How Long Does It Take to Heat Water in the Tank and What Determines DHW Comfort
How long does it take to heat water in the tank and what does DHW comfort depend on
One of the most common questions customers ask when choosing a condensing boiler with a built-in tank is simply: "How long will I have to wait for hot water?" This seemingly straightforward question actually has a surprisingly complex answer – it depends on a dozen different factors at once. The heating time of the tank and the overall comfort of domestic hot water (DHW) are not just a matter of boiler output, but also of tank volume, cold water temperature, control method, hydraulic connection, and the household's daily habits.
In this article, we'll look at the whole topic systematically, with specific numbers and practical examples, so that after reading it you can form a realistic idea of what to expect in everyday operation.
Basic principle: how a boiler with a built-in tank heats water
A condensing boiler with a built-in tank works on the principle of indirect heating. The DHW tank is not in direct contact with the flame – instead, it has a heat exchanger built inside (usually in the form of a coil or tube system) through which hot water from the boiler's primary circuit flows. This primary water transfers heat to the drinking water in the tank without the two mixing.
The result is twofold: firstly, you get hygienically clean drinking water separated from the heating water, and secondly, you have a certain "energy reservoir" available – a volume of preheated water you can draw from without the boiler having to work at full capacity at that moment.
Tank heating time: specific numbers and a formula
The time needed to heat the DHW tank can be estimated using a simple physical formula:
t = (m × c × ΔT) / (P × η)
where:
- m – mass of water in the tank (kg, practically equal to the volume in litres)
- c – specific heat capacity of water (4,186 J/kg·K, or 1.163 Wh/l·K)
- ΔT – temperature difference between the initial and required water temperature (°C)
- P – boiler heat output designated for DHW heating (W or kW)
- η – heat transfer efficiency through the exchanger (typically 0.85–0.95)
To make this less abstract, let's look at specific real-life examples that we commonly encounter:
Example 1: 21-litre tank, 20 kW boiler
A tank with a volume of 21 litres, cold water at inlet 10 °C, heated to 55 °C (ΔT = 45 °C), boiler output for DHW 20 kW, exchanger efficiency 0.90:
t = (21 × 1.163 × 45) / (20,000 × 0.90) = 1,098.9 / 18,000 ≈ 0.061 hours ≈ 3.7 minutes
This is the tank you'll find, for example, in the Protherm Tiger Condens 20/26 KKZ 21. Almost four minutes to heat the complete supply – this is a realistic situation after a large draw-off or after the system wakes up in the morning.
Example 2: 42-litre tank, 20–26 kW boiler
A 42-litre tank, the same input conditions (10 °C → 55 °C, ΔT = 45 °C), output 24 kW, efficiency 0.90:
t = (42 × 1.163 × 45) / (24,000 × 0.90) = 2,197.2 / 21,600 ≈ 0.102 hours ≈ 6.1 minutes
So for the Protherm Tiger Condens 20/26 KKZ 42, we're talking about roughly 6 minutes from cold water to a fully heated tank. This tank is very popular for 3–4 person households.
Example 3: 42-litre tank, 30–35 kW boiler
With the more powerful Protherm Tiger Condens 30/35 KKZ 42, with a maximum DHW output of around 30 kW:
t = (42 × 1.163 × 45) / (30,000 × 0.90) = 2,197.2 / 27,000 ≈ 0.081 hours ≈ 4.9 minutes
A higher output shortens the heating time by roughly 20%. At first glance not a big difference, but in practice – when the family is rushing to shower one after another in the morning – it can matter.
What actual DHW comfort in the household depends on
The heating time of an empty tank is an important figure, but in real operation what you care about is something else: whether you have enough hot water when you need it. DHW comfort is a multifactorial issue – let's look at each factor separately.
1. Tank volume and household size
The golden rule is: a tank should have at least 40–50 litres per person if you plan to take showers in quick succession. In practice this means:
- 1–2 people: a 21–35 litre tank is sufficient if the boiler is powerful enough
- 3–4 people: a 42–50 litre tank is the minimum, ideally 80–100 litres
- 5 or more people, or a bathtub: an 80–120 litre tank, or an external tank
A 42-litre tank at a temperature of 55 °C with cold water at 10 °C will give you, after mixing to a comfortable 38 °C, roughly 75–80 litres of usable water (due to mixing with tap water). One longer shower uses 50–80 litres at 38 °C, so with a 42-litre tank you can comfortably manage one shower, but for a second one right after, you'll have to wait for reheating.
We wrote about how to correctly choose the volume in the article What tank volume do I need for my house – we recommend reading it before making a final decision.
