What flow rate of hot water does a boiler with instantaneous DHW heating provide

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How much hot water flow can a boiler with instantaneous DHW heating provide?

When a customer is choosing a combi boiler with instantaneous domestic hot water (DHW) heating, one of the first questions they ask is: "How much hot water actually comes out of it?" It's a legitimate and very practical question – after all, comfort while showering, filling a bathtub, or using multiple draw-off points at the same time is what determines whether a household will be satisfied. However, the answer isn't simple. Hot water flow depends on several factors at once: the boiler's output, the temperature of the cold water entering the boiler, the required outlet temperature, and the hydraulics of the whole distribution system. In this article we'll go through all these factors in detail, with real numbers and practical examples, so you know what to actually expect from a given boiler.

How instantaneous DHW heating in a boiler works – the basic principle

Unlike a storage tank water heater, where water is heated in advance and kept in a tank, instantaneous heating works on the principle of heating the flowing water immediately. The boiler has a dedicated heat exchanger for this – usually a plate or tube type – through which cold water from the mains flows and receives heat from the boiler's primary circuit. The boiler automatically switches priority: when someone opens the hot water tap, the boiler prioritizes DHW heating over space heating and focuses fully on heating the flowing water.

This switch happens within a few seconds, but it's important to know that until the heat exchanger stabilizes, slightly cooler water may flow from the tap for a moment. This usually takes 5 to 15 seconds depending on the length of the pipework and the mass of the heat exchanger. In terms of everyday comfort, this delay is mostly negligible, but with long pipe runs, or when someone needs hot water immediately (e.g. in the kitchen for washing dishes), it can be slightly annoying.

Key formula: boiler output, temperature difference and flow rate

The whole physics of instantaneous heating can be expressed with a simple formula, which is the basis for understanding any numbers in technical data sheets:

Q = m × c × ΔT

Where:

Q = heat output of the boiler dedicated to DHW heating (in kW or W)
m = mass flow rate of water (in kg/s or l/min)
c = specific heat capacity of water ≈ 4,186 J/(kg·K), i.e. for practical calculations at temperatures of 10–60 °C we round to 4,200 J/(kg·K)
ΔT = temperature difference between the incoming cold water and the outgoing hot water (in °C)

From this formula it follows directly:

m [l/min] = (Q [kW] × 1000 × 60) / (4,200 × ΔT)

Simplified for practical calculations: m ≈ (Q × 14.3) / ΔT, where Q is in kW, ΔT in °C, and the result is in litres per minute.

Example: a boiler with a DHW output of 20 kW, cold water at 10 °C, required hot water at 45 °C → ΔT = 35 °C → flow rate ≈ (20 × 14.3) / 35 ≈ 8.2 l/min.

The same boiler in summer, when the incoming water is 18 °C (ΔT = 27 °C): flow rate ≈ (20 × 14.3) / 27 ≈ 10.6 l/min.

This is a key insight that many customers overlook: the same boiler delivers significantly more hot water in summer than in winter, because the cold water enters warmer.

Real flow rates at specific outputs – overview table

To avoid staying only with abstract formulas, take a look at specific flow rate values for various boiler outputs and temperature conditions. The outlet water temperature is set to 45 °C (a common setting for showering and washing), which is also the recommended minimum hygienic temperature.

DHW output	Cold water 8 °C (winter, ΔT=37°C)	Cold water 12 °C (spring/autumn, ΔT=33°C)	Cold water 18 °C (summer, ΔT=27°C)
18 kW	6.9 l/min	7.8 l/min	9.5 l/min
20 kW	7.7 l/min	8.6 l/min	10.6 l/min
24 kW	9.2 l/min	10.4 l/min	12.7 l/min
28 kW	10.8 l/min	12.1 l/min	14.8 l/min
30 kW	11.5 l/min	12.9 l/min	15.9 l/min

For comparison: a regular shower with a standard flow restrictor uses 6 to 9 l/min, a rain shower head with a large diameter 10 to 15 l/min, and filling a bathtub is 12 to 18 l/min. You can see where the limits of instantaneous heating lie.

