How to Choose a Low-Temperature Boiler with Instantaneous DHW Preparation: What to Watch Out For
Why choosing a combined low-temperature boiler is more complicated than it seems
When a customer comes with the request "I want a boiler that will both heat and provide hot water", it seems simple at first glance. In practice, however, this is one of those decisions where it's easiest to make a mistake – and a mistake that will bother you every day for the next fifteen to twenty years. A low-temperature boiler with instantaneous domestic hot water (DHW) preparation is a solution that combines two functions in one device, and this combination brings with it a number of technical specifics that need to be well understood before purchase.
In this category, it's not a problem to choose just any boiler – the problem is choosing the right one for your specific conditions. Far more factors than just power or price come into play. This article will guide you through all the essential criteria we actually address with every order, in the order in which they really need to be resolved.
What is instantaneous DHW heating and why does it differ from a storage tank solution
Before we move on to selection, it's important to understand the principle of instantaneous heating. Unlike a storage water heater, where water is heated gradually and kept in a tank at 55–65 °C, an instantaneous system heats water directly during draw-off – the moment you open the tap. The boiler temporarily increases output and heats the water in the primary circuit, transferring heat to the sanitary circuit via a plate heat exchanger.
The advantages are clear: no storage tank is needed, there are no storage tank heat losses, and there's no need to wait for the stored water to heat up. The disadvantage is also clear: the boiler must be powerful enough to heat the required water flow to the desired temperature in real time. If the output is insufficient, you get lukewarm water – even with the best boiler in the world. This topic is covered in more detail in the article Instantaneous DHW heating vs. storage water heater: pros and cons for a low-temperature boiler in our Knowledge Center.
Atmospheric or turbo boiler: the first and most important decision
Before you start dealing with output, price lists and features, you need to resolve one fundamental question: how will the boiler discharge exhaust gases and where will it draw combustion air from? The answer to this question determines whether you need an atmospheric or a turbo boiler – and this decision is not just about price, but primarily about technical feasibility in the given building.
Atmospheric boilers
An atmospheric boiler burns gas using air from the room in which it is installed. Exhaust gases are discharged through a traditional chimney with natural draft – the same path known, for example, from tiled stoves. Installation is simpler in houses where a chimney exists and is in good condition. However, a sufficient air supply to the boiler room must be ensured – the boiler must be able to "breathe".
Typical representatives of atmospheric boilers with instantaneous DHW preparation are, for example, Vaillant atmoTEC pro VUW SK 200/3-3 (20 kW output) or Vaillant atmoTEC pro VUW SK 240/3-3 (24 kW output). These boilers are suitable for houses with an existing functional chimney and a sufficient boiler room volume.
Turbo boilers (boilers with a closed combustion chamber)
A turbo boiler has a closed combustion chamber. It draws combustion air from the outside environment through a concentric pipe (the so-called coaxial system, colloquially "two-in-one"), discharging exhaust gases the same way. It does not need a chimney in the classic sense – an outlet through the exterior wall or roof is sufficient. This is its big advantage, for example, in renovations of apartment buildings, family houses without a functional chimney, but also in apartments in panel buildings where a shaft was built specifically for turbo boilers.
From the category of turbo boilers with instantaneous DHW preparation, we can mention, for example, Vaillant turboTEC pro VUW SK 202/3-3 (20 kW) and Vaillant turboTEC pro VUW SK 242/3-3 (24 kW). Their advantage is also greater safety – since they do not draw air from the inhabited space, there is no risk of downdraft or carbon monoxide poisoning in case of improper boiler room ventilation.
A detailed comparison of both types can be found in the article Atmospheric vs. turbo boiler with DHW preparation: which type is right for you.
