Knowledge Center

Flow-Through vs. Storage Tank DHW Heating: What's the Difference

When choosing a condensing boiler, sooner or later every customer runs into a question that looks like a minor detail at first glance but actually has a major impact on living comfort: should domestic hot water (DHW) heating be flow-through or storage tank? Both principles have their place and neither is universally "better" – it depends on the size of the household, the number of draw-off points used at the same time, and the level of hot water comfort you expect. In this article we'll explain the technical principle behind both solutions and what that means in practice.

How flow-through (combi) DHW heating works

With flow-through heating, the boiler doesn't heat water in advance into storage, but only at the moment you open the tap. Cold water from the mains passes through a plate heat exchanger inside the boiler, where it picks up heat from the heating (primary) water heated by the burner. When hot water is drawn, the boiler automatically switches priority to DHW heating, increases the burner output, and within a few seconds starts delivering heated water into the pipework. The whole process happens "on the fly" – no water is stored anywhere, the boiler acts like a very fast, powerful instantaneous water heater.

The advantage of this principle is that you theoretically have an unlimited amount of hot water – only the boiler's instantaneous output is limited, not a storage volume. The downside is that the boiler's output has to cover the entire draw-off at that moment, which directly affects how many draw-off points can run at the same time.

How storage tank heating works

Storage tank heating works on the opposite principle – accumulation. The boiler continuously heats water in a separate tank (storage tank/cylinder), either built directly into the boiler body or external, connected to a gas or electric boiler. Water in the tank is kept at a set temperature (typically around 55–60 °C), and when you draw hot water, already-heated water simply flows out of the tank – meanwhile the boiler quietly reheats the tank to top it back up. This gives you an immediate large volume of hot water, regardless of the burner's output at that moment.

If you need to connect a boiler to a storage tank as well (for example during a renovation, when you want to increase comfort without replacing the whole system), on atria.sk you'll find boilers with the option to connect a storage tank as well as ready-made boiler-and-tank sets, where the combination has already been tuned and tested by the manufacturer.

Maximum flow rate – what it really means in litres per minute

The key parameter for flow-through heating is its maximum hot water flow rate at a given temperature rise (Δt). Δt expresses the difference between the incoming cold water temperature and the outgoing hot water temperature – it is usually stated at Δt 30 K, which roughly corresponds to heating from 10 °C to 40 °C, a comfortable temperature for showering or washing.

A concrete example: the Protherm Gepard Condens 18/25 MKV boiler achieves a maximum flow rate of 12.1 l/min at Δt 30 K. What does that mean in practice? A typical shower uses roughly 6–10 l/min, a basin or sink somewhat less (usually 4–6 l/min). A flow rate of 12.1 l/min will reliably cover one shower plus using a basin at the same time, but if you wanted to shower in two bathrooms at once and fill a bath as well, the boiler's output could become the limiting factor – either there's enough water but it's noticeably cooler, or the boiler effectively allocates the flow to the first draw-off and the other points get less. That's exactly why, with flow-through heating, it's important to think in advance about how many draw-off points really run at the same time in your household.

The "cold water sandwich" phenomenon

With flow-through heating you'll sometimes come across a phenomenon called the cold water sandwich. It happens when you turn hot water on and off twice in quick succession – for example when you pause the shower for a few seconds. Between the first and second draw-off, a "cooled-down" section of water is left in the pipe between the boiler and the outlet, which hasn't had time to mix with the newly heated water. When you then open the tap again, this cooled-down section is what comes out first – you feel a brief, sometimes unpleasant cold blast before the boiler ramps back up to full output and hot water arrives again. This phenomenon doesn't occur with storage tank heating, because the tank always has a homogeneous supply of heated water ready.

Speed of hot water delivery and more distant draw-off points

With flow-through heating there is always a certain delay while the water in the boiler heats up and travels through the pipework to the outlet – so for bathrooms or kitchens further from the boiler room, you may have to wait longer for hot water at every draw-off, even a short one. With storage tank heating, hot water is ready in the tank continuously, so it reaches the appliance faster once you open the tap – the only limiting factor is the length and insulation of the pipework, not the time needed to heat the water.

Space requirements

A flow-through (combi) boiler is clearly the more space-saving solution – the whole unit fits on a wall in a small utility room, a kitchen unit, or even a flat, and you don't need any extra space at all. Storage tank heating, on the other hand, requires space for the tank itself – this can range in volume from around 50 litres (smaller built-in solutions) up to 200 litres or more for external tanks for larger households, which represents a real footprint requirement in the boiler room.

