Frequently Asked Questions about Low-Temperature Boilers with Instantaneous DHW Heating

We are still preparing products.

But you can look at other categories.

Frequently Asked Questions about Low-Temperature Boilers with Instantaneous DHW Heating

Low-temperature boilers with instantaneous domestic hot water (DHW) heating are today one of the most widespread heating solutions for households in Slovakia. Despite this – or perhaps precisely because of it – numerous questions, half-truths and myths circulate around them. Customers ask whether the boiler will manage to heat water in time for a shower, what output they actually need, why they get cold water right after hot water, what "low-temperature" means, and whether such a boiler can handle underfloor heating. In this article we answer systematically, without unnecessary simplification, based on hundreds of real orders and service cases.

What does a "low-temperature boiler with instantaneous DHW heating" actually mean?

Before we get into the questions, let's clarify the terms, because in practice they are used quite loosely. A low-temperature boiler is a boiler that can work with a low system water temperature – typically 35–55 °C instead of the classic 75–90 °C. This allows it to achieve higher efficiency (condensing boilers are an extreme case of the low-temperature principle, but low-temperature boilers are a broader category). Instantaneous DHW heating means that the boiler has no built-in hot water tank – it heats water directly in a flow heat exchanger at the moment you draw it. This solution is in contrast to tank (storage) heating, where the equipment includes a boiler cylinder with a certain volume of hot water (you can find a more detailed comparison in the article Instantaneous DHW Heating vs. Storage Tank: Advantages and Disadvantages for a Low-Temperature Boiler).

The combination "low-temperature + instantaneous DHW heating" therefore means: one compact wall-mounted appliance that both heats the house and heats flowing water on demand. In English it is called a combi boiler; here in Slovakia we call it "kombikotol" or a combined boiler.

An important design detail that many customers don't know: when you open the hot water tap, the boiler automatically switches to DHW heating mode and the heating circuit is temporarily suppressed (so-called DHW priority). Once you close the tap, the boiler returns to heating control. This cycle proceeds smoothly and you usually don't even notice it.

Why instantaneous heating and not a storage tank?

This is one of the first questions customers ask when choosing a boiler. The answer is not straightforward – it depends on the specific household. Instantaneous heating has several undeniable advantages:

Unlimited amount of hot water – as long as gas and water flow, the boiler heats. You won't run out of hot water after the third consecutive shower.
No heat losses from a tank – a tank keeps water hot even when no one is drawing it, which costs energy. Instantaneous heating only heats what is currently flowing.
Compact dimensions – a combi boiler without a tank fits into a kitchen unit or a small pantry.
Lower purchase price – you don't pay for a tank.

On the other hand, instantaneous heating also has limits you should know about: the boiler's output directly determines how much hot water you get per minute and at what temperature. More on this in the flow rate question below.

Atmospheric vs. fan-assisted (turbo) boiler: what's the difference in DHW preparation?

In terms of DHW preparation, both types work practically identically – the difference is in the combustion method and flue gas discharge. An atmospheric boiler (atmoTEC) burns gas with air from the room and discharges flue gases via natural chimney draft. A fan-assisted (turbo) boiler (turboTEC) has a sealed combustion chamber, draws air from outside via a coaxial pipe, and pushes flue gases out using a fan.

From a practical DHW heating standpoint: both types have comparable water heating output. For example, the Vaillant atmoTEC pro VUW SK 200/3-3 and Vaillant turboTEC pro VUW SK 202/3-3 are typologically identical – they differ only in the flue gas discharge system. The choice between them depends on the layout of your utility room, the condition of the chimney, and the flue routing options. You can find a more detailed comparison in the article Atmospheric vs. Turbo Boiler with DHW Preparation: Which Type Is Right for You.

How much hot water will the boiler actually deliver? Real numbers

This is probably the most common practical question and also the most frequent source of disappointment when a customer buys a boiler without checking the flow rate. The DHW flow rate depends on the boiler's DHW output and the required temperature difference between cold water and DHW.

Simple formula: Flow rate (l/min) = DHW output (kW) × 14.3 / ΔT (°C). For example, a boiler with a DHW output of 24 kW, heating cold water at 12 °C to 42 °C (ΔT = 30 °C), will deliver: 24 × 14.3 / 30 = about 11.4 liters per minute. This is a solid flow rate for 1–2 simultaneously operating outlets. For households with 3 or more bathrooms, or with a bathtub and shower running at the same time, this flow rate may be insufficient.

In practice this means: a typical household of 3–4 people with one bathroom outlet in use at a time → a DHW output of around 20–24 kW is sufficient. If you want to fill a bathtub while simultaneously showering in another bathroom, you need an output of 28–32 kW. You can find more specific examples and calculations in the article What Flow Rate of Hot Water Does a Boiler with Instantaneous DHW Heating Provide.

