Instantaneous DHW Heating in a Condensing Boiler: How It Works and What Affects Comfort
Flow-through DHW heating in a condensing boiler: how it all works and what really affects your comfort
When a customer buys a condensing boiler with flow-through domestic hot water (DHW) heating, they're usually interested in one thing: "Will there be enough hot water when I turn on the tap?" It's a legitimate question, but the answer isn't simple. Behind a comfortable shower lie several laws of physics, design solutions, and operational settings that all interact with each other. In this article, we'll break down the whole chain – from what happens the second you open the hot tap, to why a boiler sometimes doesn't deliver water as hot as you'd expect, and how to fix that.
What is DHW and why is heating it different from space heating
DHW stands for domestic hot water – the water you use for washing, showering, bathing, and similar purposes. Unlike the water in a radiator circuit, which circulates continuously in a closed system, DHW flows through the boiler only once: cold water from the mains enters, and heated water exits directly to your tap.
This is a fundamental difference. The heating circuit can be "pre-heated" slowly by the boiler, storing heat and regulating in advance. DHW must be heated instantly, on demand, without a tank (in the case of a flow-through solution), at a flow rate that can range anywhere from 2 to 15+ litres per minute. This instantaneous demand for high thermal output is the fundamental challenge of the whole flow-through heating concept.
The physics: how much energy is needed to heat water
The basic physics is simple. Heating 1 litre of water by 1 °C requires 4,186 joules (about 1.163 Wh). If you want to heat water from an inlet temperature of 10 °C to an outlet temperature of 45 °C – a difference of 35 °C – at a flow rate of 10 litres per minute, you need a thermal output of:
P = flow rate × specific heat × temperature difference = (10/60) litres/s × 4,186 J/(kg·K) × 35 K ≈ 24,400 W ≈ 24.4 kW
In other words: every 10 l/min of flow at a temperature difference of 35 °C requires almost 25 kW of net thermal output. This is precisely why condensing boilers with flow-through DHW heating usually have a nominal DHW output in the range of 20–28 kW, sometimes even more – otherwise they simply couldn't keep up.
This simple calculation also reveals another important point: the inlet temperature of cold water changes throughout the year. In summer, mains water can be 15–18 °C, while in winter it drops to 5–8 °C. To reach the same outlet temperature of 45 °C, the boiler therefore works with a larger temperature difference in winter and must deliver more energy. This directly affects the maximum flow rate the boiler is able to heat to the required temperature.
Design: what happens inside the boiler when there's a DHW demand
A modern condensing boiler with flow-through DHW heating is essentially a dual-function appliance. It contains two separate hydraulic circuits: primary (heating) and secondary (DHW). These two circuits are connected via a plate heat exchanger or a coil (coaxial) heat exchanger, but the water from them never physically mixes – it's indirect heating.
Here's what happens when you open a hot water tap:
- A flow sensor (flow meter or differential pressure switch) detects water flow in the secondary circuit.
- The boiler's control unit switches priority from heating to DHW – this is called "DHW priority".
- The boiler increases burner output to maximum for DHW (for most boilers, this means full output).
- Cold water flows through the heat exchanger, where it receives energy from the primary circuit water, which is at 60–80 °C.
- The electronic controller uses burner modulation and, if applicable, a modulating pump to maintain the DHW outlet temperature at the set point.
- When you close the tap, the boiler detects the change in flow and switches back to heating or enters standby mode.
This whole process should happen in a fraction of a second. In reality, between opening the tap and truly hot water arriving, 5–30 seconds pass – depending on the length and thickness of the pipes. This isn't a boiler malfunction, but physical reality: the water already in the pipe (which has cooled down) has to flow out before truly hot water arrives.
DHW priority: what it means for heating and why it matters
One of the most commonly misunderstood features of a combi condensing boiler is so-called DHW priority. When you open a hot tap, the boiler immediately switches almost all its output to heating water and space heating temporarily "stops". This interruption can last anywhere from 20 seconds (a quick hand wash) to 10–15 minutes (filling a bathtub). During this time, the radiators receive less heat, or none at all.
In practice, in well-designed systems this doesn't matter – radiators have sufficient thermal inertia, and a few-minute outage doesn't cause any noticeable drop in room temperature. Problems arise when:
- The household draws DHW continuously for an extended period (e.g. several people showering one after another in winter).
- The house has very low thermal storage (a lightweight new-build with a small-scale heating system).
- The boiler is undersized and can't keep up even with regular heating – in which case DHW priority makes the situation worse.
Modern boilers from Protherm and Vaillant solve this problem with intelligent control: the boiler remembers how long the DHW draw-off phase lasted, and afterwards briefly increases heating output to "catch up" on the deficit. Some models also allow setting so-called partial DHW priority – the boiler doesn't give its entire output to DHW but leaves some for the heating circuit.