Which household size suits which system

Flow-through heating is the natural choice for a smaller flat or house with one bathroom, where draw-off points are practically never used simultaneously on a large scale. For households with multiple bathrooms, a bath, or a higher number of people, where it's common for someone to be showering while someone else is washing dishes or filling a bath, storage tank heating or a boiler-plus-tank combination is more suitable – a store of hot water can cover even a short-term peak in consumption that the instantaneous output of a flow-through boiler couldn't handle.

Investment and running costs

In terms of purchase price and installation, flow-through heating usually works out cheaper – there's no cost for the tank itself, its installation, connection or extra space. Storage tank heating represents a higher initial investment (the tank, and possibly also boiler room modifications), which pays off in the form of greater comfort with higher or peak consumption. Operationally, both systems have their own specifics – see energy efficiency below.

Energy efficiency and heat losses

Storage tank heating has one unavoidable downside: water in the tank gradually loses heat to its surroundings while it "sits", even if the tank is well insulated – these are known as standby losses. The boiler therefore has to keep reheating the stock even when hot water isn't being drawn at that moment. Flow-through heating doesn't have this type of loss – heat is produced exactly at the moment of use and nothing is stored anywhere. On the other hand, flow-through heating requires the boiler to have enough instantaneous output ready to kick in fully within a fraction of a second, which places higher demands on the sizing of the burner and heat exchanger.

Summary: who each type suits

Flow-through heating is suitable if:

• you live in a flat or smaller house with one bathroom
• your draw-off points are practically never used simultaneously on a large scale
• you want the most compact solution possible with no extra space requirements
• you prefer a lower purchase and installation price
• you appreciate having theoretically unlimited hot water, only limited by instantaneous output

Storage tank heating is more suitable if:

• you have a house with multiple bathrooms or a bath
• several people live in the household and hot water use commonly overlaps (simultaneous showering, washing dishes, etc.)
• you want hot water available quickly without waiting for the boiler to ramp up
• the cold water sandwich effect bothers you and you prefer a consistent water temperature every time you open the tap
• you have enough space in the boiler room and are willing to invest more upfront in exchange for greater comfort

If you're not sure which type of heating will be optimal for your household, it's worth browsing the full range of condensing boilers, where you'll find models with flow-through DHW heating as well as models with a built-in storage tank – comparing the specific output parameters will help you pick the solution that fits your consumption exactly.

Protherm Gepard Condens vs. Viessmann Vitodens 100-W: Comparison

If you're choosing a condensing gas boiler for a flat or smaller family house with one bathroom, you'll very likely come across two names that are among the best-selling on the Slovak market: Protherm Gepard Condens 18/25 MKV and Viessmann Vitodens 100-W 25 kW. Both models are wall-hung condensing boilers with so-called flow-through domestic hot water (DHW) heating – meaning no built-in tank, water is heated continuously at the moment it's drawn. That's exactly why they are natural competitors: the target group, the way hot water is prepared, and the typical use case are almost identical. The difference lies in what's "under the hood" – the heat exchanger material, the width of the output modulation range, and the manufacturer's overall philosophy. In this article we'll put the two boilers side by side and explain what the individual technical parameters mean in practice.

Protherm Gepard Condens 18/25 MKV vs. Viessmann Vitodens 100-W – comparison table

Parameter	Protherm Gepard Condens 18/25 MKV	Viessmann Vitodens 100-W, 25 kW
Boiler type	Wall-hung gas condensing boiler, flow-through DHW heating	Wall-mounted gas condensing combi (flow-through) boiler
Rated heat output	18/25 kW (depending on model line)	3.2 – 25 kW (wider downward modulation range)
Heat exchanger material	Aluminium alloy (new design)	Inox-Radial – stainless steel
Burner	–	MatriX burner
Max. DHW flow rate (at Δt 30 K)	12.1 l/min	–
Control	Weather-compensated control option (based on outdoor temperature)	–
Weight	33.4 kg	–
Package dimensions	480 × 895 × 360 mm	–
Typical placement	Compact dimensions, low space demands for maintenance	Compact, suitable for a bathroom corner, cupboard, above a washer/dryer
Suitability	Flat/house with one bathroom, replacing an older non-condensing boiler	Flats and family houses with one bathroom

Dashes in the table mean that the given parameter isn't stated in the manufacturer's documentation for that model, not that the boiler lacks the feature – both boilers are fully-fledged condensing units with flow-through DHW heating and modulated output.

Difference in the heat exchanger – aluminium vs. Inox-Radial stainless steel

The heat exchanger is the heart of every condensing boiler – it's where heat from the flue gases is transferred into the heating water, including the latent heat released by condensing water vapour out of the flue gases (hence the name "condensing boiler"). The condensate produced in this process is mildly acidic, so the exchanger material has to withstand corrosion and deposits over the long term.