Why does hot water sometimes flow first, followed by cold water, when drawing DHW?

This phenomenon – which customers in practice describe as a "cold knee" or "the temperature drops" – is one of the most frequently reported complaints about instantaneous boilers. The explanation is actually simple and has several possible causes:

Residual water in the pipe: Hot water from the previous draw remains in the pipe between the boiler and the outlet. This flows out first (hence "hot"), then cold water from the main water supply arrives (this is the "knee"), and only after that comes new hot water heated by the boiler.
Long pipe run: The longer the route from the boiler to the outlet, the longer it takes for new hot water to arrive. In older houses without an installed DHW circulation loop, the "cold phase" can last 20–45 seconds.
DHW circulation pump: The solution is a DHW circulation loop, which continuously (or at set intervals) circulates hot water so that when you open the outlet, hot water is "ready" immediately. Not all boilers have a built-in circulation circuit – in some cases it is installed externally.
Minimum flow rate: The boiler only activates once there is sufficient water flow (typically 2–3 l/min). If you have aerators with low flow, the boiler may not start at all, or it starts with a delay.

What are the actual output differences between models? The Vaillant example

Let's look at specific examples from the range, since abstract numbers are hard to grasp. Vaillant produces the atmoTEC pro and turboTEC pro in two basic sizes for the Slovak market – with the number 200/202 and 240/242 in the designation.

The models atmoTEC pro VUW SK 200/3-3 and turboTEC pro VUW SK 202/3-3 have a DHW output of around 20 kW – suitable for an apartment or a smaller family house up to approx. 100–120 m², where you heat one zone and draw DHW from one point at a time. The hot water flow rate is around 9–10 l/min at a temperature difference of 30 °C.

The models atmoTEC pro VUW SK 240/3-3 and turboTEC pro VUW SK 242/3-3, with an output of approx. 24 kW, cover more comfortable, larger houses or households with more demanding DHW draw – two bathrooms, a bathtub. The flow rate can reach 11–12 l/min. For demanding situations where you want comfort close to that of a storage-tank system, there is the Vaillant atmoTEC plus VUW CZ/SK 240/3-5 – a model from the plus series featuring better modulation and additional functions to optimize consumption.

Is a low-temperature boiler suitable for underfloor heating?

Yes, and this is actually one of the main advantages of a low-temperature boiler. Underfloor heating typically operates with a water temperature of 30–45 °C, which is an ideal operating point for a low-temperature boiler. A classic boiler designed for 75/65 °C would work inefficiently at such a low temperature, and could face problems with condensation in the combustion chamber (in the case of a conventional, non-condensing boiler).

A low-temperature boiler is designed directly for underfloor heating. At the same time – if you have a combined system in the house (underfloor heating on the ground floor, radiators upstairs), it is advisable to use a mixing valve for the radiator circuit, because radiators usually require a higher temperature (55–65 °C). The boiler then runs at a low temperature for the floor, and the mixer boosts the temperature for the radiators. This solution is common and brings maximum efficiency to the entire system.

Why does my boiler "bang" or "sizzle" during DHW draw?

Noise during instantaneous DHW heating is another common question directed to service technicians. Possible causes:

Limescale deposits in the flow heat exchanger: If you have hard water (common in Slovakia, hardness above 14 °dH), limescale deposits build up on the internal walls of the exchanger. Water overheats in the narrowed channels and "bangs". Solution: descaling the exchanger, or installing a water softener or an inhibitor dosing device.
Excessively high water pressure: A crackling sound may be caused by a water hammer. The recommended pressure in the household water supply is 2–4 bar. If yours is higher, install a pressure reducing valve.
Air in the DHW system: During first start-up or after a repair, air may get into the flow heat exchanger, causing sizzling.

What is the correct DHW temperature setting and why?

Most boilers allow the DHW temperature to be set in the range of 35–65 °C. The question is what temperature to set. The recommendations are as follows:

At least 60 °C – at this temperature, Legionella bacteria die, which multiply at 25–45 °C. With instantaneous heating, the risk of Legionella is lower than with tanks (water is not held long-term), but manufacturers still recommend setting at least 55–60 °C.
Maximum 65 °C – a higher temperature speeds up limescale formation in the exchanger and is energy-inefficient.
With children in the household: Some families set the temperature lower (45–50 °C) for safety reasons (scald protection). In that case, however, you need to keep Legionella in mind and periodically perform "thermal disinfection" – heat to 70 °C for 30 minutes once a month.

The boiler's controller lets you set the required DHW temperature directly from the display. On older boilers without a display, it's a rotary potentiometer on the front panel.

Can I connect the boiler to a thermostat or smart control system?

