Frequently Asked Questions about Condensing Boilers with Instantaneous DHW Heating

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Frequently Asked Questions About Condensing Boilers with Instantaneous DHW Heating

Condensing boilers with instantaneous domestic hot water (DHW) heating are among the most widespread solutions for heating and hot water preparation in family houses and apartments today. Yet we receive the same questions every day – from customers who are deciding whether to buy, as well as from those who already have a boiler and something isn't quite right. This article gathers the most common of these questions and answers them with the technical depth the topic deserves. Not generally and superficially, but specifically and with figures, as required by practical customer experience.

What exactly does "instantaneous DHW heating" mean in a condensing boiler?

When we talk about a condensing boiler with instantaneous DHW heating, we mean a device in which the domestic hot water is heated directly at the point of draw-off – that is, the moment you open the tap. Cold water from the mains passes through a plate or tube heat exchanger, where it is instantly heated to the required temperature. There is no stored volume, no tank, the water is fresh, and the cold side of the exchanger is always filled with plain tap water.

This is a fundamental difference compared to storage tank solutions, where the boiler heats dozens of litres of water in advance and keeps them warm. The instantaneous principle has both advantages and disadvantages, and you can learn more about both in the article Condensing Boiler with Instantaneous Heating vs. Storage Tank DHW Heating: Which Is Better.

From this perspective, instantaneous heating is energy-efficient – the boiler does not keep dozens of litres of water hot all day long. On the other hand, it must be able to deliver power instantly and in sufficient quantity. That is precisely why these boilers are sized with a higher thermal output for DHW preparation than for heating itself – typically 18–32 kW for DHW versus 12–25 kW for heating.

How much hot water can an instantaneous condensing boiler actually deliver?

This is question number one. Customers ask whether the boiler will be "enough" for a bathroom, for two bathrooms, for three people showering one after another. The answer depends on the boiler's DHW output and the inlet water temperature.

Approximate calculations for DHW heating with an instantaneous condensing boiler (inlet water temperature 10 °C, outlet 45 °C, i.e. a difference of 35 K):

18 kW DHW output: approx. 7.3–7.8 l/min – sufficient for one shower (a standard shower cubicle needs 7–10 l/min)
24 kW DHW output: approx. 9.8–10.2 l/min – a comfortable shower, plus a smaller kitchen sink at the same time
26 kW DHW output: approx. 10.6–11 l/min – two draw-off points at once with economical use
32 kW DHW output: approx. 13–13.5 l/min – a full bathtub or two showers

Practical example: a family house with three people, morning rush hour – two people shower one after another (not simultaneously), the third waits. A boiler with 24 kW DHW output handles this without problems. If they wanted to shower at the same time, either a higher output (28+ kW) or a storage tank solution would be needed. You can find more about sizing in the article What Output of a Condensing Boiler with DHW Do I Need for My House.

Why does the hot water from the boiler "cool down" and not reach temperature quickly?

This is one of the most common complaints we hear. The customer opens the tap, waits 30 seconds, a minute... and the hot water still doesn't arrive. The boiler itself is not responsible for this phenomenon – it is the volume of piping between the boiler and the draw-off point, which is full of cold water left over from the previous draw-off.

Physically, it comes down to a simple figure: a DN 15 (¾") pipe has a cross-sectional volume of approx. 1.77 cm², which for a length of 5 metres corresponds to 885 ml of water. If the hot water line runs 8 metres from the boiler, the volume of cold water in the pipe is almost 1.4 litres – at a flow rate of 8 l/min this takes 10 seconds. But with DN 22 (pipes in the wall), for the same 8 metres it can easily be 3 litres, which at a "slower" flow rate means 20–30 seconds of waiting.

Solutions to this problem:

DHW circulation pump: keeps the water in the pipework constantly warm. It increases energy consumption (pump + heat losses of the piping) but eliminates waiting. It requires a circulation loop, and some boilers have one integrated.
Minimising distance: place the boiler as close as possible to the main draw-off points (bathroom, kitchen).
Pressure buffer tank (hydraulic balancer): a small 10–20 l tank right next to the boiler shortens the "warm-up" time for repeated draw-offs.

Is a condensing boiler with instantaneous DHW heating suitable for every house?

