How to Choose a Condensing Boiler with Instantaneous DHW Heating: What to Focus On

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How to choose a condensing boiler with instantaneous DHW heating: a comprehensive guide for homeowners and professionals

Choosing a condensing boiler with instantaneous domestic hot water (DHW) heating is a decision that will affect your household's comfort for the next fifteen to twenty years. It's not a purchase that should be made hastily or based on price alone. Over the years working with heating technology, I've seen hundreds of installations - from family homes to apartment units - and I consistently notice where people make mistakes when choosing. It's usually not a bad brand or a bad product, but a mismatch between what the boiler can do and what the household actually needs.

This article will guide you through all the key parameters to consider when choosing. If you want to know how many kW of output your house needs, check out our separate article What output condensing boiler with DHW do I need for my house. Here we'll focus on the overall selection - from technical parameters through hydraulics to what you'll find out once the boiler has been running for three years in real operation.

Instantaneous DHW heating vs. tank heating: the basic choice before selecting a boiler

Before we move on to the parameters of instantaneous boilers themselves, it's important to understand why you've decided on instantaneous heating rather than tank heating. This decision directly affects which boiler you buy, what demands you place on it, and how you size it. If you're not sure about this choice, I recommend reading the article Condensing boiler with instantaneous DHW heating vs. tank DHW heating: which is better, where both approaches are compared in detail.

In short: instantaneous heating is suitable for households where space is limited and DHW demand isn't extremely high (generally up to three bathrooms, a maximum of two simultaneous draw-off points). A tank solution is better for larger families or properties where several people often shower at the same time.

Heating output and DHW output - two different parameters

This is one of the most common sources of confusion when choosing a boiler. A condensing boiler always has two outputs - one for the heating circuit and one for DHW heating. And these are not the same figures. The DHW output is usually higher, because with instantaneous heating the boiler has to heat the water quickly - without a tank it has no choice but to run at full capacity.

An example from practice: a customer has a family house with a heat loss of 8 kW. For heating, a boiler with an output of around 10 kW (with a reserve) is enough. But for a shower with a flow rate of 8 litres per minute and a temperature difference of 35 °C, the required output is:

P = flow rate (l/min) × temperature difference (°C) × 4.187 / 60

P = 8 × 35 × 4.187 / 60 ≈ 19.5 kW

In other words, 10 kW is enough for heating, but almost 20 kW is needed for an instantaneous shower. That's why boilers with instantaneous heating have a correspondingly sized DHW output - typically 18 to 28 kW, some models even more.

What about specific models? For example, the Protherm Gepard Condens 18/25 MKV has a heating output of 18 kW and a DHW heating output of up to 25 kW - this asymmetry is typical for instantaneous combi boilers. The Protherm Puma Condens 18/24 MKV works similarly, with a DHW output of 24 kW.

Hot water flow rate - a key comfort parameter

Manufacturers state the so-called specific DHW flow rate - i.e. how many litres of hot water per minute the boiler can supply at a defined temperature rise (usually heating from 15 °C to 45 °C, i.e. ΔT = 30 °C). This figure is far more important for comfort than the output itself.

A regular shower needs a flow rate of around 7-9 litres per minute. A bath needs more - it depends on the fitting, but typically 10-15 l/min. If you want to shower two people at the same time, you need at least 14-16 l/min while maintaining a reasonable temperature. Instantaneous heating has physical limitations in this respect - so a simple rule applies: the higher the DHW output, the greater the flow rate and the better the comfort.

In practice, this means that for a one- or two-person household, a boiler with a DHW output of around 18-22 kW is sufficient. For three or more people, or if you have a larger shower head or a hydromassage unit, go for at least 24-26 kW DHW output.

Output modulation - why it matters for instantaneous heating

A condensing boiler should be able to regulate (modulate) its output. This applies to both heating and DHW. With instantaneous heating, modulation is crucial so that the boiler responds to the actual water flow rate and temperature - when you open the tap just a little, the boiler reduces its output; when you open it fully, the output increases. Without good modulation, you get an unstable temperature: the water is scalding one moment and cooling down the next.

The modulation range is stated as the ratio of minimum to maximum output, e.g. 1:5 or 1:7. A boiler with a modulation range of 1:5 and a maximum output of 25 kW can operate from 5 kW upwards - which is sufficient for instantaneous heating. The wider the range, the smoother the control and the higher the energy efficiency.

The good news: most current condensing boilers have built-in modulation. The bad news: not all of them are equally sensitive and fast. The response speed to changes in DHW flow rate is a parameter that manufacturers don't always state in the technical sheet - but you'll notice it immediately during your first shower.

