Difference Between a Boiler Ready for a Cylinder and a Combi Boiler
Boiler ready for a cylinder or a combi boiler? The key difference that will affect your entire heating system
This question is among those where customers most often hesitate – and where a wrong decision either costs unnecessary money or years of frustration with insufficient hot water. When choosing a condensing boiler, you encounter two fundamentally different philosophies of domestic hot water (DHW) preparation. And although both types may look similar on the outside – they're boilers, they hang on the wall, they burn gas – inside they work completely differently and each suits a different situation.
In this article, we'll explore both concepts in depth: what exactly "cylinder readiness" means, how a combi boiler works, why they differ in installation, operating costs, comfort and reliability. We'll also look at specific models that have proven themselves in practice, and at situations from real jobs where choosing one or the other solution was crucial.
What is a combined condensing boiler (combi)?
A combi boiler, technically referred to as a "combi" or two-circuit boiler, is a device that integrates two independent functional circuits: one for heating and one for domestic hot water (DHW) preparation. In this type of boiler, hot water is heated continuously, using the so-called flow method – meaning exactly at the moment you turn on the tap.
Inside a combi boiler there is a built-in plate or tube heat exchanger, through which cold water from the mains flows. When you turn on the hot tap, the boiler immediately switches to DHW heating mode, the burner ignites (or stays running if it was already heating), and the water passes through the exchanger, where it's heated to the required temperature. If the combi boiler is condensing, this process takes place with maximum efficiency in utilizing the condensation heat of the flue gases.
The typical thermal capacity of a combi boiler for DHW ranges between 18 and 30 kW depending on the model – this determines how much hot water the boiler can deliver per minute at the required temperature rise. For example, a boiler with a 24 kW DHW output can heat approximately 10–12 litres of water per minute with a temperature rise of 30 °C (from 12 °C to 42 °C).
Advantages of a combi boiler
- Compactness: No cylinder needed, the boiler takes up less space – ideal for small flats or plant rooms with no available space.
- Lower investment costs: No cost for a cylinder (€200–800 depending on volume and brand), accessories and cylinder installation.
- No heat losses from a cylinder: Water isn't heated "for storage", so there are no continuous losses from maintaining temperature.
- Simple installation: Fewer pipes, fewer components, faster installation.
Disadvantages of a combi boiler
- Limited DHW flow rate: With simultaneous draw-off at multiple points (shower + sink + dishwasher), the boiler's output may be insufficient.
- Waiting for hot water: It takes 20–60 seconds for the boiler to "kick in" and for the water in the pipe to heat up – depending on the distance to the draw-off point.
- Limitation for larger households: For families of 4 or more with higher hot water consumption, the comfort of a combi boiler is typically lower.
- Temperature rise depends on water pressure: With low pressure in the household supply, the resulting temperature may fluctuate.
What is a boiler ready for a cylinder?
A boiler ready for a cylinder (in English terminology "boiler-ready" or "system boiler") is a single-purpose heating boiler that doesn't prepare domestic hot water on its own. Instead, it has pre-prepared hydraulic inlets and outlets, electrical connectors and a control system designed to directly control an external hot water cylinder. The cylinder is a physically separate vessel – standing next to the boiler or in another room – which stores heated water and keeps it ready for use.
The boiler itself therefore performs two functions: it heats the interior via radiators or underfloor heating AND simultaneously recharges the cylinder. The cylinder is equipped with a heat exchanger (a coil exchanger inside the vessel's jacket), through which boiler water transfers heat to the cylinder water. The boiler and cylinder communicate via a control cable – when the water temperature in the cylinder drops below the set value, the boiler automatically switches to cylinder heating mode, usually with priority over space heating.
Such a system is considerably more flexible. You can configure the cylinder to 80, 120, 150, 200 or even 300 litres according to the actual needs of the household. The boiler's control unit usually allows you to set not only the target cylinder temperature, but also the heating priority (cylinder vs. space heating), scheduling of cylinder heating according to a daily programme, and other parameters. Many modern boilers communicate with the cylinder via a bus protocol (eBUS, OpenTherm), enabling more precise and more efficient control than simple thermostatic control.