Mostly yes, but there are situations where a storage tank solution or a combination is more advantageous. From experience we know that instantaneous heating works great if:

The household has 2–4 people with normal habits (showers, not everyone washing at once)
The boiler is placed close to the bathroom (up to 6–8 metres of piping)
The mains pressure is sufficient – at least 1.5 bar, ideally 2.5–3 bar
Water consumption is spread out over time, not concentrated in a single moment

Problematic scenarios where a storage tank should be considered:

A house with 5+ people, where everyone gets up at the same time in the morning
A large bathtub (> 150 l) – filling takes a very long time and the boiler must run at full output for dozens of minutes
Long pipe runs (> 12–15 m from the boiler) without the possibility of circulation
Low mains pressure (below 1 bar) – the boiler may not start heating

What is the difference between Protherm and Vaillant condensing boilers with instantaneous DHW?

Both manufacturers belong to the BDR Thermea group and share a common technological base, but they still differ in details. For a customer trying to decide, several parameters are important.

Protherm Gepard Condens MKV – for example the Protherm Gepard Condens 18/25 MKV is a compact wall-mounted boiler with an output of 18 kW for heating and 25 kW for DHW. It is a solid workhorse for an average family house. We appreciate its simple operation and the availability of service in Slovakia. The burner modulation is smooth, and condensation works from a return temperature of approx. 57 °C, which is standard for all condensing boilers.

Protherm Puma Condens 18/24 MKV – the Protherm Puma Condens 18/24 MKV is a step lower in the Puma range, suitable for smaller houses and apartments. A DHW output of 24 kW is enough for 1-2 draw-off points. Its advantage is a lower price and the same reliability of the basic components (exchanger, burner, electronics).

For comparison with Vaillant: the Vaillant VUW 236/5-3 ecoTEC pro and Vaillant VUW 26CS/1-5 ecoTEC plus IoniDetect offer combustion diagnostics via an ionisation sensor (IoniDetect), a feature of the more premium ecoTEC plus range. In practice, this means the boiler can continuously monitor the quality of the combustion process and adjust the gas/air ratio without the need for manual servicing. For a household, this can bring a 2–5% saving in gas consumption compared to boilers without this function.

You can find a more detailed comparison of these two brands in a separate article Protherm vs. Vaillant Condensing Boilers with DHW: Comparison of Models and Parameters.

Can I connect an external DHW storage tank to an instantaneous condensing boiler?

Yes, most condensing boilers with instantaneous DHW heating also allow the connection of an external storage tank – but be careful, it's not always as simple as it seems. The boiler must have a hydraulic circuit adapted for storage tank heating, meaning it must have an outlet and inlet for the storage circuit, or at least a 3-way valve outlet for the tank.

Why would anyone want to connect a storage tank to an instantaneous boiler at all? The most common reason: a family house where a lot of hot water is needed at once in the morning (two showers, a bath), and the instantaneous output is not enough. The tank (80–120 l) is preheated at night or in the early morning (when heating is not active), and in the morning a reserve is available. Disadvantage: the boiler must "switch" between heating and tank heating, and if both circuits demand output simultaneously, the boiler will prioritise the tank (so-called tank priority) – heating will temporarily stop.

In practice, however, we encounter this solution more often with customers who originally planned an instantaneous boiler but found after the first winter that it did not suit their comfort needs. We recommend considering this before purchase rather than addressing it afterwards – a storage tank connection requires space, additional piping, and extra installation costs.

How is condensation in the boiler related to DHW heating?

A condensing boiler uses the latent heat contained in the water vapour of the flue gases. A regular (non-condensing) boiler releases these flue gases through the chimney at a temperature of 160–200 °C – this heat is simply lost. A condensing boiler cools the flue gases down to 50–60 °C, during which the water vapour condenses and releases additional heat into the system. This "bonus" heat represents 10–15% extra compared to the calorific value of the gas.

An important principle applies to DHW heating: the lower the inlet water temperature, the better the condensation works. For heating, this is ensured by a low-temperature system (underfloor heating, or radiators sized for 55/45 °C instead of the old 80/60 °C). But for instantaneous DHW heating, the inlet temperature is that of the mains water supply – i.e. cold water from the network, typically 10–15 °C in winter. This is ideal for condensation! The boiler "condenses" during DHW heating at all times, because the cold water always ensures the necessary cooling of the flue gases.