DHW output: why manufacturers list two figures
On every condensing boiler with flow-through DHW heating, you'll usually find two output values in the technical documentation – for example "18/25 kW" or "20/26 kW". The first figure is the heating output, the second is the DHW output. Why is the DHW output higher? Because during DHW heating, the boiler operates briefly at a higher output, which is thermodynamically manageable and safe for a condensing boiler, since combustion modulates over a wide range.
For example, the Protherm Gepard Condens 18/25 MKV has a heating output of 18 kW but reaches 25 kW for DHW heating. Other models work similarly – the Protherm Puma Condens 18/24 MKV offers 18 kW of heating output and 24 kW of DHW output. This figure directly determines how much hot water the boiler can produce per minute.
Approximate calculations of maximum DHW flow rate at different outputs (inlet water temperature 10 °C, outlet 45 °C, Δt = 35 °C):
| DHW output (kW) | Max. flow rate (l/min) | Typical use |
|---|---|---|
| 18 kW | ~7.3 l/min | Shower (single point) |
| 21 kW | ~8.5 l/min | Shower + sink simultaneously |
| 24–25 kW | ~9.7–10.1 l/min | Shower + kitchen, comfortable |
| 28–30 kW | ~11.3–12.1 l/min | Larger family, 2 draw-off points |
These values apply under ideal conditions. In practice, you need to account for cooler inlet water in winter (5–8 °C), which means the actual flow rate will be 10–15 % lower than in the table above.
Why cold water flows at first – and what to do about it
This is a topic customers mention very often, and one that's a source of many misunderstandings. The situation looks like this: you open the hot tap, you wait... 10 seconds... 20 seconds... 30 seconds... and cold water is still flowing. The boiler is working correctly, but the hot water is still on its way.
The problem isn't the boiler, but the pipework. Every pipe between the boiler and the draw-off point contains a certain volume of water that has cooled down to room temperature since the last draw-off. This cooled water has to flow out before truly hot water arrives. The longer the pipe and the larger its diameter, the longer the wait.
A real-world example: a boiler in a basement boiler room, a bathroom on the second floor, pipe distance ~12 metres, DN 15 (3/4"). Pipe volume = 12 m × π × (0.0075 m)² ≈ 2.12 litres. At a flow rate of 8 l/min, that means a wait of ~16 seconds. If the pipe is DN 22 and longer, you could wait a minute or more.
There are solutions:
- DHW recirculation pipe – a separate return pipe with a circulation pump that keeps the water in the pipe hot. The most comfortable solution, but it increases energy consumption. Many condensing boilers (including Vaillant and Protherm models) have a dedicated inlet for recirculation, or even a built-in circulation inlet.
- Local storage tank at the draw-off point – a small 5–10 l tank that delivers water instantly. Limited capacity.
- Reducing pipe diameter – during renovations, it's sometimes possible to replace a DN 22 distribution system with DN 15 or even 12×1 mm copper or plastic piping, which significantly reduces the volume of cold water sitting in the pipe.
Burner modulation and maintaining a constant DHW temperature
One of the key features of a modern condensing boiler is modulation – the ability to continuously vary thermal output from minimum to maximum. For DHW heating, this feature is extremely important, because the flow rate of hot water is not constant. When you turn the tap slightly, the flow decreases. When you open another tap as well, the flow increases. The boiler must respond to these changes instantly.
Older types of control worked in a binary fashion – the boiler was either firing at full power or switched off. The result? Unstable DHW temperature, fluctuations of ±5–8 °C, unpleasant temperature shocks while showering. Modern condensing boilers modulate typically within a range of 1:5 to 1:8 – that is, from 20 % to 100 % of output. Some premium models have an even wider modulation range.
As a result, at a constant flow rate the DHW outlet temperature is stable within a range of ±1–2 °C. This is the comfort standard customers are used to today.
A practical example: a customer in Bratislava, a family house, 2 bathrooms. They use thermostatic shower valves set to 38 °C. The boiler maintains DHW at 45 °C, and the thermostatic mixer adds cold water. The result: exactly 38 °C without fluctuation, comfort comparable to an electric water heater – but without waiting time and without a storage tank.
Comfortable shower flow rate vs. boiler capacity
A typical showerhead has a flow rate of 8–12 l/min at normal mains pressure. A rain shower with a larger diameter can have 15–20 l/min. These are figures you need to keep in mind when sizing a boiler.
If a family has 4 members who tend to shower one after another without a break, the boiler must be able to maintain the required flow rate over an extended period. Flow-through heating has no storage tank, so there's no issue of "running out of hot water" – the boiler heats continuously as long as it has gas and water. The only limitation is the maximum flow rate the boiler can handle at the required temperature.