What these numbers mean in practice – household scenarios

Scenario 1: Two-person household, small apartment, one bathroom tap

A typical two-person household in an apartment usually has one bathroom with a bathtub or shower and a kitchen sink. It never uses all draw-off points at once. In this case, a boiler with a DHW output of around 18–20 kW is sufficient, which corresponds to a flow rate of 7–9 l/min in winter. This covers a comfortable shower (6–8 l/min with a standard flow restrictor). Filling the bathtub will be slower – be prepared for that. A 150-litre bathtub takes over 18 minutes to fill at 8 l/min, but that's acceptable.

Scenario 2: Family of four, detached house, two bathrooms

Here things get more complicated. If the boiler needs to cover, for example, simultaneous showering and dishwashing (10–13 l/min combined), a 20 kW boiler is not enough under winter conditions. At least 24 kW is needed, ideally 28 kW. This is why boilers in this series come in various output variants – for example, the Vaillant atmoTEC pro VUW SK 200/3-3 is sized for smaller households, while the Vaillant atmoTEC pro VUW SK 240/3-3 with higher output covers more demanding draw-off situations.

Scenario 3: Holiday house, seasonal operation

In a cottage or weekend house, where most usage occurs in summer and autumn, DHW flow can be surprisingly comfortable even with a smaller boiler. The reason is simple: the summer temperature of cold water from the mains is 16–20 °C, so the temperature rise is small and a 20 kW boiler will easily deliver 10–11 l/min. Neither filling a bathtub nor showering is a problem.

Scenario 4: Older house, low pressure in the distribution system

This is a classic problem we encounter in older detached houses. The boiler itself may have an output of 28 kW, but if the incoming cold water pressure is only 1 bar (instead of the usual 2.5–4 bar), physically not enough water flows through the heat exchanger. Result: the outlet temperature jumps, the water is sometimes hot, sometimes lukewarm, and the boiler behaves unstably. The solution is either to improve the water pressure (a pressure tank, adjusting the supply), or to consider a storage tank solution. You can read about the differences between instantaneous DHW heating and a storage tank water heater in the article "Instantaneous DHW heating vs. storage tank water heater: pros and cons for a low-temperature boiler".

Technical parameters of boilers – where to find the actual DHW flow rate

Manufacturers usually state the DHW flow rate in technical data sheets as a value at specific conditions – most often at a temperature rise of ΔT = 30 °C or ΔT = 35 °C. It's important to know what that value means. If a data sheet states "DHW flow rate = 10.5 l/min", always check at what ΔT this value was measured.

For example, the Vaillant turboTEC pro VUW SK 202/3-3 boiler is a turbo variant with forced flue gas discharge. Although the principle of DHW heating doesn't differ from the atmospheric variant, the condensing effect and better utilization of combustion heat don't apply here – this boiler is low-temperature, not condensing. This means that the maximum output for DHW is given by the rated burner output, not by condensing enhancement. This sometimes causes misconceptions among customers, as they confuse low-temperature boilers with condensing ones.

For the Vaillant turboTEC pro VUW SK 242/3-3 and Vaillant atmoTEC plus VUW CZ/SK 240/3-5 models, a similar logic applies – a higher numerical output (24x = approx. 24 kW) directly affects the available DHW flow rate. The atmoTEC plus series brings several improvements in control and comfort over the basic pro series, which you can read more about in the article "Vaillant atmoTEC pro vs. atmoTEC plus: what's the difference and which to choose".

Effect of DHW temperature setting on flow rate – a very underestimated factor

Many people set the boiler to the maximum DHW temperature (60–70 °C), planning to mix it with cold water at the tap afterward. Seemingly logical – I'll have more hot water in the tank. But this doesn't apply with instantaneous heating – there's no tank here. Setting a higher DHW temperature on an instantaneous boiler means a HIGHER ΔT, which at the same boiler output means a LOWER hot water flow rate.