The Protherm Gepard Condens 18/25 MKV uses an aluminium alloy exchanger in a new design. Aluminium exchangers are a proven, more affordable solution that has been used in condensing boilers for years – with normal operation and regular servicing they deliver reliable performance for the boiler's whole expected lifespan. The new design in this model brings improvements over older aluminium exchangers, but in principle it's still aluminium as the material.

The Viessmann Vitodens 100-W is built around an Inox-Radial stainless steel exchanger. Stainless steel is more resistant than aluminium to corrosive attack from condensate and any impurities in the flue gases, which in practice translates into higher durability and a longer expected exchanger lifespan. This type of construction is also why the Viessmann brand has long been seen as the choice for customers who prioritise material quality over a lower purchase price.

Put simply: aluminium is a proven, more economical solution; stainless steel is an investment in greater durability and lifespan. Both approaches have their place – it depends on what matters most to the individual customer.

Output modulation range – why the lower limit matters too

Output modulation means the boiler's ability to smoothly reduce and increase its output according to current heat demand, instead of just running at 100% and then switching off completely. The wider the modulation range – i.e. the lower the boiler can "throttle back" its output – the more precisely it can match the household's actual needs, especially during transitional periods (spring, autumn, mild winter days) when only a fraction of maximum output is needed.

The Viessmann Vitodens 100-W offers a range of 3.2 to 25 kW, a significantly wider downward modulation range compared to typical condensing boilers in this output class. In practice this means smoother temperature control and less so-called cycling – repeated switching of the burner on and off at short intervals. Less cycling is beneficial for two reasons: it reduces wear on components (ignition, fan, pump) and contributes to a more stable feeling of warmth in the room without fluctuations.

The Protherm Gepard Condens 18/25 MKV operates in the 18/25 kW output class with a narrower modulation range. That doesn't mean the boiler performs poorly – for normal operation in a smaller flat or house with one bathroom, this output reserve is entirely sufficient and the boiler handles seasonal swings in heat loss without any problems. The difference shows up more in the fineness of control at very low heat demand, where the Viessmann's wider modulation range gives somewhat smoother operation.

Which model suits whom

Both boilers are designed for the same typical use case – a household with one bathroom, where flow-through DHW heating without a tank is sufficient. The decision between them is therefore more a matter of priorities than technical suitability.

The Protherm Gepard Condens 18/25 MKV is a typical choice where the priority is a simple, affordable replacement of an old non-condensing boiler with a modern condensing solution. Thanks to its compact dimensions (480 × 895 × 360 mm) and weight of 33.4 kg, it fits well even where an older, less efficient boiler used to sit, and doesn't require major changes to the existing system. The weather-compensated control option based on outdoor temperature is a nice bonus for those who want heating to automatically adapt to the weather without manual adjustments.

The Viessmann Vitodens 100-W 25 kW will appeal more to more demanding customers who value material quality and a longer expected lifespan of the heat exchanger, or smoother control thanks to its wider modulation range. Its compact construction also lets it be placed in tighter spaces – a bathroom corner, a cupboard, or above a washer and dryer, for example – which is especially valuable in smaller flats where every centimetre of space counts.

Summary and recommendation

If you're looking for a reliable, more affordable replacement for an old boiler in a flat or house with one bathroom, the Protherm Gepard Condens 18/25 MKV is a proven choice with a good balance of features and ease of replacement. If, on the other hand, you prefer the more durable Inox-Radial stainless steel exchanger, a wider output modulation range, and are willing to invest a bit more for these features, the Viessmann Vitodens 100-W 25 kW is the natural choice.

You'll find both models in the atria.sk range: Protherm Gepard Condens 18/25 MKV and Viessmann Vitodens 100-W, 25 kW, DHW. If you're not sure which output or exchanger type exactly matches your hot water usage and your household's heat loss, feel free to contact our advisors at atria.sk – we're happy to help you choose a boiler tailored exactly to your needs.

Installing a Condensing Boiler with Flow-Through DHW Heating

Buying a condensing boiler with flow-through domestic hot water heating is only the first step. The installation itself must be carried out by a professionally qualified company authorised to install and service gas appliances – this isn't just a formality, it's a legal requirement as well as a matter of your safety. Gas equipment burns a flammable medium, operates under pressure, and is connected to a chimney system or facade flue outlet. An installation mistake can cause a gas leak, flue gases being drawn back into the room, or a fault that only shows up after months of operation. That's why, when accepting the finished work, always ask for the inspection report and the commissioning certificate – without them, in the event of an insurance claim or complaint, it will be difficult to prove that everything was done correctly.

