drops. This is not a malfunction – it's a physical limitation of the instantaneous principle.
A 150 to 200-liter tank, on the other hand, absorbs this morning load without hesitation. The tank has been heated since 5:00 am, the entire volume is at 55 °C, and four showers can run comfortably without anyone noticing a temperature change. Meanwhile, the boiler continues heating the tank. For families with multiple people, a storage tank is effectively the standard.
On the other hand: a two-room apartment, a married couple, one bathroom. An instantaneous combi boiler is ideal here – a tank would be unnecessary, would take up space, and add heat losses. Similarly for a cottage used only on weekends – a tank would sit unused all week, and its contents would have to be reheated from scratch each time.
Choosing Boiler Output for Instantaneous Heating: Why Combi Boilers Are "Oversized" for Heating
This is one of the most frequently misunderstood points. A customer comes in saying their house needs 12 kW for heating, but the installer fits a 24 kW boiler. The customer is confused – why such output?
The answer is simple: the output required for instantaneous DHW heating follows a completely different formula than heating output. The thermal output needed to heat the water flow is calculated using the formula:
P [kW] = flow rate [l/min] × temperature difference [°C] × 0.07
So for a flow rate of 12 l/min and heating from 10 °C to 42 °C (a difference of 32 °C), we need: 12 × 32 × 0.07 = 26.9 kW. This is the output the boiler needs to provide one comfortable shower and kitchen use at the same time.
This is exactly why combi boilers are sold with outputs of 20, 24, 28, 32 kW – not because homes need that much heating output, but because hot water demands it. This excess output relative to heating needs leads to so-called short cycling (frequent short on/off periods), which in condensing boilers is partly compensated for by a modulating burner.
With a storage tank solution, you size the boiler for heating only (e.g., 15 kW for 120 m²), which is cleaner from a control perspective and, in certain configurations, also more economical. You can find more about proper boiler output in the topic What Condensing Boiler Output Do I Need for My Home in our Knowledge Center.
Tank Heat Losses: Myths and Reality
Opponents of the storage tank solution point to heat losses from the tank – water in the tank cools down, the boiler has to reheat it, and that costs money. This is true, but it's important to put it into proper perspective.
Modern DHW tanks with quality thermal insulation (polyurethane foam 50–80 mm thick) lose only 1 to 3 kWh per day, which at a gas price of €0.06/kWh represents €0.06 to €0.18 per day, or roughly €22 to €66 per year. That's not negligible, but with a properly sized tank and quality insulation, these losses stay at the lower end.
On the other hand: a combi boiler goes through a burner start-up sequence every time the tap is opened, which is also not energy-free. With low draw-offs (washing hands, briefly turning on the tap), the boiler ignites, heats only a small amount of water, and shuts off – with a relatively low overall efficiency for that specific event. The sum of these small losses can, in some households, be comparable to tank losses.
From an energy perspective, neither principle is dramatically better – what matters is choosing the right one for the specific consumption profile. Remember this as a basic rule.
Specific Products for Both Approaches
For those looking for a combi solution (instantaneous DHW heating), a typical example is the wall-mounted condensing boiler with instantaneous DHW heating BOSCH Condens GC2300iW 22/25 C. This model covers most apartments and smaller family houses – 22 kW output for heating, 25 kW for DHW, a modulating burner, condensing technology with efficiency up to 109% (based on Hs). The boiler has a built-in 3-speed circulation pump with an air separator, an electronic igniter, and a built-in expansion vessel. Installation is simple – a single unit on the wall, without external tanks.
For those who only need heating and plan to use a storage tank, a suitable choice is, for example, the gas boiler BOSCH Condens GC2300iW 15 P – a single-function wall-mounted condensing boiler with 15 kW output, designed purely for heating with a direct connection to an external DHW tank. The advantage is that the 15 kW output is close to the actual heating demand of the house, so the boiler cycles less. For larger houses with higher heating demand, there is analogously the BOSCH Condens GC2300iW 24 P with 24 kW output, or the premium BOSCH Condens GC8700iW 30 P with 30 kW and advanced modulation for larger properties.
All these boilers can also be controlled via a smart thermostat – for example, the Bosch Easycontrol CT 200 is an intelligent internet-connected controller compatible with a wide range of Bosch boilers. You can find more about smart control in the topic Intelligent Control of a Condensing Boiler: Regulation and Smart Thermostats in our Knowledge Center.