Boiler output: how to correctly determine it for both DHW heating and space heating
This is the critical point where mistakes are most often made – and where a mistake hurts the most. A combined boiler must cover two functions at once, or at least be able to switch between them with sufficient reserve. In practice, simple physics applies to instantaneous DHW heating: if you want to heat a flow of 10 liters of water per minute from 12 °C (typical cold water temperature in winter) to 42 °C (a pleasant shower), you need an output of at least 21 kW just for DHW.
The calculation is as follows: output [kW] = flow [l/min] × temperature difference [K] × 0.07. For 10 l/min and a difference of 30 K, we get: 10 × 30 × 0.07 = 21 kW. This means that a 20 kW boiler simply isn't enough at full shower flow – you'll get lukewarm water. That's why most households with typical water consumption (shower + washbasin) are better served by a 24 kW boiler, not a 20 kW one.
However: a higher boiler output for DHW does not automatically mean that the boiler is also suitable for heating in the given house. A family house with a heat loss of 8 kW does not need a 24 kW output in the heating circuit – the boiler will cycle (turn on and off in short cycles), which wears it out. That's why it's important that the boiler's output in heating mode is modulatable – ideally down to 30–40% of nominal output.
A detailed calculation procedure can be found in the article What boiler output with DHW heating do I need for my house, which also includes specific tables for various types of houses.
| Household type | DHW draw-off | Required DHW output | Recommended boiler |
|---|---|---|---|
| Apartment / 1-2 people | 6–8 l/min | 13–17 kW | 20 kW |
| Family house / 3-4 people | 9–12 l/min | 19–25 kW | 24 kW |
| Larger house / 5+ people | 12–16 l/min | 25–33 kW | storage tank or 2 boilers |
Low-temperature boilers and their limitations – what this means in practice
A low-temperature boiler is not a condensing boiler. This is an important distinction that customers often confuse. A low-temperature boiler operates with a water temperature in the heating circuit of roughly 40–80 °C, and its efficiency increases at lower return temperatures – but not as dramatically as with a condensing boiler. It cannot condense exhaust gases, has no condensate trap, and does not require heat exchanger materials resistant to condensate.
For heating, low-temperature boilers are suitable primarily for:
- older heating systems with cast-iron or steel radiator networks, designed for temperatures of 70/50 °C or 75/60 °C
- renovations where the entire heating system is not being replaced
- houses with poorer thermal insulation, where it's not possible to reduce the heating system's design temperatures below 55 °C
Conversely, for underfloor heating (design temperatures of 35/28 °C) and well-insulated houses, a condensing boiler is more advantageous. A low-temperature boiler will work in such a house, but it doesn't fully utilize the potential of a low-temperature system in terms of efficiency.
What to compare between models: specific technical parameters
Once you know what type (atmo/turbo) and what output you need, it's time to compare specific models. Here are the parameters that are really worth looking at – not just price and brand:
Minimum flow rate to start DHW heating
The boiler does not start preparing DHW every time the tap is opened – it activates only when the water flow reaches a certain minimum value (usually 2.0–3.0 l/min). If this threshold is too high, a situation may arise where the boiler doesn't respond at all when the tap is only slightly opened. This can be unpleasant with older taps or for people who prefer a gentle flow. Look for the lowest possible minimum flow value – around 2.0 l/min is standard, some models achieve as low as 1.5 l/min.
Maximum sanitary flow at Δt 30 K
This is a parameter given by the manufacturer that directly relates to the boiler's "capacity" for DHW heating. For example, a 24 kW boiler with a sanitary flow of 11.4 l/min at Δt 30 K will actually give you a comfortable shower (where 8–10 l/min is enough). If the parameter is lower, e.g. only 8 l/min, you'll notice that the water isn't warm enough when the shower tap is fully open. More on this topic in the article What hot water flow will a boiler with instantaneous DHW heating provide.
Burner modulation range
Quality boilers modulate burner output in a range of roughly 1:4 to 1:5 – meaning a boiler with a nominal output of 24 kW can also heat at 5–6 kW. This is essential for everyday operation: for most of the heating season, the boiler doesn't heat at full output, but at partial output. The greater the modulation range, the less the boiler cycles, and the longer its service interval. Boilers with a narrow modulation range (e.g. 1:2) turn on and off every few minutes, unnecessarily wearing out the burners, ignition electrodes and pump.