This article is meant to give you an overview of what installing a condensing boiler with flow-through DHW heating technically involves – so you know what to ask the installation company in advance and what to expect from them.

Condensate drainage

A condensing boiler works by also using the heat that would otherwise escape up the chimney as water vapour. As this vapour cools, it condenses back into water directly inside the boiler's heat exchanger – and this is exactly the property that gives condensing boilers their high efficiency. The by-product, however, is condensate, an acidic liquid with a pH of roughly 3 to 5 (similar to, say, vinegar or acid rain), which needs to be drained away somewhere.

• At normal outputs (typical for a family house or flat), draining to the nearest waste outlet is enough – a basin, a small tray, or directly into the siphon under the boiler. The amount of condensate is on the order of a few litres a day, so an ordinary household drain handles it without any problem.
• At higher outputs (typically in larger apartment buildings, commercial premises, or with several boilers connected in cascade) a neutralisation unit is required – a container with a neutralising filling that reduces the acidity of the condensate before it's discharged into the sewer. The exact output threshold above which neutralisation is mandatory is set by the project and the local sewer operator; the installation company should be able to confirm this for you.

An important detail worth asking the installer about: what material is the condensate drain pipe to the waste outlet made of? A plastic (e.g. PVC or PP) waste pipe will withstand acidic condensate without any problem over the long term. However, if the condensate passes through an older metal or cast-iron waste or sewer pipe, it risks gradually eating into it over the years – the corrosion may not show up immediately, but over long-term operation it shortens the pipe's lifespan. The solution is either to reroute the pipe so it only runs through plastic material, or at least to dilute the condensate sufficiently with other waste water before it enters the metal pipe (for example by connecting it after a basin's siphon).

Flue gas discharge (flue pipe)

The second key decision when installing is how the flue gases are discharged. For boilers with flow-through heating, which are often installed in flats and smaller spaces, you'll typically come across two solutions in practice.

Concentric discharge through the facade

This is a twin-wall pipe – flue gases leave through the inner pipe, while combustion air is drawn in directly from outside through the outer sleeve. It exits through a short penetration through the external wall, with no need to connect to a chimney. This solution is now the most common choice for flow-through boilers in flats, because:

• it doesn't need combustion air supplied from the room – suitable for today's airtight, well-insulated or renovated flats with plastic windows, where there would otherwise be a risk of insufficient air for combustion,
• it doesn't require a working chimney or any chimney modifications,
• installation is faster and cheaper, since it's just a short penetration through the facade.

You just need to keep to the prescribed minimum distance of the outlet from windows, doors, ventilation openings and the neighbouring property boundary – more on that in the common mistakes section below.

Discharge through an existing chimney

If a house or apartment building has a working chimney (for example from the original solid-fuel boiler or an older atmospheric gas boiler), it can also be used – usually a plastic or stainless-steel liner of the appropriate diameter is inserted into it, suited to condensing operation (resistant to moisture and the mildly acidic condensate that also forms inside the chimney itself). This solution is chosen especially where routing the discharge through the facade isn't possible or suitable, or for aesthetic or structural reasons. It does, however, require a professional assessment of the chimney's condition and diameter before installation. You'll find specific components for both types of flue discharge in the Flue gas discharge, flue pipes category.

Gas connection and pressure

Before the boiler itself is connected, the gas pipework in the house or flat must be in order – meaning a valid gas equipment inspection and a completed pressure test of the pipework. If the pipe route is changed or extended (for example when moving the boiler to a different location), the pressure test needs to be carried out again.

Equally important is correctly sizing the gas pipe diameter for the boiler's output. Condensing boilers with flow-through heating generally have a higher peak output than purely space-heating boilers without DHW preparation – they need to deliver significantly more heat for a short time in order to heat the flowing water to the required temperature in real time. If the supply pipe is undersized, the boiler may not get enough gas flow at full output, which shows up as fluctuating hot water temperature or insufficient output at higher draw-off (for example when two outlets are used at the same time). Correct pipe sizing is part of the project documentation, which the installation company should check before installation.

Installation location and space requirements

One of the advantages of boilers with flow-through heating is their compactness – there's no bulky hot water tank, so they fit even where a classic boiler with a tank wouldn't have a chance: in a bathroom, a cupboard, a hallway, or a kitchen unit. Even so, a few rules still apply here:

• Minimum clearances from flammable materials – wooden furniture, cladding or curtains in the immediate vicinity of the boiler may require fire-resistant insulation or a larger clearance; the exact values are set by the manufacturer in the installation manual.
• Space for future servicing – the boiler is serviced regularly (usually once a year), and some parts (exchanger, pump) may need to be replaced over its lifetime. If the boiler is built into a cabinet with no free access from the sides and below, the service technician simply won't be able to reach it – so check with the installer in advance whether there will be enough room around the boiler even a few years down the line.
• Access for both flue gas and condensate discharge – the installation location must also allow a short, direct route to the facade or chimney, as well as a sloped condensate drain to the waste outlet (more on this below).