A sample sizing example for a flat vs. a family house:
- Studio/1-bedroom flat, 1 person: 18–20 kW of DHW output is sufficient. A shower at 8 l/min, Δt 35 °C = 19.7 kW.
- 2–3-bedroom flat, 2–3 people: 24–25 kW of DHW output is a safe choice. Shower + sink = ~12 l/min = ~24.6 kW.
- Family house, 4–5 people, 2 bathrooms: consider 28–30 kW DHW output or a storage-tank solution.
For larger family homes with genuinely high DHW demand, we also recommend looking at a comparison of flow-through vs. storage-tank DHW heating – this topic is covered in a separate article in our Knowledge Centre: Condensing boiler with flow-through heating vs. storage-tank DHW heating: which is better.
Specific models: what they offer in practice
Let's take a closer look at a few specific models that cover a wide range of needs:
The Protherm Gepard Condens 18/25 MKV is one of the most popular models for flats and smaller family houses. With a 25 kW DHW output, it can comfortably handle a shower cubicle even with simultaneous use of a sink. It has a fully modulating burner, electronic ignition, and a built-in flow meter for precise detection of DHW draw-off. For most Slovak households in a flat or smaller house, it's a safe choice.
The Protherm Panther Condens 20/26 KKV pushes the DHW output to 26 kW, which at an inlet water temperature of 10 °C corresponds to a flow rate of ~10.5 l/min. The model has an extended modulation range and is suitable for larger flats or smaller family houses where a bathroom and kitchen are used simultaneously.
From the Vaillant range, the Vaillant VUW 236/5-3 ecoTEC pro is worth noting – a model with a 26 kW DHW output and a wide modulation range. Vaillant is traditionally distinguished by its sophisticated electronic control, which significantly reduces temperature fluctuations during changing draw-offs. Suitable for more demanding customers who value consistent comfort.
For those looking for a premium segment with modern diagnostics, there's the Vaillant VUW 26CS/1-5 ecoTEC plus IoniDetect. IoniDetect technology analyses the flame in real time using an ionisation sensor and optimises combustion to a level conventional boilers cannot reach. The result is not only long-term reliability, but also minimal emissions and high efficiency specifically during flow-through DHW heating.
Hydraulic balancing and mains pressure: hidden comfort factors
Customers sometimes wonder why, with the shower mixer set to 40 °C, the water is sometimes 35 °C and sometimes 44 °C. The boiler is set the same, the burner is modulating... so where's the problem?
The answer often lies in cold water pressure. If a washing machine, dishwasher, or toilet flush is running elsewhere in the flat, the cold water pressure in the mains drops. The thermostatic mixer tries to maintain the hot/cold ratio – but at lower cold water pressure, it adds less cold water, which pushes the resulting temperature up. If the boiler also can't respond fast enough through modulation, the DHW temperature will fluctuate.
Solutions include:
- Installing a pressure-reducing valve with a consistent setting (~2–3 bar for the DHW distribution system).
- Using thermostatic mixers with pressure balancing, which automatically respond to changes in cold water pressure.
- Proper hydraulic balancing of the entire DHW distribution system – so that all draw-off points have similar and stable pressure.
This topic is also covered in detail in the article Installing a condensing boiler with flow-through DHW heating: procedure and requirements in our Knowledge Centre.
Condensation and DHW: how they're related
A condensing boiler achieves its high efficiency (up to 109 % Hs) precisely by making use of the latent heat released when water vapour in the flue gases condenses. This condensation occurs when the return temperature is sufficiently low – usually below 55 °C, ideally around 35–45 °C.
However, during DHW heating the boiler operates with higher primary circuit temperatures (~70–80 °C) in order to be able to heat the water quickly through the exchanger. This means that during the DHW heating phase, the condensing effect is partially suppressed – the boiler behaves more like a conventional boiler, not a fully condensing one.
This is why experts say that the actual savings of a condensing boiler are realised primarily during the heating phase, not during DHW heating. That's why, when planning consumption, it's important to distinguish between these two operating components. You can read more about setting temperatures for optimal savings in the article Setting the temperature and pressure in a condensing boiler with DHW for optimal savings.
What specifically affects comfort with flow-through heating: a summary of factors
To conclude this section, let's summarise all the factors that a customer (and installer) should be aware of:
- Boiler DHW output (kW) – directly determines the maximum hot water flow rate. A basic parameter when choosing a boiler.
- Cold water inlet temperature – drops in winter, reducing the actual available flow rate.
- Modulation range – the wider it is, the more stable the DHW temperature during changing draw-offs.
- Length and diameter of DHW pipework – affects the waiting time for hot water.
- Cold water mains pressure – pressure fluctuations cause temperature swings at thermostatic mixers.