Gas connection pressure range
Most boilers are primarily designed for natural gas (G20) at an operating pressure of 20 mbar. If you have propane-butane or LPG, a different set of nozzles and settings is required – check with your dealer whether the boiler is suitable or whether a conversion kit is available. In practice, we've encountered cases where a customer bought a boiler and only discovered during installation that it wasn't ready for propane.
Built-in frost protection and pump protection
Features that seem obvious but aren't: frost protection (the boiler starts up when the water temperature drops below 5–7 °C regardless of the thermostat) and pump protection (automatic pump activation once every 24 hours in summer to prevent it from seizing up). In the event of a long-term absence from a cottage or a power outage, these functions can determine whether the system freezes and cracks or not.
Electronics, diagnostics and communication
Modern boilers (even low-temperature ones) have a display and error codes. The Vaillant atmoTEC pro boiler, for example, displays diagnostic codes that allow a service company to quickly identify the problem. Some models even communicate via eBUS or VR-Net buses, enabling connection to smart controls (e.g. VRC 700 or eRELAX with OpenTherm). This is an advantage for larger installations or when remote control is required.
Vaillant atmoTEC pro vs. atmoTEC plus: when is it worth paying extra
Within Vaillant atmospheric boilers, there are two lines – pro and plus. Vaillant atmoTEC plus VUW CZ/SK 240/3-5 is a higher-class version with extended diagnostics, a more advanced system interface for smart control, and a longer warranty on some components. For a standard family-house installation, the pro model may be entirely sufficient, but for a larger installation or for a customer planning to integrate smart control (e.g. eRELAX), it's worth investing in the plus line. A detailed comparison of parameters can be found in the article Vaillant atmoTEC pro vs. atmoTEC plus: what's the difference and which to choose.
What to watch out for during installation and design
Choosing a boiler without considering the installation conditions is like buying a car without knowing whether your garage door is tall enough. Here are practical points that determine whether installation goes smoothly or you run into problems:
Gas pressure and supply pipe sizing
A 24 kW boiler at full output consumes roughly 2.5 m³/h of natural gas. This needs to be taken into account when designing the gas connection and pressure regulator. If the gas connection is undersized (e.g. originally designed for an old 16 kW boiler), a pressure drop can occur at full output, and the boiler will shut down for safety reasons. A revision technician should check the connection capacity before installation.
Water pressure in the plumbing system
Instantaneous DHW heating is sensitive to pressure in the sanitary circuit. The boiler needs a minimum inlet pressure (usually 1.0–1.5 bar) to be able to detect flow and start heating. In some older houses with a low pressure head or with a shared plumbing network, the pressure may be too low. In such a case, the solution is either a hydrophore or storage-tank heating.
Space and ventilation of the boiler room for atmospheric boilers
An atmospheric boiler needs a sufficient volume of air in the installation room and a guaranteed supply of fresh air. According to the STN EN standard, a minimum volume of 4 m³ per 1 kW of input is required, or a permanent supply of outside air with an opening area of at least 1.5 cm² per 1 kW of input. If the boiler room isn't sufficiently ventilated, the boiler's safety devices will keep shutting it down, and in extreme cases there is a risk of CO poisoning.
Heating circuit connection and pipe sizing
A combined boiler has connections for the primary circuit (flow and return), standardly DN 3/4" or 1". Sanitary connections are mostly DN 1/2". During renovations, it's not uncommon for the original piping to be made of soft steel, and when disconnecting the old boiler, it turns out that the fittings are rusted or cracked. We recommend planning for the replacement of at least the elbows and fittings in the immediate vicinity of the boiler with every renovation.