Advantages of a boiler ready for a cylinder
- Unlimited DHW comfort: Once the cylinder is full, hot water flows at full rate regardless of the boiler's output.
- Simultaneous draw-off at multiple points: Shower, bath, sink and dishwasher can all run at once without a drop in temperature.
- Volume flexibility: You can choose the cylinder size exactly according to the number of people and household habits.
- Possibility of solar pre-heating: Most cylinders have a second exchanger for a solar collector system – the boiler then only tops up what the sun couldn't provide.
- Stable water temperature: The temperature doesn't change with fluctuations in mains water pressure.
- Lower boiler wear: The boiler cycles less (fewer starts/stops), because it recharges a larger volume at once.
Disadvantages of a boiler ready for a cylinder
- Higher investment: Cost of the cylinder (€200–800 + installation), more space required in the plant room.
- Cylinder heat losses: Maintaining the water temperature in the cylinder generates constant heat losses of 1–3 kWh/day, depending on insulation and volume.
- The cylinder must be periodically heated to 60 °C: Legionella prevention – a thermal disinfection cycle is required once a week.
- More complex installation: More pipes, a three-way valve, control cable, larger plant room.
Key technical differences – overview
| Parameter | Combi boiler | Boiler with cylinder |
|---|---|---|
| DHW preparation | Flow-based, directly in the boiler | Cylinder-based, in an external vessel |
| Output for DHW | Limited by boiler output (18–30 kW) | Unlimited (depends on cylinder volume) |
| Space required | Just the wall-mounted boiler | Boiler + cylinder (80–120 cm of floor space) |
| Investment cost | Lower (boiler only) | Higher (boiler + cylinder + installation) |
| DHW heat losses | None (heating only on demand) | 1–3 kWh/day (cylinder insulation) |
| Comfort at higher demand | Limited (fluctuating temperature and flow rate) | High (stable temperature, high flow rate) |
| Solar integration | No (possible only with an ext. cylinder) | Yes (bivalent cylinders) |
| Legionella prevention | Not required | Mandatory 60 °C thermal cycle |
| Suitability for flats | Excellent | Limited (space) |
| Suitability for houses | Adequate (smaller houses, 1–3 people) | Excellent (4+ people, higher demand) |
Specific models from practice: which boilers belong to which category?
On the Slovak market, we encounter both types from established manufacturers. Let's look at specific models that are commonly available and used in practice.
Boilers ready for a cylinder (single-purpose, system boiler)
This category includes boilers typically denoted by the letter "K" or "U" (from the German "Umlauferhitzer" – circulation heater without a cylinder) in the type designation. They don't contain a DHW exchanger, but have an outlet for a cylinder with a three-way valve, or are prepared for its connection.
An example is the Vaillant VU 246/5-3 ecoTEC pro – a single-purpose condensing boiler with an output of 5.6–24 kW, which doesn't heat DHW directly, but is equipped with a hydraulic outlet and electronics for a cylinder. In practice, I typically configure it with Vaillant uniSTOR cylinders of 120 or 150 litres. The boiler supports communication via the eBUS protocol and allows you to set cylinder priority, heating schedule and temperature hysteresis exactly according to the family's habits.
Similarly, the Vaillant VU 25CS/1-5 ecoTEC plus IoniDetect works – a premium single-purpose boiler with the IoniDetect system, which monitors combustion quality and automatically optimises the gas/air ratio. This model is intended for larger houses, where a 150–200 litre cylinder serves a family of 4–6 people. The IoniDetect technology is valuable in practice for long-term operation: the boiler itself tells you if something changes in combustion quality, which significantly reduces the risk of faults.
From the Protherm portfolio, an example is the Protherm Panther Condens 15 KKO – a single-purpose Protherm condensing boiler with a hydraulic outlet for a cylinder and two-way circulation. "KKO" in the designation means that the boiler is intended exclusively for heating and cylinder-based DHW heating via an external cylinder, not flow-based. In practice, I typically pair it with cylinders from the Protherm and Aqua Silber ranges, depending on the size of the household.