2. Boiler output for DHW heating (peak output)
Boiler manufacturers state the output for DHW and the output for heating separately – and this is no coincidence. Condensing boilers with a tank prioritize DHW heating: when the tank cools down and its heating starts, the boiler works at full DHW output and heating is temporarily suppressed (for a few minutes). That's why sufficient output is important – with an undersized boiler, reheating takes longer, and if you're heating simultaneously in the morning, you may notice a drop in room temperature.
The Vaillant ecoTEC plus in the model VUI 26CS/1-5 ecoTEC plus IoniDetect has a DHW output of 26 kW, which with a 42-litre tank means a real heating time of about 5.5–6 minutes. This combination with the sensoCOMFORT 720 controller also allows prediction of draw-offs and intelligent preparation of the tank before the morning peak.
3. Cold water inlet temperature
This is a factor often forgotten in discussions about boilers – yet it's essential. In summer months, tap water temperature is typically 15–18 °C, while in winter it can drop to 5–8 °C. The difference from the target temperature of 55 °C is therefore 37–40 °C in summer and 47–50 °C in winter. With winter's colder inlet water, a tank of the same volume heats up 15–20% longer than in summer.
Practical consequence: if the tank suits you in summer, in winter (when DHW demand is usually also higher) you may find it insufficient – and that's not a malfunction, it's physics.
4. Set tank temperature
Most boilers allow setting the tank temperature in the range of 40–70 °C. The higher the temperature, the:
- longer the heating time
- greater the usable volume (at 65 °C the tank yields more usable water after mixing than at 45 °C)
- higher the energy consumption
- better the hygienic protection (Legionella does not survive above 55 °C)
Hygiene standards recommend a minimum of 60 °C in the tank to eliminate the risk of Legionella. If you set 45 °C, the tank heats up faster, but you must be careful with longer boiler shutdowns (holiday mode). The optimal compromise is 55–60 °C.
5. Quality and area of the heat exchanger
Different manufacturers and models have different internal tank exchanger designs. A higher-quality coil with a larger area – measured in m² – transfers heat more efficiently and faster. For example, Protherm Tiger Condens KKZ series boilers have a stainless steel exchanger with an area optimized for fast heating, which shows in good draw-off capacity (the so-called NL25 parameter – output in litres per minute at a 25 °C temperature drop).
6. Hydraulic connection and DHW circulation
Family houses often have a DHW circulation loop installed – hot water continuously circulates through the pipes being reheated, so you get hot water at the tap immediately without a long wait for flow. This solution dramatically increases comfort, but has two drawbacks: higher energy consumption (constant heat losses from the piping) and a smaller effective tank volume (circulation slightly cools it down). If you have long piping runs (typically an older house where the bathroom is 8–15 metres from the boiler room), circulation is worth it. A modern smart controller, such as the one in the Protherm Tiger Condens 20/26 KKZ 42 with a smart controller set, can control circulation intelligently – running it only at set times or on demand via the app, saving energy.
Specific operating scenarios and their impact on waiting time
Theory is one thing, practice another. From on-the-ground experience, we know that DHW comfort depends on the specific habits of the family. Here are typical scenarios:
Scenario A: Morning peak (3 people, showers one after another)
Parents get up at 6:00, the child at 7:00. The boiler has a 42-litre tank preheated to 55 °C. The first shower (8 minutes, ~50 litres at 38 °C) largely depletes the tank. The boiler immediately starts reheating. At an output of 24 kW, the tank is full again within 6 minutes. The second person gets into the shower 10–15 minutes later – no problem. The third person an hour later – the tank has long since been heated.
Conclusion: a 42-litre tank with a 24 kW boiler handles a 3-person household without stress, as long as showers aren't taken immediately one after another.
Scenario B: Evening children's bath + adults' shower
A bath for children (volume approx. 150 litres, but actual hot water draw-off after mixing approx. 100 litres) is a critical draw-off. A 42-litre tank is not enough – the boiler must operate in flow mode or reheat the tank quickly. In this scenario, the 42-litre tank proves to be a limiting factor. Recommended solution: an 80-litre tank, or an external tank. If you invest in a boiler with a 42L tank, it's a good idea to fill the bath gradually (not fill 150L at once), or set the tank temperature to 65 °C, which increases the volume of usable hot water after mixing.
Scenario C: Cottage or house with irregular use
A holiday home where the family arrives for the weekend. The tank has been cold all week. After arrival, you need to wait 5–8 minutes (depending on the model) and hot water is available. Here, the compactness of a boiler with a built-in tank is an advantage – no external 120-litre tank that would need an hour to heat up. You can find more about deciding between a built-in and an external tank in the article Condensing boiler with a tank vs. boiler with an external tank: which is more worthwhile.