A more detailed overview of the installation steps can be found in the article Installation of a combined low-temperature boiler with instantaneous DHW preparation.
Long-term operation, service and reliability: what really matters
A boiler isn't a one-off purchase. It's a device you'll live with for fifteen to twenty years – and one a service company will work on every year or every two years. That's why factors not directly related to technical parameters, but to long-term operation, are also important when choosing:
Availability of the service network and spare parts
Vaillant is one of the global players with a dense service network in Slovakia. Spare parts (ignition electrodes, gas valves, heat exchanger plates, NTC sensors) are typically available in stock at most authorized partners. From experience, we know that with less widespread brands, it sometimes happens that a customer waits three to four weeks for a spare part – which is a problem in the middle of the heating season.
Frequency and scope of servicing
A low-temperature boiler with instantaneous DHW heating should be serviced once a year. Servicing includes checking and cleaning the burner, replacing seals, checking the pressure in the expansion tank, checking the ionization electrode and the DHW flow sensor. For atmospheric boilers, this also includes checking the draft and the condition of the chimney. Details on what and how often to check can be found in the article Maintenance and servicing of combined low-temperature boilers: what and how often.
Water hardness and heat exchanger protection
The plate heat exchanger for DHW heating is sensitive to limescale. In very hard water (above 400 mg/l CaCO₃, i.e. above 22.4 °dH, which is common, for example, around Bratislava or Nitra), limescale builds up on the exchanger plates, reducing hot water flow and increasing the plate temperature – which accelerates their degradation. The solution is a softening filter or an inhibitor dosing device. Some customers deal with this through periodic (once every 2–3 years) chemical flushing of the exchanger with citric acid.
Safety features and certification
The boiler must be certified for placing on the EU market (CE marking) and must meet the requirements of the EN 677 standard (for low-temperature boilers) and EN 483 (for combined boilers). Also check whether the boiler has certification from the gas utility for the given country – in the case of Vaillant boilers, this is standardly ensured, but with less well-known brands, it happens that certification is missing or invalid for Slovakia.
Most common mistakes when choosing: practical examples
Over years of practice, we've seen recurring mistakes customers make when choosing a combined boiler. Here are the most common ones:
- Choosing output based on the original boiler: "We had 18 kW, so let's go with 18 kW again." However, the original boiler may have been oversized, or it may have been sized for a storage tank, not instantaneous heating. Result: lukewarm water in the shower.
- Ignoring the minimum flow rate: The customer had low-flow taps and a boiler with a start-up threshold of 3.0 l/min. When washing hands, the boiler didn't respond at all. Solution: replacing the taps or a different boiler.
- Forgetting about the expansion tank: Low-temperature boilers tend to have a smaller built-in expansion tank (2–8 liters). For a larger heating system, an external expansion tank is needed. If this is underestimated, the safety valve keeps discharging and the system loses water.
- Uninspected chimney condition: The customer bought an atmospheric boiler, but the chimney was damp, partially blocked, and the draft was insufficient. The boiler kept shutting down due to a draft fault. Solution: chimney inspection and lining, which doubled the total costs.
- Overpaying for features that won't be used: A customer at a cottage they visit four times a year bought a boiler with advanced smart control and a wifi module. Yet a simple room thermostat with a weekly program would have been enough.
Frequently asked questions about low-temperature boilers with instantaneous DHW preparation
Can I fill a bathtub with an instantaneous boiler and supply several draw-off points at the same time?
It depends on the boiler's output and the required flow. Most 24 kW boilers provide a comfortable flow of 10–12 l/min, which covers showering or filling a bathtub – but not both at the same time. If you need several draw-off points simultaneously (e.g. two showers), an instantaneous boiler with a typical output is not enough. In that case, a boiler with an external storage tank or a larger condensing boiler with an integrated storage tank is more suitable. More information in the article Frequently asked questions about low-temperature boilers with instantaneous DHW heating.