Example: a 24 kW boiler, DHW temperature set to 60 °C, cold water 10 °C → ΔT = 50 °C → flow rate = (24 × 14.3) / 50 = 6.9 l/min. The same output setting with DHW at 45 °C → ΔT = 35 °C → flow rate = (24 × 14.3) / 35 = 9.8 l/min. The difference is huge.

For maximum comfort with instantaneous heating, we recommend setting DHW to 45–50 °C (which is also the hygienic standard preventing Legionella growth), not higher. If you need a higher temperature because of Legionella (recommended weekly regeneration to 60 °C), let the boiler's thermal regeneration function do it automatically instead of permanently setting the temperature to 60 °C. Most modern boilers, including the Vaillant pro/plus series, have this function.

Simultaneous draw-off points – where instantaneous heating hits its limits

This is a painful spot of instantaneous heating that needs to be discussed openly. When the shower is running and someone opens the tap in the kitchen, the total hot water flow is split between the two draw-off points. The boiler is still at the same output, but there's twice as much water, meaning the temperature at each outlet drops.

More precisely: the boiler mostly regulates the outlet temperature based on flow rate. At a higher flow rate (two points at once), the control tries to maintain the set temperature, but hits the physical limit of the output. The result is either a drop in DHW temperature at both points, or the flow rate is hydraulically limited (if there are pressure or flow regulators on the outlets in the household).

From practice: a family of four, where two adults get up at the same time in the morning and want to shower simultaneously, needs either a boiler with an output of at least 28–30 kW, or a combined system with a DHW storage tank. Instantaneous heating alone is not enough for such a scenario in winter conditions with a 20 kW boiler. This topic is covered in more detail in the article "How to choose a low-temperature boiler with instantaneous DHW preparation: what to watch out for".

Output modulation – modern boilers adapt

An important aspect to understand: modern boilers with instantaneous DHW heating are not simply on/off at full power. Most current models, including the entire Vaillant atmoTEC pro and turboTEC pro range, feature output modulation – the burner regulates the combustion intensity according to the current need. At a low DHW flow rate (e.g. just washing hands, 2–3 l/min), the boiler runs at minimum output; at full flow, it goes to maximum.

Advantage: better efficiency, smaller temperature fluctuations, longer burner lifespan (fewer switching cycles). Disadvantage: at very low flow rates (below 2 l/min), some boilers won't activate heating at all – their minimum activation flow rate is usually 2–3 l/min. So if you want hot water for washing but have outdated taps with low flow or a water-saving aerator limiting flow to 1.5 l/min, the boiler simply won't recognize it – the water will stay cold. The solution is to replace the aerator with one that has a higher minimum flow rate.

Water pressure, pipe sizing and their effect on flow rate

A boiler may have an output of 28 kW, but if the DHW distribution system is sized incorrectly, the actual flow rate will be lower. The most common mistakes we see in practice:

Too long a hot water pipe run with a small pipe diameter – hydraulic losses increase with length and decrease with diameter. An old house with 3/8" (DN10) copper pipes over a long run can reduce the flow rate by as much as 30–40 % compared to the theoretical value.
Clogged pipework or heat exchanger – limescale and sediment narrow the flow cross-section. This is the most common cause of deteriorated DHW flow in boilers 5–10 years old. Solution: regular cleaning of the DHW heat exchanger and descaling. You can read more about maintenance in the article "Maintenance and servicing of combined low-temperature boilers: what and how often".
Low inlet water pressure – below 1.5 bar, the flow is limited by physics itself. The boiler cannot create pressure – it only transfers heat to the flowing water. If the water doesn't flow fast enough, there's nothing it can do about it.
Forgotten shut-off valves, partially closed taps – it sounds trivial, but we see it surprisingly often on jobs. After installation or servicing, valves remain at 3/4 output, the customer complains about low flow, and the solution is trivial.

Atmospheric vs. turbo boiler – does it affect DHW flow rate?

This is a question customers ask often. The answer is: directly no, but indirectly yes. The type of flue gas discharge (atmospheric chimney vs. turbo sealed chamber with concentric supply/discharge) does not directly affect the DHW output at the same rated boiler output. A 20 kW atmospheric boiler and a 20 kW turbo boiler have the same heat output available for DHW.