Scenario D: Two-generation household, 6 people
Here, a built-in 42-litre tank simply isn't enough. You need either a very powerful boiler (30–35 kW) with the largest possible built-in tank, or a combined solution of a boiler + an external tank of 100–150 litres. For such households, the strongest argument for the Protherm Tiger Condens 30/35 KKZ 42 is precisely its higher DHW heating output, which shortens reheating times.
Priority DHW heating: what it means in practice
Most condensing boilers with a tank operate in a so-called DHW priority mode. If the tank temperature drops below the set value (typically 5–10 °C below the target), the boiler switches full output to heating the tank and heating is temporarily interrupted. In a well-insulated house you will practically not notice this – a 5–8 minute interruption of heating in a well-insulated room causes only a fraction of a degree drop in temperature. In an old, uninsulated house on a frosty night, it may be more noticeable.
Modern controls – such as sensoCOMFORT 720 combined with the Vaillant VUI 26CS/1-5 ecoTEC plus – can mitigate this conflict by preheating the tank ahead of predicted peaks (in the morning before waking up, in the evening before bathing), so it doesn't need to intervene in heating during the hottest part of the day.
The role of controls and "smart" functions in DHW comfort
Modern boilers with a built-in tank offer increasingly sophisticated options for controlling DHW preparation. Basic functions are available even in simple models, but the difference between a basic and an advanced controller can be very significant in practice.
Time programs for tank heating
Instead of keeping the tank at temperature continuously (which is the most energy-costly), you can set time windows during which the tank should be heated. Typically: 5:30–7:00 (morning hygiene), 11:00–12:00 (lunch), 18:00–21:00 (evening bathing). Outside these windows, the tank "rests" and its temperature drops slightly – which is beneficial, but requires some discipline in household habits.
ECO button and holiday mode
Almost all boilers have a holiday or ECO mode, in which the tank maintains only a minimum temperature (typically 20–35 °C). After returning and switching back to normal mode, heating from cold to 55 °C takes those 5–8 minutes – manageable without stress for most households. Note: with a longer holiday mode, we recommend running a manual "hygienic heating" cycle to 65–70 °C once every 1–2 weeks to eliminate bacteria.
Intelligent pre-programming via smart controls
The Protherm Tiger Condens 20/26 KKZ 42 with a smart controller set offers the option to control it via a mobile app. You can remotely turn on tank heating before arriving home – hot water will be ready when you enter the house. For cottage owners or people with irregular work schedules, this represents a significant improvement in quality of life.
The impact of boiler and tank aging on heating time
After several years of operation, tank heating time may be longer than at first startup. The most common causes are:
- Limescale on the tank's heat exchanger: in areas with hard water (most of Slovakia, hardness above 15 °dH), scale builds up on the surface of the coil inside the tank. A 1 mm layer of scale reduces heat transfer by 10–20%, a 3 mm layer by 30–40%. Result: heating takes longer, and the boiler "works" for the same result with higher gas consumption.
- Primary circuit circulation pump failure: if the pump loses output, the primary water flow through the tank exchanger decreases and heat transfer deteriorates.
- Three-way valve fouling: the three-way valve switches between heating and the tank. In case of partial failure, it may not switch correctly, and the tank heats less efficiently.
Regular servicing – recommended once a year – includes checking these components. You can find more about maintenance in the article Maintenance and servicing of a condensing boiler with a tank: what and how often to check.
Hard water, softening, and their impact on the tank
The issue of hard water in the DHW tank deserves special attention. In Slovakia, most supplied areas have water hardness of 10–30 °dH (German degrees). The DHW tank is particularly prone to scaling, because the water inside it is static and heated – both conditions accelerate the precipitation of calcium carbonate.
Practical recommendations:
- At a hardness above 20 °dH, consider installing a softening filter on the cold water inlet to the tank.
- Keep the tank temperature at 55–60 °C, not 65–70 °C – higher temperatures accelerate scale build-up.
- Plan tank descaling (chemical or pressure cleaning) every 2–3 years at a hardness above 15 °dH.
- Stainless steel tanks are more resistant to corrosion than enamelled ones, but scale builds up equally.
Comparison: built-in tank vs. instantaneous DHW heating
DHW comfort from a tank boiler vs. an instantaneous (dual-flow, so-called "combi") boiler is a fundamental difference, and customers ask us about it very often:
- Instantaneous boiler: hot water immediately, unlimited volume (as long as water flows), but with multiple simultaneous draw-offs (shower + washing machine) the output may be insufficient. Requires higher maximum output (28–35 kW).