Yes, and nowadays customers are asking for this more and more often. Modern combi boilers from the Vaillant atmoTEC/turboTEC series have an eBUS communication interface, through which various types of thermostats can be connected – from simple on/off thermostats to the Vaillant multiMATIC or third-party systems compatible with the eBUS protocol. A smart thermostat can regulate heating based on occupancy, outdoor temperature and your habits, which saves 10–20% on heating costs over the long term. DHW control via smart systems allows, for example, setting the boiler not to heat water at night, or starting heating before the expected morning draw.

What does "output modulation" mean and why is it important?

Modulation is the boiler's ability to vary its output smoothly – not just "on/off". A boiler without modulation (two-position) burns either at full power or not at all. This causes frequent cycling (turning on and off), burner wear, and uneven temperature in the system. A modulating boiler reduces output according to current demand – in mild frost it burns at 30%, in severe frost at 100%.

Modulation is also key for instantaneous DHW heating: if you draw water slowly (e.g. filling a basin), the boiler lowers its output and maintains the temperature through consistent control. If you draw quickly, it goes to full power. Modern Vaillant boilers modulate output in a ratio of 1:5 to 1:7, which is a solid range. This means a boiler with a maximum output of 24 kW can also work at 3.5 kW without shutting down.

What is the minimum heating temperature at which the boiler works efficiently?

A low-temperature boiler is designed for heating water temperatures in the range of 40–75 °C, with typical efficient operation at 50–65 °C. If you pushed it to temperatures below 40 °C without special equipment, condensation of flue gases could occur in the combustion chamber, which is not desirable in a non-condensing boiler – it can lead to corrosion of the heat exchanger and chimney. Condensing boilers are designed precisely for condensing operation, but standard low-temperature boilers (atmoTEC, turboTEC) are non-condensing – this must be taken into account when setting up the system.

In practice: if you have underfloor heating designed for 35/30 °C, it is worth considering a condensing boiler. A non-condensing low-temperature boiler will work at 35 °C, but not optimally. For a combination of underfloor heating + radiators, a low-temperature boiler is an excellent component when used with a mixing valve.

How does frost protection work in the boiler?

All modern Vaillant boilers have a frost protection function. If the water temperature in the boiler drops below a set limit (typically 5–8 °C), the boiler starts automatically and briefly heats the system. This function only works if the boiler is connected to the electrical grid and has enough gas. If you are leaving for a longer period, set the boiler to "Frost protection" mode instead of switching it off completely. If the absence is long-term, it is advisable to drain the system or add antifreeze to the water (though this has further implications for service and warranties).

How often should a combi boiler with instantaneous DHW heating be serviced?

The recommended interval for annual service is once a year – usually before the heating season. Service includes inspection and cleaning of the burner, heat exchanger, ignition electrode, ionization probe, checking the tightness of the gas piping, output adjustment, seal replacement, and sludge removal. For the DHW flow exchanger, checking for limescale is important – with hard water, descaling is recommended every 2 years. Neglecting service leads not only to lower efficiency but also to premature wear of components and loss of warranty. More information can be found in the article Maintenance and Service of Combined Low-Temperature Boilers: What and How Often.

What are the most common error codes and what do they mean?

Vaillant boilers display error codes on the display. From experience, the most common are:

F.22 – low water pressure in the system (below 0.8 bar). Solution: add water to the system via the filling valve, set the pressure to 1.2–1.5 bar when cold.
F.28 / F.29 – the boiler failed to ignite (ignition failure). Cause: gas supply interruption, closed shut-off valve, contaminated ionization probe.
F.73 / F.74 – water pressure sensor fault. Usually requires a service intervention.
F.75 – the pressure sensor did not register a pressure increase when the pump started – possible pump or sensor fault.

A detailed overview of error codes and their solutions can be found in the article Common Faults of Vaillant atmoTEC and turboTEC Boilers and Their Solutions.

Is installing a combi boiler with instantaneous DHW demanding?

Installation of a combi boiler must be carried out by a professionally qualified person (a gas fitter, or a technician certified to install gas appliances). This involves work on the gas installation, which is legally subject to certification. In addition to gas, the heating circuit (primary circuit), DHW supply and discharge, condensate drainage (if relevant), electrical connection, and flue gas discharge must all be correctly connected. A detailed procedure and what to watch out for during installation can be found in the article Installation of a Combined Low-Temperature Boiler with Instantaneous DHW Preparation. Also interested in choosing the right model? Read How to Choose a Low-Temperature Boiler with Instantaneous DHW Preparation: What to Watch Out For.

How much does operating a boiler with instantaneous DHW heating cost compared to a storage tank solution?