Indirect effect: turbo variants (e.g. Vaillant turboTEC pro VUW SK 202/3-3 or Vaillant turboTEC pro VUW SK 242/3-3) have a sealed combustion chamber, which allows them to manage combustion air more efficiently and achieve a slightly better degree of fuel utilization. In practice this can mean 2–4 % higher efficiency, which positively affects gas consumption during intensive DHW use. It has no significant effect on the flow rate in litres itself. A detailed comparison can be found in the article "Atmospheric vs. turbo boiler with DHW preparation: which type suits you better".

How to verify whether the boiler's flow rate suits your household

Before buying a boiler, we recommend doing a simple inventory of draw-off points. Procedure:

List all DHW draw-off points in the house (shower, bathtub, bathroom sink, kitchen sink, possibly dishwasher).
Measure the current flow rate of each draw-off point (plastic bottle + stopwatch, or buy a cheap flow meter for 5 euros).
Decide which combinations may run simultaneously (typically shower + kitchen tap).
Add up the flow rates for the worst-case simultaneous scenario.
Find out the average cold water temperature in your region in winter (ask the water utility company, or measure with a thermometer).
Calculate the required output using the formula: Q = m × ΔT / 14.3 (where m is the total simultaneous flow rate in l/min, ΔT = required DHW temperature minus cold water temperature).
Compare the result with available boiler outputs and choose one with a slight reserve (+20 %).

This procedure will give you a much more accurate result than any blanket recommendation like "20 kW is enough for a 3-room apartment." More about sizing the output can be found in the article "What boiler output with DHW heating do I need for my house".

Common myths about DHW flow rate in combi boilers

Myth 1: "Higher boiler output = always enough hot water." Not necessarily. Output is one of the variables, but the cold water temperature, pipe sizing, water pressure and DHW temperature setting are equally important.

Myth 2: "Instantaneous heating is always worse than a storage tank." No – instantaneous heating has the advantage of an unlimited amount of hot water (as long as the boiler is burning gas, the water is hot). A storage tank can run out. However, a storage tank can simultaneously supply more draw-off points with a larger flow in a short time. It depends on the usage scenario.

Myth 3: "In summer the boiler unnecessarily burns gas for heating when heating DHW." Modern combi boilers have a summer mode (switching to DHW only), during which the heating circuit stays cold and the boiler only takes care of DHW preparation. No "unnecessary" losses into the heating circuit occur.

Myth 4: "The higher the DHW temperature, the more hot water I have." The opposite is true – a higher temperature means a higher ΔT, which means a lower DHW flow rate at the same output. Set the temperature to 45–50 °C, not 65 °C.

Practical tips for maximizing DHW comfort with instantaneous heating

Set the DHW temperature to 45–50 °C, not higher. At this setting you achieve the highest flow rate while maintaining a hygienically safe temperature.
Install a circulation loop on the boiler's DHW outlet (if the house is large) – water will be instantly hot even at distant draw-off points, and you won't be paying for "letting cold water run down the drain."
Before buying, check the water pressure in the household. The ideal is 2.5–4 bar at the boiler inlet. Below 1.5 bar, consider increasing the pressure or a storage tank solution.
Regularly descale the DHW heat exchanger (every 2–3 years with hard water). A clogged heat exchanger can reduce flow by as much as 30–40 %.
Don't use overly restrictive aerators and water-saving flow limiters on showers if the boiler has a minimum activation flow rate of 2–3 l/min. There's a risk of heating not activating.
If you have two bathrooms, consider a boiler with an output of at least 24–28 kW, not a cheaper 18 kW model.

Frequently Asked Questions (FAQ)

Why does the water from the tap first run cold and only turn hot after a while?