- Tank boiler: the tank absorbs peak demands, doesn't depend on the boiler's instantaneous output, but once the tank is depleted you need to wait 5–8 minutes. Advantageous for short-term intensive draw-offs (shower) and in households with predictable habits.
For a 3–4 person family in a family house, where the bathtub isn't used every day, a tank boiler with a 42-litre tank is an excellent solution. For families with higher consumption or unpredictable draw-offs, an external tank of 80–120 litres is more suitable. You can find a comparison of both approaches in the article Condensing boiler with a tank vs. boiler with an external tank: which is more worthwhile.
Energy efficiency and DHW comfort: how they're related
Condensing boilers with a tank have one interesting feature: they achieve higher condensing efficiency when heating DHW than when heating spaces. The reason is simple – the tank's return is cold (30–40 °C), which is an ideal temperature for condensing the flue gases. Condensing efficiency for DHW can reach 107–110% (relative to gas calorific value), while for heating at 55/45 °C it's 95–100%.
Practical consequence: heating the DHW tank isn't just comfortable, it's also energy efficient. Annually, DHW makes up 25–40% of a household's total gas consumption, so efficient DHW preparation has a measurable impact on the annual energy bill.
Frequently asked questions (FAQ)
Why does it take longer for hot water to arrive in the morning, even though the tank is heated?
The tank may be heated, but the hot water has to "travel" from the boiler room to the tap through cold piping. If you don't have a circulation loop, the water in the pipe has cooled down in the meantime, and you need to wait for the cold water to flow out before hot water arrives from the tank. Solution: a circulation pump on the DHW distribution, or accepting that cold (or lukewarm) water flows for the first 20–40 seconds.
Can I increase DHW comfort without replacing the boiler – just by adjusting settings?
Yes, in many cases it's enough to adjust the settings: increase the set tank temperature to 60–65 °C (larger effective volume after mixing), set tank preheating before the morning peak, or install a smart controller with time programs. Sometimes the result is surprisingly better without any hardware investment.
How do I know if my tank has become scaled with limescale and heats up more slowly?
Reliable indicators are: significantly longer heating time compared to the past (30% or more), increased gas consumption for the same comfort, or possibly sound effects (cracking, knocking) during tank heating. A service technician can check the condition of the exchanger by pressure testing or with an endoscope. In areas with hard water, we recommend a preventive check every 2–3 years.
Is a 21-litre tank really enough for a couple?
For a couple with normal habits (showers, not baths), yes – provided the boiler has an output of at least 20 kW and showers aren't taken immediately one after another. The Protherm Tiger Condens 20/26 KKZ 21 model is designed exactly for this kind of use: compact, space-saving, and 3–4 minutes to a fully heated tank is realistic. If in doubt, it's better to opt for the 42-litre version.
Is it worth setting the tank to 70 °C for a bigger "energy reservoir"?
Physically, yes – at 70 °C the tank yields more usable water (after mixing to 38 °C) than at 55 °C. But energy efficiency decreases (higher temperature = higher tank heat losses), scale build-up accelerates, and there's also a safety aspect (risk of scalding). The optimal temperature from a comfort/efficiency/hygiene point of view is 55–60 °C. We recommend 70 °C only for weekly hygienic heating, not permanently.
What happens to hot water if the boiler breaks down?
In the event of a boiler failure, the tank cools down quickly – usually within 4–8 hours, depending on its thermal insulation. Better-insulated tanks (insulation thickness 50–80 mm) lose temperature more slowly. In an emergency, you can temporarily heat the tank with an immersion electric heater, if the tank has an electric connection port (some models offer this as an accessory).
Conclusion: what this means for your choice
Tank heating time – typically 4–8 minutes for boilers with a 21–42 litre tank – is just one link in the chain of factors that determine DHW comfort. Equally important are: sizing the tank to the actual household size, boiler output, set tank temperature, water hardness, hydraulic connection, and quality of control.
From the category of condensing boilers with a built-in tank, the rule is: for most 3–4 person households in an ordinary family house, a 42-litre tank with a 24–26 kW boiler is a very well-sized solution that provides comfortable heating without long waits. If you have a larger family, unusual habits (daily bath, simultaneous showers), or very hard water, it's worth considering either a more powerful model or discussing an external tank.
For a more detailed comparison of specific models, see the article Protherm Tiger Condens vs. Thermona THERM vs. Vaillant ecoTEC: comparison of models with a tank, and if you're choosing from scratch, we also recommend How to choose a condensing boiler with a built-in tank: what to watch out for.
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.