Combi boilers – continuous DHW heating
Protherm's Gepard Condens combi boilers are a great example of the flow-based concept. The Protherm Gepard Condens 12 MKO, with a modulating output of 5–12 kW for heating and up to 18 kW for DHW, is a typical representative for smaller flats and houses with one bathroom. "MKO" in the designation stands for: M = modulating, K = condensing, O = open (overflow) system or "open" circuit – depending on the manufacturer's context, but in this case it refers to continuous heating without a cylinder.
The Protherm Gepard Condens 25 MKO, with an output of 8–25 kW, is a stronger model in the same series, suitable for houses with 4–5 draw-off points, but still works on the flow-heating principle. At 25 kW DHW output, it can heat 10–11 litres per minute with a 30 °C temperature rise – which is the threshold where a larger household (2 simultaneous showers) may notice limitations.
Practical scenarios from real jobs
Scenario 1: Three-room flat in a block of flats, a married couple without children
Here the choice is clear: a combi boiler. The couple doesn't shower simultaneously, DHW demand is low, and the utility room behind the kitchen is one square metre. The combination of a Protherm Gepard Condens 12 MKO + thermostat is ideal. A cylinder here would take up space, generate unnecessary heat losses and significantly increase installation costs without real comfort benefit.
Scenario 2: Family house, parents + 3 children, three showers in quick succession in the morning
Here a combi boiler fails. In the morning, when 5 people are getting ready for work and school, DHW demand is extremely concentrated. A 24 kW combi boiler can heat 10 l/min, but with three consecutive showers, each lasting 8–10 minutes – the third person enters the shower to cold water first. Solution: Vaillant VU 246/5-3 ecoTEC pro + a 150 litre cylinder. The cylinder fills overnight or early in the morning, and the family has a peaceful morning routine without compromises.
Scenario 3: Older house with underfloor heating, owner considering solar panels
Here a cylinder system is not only better, but practically the only sensible choice. Solar collectors (3–4 flat collectors facing south) can cover 70–80 % of DHW heating in the summer period. However, a bivalent cylinder (with two exchangers) is essential for the solar station – the lower one for solar, the upper one for the boiler. The combination of a Vaillant VU 25CS/1-5 ecoTEC plus IoniDetect + bivalent 200 litre cylinder + 4 solar collectors provides an excellent energy balance here.
Scenario 4: Holiday cottage, seasonal operation, minimal occupancy
A combi boiler is ideal here. The cottage is occupied sporadically, and a cylinder would be empty most of the year, or would have to be heated "for nothing" (legionella prevention). A combi boiler simply turns on upon arrival and works instantly. The Protherm Gepard Condens 25 MKO can handle a family of four for the weekend without any issues.
Hydraulic layout: how does the installation differ?
In practice, installing a combi boiler requires connecting five basic pipes: gas supply, heating circuit flow and return, cold water supply and hot water outlet. The whole installation is relatively simple and an experienced plumber can handle it in half a day.
With a boiler ready for a cylinder, the situation is more complex. In addition to the five basic connections, there are: cylinder circuit flow and return (usually via a three-way switching valve – either built into the boiler or external), the cylinder control cable, possibly a cylinder temperature sensor (NTC resistor), and if it's a bivalent cylinder, also an outlet for the solar circuit. Installation typically takes a full day, or more for larger systems.
The three-way valve is a key component here – it's what physically switches where the boiler water flows: either to the heating circuit or to the cylinder exchanger. Most modern boilers ready for a cylinder have the three-way valve integrated directly into the hydraulic block (e.g. the Vaillant ecoTEC series), which simplifies installation and reduces the risk of leaks at an additional joint.