When to Clearly Choose Instantaneous Heating (Combi Boiler)
In practice, it's worth going the combi boiler route in these situations:
- An apartment or small family house with 1–4 people without a need for simultaneous draw-off from multiple bathrooms at once
- Limited space – there's no boiler room available, and a tank wouldn't fit logistically or spatially
- Low investment – a tank, its installation, and accessories add €800 to €1,500 to the installation cost
- Apartment renovation, where the original boiler was also a combi unit and the DHW pipework is sized for direct heating
- Recreational properties – cottages and cabins with irregular use, where a tank would lose heat during periods of inactivity
- Households with an irregular schedule, where hot water is consumed at very spread-out times throughout the day rather than in a block
When to Clearly Choose Storage Tank Heating
- A larger family (5 or more people) with a morning peak in hot water consumption
- Multiple bathrooms with the possibility of simultaneous draw-off – typically family houses with 2+ bathrooms
- Solar collectors or a heat pump for DHW preparation – a tank is essential in this case, as it serves as an energy accumulator
- A house with low heating demand (a well-insulated new build, low-energy house) – where a combi boiler would still have to be oversized due to DHW and would cycle a lot
- Combined DHW preparation from multiple sources – boiler + solar + electric immersion heater in the tank
- A large bathtub or jacuzzi – a bathtub volume of 200–300 liters simply can't be comfortably heated in real time by an instantaneous boiler
Storage Tank: How to Size It Correctly
If you decide on storage tank heating, the size of the tank is crucial. A tank that's too small will run out quickly, while one that's too large costs more unnecessarily and has greater heat losses. The general rule is 40 to 60 liters per person. For a family of four, that means 160 to 240 liters – in practice, a 200-liter tank is most often chosen.
Another important parameter is the output of the tank's heat exchanger. The tank must have an exchanger large enough (heating surface of 1.5 to 3 m²) for the boiler to heat the tank within a reasonable time – typically 30 to 45 minutes from cold. If the exchanger is undersized, the boiler works for a long time, and with low-temperature heating (underfloor, 35/30 °C), the tank may not reach the required temperature at all. This is a technical trap you can fall into with cheap tanks.
Direct (upright) tanks typically take up floor space of 60 × 60 cm for a volume of 120–200 l. At 300 l, this is already 70 × 70 cm. There are also tanks designed to be placed under/next to the boiler (so-called compact units or boiler+tank combinations as a single unit), but these are mostly limited to a volume of 80–130 liters.
Hygienic Safety of the Storage Tank: Legionella Is Not a Myth
DHW tanks carry a hygienic risk – bacterial contamination, particularly by the bacterium Legionella pneumophila. This bacterium multiplies at temperatures of 25 to 50 °C. A DHW tank set to 45 °C is an ideal environment for it, especially if it's unused for long periods or has cold dead zones.
The solution is twofold. First: set the tank to at least 55–60 °C – Legionella doesn't multiply at these temperatures. Second: the control unit of the boiler (or tank) has a thermal disinfection program that heats the tank to 70 °C for 1 hour once a week. This program consumes a bit more energy, but it's a necessary preventive measure, especially important for tanks larger than 200 liters and during longer summer periods when the tank isn't used for heating.
Instantaneous boilers eliminate this risk entirely – water isn't stored, it flows immediately before use and doesn't sit in warm pipes long enough for bacteria to multiply.
Practical Scenario 1: Apartment in a Prefab Building, 2 People
A two-room apartment, 55 m², gas heating, old atmospheric boiler due for replacement. The apartment has one bathroom, one toilet, and a kitchen. Hot water consumption is low – one shower in the morning, one in the evening, ordinary cooking and washing up. No renovations are planned.
Recommendation: a 20–22 kW combi boiler. A tank here would take up valuable space (in a prefab apartment every square meter counts), wouldn't be sufficiently utilized, and would add costs. Instantaneous heating is perfectly sufficient here.
Practical Scenario 2: Family House, 5 People, 2 Bathrooms
A 160 m² family house, a family of four plus one teenager. Two bathrooms on two floors, a large shower and a bathtub. Between 6:30 and 7:30 in the morning, four people shower one after another. The house has a garage with a boiler room, where a 200-liter tank fits comfortably.
Recommendation: a 15–18 kW heat-only condensing boiler + a 200-liter DHW tank with a coil exchanger. The tank absorbs the morning load without any problem, and the boiler is correctly sized for heating and cycles less.
Practical Scenario 3: Low-Energy House, Heat Pump + Gas Boiler in Bivalent Mode
A 140 m² new build with excellent thermal properties (U-values below 0.2 W/m²K). Heating demand is only 6–8 kW. The owner wants an air-to-water heat pump as the primary source, with a gas boiler only as a backup during freezing weather. For DHW, solar collectors are planned.
Recommendation: a 300-liter DHW tank with two exchangers (one for solar, one for the boiler/heat pump) is an absolute necessity here. A 15 kW gas condensing boiler, purely for heating (without DHW heating), is the right choice – the tank covers DHW and the boiler is correctly sized as a backup. A combi boiler wouldn't make sense here.
Combined Solutions: Combi Boiler + Storage Tank?
There's also a hybrid approach – a combi boiler with an external storage tank. Technically it's possible, but in practice rather pointless. If you have a tank, you don't need the instantaneous exchanger in the boiler – you'd be paying unnecessarily for equipment you won't use. It's only worthwhile in a situation where you're adding a tank later to an existing combi boiler (e.g., the family has grown and you want to avoid comfort issues) but don't want to replace the boiler. In that case, you can connect an external tank to the existing combi boiler, with the boiler heating the tank via the tank's exchanger or by direct filling (depending on the hydraulics).