The comparison of operating costs depends on several factors. As a guideline: instantaneous DHW heating consumes gas only when water is flowing. A storage tank consumes gas even to maintain temperature (standby losses of 50–100 W continuously). For a typical four-room household with a DHW draw of around 100 liters/day, instantaneous heating can be 10–15% more economical. If the draw is small (e.g. 1–2 people), the difference can be even greater in favor of instantaneous heating. Conversely, for large households with high simultaneous DHW draw, an 80–150 l storage tank combined with a heating boiler may be more comfortable. A more detailed comparison can be found in the article Instantaneous DHW Heating vs. Storage Tank: Advantages and Disadvantages for a Low-Temperature Boiler.

How long does a combi boiler with instantaneous DHW preparation last?

With proper installation, regular servicing and good-quality water, the typical service life of a Vaillant atmoTEC or turboTEC boiler is 15–20 years. The critical components in terms of lifespan are: the DHW flow heat exchanger (with hard water and no descaling, it can fail after 8–10 years), the burner (usually 15+ years with servicing), and the electronic control board (10–15 years). Investing in a boiler is therefore a long-term decision, and it is worth consulting an expert about the model and output – see the article What Boiler Output with DHW Heating Do I Need for My House.

Frequently Asked Questions (FAQ)

Why doesn't my boiler with instantaneous DHW heating deliver sufficiently hot water?

The most common cause is a low water flow rate through the exchanger (aerators, clogged filters) or, conversely, too high a flow rate relative to the boiler's output. Check the DHW temperature setting on the controller – increase it to 55–60 °C. If you have hard water, check the flow exchanger for scale deposits (signs: decreased DHW flow, noisy operation). Insufficient gas pressure in the building's gas supply can also be a problem – with several simultaneous draws in the building, pressure can drop.

Can I have heating and DHW draw running at the same time?

Yes, but during a DHW draw the boiler temporarily prioritizes DHW heating (DHW priority) and briefly suppresses the heating circuit. Once the DHW draw ends, the boiler continues heating. In practice you won't notice this – the heating interruption lasts only a few seconds to minutes, and the thermal inertia of the system evens it out.

What pressure should the boiler have in the system?

The recommended operating pressure of the heating system is 1.0–2.0 bar when cold (water temperature around 20 °C). After heating up to operating temperature, the pressure naturally rises by 0.2–0.5 bar. If the pressure repeatedly drops, there is a leak in the system – call for service. Always top up pressure when the system is cold, slowly, via the filling valve.

Is it a problem if the boiler keeps working in summer (outside the heating season)?

No, on the contrary – in summer the boiler works exclusively for DHW heating, and the heating system's circulation pump only runs briefly at intervals (to protect the pump from seizing due to inactivity). This is a standard summer mode. If you want to minimize the pump's electricity consumption in summer, set the summer mode in the boiler's menu.

What should I do if my boiler keeps cycling (briefly ignites and then shuts off)?

So-called short cycling is a phenomenon where the boiler ignites for a few seconds and then shuts off immediately – and this repeats very frequently. Causes: the boiler is oversized (too high an output for the given system), a faulty thermostat, too little water in the system, or closed radiator valves. Short cycling wears out the boiler and increases consumption. The solution may be installing a larger expansion vessel, a hydraulic separator, or setting a minimum boiler run time.

Can I connect solar collectors to a low-temperature boiler with instantaneous DHW heating?

Direct connection of solar collectors and instantaneous DHW heating is not a common solution – the flow heat exchanger is not designed to prepare water from a solar circuit. The standard solution is: solar collectors heat the water in a storage tank (a bivalent cylinder), and the boiler only tops up this tank when the sun is not enough. In that case, you wouldn't be making full use of a boiler with instantaneous DHW heating – a heating-only boiler (without DHW) or a boiler with storage tank DHW heating would be more suitable. Solar systems therefore require a differently configured system.

Conclusion: key takeaways from this article

A low-temperature boiler with instantaneous DHW heating is an elegant, compact and energy-efficient solution for most typical Slovak households – apartments and family houses up to 200 m² with 1–2 bathrooms and a standard hot water draw. Its advantages are an unlimited volume of hot water, lower operating costs compared to a storage tank, and simpler installation. The limits – primarily the maximum DHW flow rate and comfort's dependence on correctly chosen output – can be solved by selecting the right model and possible additional installations (circulation, water softener).

If you're not sure about your choice, browse the other articles in this Knowledge Center – you'll find detailed guides on choosing output, comparisons of boiler types and specific models. If you're looking for a proven choice with a long history on the Slovak market, take a look at the entire category of low-temperature boilers with instantaneous DHW heating – from simple atmoTEC pro models to the more technologically advanced Vaillant atmoTEC plus VUW CZ/SK 240/3-5.

Do you have a question on 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.