Result: a condensing boiler achieves a seasonal efficiency (SCOP) of typically 95–109% during instantaneous DHW heating (related to the lower heating value of gas, Hs). This figure above 100% is not a mistake – it results from the inclusion of the condensation heat that a regular boiler "loses".

What affects hot water comfort with an instantaneous condensing boiler?

From experience, we know that customers most often complain about these phenomena:

1. Fluctuating water temperature during showering – if someone turns on water in the kitchen or flushes the toilet, the temperature in the shower changes. The cause is simple: the boiler regulates output according to the total flow. If the flow suddenly increases (flushing = supply of cold water into the system), the boiler needs a few seconds to adjust its output. Modern boilers with fast electronics (e.g. ecoTEC pro or Panther Condens) handle this regulation within 1–2 seconds, which is practically imperceptible.

2. Minimum flow rate to start heating – all instantaneous boilers have a defined minimum flow rate at which heating starts. Typically this is 1.5–2.5 l/min. If you only open the tap to a trickle, the boiler will not start and cold water flows out. This is not a fault but a protective function.

3. Hydraulic pressure problem – with low mains inlet pressure (< 1.5 bar), the flow rate is limited and the boiler may not reach the required temperature. Solution: a pressure reducer + accumulator, or a booster pump station.

4. Limescale build-up in the DHW exchanger – in areas with hard water (hardness > 14 °dH), limescale gradually builds up in the exchanger. This reduces heat transfer and can lead to overheating and failure. We recommend installing a water softener or regular descaling service of the exchanger every 2–3 years. You can read more about maintenance in the article Maintenance and Service of a Condensing Boiler with DHW: What and How Often to Check.

What are the requirements for flue gas and condensate discharge for these boilers?

Condensing boilers with a closed combustion chamber (room-sealed boilers) use coaxial or twin-pipe flue gas discharge systems. The coaxial system (pipe within a pipe – DN 60/100 or DN 80/125) discharges flue gases through the outer pipe and draws in fresh air through the inner one. This type is most common in installations in apartments and houses without a traditional chimney.

Important technical parameters of flue gas discharge:

Maximum length of DN 60/100 coaxial duct: typically 5–8 metres (depending on the manufacturer and boiler output)
Every 90° bend shortens the maximum length by 1–1.5 m of equivalent length
Flue gas temperature at the outlet of a condensing boiler: 50–80 °C (compared to 160–200 °C for a non-condensing one)
Flue gas condensate: an acidic solution (pH 3.5–5.5), discharge must go to the sewer via a neutralisation box or directly (depends on local regulations – in Slovakia direct discharge into the sewer is permitted for household boilers up to 200 kW)
Amount of condensate: approx. 1–3 litres per hour of operation during heating, less during DHW heating (shorter cycles)

A mistake we encounter during installations: the customer buys a boiler and then it turns out that the wall for the coaxial duct passage is 60 cm of brick masonry thick. The solution is either an angled drilled passage, or an extension to a twin-pipe system through an existing chimney flue. In the article Installation of a Condensing Boiler with Instantaneous DHW Heating: Procedure and Requirements you will find the whole procedure, including these situations.

What is the difference between the Protherm Gepard, Puma, and Panther Condens models?

Protherm offers several ranges of condensing boilers with instantaneous DHW, which differ in output, features, and price. From experience, we categorise them as follows:

Puma Condens MKV – the entry-level range of condensing boilers. The Protherm Puma Condens 18/24 MKV is a compact model suitable for apartments and smaller family houses (up to 120 m², up to 3 people). Heating output 18 kW, DHW 24 kW. Simple operation, reliable, without unnecessary extra functions.

Gepard Condens MKV – mid-range class. The Protherm Gepard Condens 18/25 MKV has a somewhat higher DHW output (25 kW) and better burner modulation across the entire output range. Suitable for family houses up to 150 m², 3–4 people. The electronics are more advanced, with wider compatibility with thermostats and smart heating control.

Panther Condens KKV – the premium range. The Protherm Panther Condens 20/26 KKV has a heating output of 20 kW and DHW output of 26 kW. Besides the higher output, it offers better control, a higher degree of condensation at partial load, and compatibility with eBus thermostats and solar systems. It is intended for houses with higher demands or where long-term operating economy is a priority.

Does a condensing boiler need to be serviced every year?