- DHW priority and its setting – with prolonged DHW draw-off, it can temporarily affect heating output.
- DHW recirculation piping – its presence or absence significantly affects comfort (waiting for hot water).
- Water quality (hardness) – hard water forms limescale in the plate heat exchanger and worsens heat transfer. Recommended hardness for boilers is up to 3 mmol/l (17°dH). More on this in the article Maintenance and servicing of a condensing boiler with DHW.
Frequently asked questions (FAQ)
Why does my boiler stop heating DHW – cold water flows after a few minutes?
This is a relatively common problem with several possible causes. The most common is overheating of the DHW heat exchanger caused by limescale – deposits reduce heat transfer, the boiler overheats, and the safety thermostat shuts it down. Another possibility is a clogged flow meter or flow sensor, which stops detecting flow, making the boiler think the tap has been closed. In some cases, it's an issue with the control system or burner modulator. In any case, diagnosis by a service technician is needed. More in the article Common faults in condensing boilers with flow-through DHW heating and how to fix them.
Can I set the DHW temperature to 60 °C to make sure I have enough hot water?
You can, but it doesn't make sense on a flow-through boiler. On a storage-tank boiler, 60 °C is set to prevent Legionella in the tank. With flow-through heating, water isn't stored – it passes directly through the exchanger to the tap. Setting 60 °C as the DHW setpoint will only mean you have to add more cold water at the tap, which actually wastes energy. The optimal setpoint for flow-through heating is 42–48 °C, ideally 45 °C.
Why does the water temperature fluctuate while I'm showering – sometimes hot, sometimes cold?
The most common cause is a change in cold water mains pressure (a washing machine running, a flush, another draw-off elsewhere in the house) combined with a thermostatic mixer that can't respond fast enough. It could also be unstable burner modulation (an older or damaged modulator). In some cases, the cause is the so-called "cold water sandwich" effect – during short interruptions in draw-off (e.g. you shower, then pause for a moment), a column of water that has had time to cool remains in the pipe. Solutions include thermostatic mixers with pressure balancing and properly set mains pressure.
How many litres of hot water per minute will a 24 kW DHW boiler give me?
At a cold water inlet temperature of 10 °C and a required outlet temperature of 45 °C (temperature difference of 35 °C), this is: 24,000 W / (4,186 J/(kg·K) × 35 K) ≈ 0.164 kg/s ≈ 9.8 l/min. In summer, when the inlet water is 15 °C (difference of only 30 °C), it will be ~11.5 l/min. In winter, with an inlet of 5 °C (difference of 40 °C), it drops to ~8.6 l/min. That's why manufacturers usually state flow rate at reference conditions (Δt = 30 °C) – read the technical data sheets carefully.
Is flow-through DHW heating in a condensing boiler more economical than an electric water heater?
Yes, in absolute terms of energy consumption cost, a gas condensing boiler is significantly cheaper than direct electric heating. The price of 1 kWh from gas in Slovakia today is about €0.07–0.10, while the price of 1 kWh of electricity is €0.18–0.25. A condensing boiler also has an efficiency of ~98–109 % (calculated on calorific value), whereas a direct electric water heater has 100 % conversion, but the electricity itself is more expensive. The difference in annual operating costs for DHW heating for a typical family of 4 can be €200–400 in favour of the gas condensing boiler.
Does it make sense to add an external DHW storage tank to an existing condensing boiler with flow-through heating?
Yes, under certain circumstances it makes a lot of sense. If you have a flow-through boiler with a DHW output of 24–26 kW, but the family has 5–6 members and simultaneous DHW draw-offs are pushing the boiler's capacity to its limit, adding a 120–150 litre tank will significantly improve comfort. The boiler pre-heats the tank during periods of low demand, and the tank then absorbs peak demands. Not every boiler is suited for this – consult a technician before making such a modification, and take a look at the article Condensing boiler with flow-through heating vs. storage-tank DHW heating: which is better.
Conclusion: flow-through DHW heating isn't magic, it's physics
Flow-through domestic hot water heating in a condensing boiler is an elegant, compact, and – when properly sized – extremely comfortable solution. The key is understanding that comfort rests on three pillars: sufficient DHW output from the boiler, stable distribution hydraulics, and quality components (thermostatic mixers, correct mains pressure, and possibly recirculation). When these three things line up, flow-through heating in operation is not inferior to a storage-tank solution, and thanks to its smaller footprint and lower heat loss, it even has real advantages over it.
If you want guidance on which model is suitable for your specific case, we recommend visiting other articles in this Knowledge Centre – especially How to choose a condensing boiler with flow-through DHW heating: what to look for and What output condensing boiler with DHW do I need for my house. If you have specific technical questions about models, you'll find the parameters directly on the product pages in the category of condensing boilers with flow-through DHW heating.
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