Condensing efficiency and ErP class: what really affects consumption

Condensing boilers work on the principle of using the latent heat from the condensation of water vapour in the flue gases. Efficiency exceeds 100% (relative to the calorific value of the fuel) - typically ranging between 108 and 111% in low-temperature mode. When heating DHW, conditions for condensation are less favourable, because the boiler operates at higher outputs and the return temperature is higher. Nevertheless, a condensing boiler is still more economical when heating DHW than a non-condensing one.

The ErP (Energy related Products) class rates the boiler according to its seasonal energy efficiency. For condensing boilers, class A or A+ is standard. It's important to know that this class rates the boiler in combination with the controller - a boiler with intelligent control can achieve a higher class than an equally powerful boiler without one.

Proper condensation during heating requires a return temperature below 57 °C (ideally 30-45 °C). For instantaneous DHW heating, the boiler temporarily operates in a higher temperature range and condensation is limited. This is normal - overall it has a minimal effect on the bill, because DHW heating usually accounts for 20-35% of total consumption in a family house (the rest is space heating).

Hydraulic connection and mains water pressure

Instantaneous DHW heating depends directly on the pressure and quality of the water in the mains supply. The boiler itself is not a pump - cold water from the mains flows into the inlet and the boiler heats it as it passes through the heat exchanger. The minimum pressure for proper operation of instantaneous heating is usually 1.0-1.5 bar, with the optimum being 2.0-3.5 bar.

If you have low water pressure in your house (for example, an older house in the countryside, or a house at the end of a water supply branch), instantaneous DHW heating can be problematic. In that case, it's better to consider a tank solution or the installation of a pressure tank (hydrophore). From experience, I know that low mains water pressure is actually the most common cause of comfort complaints - not the boiler, but the mains water.

Another important aspect is water hardness. Hard water (over 20 °dH) causes limescale to build up in the DHW heat exchanger. With instantaneous heating, this problem is more serious than with tank heating, because the heat exchanger is small, compact and has a high heat flux. I always recommend installing a mechanical impurity filter, and in areas with hard water, also a water softener or scale inhibitor.

Design: plate vs. coil DHW heat exchanger

Inside the boiler, the DHW heat exchanger is designed either as a plate heat exchanger or as a coil (spiral) heat exchanger. Each type has its advantages and disadvantages:

Plate heat exchanger - high heat transfer efficiency, compact dimensions, quick response. Disadvantage: prone to clogging with hard water and impurities, harder to replace.
Coil (spiral) heat exchanger - more robust, less prone to clogging, but larger and heavier. Combined with suitable insulation, it can function as a mini-tank.
Primary circuit with external heat exchanger - some boilers have a separate DHW heat exchanger outside the main body. This type allows for easier replacement, but is more expensive to manufacture.

When choosing, don't focus only on the type of heat exchanger, but also on the availability of spare parts and the service network. For example, the Protherm Panther Condens 20/26 KKV is a model from the top-of-the-line series with well-available spare parts and a well-developed service network in Slovakia. This isn't a minor detail - a boiler that breaks down and waits a month for a part is just as problematic for a household as a poor-quality boiler.

Controls and connection to smart systems

Modern condensing boilers are equipped with digital controls, which can affect consumption by as much as 15-25% compared to simple thermostat control. What to focus on:

Weather-compensated control - the boiler regulates the flow temperature depending on the outdoor temperature. This is the basis for economical operation. Most current condensing boilers have this feature, or allow it to be connected.
Connection of an internet thermostat - via OpenTherm or a proprietary protocol (eBUS for Vaillant, Bus for Protherm). Allows control via smartphone, weekly programmes, and absence detection.
Flame ionisation detection - for example the IoniDetect system in Vaillant units. The boiler continuously evaluates combustion quality and optimises the air/gas ratio. Result: a more stable flame, longer burner life and slightly lower gas consumption.
Diagnostics and error codes - a good boiler should display clear error messages and ideally allow remote diagnostics by service technicians.

When it comes to smart control, the Vaillant VUW 26CS/1-5 ecoTEC plus IoniDetect is worth attention - this model has built-in ionisation detection, is ready for connection to the sensoNET system, and supports weather-compensated control. In practice, this means that if you add a suitable internet thermostat, you'll achieve measurable percentage savings on your annual gas bill.

For those looking for a combination of reliability and modern control at a reasonable price, the Vaillant VUW 236/5-3 ecoTEC pro is also interesting - a mid-range model in the ecoTEC line, offering solid modulation, connectivity to OpenTherm thermostats and simple operation. It's well suited to a standard family house where you don't want complicated systems, but you do want a reliable and economical boiler.