Electrical connection and fuses

The boiler needs a standard 230 V socket with correctly wired earthing – without earthing, protection against fault currents and some of the boiler's safety functions may not work. It's recommended that the circuit also be protected by a residual current device (RCD), which will cut the power in the event of an insulation fault before an injury can occur. If the socket by the boiler is old, undersized, or lacks an earthing pin, it's better to have it replaced before installation – making changes after the boiler is already hung is inconvenient and extends the installation time.

Commissioning and first start-up

Connecting the boiler is only half the job – the other half is commissioning it correctly. The installation company should:

• Bleed the whole heating system – air in the radiators or pipework causes bubbling, uneven heating and puts strain on the circulation pump.
• Set the heating water pressure – typically in the range of 1 to 2 bar (depending on the height and layout of the system); the boiler should have a built-in pressure gauge that lets you check this value yourself on an ongoing basis afterwards too.
• Set the boiler's operating parameters according to the type of heating system (radiators vs. underfloor heating), the weather-compensation curve if connected, or link the boiler to a room thermostat or control unit – you'll find suitable accessories in the Boiler controls category.
• Explain the basic operation to you – how to top up the water pressure, what the error codes on the display mean, and when it's time for a service.

Common installation mistakes to avoid

Most of the problems that condensing boiler owners run into later don't stem from a fault in the boiler itself, but from incorrect installation. The most common ones are:

• Undersized gas pipe – as mentioned above, this causes fluctuating output and hot water temperature at higher draw-off.
• Incorrect slope of the condensate pipe – the condensate drain must have a constant fall towards the waste outlet, at least a few percent. If it runs horizontally or even against the fall, condensate collects in it, can get blocked by deposits, and in frosty weather (especially if part of the route runs outdoors) can freeze in it – frozen condensate then shuts the boiler down for safety reasons.
• Incorrectly positioned outdoor flue terminal – the outlet of the concentric flue discharge must be far enough from windows, doors, balconies and the neighbouring property boundary. If placed too close to a window (your own or a neighbour's), flue gases can enter when airing the room, which is not only unpleasant but also potentially dangerous. The exact minimum distances are set by the technical standard and the boiler manufacturer's installation manual, and keeping to them is one of the things the installer should think through before drilling a hole in the facade.

If you're considering a boiler with flow-through heating, you'll find an overview of the available models in the Boilers with flow-through DHW heating category, and a wider selection of condensing boilers including tank variants in the Condensing boilers category.

Conclusion

Installing a condensing boiler with flow-through DHW heating is professional work involving several interlinked steps – from condensate drainage, through choosing and implementing the flue gas discharge, checking the gas supply, right up to the electrical connection and final commissioning setup. Before ordering the installation, get a specific quote drawn up that also takes these details into account (type of flue, route lengths, any condensate neutralisation needed) – this way you'll avoid unpleasant surprises after signing the contract. And always check that the company you choose holds a valid authorisation to install and service gas appliances. That's the only guarantee that your new heating will be safe, reliable, and will serve you well for years to come.

Troubleshooting Common Problems with Flow-Through Heating

Flow-through domestic hot water (DHW) heating is now the standard in the large majority of condensing as well as classic boilers – water isn't heated into a tank, but directly as it flows through a plate heat exchanger the moment you open the tap. This technology saves space and energy, but it also has its own quirks that an average user often perceives as a fault, even though they're actually physical limitations of the system. In this article we go through the most commonly reported problems the way we handle them in our service department – always in the order problem → cause → solution.

1. "Cold water sandwich" – why cold water briefly appears when you turn the tap back on

The phenomenon that English terminology aptly calls the cold water sandwich is familiar to almost every user of a boiler with flow-through DHW heating. The situation is typically like this: you're showering, you stop the water for a few seconds (for example while you shampoo your hair), and when you turn the tap back on, instead of hot water you're briefly surprised by a cold stream, before the temperature settles again.