Is the DHW temperature adjustable, and can it be set to 60 °C for disinfection?
Most combined boilers with instantaneous heating have an adjustable DHW temperature in the range of 35–60 °C. Some models allow an even higher setting range, but a temperature of 60 °C with instantaneous heating and full flow may be unattainable for the given output – it depends on the inlet cold water temperature and the flow rate. In winter, when the inlet water is 8–10 °C, an output of around 29 kW is needed to reach 60 °C at a flow of 10 l/min. In practice, the flow is automatically limited via thermostatic taps or a mixing valve.
How long does it take for hot water from the boiler to reach a more distant draw-off point?
The boiler starts heating water immediately after detecting flow – but the hot water physically has to travel through the pipe to the tap. If the draw-off point is 10 meters from the boiler and the pipe is DN 15 (3/4"), the volume of cold water in the pipe is roughly 1.5 liters – which you have to "flush out". At a flow of 8 l/min, this takes about 10–12 seconds. For greater distances or thicker pipes, the wait can reach 30–40 seconds. The solution for larger houses is a DHW circulation loop with a circulation pump – but this requires a return pipe and is more energy-intensive.
Can I install the boiler myself, or do I need a qualified technician?
Gas appliances may only be installed by a person with professional competence under Act No. 56/2012 Coll. and Decree 508/2009 Coll. (in practice: a gas equipment revision technician or a gas equipment installer with the appropriate certificate). A lay installation is not only dangerous but also legally problematic – insurance does not cover damage caused by gas in such cases. Vaillant also makes the warranty conditional on installation by an authorized partner. The warranty is also void if the first start-up is not performed by a professional.
How much energy does the boiler consume in standby mode?
Modern Vaillant low-temperature boilers have very low electrical energy consumption in standby mode – usually under 5 W. During operation, electrical consumption rises to 80–150 W (pump, fan in turbo boilers, electronics). So the boiler's annual electrical consumption is roughly 100–250 kWh – which, at an electricity price of €0.25/kWh, represents €25–60 per year.
Do I need to address hydraulic separation of systems when replacing an old boiler with a new one?
It depends on the specific situation. If you're replacing the boiler 1:1 (same type, same installation), hydraulic separation is not needed in most cases. However, if you're switching from a storage-tank boiler to an instantaneous one, or if you're adding new heating circuits (e.g. underfloor heating), a hydraulic separator or a distribution manifold may be needed. This should be assessed by a designer or an experienced service technician during an on-site inspection. Never plan an installation based on a photo alone – an on-site inspection is essential.
Conclusion: a systematic step-by-step selection process
Choosing the right low-temperature boiler with instantaneous DHW preparation is not a matter of chance or good marketing. It's the result of a systematic assessment of technical conditions, a correct output calculation, and a realistic evaluation of long-term operating costs. To summarize it in a few steps:
- 1. Verify technical feasibility: Is a chimney available (atmo) or is a coaxial outlet possible (turbo)? What is the gas pressure and connection capacity?
- 2. Calculate the required output: Separately for heating (the building's heat loss) and separately for DHW (required flow). The boiler's final output must cover the larger of the two – in practice, this is usually DHW.
- 3. Compare models by technical parameters: Not just output, but also the modulation range, minimum flow to start DHW, availability of controls, and the service network.
- 4. Consider the installation conditions: Chimney condition, water pressure, water hardness, expansion tank size, layout of DHW draw-off points.
- 5. Ensure professional installation and initial commissioning: This is not the place to save money – an installation mistake will cost you more than the price of the boiler.
The category of low-temperature boilers with instantaneous DHW preparation contains proven models that, thanks to their output, reliability and service availability, rank among the most popular solutions on the Slovak market. Whether you choose the atmospheric atmoTEC pro 24 kW for a house with a chimney, or the turbo boiler turboTEC pro 24 kW for a modern renovation without a chimney, it's important that the choice is based on real technical data – not just price or availability.
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 advise.