This is a natural characteristic of instantaneous heating. The DHW heat exchanger in the boiler has a certain mass and needs to heat up. During those 5–15 seconds, while the burner ramps up to output and the heat exchanger heats up, the water that remained in the pipe from the previous draw-off flows out – and it's cold or lukewarm. The longer the hot water pipe run from the boiler to the draw-off point, the longer the heating-through takes. The solution is a DHW circulation loop with a small circulation pump that keeps the water in the pipes constantly hot.

Can I shower and heat with the space heating system at the same time?

Yes, but when DHW is drawn, the boiler automatically prioritizes hot water heating over space heating. Space heating is temporarily "paused" – the control returns to space heating as soon as the DHW draw-off ends. In practice this means that during a short DHW draw-off (a 5–10 minute shower), the radiators cool down slightly but even out again within a few minutes. With old cast-iron radiators with large thermal mass, this is not noticeable at all.

Why is the hot water flow rate higher in summer than in winter?

Because the temperature of cold water from the mains is higher in summer (15–20 °C compared to 6–10 °C in winter). The boiler only needs to heat the water by a smaller temperature difference, so at the same heat output it can "serve" a larger volume of water per minute. Mathematically: flow rate = output / (4,200 × ΔT). The smaller the ΔT, the greater the flow rate.

What is the DHW output of the Vaillant atmoTEC pro 240 boiler?

The Vaillant atmoTEC pro VUW SK 240/3-3 model has a rated heat output ranging from approx. 10–24 kW (modulated). For DHW heating, the maximum output at this level is available, which under normal conditions (cold water 10 °C, DHW 45 °C, ΔT=35 °C) corresponds to a flow rate of approx. 9.8 l/min. Under summer conditions (cold water 18 °C, ΔT=27 °C) it's up to 12.7 l/min. Always verify the exact values in the manufacturer's current technical data sheet, as they may differ between production series.

Is instantaneous DHW heating suitable for a household of 4–5 people?

Yes, if the boiler is correctly sized. For 4–5 people with one bathroom and kitchen, a boiler with a DHW output of 24–28 kW is sufficient, provided not everyone draws hot water simultaneously. If the house has two bathrooms and two people shower simultaneously in the morning, we recommend 28–30 kW. If draw-off comfort is a priority, or it's a family of five with two bathrooms, consider a storage tank solution or a combi boiler with an integrated mini-tank.

Why does the hot water in the shower fluctuate – sometimes hot, sometimes lukewarm?

DHW temperature during instantaneous heating can fluctuate for several reasons: fluctuations in water pressure (e.g. when someone flushes a toilet or turns on the dishwasher), control hysteresis of an older boiler, a clogged DHW heat exchanger (limescale deposits), or too low inlet water pressure. With modern boilers with electronic control (like the Vaillant pro/plus series), temperature fluctuations are usually minimal – within ±2 °C. If the fluctuation is significant (±5 °C or more), it's likely a hydraulic or servicing issue. We recommend checking the water pressure and the condition of the heat exchanger.

Conclusion – realistic expectations are the basis of satisfaction

The hot water flow rate of a boiler with instantaneous DHW heating is not a fixed value – it's the result of an interaction between the boiler's output, the cold water temperature, the temperature setting, the distribution hydraulics, and simultaneity of draw-off. A typical low-temperature combi boiler with an output of 20 kW delivers 7.5–8.5 l/min in winter conditions, and up to 10–11 l/min in summer. A boiler with an output of 24 kW provides 9–13 l/min depending on the season.

For a one- to two-person household in an apartment or small house, instantaneous heating at 20 kW is fully sufficient. For a larger family with multiple bathrooms and a requirement for simultaneous draw-off points, you should reach for a 24–28 kW variant, or consider a storage tank solution. The key is a realistic calculation before choosing a boiler, not a guess based on the number of rooms.

When choosing a specific model, a comparison of available boilers in the category of low-temperature boilers with instantaneous DHW heating will help you. All models are sized with different output levels precisely to cover various household sizes and draw-off scenarios – from the compact Vaillant atmoTEC pro VUW SK 200/3-3 for smaller apartments to more powerful models such as the Vaillant atmoTEC plus VUW CZ/SK 240/3-5 with extended functionality for more demanding requirements.

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.