Cause

It's a combination of two physical effects:

• Residual cold water in the pipe – while the tap was closed, the section of pipe between the boiler's exchanger and the draw-off point (the tap, the shower head) contains water that has already cooled down to the temperature of the surrounding pipe. This water has to "flow through" first, before genuinely heated water from the boiler arrives again.
• Burner and control lag – when the flow switch triggers, the boiler needs a short time to start the burner and stabilise the outlet temperature at the required value. While the burner ramps up to full output and the control "catches up" to the target temperature, water at a lower-than-expected temperature may flow for a moment.

Solution

• Modern condensing boilers have a preheat (micro-accumulation) function – a small volume of water in the exchanger or a micro-tank is kept warm even when not drawing water, so the delay is shorter when you open the tap again. If your boiler supports this function, it's worth keeping it switched on (at the cost of slightly higher standby consumption).
• We recommend a simple habit for users: during a short pause in drawing water (for example while showering), don't close the tap completely, just turn the flow down to a minimum – this keeps the burner running and the "sandwich" doesn't occur.
• If the effect is extremely pronounced (cold water lasting longer than 5–10 seconds), the problem is no longer purely physical – we recommend a service check of the preheat setting and the burner's response time.

2. Fluctuating water temperature when using several draw-off points at once

A classic situation: someone is showering while hot water is also turned on in the kitchen sink at the same time – the shower temperature drops immediately or the pressure falls significantly.

Cause

Every flow-through boiler has a defined maximum DHW flow rate at which it's still able to heat water to the set temperature (usually stated in litres per minute at a given temperature rise). When the combined draw-off from all open points exceeds this capacity, the boiler can't deliver enough heat for the whole volume of water flowing through – the result is either a drop in temperature, or (with some control systems) a drop in pressure/flow to keep the temperature steady.

Solution

• The simplest solution is organisational – avoid drawing from two or more points at the same time, especially a shower and a kitchen sink simultaneously.
• If a household regularly needs simultaneous hot water draw-off in several bathrooms (typically in larger family houses or multi-generational living), flow-through heating may no longer have enough capacity, and it's worth considering switching to a combined system with a storage tank, which can cover a temporary higher draw-off from its accumulated volume of hot water.
• When choosing a new boiler with flow-through heating, it's worth going by your household's actual needs, not just the cheapest model – you'll find an overview of suitable types in the Boilers with flow-through DHW heating category.

3. Insufficient hot water flow (weak stream from the shower or tap)

If hot water flows noticeably weaker than cold water from the very same tap, the cause is almost always limited flow somewhere in the system – not the boiler's output itself.

Most common causes

• Clogged filter at the cold water inlet to the boiler – mechanical impurities (sand, pipe rust, deposits) gradually get caught in the filter screen and progressively restrict flow.
• Heat exchanger scaled up with limescale – especially in areas with harder water, a layer of limescale gradually builds up on the inner walls of the plate exchanger, narrowing the flow channels.
• Incorrectly set maximum output on the boiler's control panel – some models allow you to limit the DHW heating output, which in practice shows up as a permanently weaker hot water flow regardless of how many taps are open.

Solution

• The filter at the cold water inlet can be cleaned yourself fairly easily (after shutting off the water supply) – we recommend checking it at least once a year, more often in areas with poorer water quality.
• Descaling the exchanger is a more specialised job (chemically flushing the circuit with a suitable descaling agent) and should be done by a service technician during the regular annual inspection, especially where local water hardness is higher.
• The maximum output setting on the panel can be checked by the user themselves according to the model's manual, or verified by a service technician during a visit.

4. The boiler keeps switching on and off during hot water draw-off (cycling)

Instead of heating smoothly, the burner "hops" – switching on and immediately off, with the water temperature swinging up and down at short intervals.

Cause

Every boiler has a defined minimum flow rate at which the flow sensor registers a draw-off at all and allows the burner to start. If the tap is opened only minimally (for example when topping up a pot), the actual flow may hover right around this threshold – the sensor alternately "sees" and "doesn't see" sufficient draw-off, so the burner switches on and immediately off again. The same symptom also appears when the flow sensor itself is faulty or dirty and incorrectly evaluates the actual flow.

Solution

• If cycling only appears with a very slow draw-off (for example a tap opened just slightly), this is closer to normal behaviour right at the minimum flow threshold – simply open the tap more.
• If cycling also occurs with the tap fully open as normal, it's highly likely a fault or contamination of the flow sensor – a service check and possible cleaning or replacement of the sensor is needed.
• In some cases a service adjustment of the control's sensitivity/hysteresis can help, if the given boiler model allows it – however, this belongs strictly in the hands of a qualified technician, not DIY adjustments.

5. High water and gas consumption when heating DHW

Users sometimes notice that before genuinely hot water appears at the tap, a surprisingly large amount of cold water runs "needlessly" – and this shows up as higher consumption of both the water itself and the gas needed to reheat it.