Watch out for one detail from practice: some boilers ready for a cylinder require an external pump circuit (cylinder circulation pump) for certain types of cylinders – it depends on the hydraulic architecture. Always check the technical documentation of the specific boiler and cylinder, or consult with the technician designing the system. You can find more on this topic in the article Installing a cylinder with a condensing boiler – procedure and requirements in this Knowledge Centre.
Energy savings: where's the truth?
Energy efficiency of both solutions is a topic where I encounter a lot of myths. Let's define it precisely.
A combi boiler has no cylinder heat losses. That's factually true. But there's another side to that coin: flow-based DHW heating typically takes place at higher boiler water temperatures (55–65 °C), which slightly reduces the boiler's condensing efficiency (condensation occurs more intensively at lower return temperatures). For the boiler, flow-based DHW heating is therefore less condensing-efficient than heating a cylinder.
A cylinder system has heat losses through the vessel's insulation – these are real losses, typically 0.5–2.5 kWh/day depending on the quality of the cylinder insulation and the temperature difference between the cylinder and its surroundings. Modern cylinders with 50–80 mm thick polyurethane insulation have losses below 1 kWh/day. On the other hand, a cylinder allows heating at a lower boiler water temperature (55 °C is sufficient) and longer heating intervals – the boiler works in long stretches with full modulation, which is very condensing-efficient.
The real difference in annual energy consumption between the two systems for the same house and the same DHW demand is usually less than 5 %. A more decisive influence on consumption is the overall control system, the thermal insulation of the house and correct boiler sizing. You can read more on this topic in the article Why cylinder heating saves energy compared to flow heating.
Legionella and cylinder hygiene: an underappreciated topic
One of the disadvantages of a cylinder system, which is rarely sufficiently highlighted during sales, is the risk of the bacterium Legionella pneumophila multiplying in the cylinder. This bacterium multiplies at temperatures of 25–50 °C and can cause a serious respiratory illness (Legionnaires' disease). Strict hygiene requirements therefore apply to hot water cylinder systems.
Prevention is simple, but must be consistent: the cylinder must be heated to at least 60 °C at least once a week, and this temperature must be maintained for at least 1 hour. Modern boilers (Vaillant ecoTEC, Protherm Panther Condens) have this thermal disinfection cycle programmed directly into the control unit – it's usually set for night hours, when electricity or gas rates are lower (if you have a dual-rate tariff) and when there's no DHW demand.
This problem doesn't exist for a combi boiler – water flows through the exchanger at a temperature of 55–65 °C directly upon demand, and stagnation in a cylinder doesn't occur. From a hygiene perspective, a combi system is trouble-free, which is appreciated, for example, by holiday facilities with intermittent operation.
How to correctly size a cylinder for your chosen boiler?
If you've decided on a cylinder solution, the key question is: what cylinder volume do you need? In practice, I use the following rule of thumb:
- 1–2 people: 80–100 litre cylinder
- 2–3 people: 100–120 litre cylinder
- 4–5 people: 150–200 litre cylinder
- 6 or more people, or a house with a bathtub + shower cubicles: 200–300 litre cylinder
Boiler output vs. cylinder volume must be properly matched. A 15 kW boiler can heat 150 litres of water from 12 °C to 60 °C in approximately 27–32 minutes (at 100% efficiency and a 48 °C temperature rise). A 25 kW boiler can do the same in 16–20 minutes. If the cylinder takes too long to recharge (more than 30–45 minutes), it's either too large for the boiler's output, or the boiler's output is undersized.
Read more about sizing in the article What cylinder volume do I need for my boiler, where you'll also find more precise calculation methods.
Service and failure rates: what does practice show?
From the perspective of service and long-term operation, each system has typical weak points. A combi boiler has a built-in DHW plate exchanger, which is prone to limescale build-up with hard water (hardness above 10 °dH) and to fouling from impurities in the water supply. In areas with hard water (most of Slovakia), I recommend installing a magnetic filter on the cold water supply in every combi boiler and having the exchanger flushed with a descaler every 2–3 years. A neglected DHW exchanger is one of the most common causes of combi boiler faults – the exchanger then leaks, the boiler shuts off via a safety cut-out, or water "hisses" from the air vent.