Yes, even if it "works without problems". Annual servicing of a condensing boiler is not just a formality – it has a technical justification and an impact on lifespan, safety, and warranty conditions.

What is done during the annual service:

Inspection and cleaning of the burner (accumulated dirt affects combustion)
Flue gas analysis – CO, CO₂, O₂ ratio (reveals inefficient combustion or dangerous leaks)
Inspection and cleaning of the condensate trap (a trap left without water causes flue gases to leak into the room)
Checking the pressure in the expansion vessel and topping up the system pressure
Visual inspection of seals, condensate drainage, and the flue gas system
Testing of safety features (STB, safety valve)
Checking the DHW plate exchanger for limescale deposits

From long-term experience: boilers that receive regular servicing last 15–20 years without major repair. Boilers without servicing tend to break down in the 8th–10th year of operation with faults that servicing would have caught early (blocked circulation pump, worn burner seal, clogged condensate trap).

Why does the boiler keep running in summer instead of only switching on for DHW draw-off?

This is a very common observation by customers after their first summer with a condensing boiler. The boiler runs even though no radiators are heating and hot water is not currently being drawn. The reason is usually one of the following:

Anti-legionella programme: once a week the boiler heats water to 65–70 °C to kill any Legionella bacteria in the pipework. This takes 20–40 minutes and happens automatically. This is intentional, not a fault.

DHW circulation pump in a circulation setup: if the boiler is connected with a DHW circulation pump, the pump runs on a schedule and periodically pumps water through the pipework, while the boiler maintains the temperature. In summer you see this as the boiler "unnecessarily" running.

Incorrectly set heating temperature in summer: if the summer mode (DHW only) is set incorrectly on the boiler, or if the thermostat has not switched off heating, the boiler may occasionally try to heat the circuits. Solution: set the summer mode according to the manual or via the thermostat.

You can read more about settings and operation optimisation in the topic Setting Temperature and Pressure in a Condensing Boiler with DHW for Optimal Savings.

Can a condensing boiler with instantaneous DHW replace an old combi boiler with a hot water cylinder?

Yes, and this is one of the most common renovation scenarios. An old gas boiler with an 80–120 l storage cylinder is replaced with a compact condensing boiler with instantaneous DHW – saving space, increasing energy efficiency, and getting rid of the large water tank, which used to consume dozens of kWh per month just to maintain its temperature through heating and cooling cycles.

A practical example from experience: a house originally had a 24 kW non-condensing boiler + an 80 l electric cylinder. The customer consumed approx. 2,200 m³ of gas + 180 kWh of electricity annually (for the cylinder) for heating + DHW. After replacement with a 20/26 kW condensing boiler with instantaneous DHW: gas consumption dropped to 1,750 m³ (a saving of approx. 20% on heating + 100% on electricity for the cylinder). The investment in the boiler paid off in 5–7 years, depending on energy prices.

Important for such a renovation: the heating pipework needs to be checked. If the radiators are sized for 80/60 °C (old system), a direct replacement condensing boiler will work, but may not fully condense (the return temperature will be too high). Solution: lower the set heating curve temperature and check whether the radiators are oversized – it often turns out that old radiators are oversized and work great with a condensing boiler even at 55/40 °C.

What to do when cold water flows from the tap even though the boiler is running?

This is one of the typical service scenarios. The boiler is running, shows correct temperature and pressure, but cold or lukewarm water flows from the tap. The causes are usually as follows:

Clogged cold water filter: there is a mesh filter at the cold water inlet to the boiler. In areas with turbid mains water, it gets clogged and restricts flow below the minimum needed to start the burner.
Faulty 3-way valve: the valve switches between heating and DHW. If it is stuck in the "heating" position, the DHW circuit does not receive hot water from the primary circuit.
Limescale in the DHW exchanger: the DHW plate exchanger is clogged to the point that water does not flow through it fast enough. The boiler starts, but the flow is limited and the temperature low.
Faulty DHW NTC sensor: the boiler measures the temperature incorrectly and ends the heating prematurely.

You can find more about these faults and their solutions in the article Common Faults of Condensing Boilers with Instantaneous DHW Heating and Their Solutions.

What is the lifespan of a condensing boiler with instantaneous DHW and what affects it?

A condensing boiler has a projected lifespan of 15–20 years. In practice, we encounter boilers that operate for 22–25 years (with some component replacements), as well as boilers that break down within 7 years. What makes the difference?