Dimensions, installation and flue gas routing

Condensing boilers with instantaneous heating are usually wall-mounted, compact units. Standard dimensions are around 400 × 700 × 300 mm (w × h × d), but this depends on the manufacturer and model. Before buying, check:

Whether the boiler will fit into your existing kitchen or utility room, including the required safety clearances.
Where the flue gas and condensate outlets are - the boiler must have a condensate drain to the sewer (ideally via a condensate neutraliser, which is required by law in many municipalities).
Type of flue connection - boilers with a closed combustion system (C-type, i.e. drawing combustion air from outside via a coaxial pipe) are the most common and suitable for modern low-energy houses. Open-system boilers (B-type) need an air supply from the room and are less common.

For details on installation and requirements, I recommend reading the article Installing a condensing boiler with instantaneous DHW heating: procedure and requirements.

Service network, warranty terms and real operating costs

The price of the boiler is only one component of the total cost. You also need to add:

Installation - depends on complexity (replacing an old boiler: €200-400, new installation including flue routing: €400-800).
Annual service - a condition for maintaining the warranty with most manufacturers. Cost of a service inspection: €80-150 depending on scope and region.
Spare parts - for boilers from reputable brands (Vaillant, Protherm/Bosch), parts are readily available and reasonably priced. With less well-known brands, this can be a problem.
Gas consumption - with a properly sized and set boiler and weather-compensated control, annual consumption can be 15-25% lower than with an old non-condensing boiler.

Warranty terms vary: the standard warranty is 2 years, but many manufacturers offer an extended warranty (5 years or more) upon product registration and regular servicing. For example, Vaillant offers up to a 5-year warranty when conditions are met, and Protherm offers something similar. Always read the extended warranty terms - they usually require an annual service inspection by an authorised technician.

For more detailed information on maintenance and service intervals, see the article Maintenance and servicing of a condensing boiler with DHW: what to check and how often.

Protherm vs. Vaillant: how to choose the right brand

Two players dominate the Slovak market in the category of condensing boilers with instantaneous DHW heating: Protherm (part of the Vaillant Group) and Vaillant itself. They are sister brands under the same roof, but with different positioning and pricing.

Protherm (Gepard, Panther, Puma series) is more affordable, works reliably, and has a well-developed service network. It's suitable for customers looking for a good decision without paying extra for a premium badge. Vaillant (ecoTEC pro, ecoTEC plus series) is more technologically advanced, has a better display and interface, a higher degree of automation, and usually also a higher price. For a customer who wants the best control and full integration into a smart home, Vaillant is the better choice.

For a detailed comparison, I recommend reading the article Protherm vs. Vaillant condensing boilers with DHW: comparison of models and parameters, where specific models are compared both in tables and from a practical perspective.

Practical scenarios from experience: who should choose which boiler

Over the years, several typical situations have emerged:

Scenario 1: Apartment building, 2-room flat, 2 people. Heat loss of the flat approx. 4-5 kW, one bathroom, shower. Ideal boiler: 15-18 kW (heating) / 18-22 kW (DHW). I recommend the Protherm Puma Condens 18/24 MKV or a similar model in this class - compact, reliable, and reasonably priced.

Scenario 2: Family house, 4 people, 2 bathrooms, 1 shower + 1 bath. House heat loss approx. 10-12 kW, occasional simultaneous DHW draw-off. Ideal boiler: 20-24 kW (heating) / 24-28 kW (DHW). Here I would recommend the Protherm Panther Condens 20/26 KKV or Vaillant VUW 26CS/1-5 ecoTEC plus, depending on how much the customer wants to invest in smart controls.

Scenario 3: Older family house, poor insulation, 5 people, 2 bathrooms. Heat loss approx. 14-16 kW. Here instantaneous heating hits its limits - with simultaneous draw-off, the water may run lukewarm. Solution: either a more powerful boiler (28+ kW DHW), or a tank-based solution. This is a case where careful consideration is needed when choosing.

Scenario 4: Low-energy house, 3 people, 1 bathroom. Heat loss only 4-6 kW. Note - here the modulation range is important, because the boiler needs to be able to go down to 2-3 kW so it doesn't operate in a cycling regime. Look for a boiler with a minimum output below 4 kW and proper weather-compensated control.

What to check when accepting the boiler and at first start-up

This is an area where customers often rely solely on the installer and regret it later. Here are a few things you should check when accepting the boiler:

Pressure setting of the expansion vessel (2/3 of the operating pressure, usually 1.5 bar).
DHW temperature setting - manufacturers recommend 45-55 °C. A higher setting means higher consumption but a lower risk of legionella. If set below 50 °C, the boiler should have a thermal disinfection function (periodic heating to 60-65 °C). More on this topic in the article Setting the temperature and pressure of a condensing boiler with DHW for optimal savings.
Type and slope of the flue pipe - there must be no upward slope towards the boiler, so that condensate can drain freely.
Presence of a condensate neutraliser - a condensing boiler produces condensate with a pH of 3-5, which must not go directly into the sewer without neutralisation.
Correct setting of the weather compensation curve - depends on the thermal insulation of the house and the type of heating elements.