With a cylinder system, the DHW exchanger isn't part of the boiler but of the cylinder – and the cylinder exchanger is much less prone to fouling, because the water in the cylinder circulates slowly and most limescale settles at the bottom of the vessel, not on the exchanger. Cylinders need to be descaled every 3–5 years and the magnesium anode, which protects the steel vessel from corrosion, should be checked. You can learn more about typical faults and their solutions in the article Common condensing boiler faults when operating with a cylinder.
Frequently asked questions (FAQ)
Can I subsequently connect a cylinder to an existing combi boiler?
It depends on the model. Some combi boilers are hybrid – they have both an integrated DHW exchanger and an outlet for an external cylinder. However, most standard combi boilers don't support a cylinder, because they lack a three-way valve and the control logic for a cylinder. Before buying a cylinder, always check the boiler's documentation to see whether a cylinder is supported. If not, it's usually more economical to replace the boiler with a system boiler (ready for a cylinder) than to modify a combi boiler. More advice can be found in the article How to connect a cylinder to a Protherm Panther Condens or Gepard Condens.
Why does a boiler ready for a cylinder have a lower heating output than a combi boiler in the same series?
It's not a rule, but manufacturers' sizing logic. Combi boilers tend to have a higher maximum output so they can efficiently heat DHW using the flow method. System boilers (with a cylinder) are dimensioned primarily for heating output – and since they heat the cylinder at a lower output (the cylinder stores heat over a longer period), they don't need such a high peak output. The cylinder acts as a thermal battery that evens out the discontinuity of heating.
What's better for underfloor heating – a combi boiler or a boiler with a cylinder?
For underfloor heating, a system boiler with a cylinder is always better. Underfloor heating requires a low water temperature (30–45 °C), which is ideal for a condensing boiler. If a combi boiler had to simultaneously supply underfloor heating (low temperature) and heat DHW using the flow method (which needs a higher temperature), it would have to compromise by operating at higher temperatures, which reduces the condensing effect. A system boiler operates at optimal temperatures for underfloor heating and recharges the cylinder at a target temperature of 55–60 °C – without compromises.
Is there a difference in warranty and servicing between a combi boiler and a system boiler?
The basic warranty period is usually the same (2–5 years depending on the manufacturer and the conditions of an extended warranty). The difference is that with a cylinder system, the service technician must also service the cylinder, which may increase the cost of service inspections somewhat. On the other hand, the overall system is divided into more components, so a fault in one doesn't necessarily take out the whole system – if the cylinder fails, the boiler still provides heating. Read more about proper servicing in the article Maintenance and servicing of a condensing boiler with a hot water cylinder.
Can a boiler ready for a cylinder be connected to a heat pump?
Yes, and that's actually one of the great advantages of system solutions. Modern boilers ready for a cylinder are designed to work in a hybrid system – for example, an air-to-water heat pump primarily heats the cylinder, and the boiler only kicks in during frosts, when the heat pump doesn't have sufficient output (the so-called bivalent point, typically around -5 °C to -10 °C). Such a solution significantly reduces annual gas consumption.
What to do in summer when I don't need heating but want hot water from the cylinder – does the boiler have to run at maximum?
No. Modern boilers ready for a cylinder have a "summer mode" – the boiler works only for the cylinder, and the heating circuit is deactivated. The burner modulates to the low output needed for the cylinder circuit (typically 5–8 kW for a 150 l cylinder), which is very condensing-efficient. In practice, this means that in summer the boiler works only 2–4 times a day for 15–25 minutes, which is very energy favourable. If you also have solar collectors, the boiler may be on standby for several days from June to August – solar power can fully cover the cylinder without the burner starting at all.
Conclusion: Which solution should you choose?
Choosing between a combi boiler and a boiler ready for a cylinder isn't a question of "which is better", but "which is better for the specific situation". A combination of a small flat, a single-person household and limited space clearly points to a combi boiler. A family house, four or more people, planned instal
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