Cause

This is a direct consequence of the volume of water in the supply pipe between the boiler and the draw-off point. The longer this pipe is (typically when the boiler is located far from the bathroom, for example in a basement or utility room on the opposite side of the house), the larger the volume of already-cooled water that has to be "pushed out" first at every new draw-off, before genuinely heated water from the boiler starts flowing. This effect multiplies with every single draw-off during the day.

Solution

• For new builds or major renovations, the most effective solution is a circulation pipe with a circulation pump – hot water then constantly circulates as close as possible to the draw-off points, and reaches the tap itself practically instantly with no loss over a long route. Circulation is especially worthwhile in larger houses with multiple bathrooms or long pipe runs.
• Where circulation isn't feasible, it at least helps to place the boiler as close as possible to the most frequently used draw-off point (usually the main bathroom), which minimises the volume of "dead" cold water in the pipe.
• When choosing suitable controls that can also manage the circulation pump on a schedule or on demand, we recommend checking the Boiler controls category.

6. Odour or discolouration of water from the boiler

After a longer absence (holiday, a cottage over winter), smelly or slightly discoloured water sometimes appears the first time you draw water.

Cause

Water that has stagnated for a long time in the exchanger and supply pipe without moving can:

• develop a characteristic musty smell as a result of standing for a long time without flow,
• release small deposits or traces of corrosion from the inner surfaces of the pipe and exchanger, which shows up as slight cloudiness or a reddish tinge.

Solution

• After every longer shutdown, we recommend letting the water run freely at all draw-off points for a few minutes before using it for personal hygiene or in food – in the vast majority of cases, the odour and any cloudiness disappear quickly.
• If the problem recurs outside of longer shutdowns, or the odour/discolouration persists even after running the water for longer, this is a sign that the exchanger and internal pipework need a service check, not just a normal operating phenomenon.
• Regular annual servicing, including checking the condition of the exchanger, significantly reduces the risk of deposits building up enough to cause visible water quality problems.

When you can manage on your own and when to call a service technician

Most of the minor phenomena described above – a brief "cold water sandwich", a slight pressure drop with simultaneous draw-off, needing to run water after a shutdown – can be solved or at least eased by the user themselves with a simple change of habits, or by cleaning an easily accessible filter. On the other hand, any intervention in the burner, the flow sensor, the heat exchanger, or the control settings belongs strictly in the hands of a qualified service technician – an incorrect intervention here can affect not just comfort, but also the safety of the boiler's operation.

The best prevention for most of the problems described above remains a regular annual service inspection, during which a technician checks and, if needed, cleans the filter, the exchanger and the flow sensor before minor deposits or contamination show up as a visible fault. When choosing a new boiler with quality flow-through heating that's more resistant to most of these effects, we also recommend browsing the wider range in the Condensing boilers category.

What Output and Flow Rate You Need Based on the Number of Bathrooms

When choosing a condensing boiler with flow-through domestic hot water (DHW) heating, most customers look almost exclusively at one figure – the heating output in kilowatts (kW). That's understandable, since this is the number most visible in the product name and in price lists. But with flow-through heating, a completely different parameter handles a completely different job: the maximum DHW flow rate in litres per minute (l/min) at a given temperature difference Δt. And it's exactly this parameter that decides whether you enjoy a hot shower in the morning, or end up under a cold stream of water while someone else in the household is doing the dishes.

Output (kW) and flow rate (l/min) – not the same thing

A boiler's heating output determines how quickly and efficiently it can heat the air in your rooms via radiators or underfloor heating. It's a value calculated based on the building's heat losses – its floor area, insulation, number and size of windows, and climate zone.

DHW flow rate, by contrast, tells you something completely different: how many litres of water the boiler can heat to the required temperature per minute as it flows through its exchanger. This value is always stated for a specific temperature difference Δt (for example Δt 30 K means heating water from, say, 10 °C to 40 °C). The higher the required temperature rise, the lower the achievable flow rate – and vice versa.

A boiler with flow-through DHW heating has no hot water tank, so it heats water "on demand" – the moment you open the tap. That's an advantage in terms of saving space and energy (water isn't needlessly heated in advance and doesn't cool down sitting in a tank), but it also means a limit: if you need hot water at two points at the same time (for example a shower and a kitchen sink), the boiler has to split its maximum flow rate between both draw-offs. That's exactly why, when choosing a boiler with flow-through heating, it's crucial to know your household's real needs in litres per minute, not just the heating output in kW.