Installation quality: correct condensate drainage, correct system pressure (1.2–1.8 bar when cold), correct expansion of the flue gas pipe – all of this extends the lifespan
Water quality: hard water damages the DHW plate exchanger. A softener or regular exchanger servicing is essential
Cycling frequency: a boiler that switches on and off 8 times an hour (oversized output) wears out the ignition, seals, and exchanger faster than a boiler running smoothly at low output. A hydraulic pressure balancer (buffer) or correctly sizing the boiler solves this
Regular servicing: as mentioned above – boilers with servicing last considerably longer

Questions and Answers (FAQ)

Do I have to demolish walls and change pipework when installing a condensing boiler with instantaneous DHW?

Not always. If you are replacing an old gas boiler in the same location, the gas, heating, and water pipe connections usually remain. The main new thing is usually just the flue gas discharge – a condensing boiler doesn't need a traditional chimney, a coaxial outlet through the wall is enough. If you're installing the boiler in a new location, new pipework will be needed, but even then it is a smaller intervention than replacing the entire heating system.

Why does a condensing boiler emit visible steam from the outdoor flue outlet?

This is normal and even desirable – it's a sign of condensation. The flue gases exit at a temperature of 50–80 °C, which is much cooler than the outdoor air in winter weather. The water vapour in the flue gases condenses immediately and forms a visible cloud of steam. Neighbours may find this concerning, but physically it is proof of a properly functioning condensation process. The opposite would be a problem – if no steam is coming out, the boiler may not be condensing (return temperature too high).

Can I connect solar collectors with a condensing boiler with instantaneous DHW?

Technically yes, but it requires a change in the connection setup. An instantaneous boiler does not have a storage tank on its own, so there is nowhere to store solar energy. The solution is a combination: a DHW storage tank (80–200 l) heated by solar collectors, with the boiler with instantaneous heating acting as a backup heater – when the solar system isn't enough, the boiler brings the water up to the final temperature. Some boilers have this mode integrated (e.g. Vaillant ecoTEC plus with the auroMATIC controller). In an average family house with 2–3 solar collectors (5–7 m² of absorber) and a 120 l tank, solar can cover 60–70% of annual DHW consumption.

What is "flow comfort" and why do some boilers advertise it as a feature?

Flow comfort (comfort mode) is an operating mode in which the boiler keeps the primary circuit permanently at a higher temperature (e.g. 60 °C), even when there is no DHW draw-off at the moment. Result: when you open the tap, hot water is available almost instantly, because the DHW exchanger is already preheated. You pay for this with slightly higher gas consumption (approx. 3–6% extra). Customers with longer pipe runs or higher comfort demands appreciate this feature. Boilers without this function must first heat the primary circuit at every draw-off – this takes an extra 5–15 seconds.

Does a condensing boiler need to be insured, and how does the warranty work?

The warranty period depends on the manufacturer and on who carried out the installation and first service. Standard warranty is 2 years, but by registering the product with an authorised dealer and having it installed by an authorised Protherm or Vaillant service, you can get an extended 5-year warranty. This usually requires an annual service by an authorised technician. Household insurance should cover damage caused by a boiler malfunction (flooded room, gas leak), but that depends on the specific insurance policy – we recommend checking the insurance terms.

What gas pressure is needed for a condensing boiler with instantaneous DHW to work properly?

The standard pressure of natural gas in a household connection is 20–25 mbar (2–2.5 kPa). Condensing boilers are designed for this pressure. If the pressure is lower (for example, at the end of a long network or during peak winter demand), the boiler may not reach full output – this shows up as lower DHW temperature or partial blocking under high demand. Gas pressure is checked by a technician during installation using a manometer at the test point. Propane-butane versions of boilers are designed for a pressure of 37 mbar and are not interchangeable with natural gas versions without a conversion kit.

Conclusion

Condensing boilers with instantaneous DHW heating currently represent the most sensible choice for most new builds and renovations in family houses and apartments. Their combination of compactness, energy efficiency, and comfort is hard to beat – provided the system is correctly sized, installed, and maintained.

The most common problem we encounter is not a bad boiler, but a poor choice of output, insufficient system pressure, or neglect of regular servicing. That's why we recommend

Do you have a question about this topic?

Can't decide or are you dealing with a specific situation in your household? Write to us - we'll be happy to help.