Most common mistakes when choosing

To conclude this large section drawn directly from practice - mistakes that keep recurring:

Underestimating DHW output - a customer buys a boiler with 18 kW DHW output and then wonders why the shower water is lukewarm. DHW output determines comfort, not just the heating output.
Ignoring water quality - without a filter and in hard water areas, the DHW heat exchanger can scale up within two years. Servicing then costs €200-400.
Low mains water pressure - instantaneous heating requires minimum pressure. If the mains pressure in your area is only 1 bar, instantaneous heating may not work reliably.
Choosing a boiler without regard for the service network - a cheap boiler from an unknown brand can be more expensive in the long run if there are no parts or technicians available for it.
Omitting weather-compensated control - without it, the boiler doesn't take outdoor temperature into account and operates at an unnecessarily high temperature during transitional periods, which directly increases annual consumption.

Frequently asked questions about condensing boilers with instantaneous DHW heating

Can an instantaneous condensing boiler fill a bathtub full of hot water without a drop in temperature?

It depends on the DHW output and the flow rate of the fitting. A boiler with 25-28 kW DHW output can fill a bath continuously at a flow rate of 10-12 l/min and a temperature of 45 °C without a temperature drop. A problem arises if you open the tap fully (15-18 l/min) - at this flow rate, the temperature may drop. Solution: turn the tap down slightly and extend the filling time. Or consider a tank-based boiler.

Can I connect an instantaneous condensing boiler to solar collectors?

Direct DHW heating in an instantaneous boiler is not suitable for combination with solar pre-heating in the sense of a storage tank. Solar systems are typically paired with a tank boiler or a bivalent tank. However, there are solutions where solar heats the heating circuit (underfloor heating) and the instantaneous boiler handles DHW separately - this works and makes energy sense.

What is boiler "cycling" and why should it be avoided?

Cycling occurs when the boiler switches on and off very briefly - shorter than 3-5 minutes. The cause is a minimum boiler output that is too high relative to the actual needs of the house. Every start-up wears out the igniter, valves and circulation pump. Solution: correct boiler sizing, use of a hydraulic separator or larger expansion vessel, or choosing a boiler with a lower minimum output.

How much does it cost to run a condensing boiler with instantaneous DHW heating annually?

For a standard family house (150 m², 4 people, heat loss 10 kW), annual natural gas consumption with a condensing boiler and weather-compensated control is approx. 1,200-1,800 m³/year (depending on house insulation and climatic conditions). At a gas price of €0.08-0.10/kWh, this comes to roughly €900-1,500/year. DHW heating accounts for about 20-30% of that. Compared to an old boiler, this represents savings of 15-25%.

Is an annual service inspection necessary if the boiler is working without problems?

Yes, for several reasons: maintaining the manufacturer's warranty, safety (detection of gas and CO leaks), equipment lifespan, and operating efficiency. During the inspection, the burner is cleaned, and the flue path, air/gas combustion ratio, expansion vessel, water circuit and DHW heat exchanger are checked. A boiler without an inspection can increase consumption by 5-10% per year due to a dirty burner and heat exchanger.

What does "closed combustion chamber" (C-type) mean and why is it advantageous?

A boiler with a closed combustion chamber (marked C) draws combustion air from outside via a coaxial or separate pipe - i.e. not from the interior. Advantages: the boiler can be installed even in well-sealed (low-energy) buildings where there isn't enough air for combustion. There's no need to provide an air supply to the room. The risk of flue gases being drawn back into the interior is eliminated. Today, C-type is the standard for condensing boilers.

Conclusion: the right choice deserves time and an expert view

Choosing a condensing boiler with instantaneous DHW heating isn't rocket science, but it does require answering a few specific questions: What is my house's heat loss? How many people in the household shower, and how often simultaneously? What is the mains water pressure and hardness? What are my requirements for control? How much am I willing to invest in servicing?

Once you know the answers to these questions, the choice narrows down to a few specific models, among which you choose based on brand, price and preferences. All the models in the condensing boilers with instantaneous DHW heating category at Atria.sk are proven products from reputable brands - Protherm and Vaillant - with service available throughout Slovakia. The difference between them isn't in quality, but in parameters and price class, which should match your actual needs.

If you'd like more information about how instantaneous heating works, I recommend the article Instantaneous DHW heating in a condensing boiler: how it works and what affects comfort. And if you're interested in common faults and their solutions in operation, check out Common faults in condensing boilers with instantaneous DHW heating and how to fix them. We collect answers to more questions in the section Frequently asked questions about condensing boilers with instantaneous DHW heating.

Do you have a question on this topic?

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