What Affects the Right Choice: A Clear Decision Framework
To conclude the theoretical section, let's summarize what really determines the choice. In every project, you need to answer four questions:
- How many people will live in the house/apartment, and how many bathrooms does the property have? – More people and more bathrooms = a storage tank.
- Is there space available for a tank? – If not, a combi boiler is the only sensible choice.
- Will the house be combined with an alternative energy source (solar, heat pump)? – If so, a tank is essential.
- What is the house's heating demand? – If it's very low, a heat-only boiler with a tank allows for better sizing; if it's standard, both approaches are equally valid.
You can find more about choosing a boiler in a broader context (not just DHW, but also the type of heating system, fuel base, chimney) in the topic How to Choose a Condensing Boiler: What to Focus On Before Buying, and a comparison of wall-mounted and floor-standing solutions in the topic Wall-Mounted vs. Floor-Standing Condensing Boiler: Which Is More Suitable.
Frequently Asked Questions (FAQ)
Is an instantaneous combi boiler suitable for a family with three children in a family house?
In most cases, yes – but on the condition that showers aren't taken at the same time. If the house has only one bathroom and showers are taken one after another (even in quick succession), a 24–28 kW combi boiler will handle it without any problem. If the house has two bathrooms and there's a real chance that two people will shower simultaneously, a storage tank is the more comfortable choice. In practice, for families with 3+ children, we recommend a tank as the preferred option.
What is the lifespan of a DHW tank, and when does it need to be replaced?
A quality steel tank with an internal enamel coating and a magnesium anode has a lifespan of 15 to 25 years with regular anode replacement (every 2–4 years, or depending on water hardness). The anode is a sacrificial metal rod that corrodes instead of the tank. If it's neglected, the tank starts corroding from the inside and water leakage can occur without warning. Always handle anode replacement during the boiler's annual service.
Can I connect solar collectors for DHW heating to an existing combi boiler?
Not directly, technically speaking – a solar system requires a tank with a dual exchanger, where one circuit is for solar and the other for the boiler. You would therefore need to add a tank to the existing combi boiler and adjust the hydraulics. Most installers handle this by having the combi boiler continue operating, but the tank takes over primary responsibility for DHW, with the boiler only topping up when solar isn't sufficient. This is feasible, but hydraulically more complicated – we recommend consulting a designer before installation.
What does it mean that a combi boiler "cycles," and is that a problem?
Cycling (short cycling) is a phenomenon where the boiler briefly ramps up to full output, quickly heats a small amount of water, shuts off, then turns back on a few minutes later, and so on. This happens when the boiler is oversized relative to actual demand – which is common with combi boilers in low-energy houses. Excessive cycling prematurely wears out the igniter, circulation pump, and valves. Modern condensing boilers with a modulating burner (modulation ratio of 1:5 or 1:7) significantly reduce cycling – they can operate at as low as 20% of nominal output. For houses with low heating demand, it's advisable to underconfigure the boiler (set the maximum output lower via software) or consider adding a hydraulic separator.
Is it worth buying a larger tank "just in case" with a storage tank solution?
Not always. A larger tank has greater heat losses, costs more, is harder to replace, and requires a longer heating time. For a family of four, a 200-liter tank is optimal; 300 liters would only be justified for five or more people, or with a large bathtub (over 200 l in volume). Furthermore: a tank with a larger volume must also have a more powerful exchanger, otherwise the boiler won't heat it in time. If the boiler output and tank volume are mismatched (small boiler + large tank), the result paradoxically worsens comfort.
How does the installation of a combi boiler differ from a storage tank solution in terms of effort and cost?
Installing a combi boiler alone typically takes an experienced installer 4 to 8 hours (including flue gas venting, gas inspection, and setup). Adding a tank extends the installation by another 2 to 4 hours and adds material costs of €800 to €1,500 (tank + tank circulation pump + fittings + wiring). The total installation of a storage tank solution thus comes out roughly €1,000 to €2,000 more expensive, which needs to be considered when deciding – especially in apartments, where the financial argument strongly favors the combi boiler.
Conclusion
The choice between instantaneous and storage tank domestic hot water heating is not just a technical question – it's above all a question of lifestyle, number of people, available space, and long-term plans for the property. An instantaneous combi boiler is an elegant, compact solution for apartments and small households, where a tank would be an unnecessary investment. Storage tank heating, on the other hand, is a more reliable and comfortable solution for larger families, houses with multiple bathrooms, and for anyone planning to combine heating with renewable energy sources.
If you're not sure which solution is right for your specific case, take a look at other topics in our Knowledge Center as well – for example, How Condensing Boiler Installation Proceeds and What to Prepare or Flue Gas Venting and Air Supply for a Condensing Boiler: What You Need to Know. A correctly chosen boiler with a suitable DHW preparation system will save you not only money but, above all, years of trouble-free operation.
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 help.