Approximate flow rate by number of bathrooms

Below is an approximate guide to how many litres of hot water per minute your boiler should realistically be able to handle, based on your household's layout:

• 1 bathroom (shower or bath) – a flow rate of roughly 10 – 12 l/min at a typical Δt is enough. This covers a comfortable shower as well as filling a bath without much waiting.
• 1 bathroom + kitchen (simultaneous draw-off) – if it's common in your household for someone to shower while dishes are being washed or water is being drawn in the kitchen at the same time, it's worth planning for a higher flow reserve, ideally a boiler with an output range closer to the upper end of the range on offer (e.g. a 24 – 25 kW variant), so the drop in pressure/temperature during simultaneous draw-off isn't noticeable.
• 2 bathrooms used at the same time – here, flow-through heating alone generally can't cover comfort without compromises (a drop in temperature or water pressure). In this case it's more sensible to consider switching to a storage tank system, where the boiler continuously reheats a tank with a larger water volume that can cover short-term peak draw-offs at several points at once. You'll find an overview of suitable solutions in the Boiler-and-tank sets category.

If you're not sure which category your household falls into, the safest option is to consult a technician directly before buying – but keep in mind that the approximate values above come from common practice, not from a precise calculation for your specific house.

Comparison of five condensing boilers with flow-through DHW heating

We've included five popular models from the condensing boilers range in this comparison, specifically from the boilers with flow-through DHW heating line. Each has slightly different strengths, so it's worth looking at them side by side.

Model	Output	Key feature	Suitable for
Protherm Gepard Condens 18/25 MKV	18/25 kW	Max. DHW flow rate 12.1 l/min at Δt 30 K, aluminium exchanger, weight only 33.4 kg	1 bathroom, flats and smaller households
Viessmann Vitodens 100-W, 3.2-25 kW DHW	3.2 – 25 kW	Inox-Radial stainless steel exchanger, MatriX burner, very wide modulation range	Flats and family houses, households with variable heat demand
ATTACK CONDENSING 20 Premium	4.5 – 20 kW	High manufacturer-stated efficiency of 98.5 – 109.5%	Households focused on running economy
Buderus Logamax plus GB072-24K	24 kW	Proven condensing technology with flow-through DHW heating	1 bathroom + kitchen, reliable year-round use
Baxi Luna Duo-tec+ 24	24 kW	Backlit LCD display for easy operation and diagnostics	1 bathroom + kitchen, households that value simple operation

Difference in exchanger type – aluminium vs. Inox-Radial stainless steel

The heat exchanger is the heart of every condensing boiler, and its material directly affects the unit's durability and lifespan. The Protherm Gepard Condens 18/25 MKV uses an aluminium exchanger – a proven, lightweight (total boiler weight only 33.4 kg) and affordable solution that, with proper operation and maintenance, reliably serves for years.

The Viessmann Vitodens 100-W, by contrast, is built around the Inox-Radial stainless steel exchanger. Stainless steel is generally more resistant to corrosion caused by condensate (which is mildly acidic in condensing technology) and to thermal stress from frequent temperature cycling. Combined with the MatriX burner, this is a construction aimed at long-term output stability even under intensive use.

Neither approach is universally "better" – the choice should be based on whether you prefer a lower purchase price and simplicity (aluminium exchanger), or greater material durability in the long run (stainless steel exchanger).

Difference in output modulation range

Output modulation means how far a boiler can smoothly reduce its output downward as actual heat demand falls – for example during spring or autumn days, when only a little extra heating is needed. The wider the modulation range, the more precisely the boiler can adapt to current demand without needless switching on and off (so-called cycling), which saves wear on the burner as well as fuel.

The Viessmann Vitodens 100-W offers a range of 3.2 – 25 kW, one of the widest ranges in this comparison – so it can work even at very low output during transitional periods. The ATTACK CONDENSING 20 Premium, with a range of 4.5 – 20 kW, goes in a similar direction, albeit with a lower maximum output ceiling. Models with a fixed stated output (e.g. 24 kW with no lower range given), by contrast, can be just as reliable during the heating season, but in very mild weather they rely more on short on/off cycles than on smoothly "throttling" their output downward.

Practical conclusion

The approximate flow-rate values by number of bathrooms and the model comparison above will help you narrow down your choice, but they're no substitute for an accurate heat-loss calculation. Your actual output requirement depends on your specific building's heat losses, the number and simultaneity of hot water draw-offs, and the type of heating system (radiators vs. underfloor heating). That's why, before committing to a purchase, we recommend having the exact output and connection type (flow-through heating vs. storage tank system) confirmed by a technician or through a heat-loss calculation – especially if your household uses two bathrooms at the same time, where it's worth considering a boiler-and-tank set instead of a purely flow-through solution.