Setting Temperature and Pressure in a Condensing Boiler with DHW for Optimal Savings

We are still preparing products.

But you can look at other categories.

Setting the Temperature and Pressure in a Condensing Boiler with DHW for Optimal Savings

A condensing boiler with instantaneous domestic hot water heating is today one of the most common solutions for heating and hot water in family houses and apartments. Nevertheless, most owners of such boilers have never optimized their settings – either they left the technical configuration to the installer at first startup, or they simply turned the dial to a higher temperature and thereby killed off savings worth hundreds of euros a year. This situation is well known to me from practice: I come to service a customer's boiler that has been running since installation five years ago without a single change to the settings, the boiler water is set to 80 °C, the DHW to 60 °C, and the pressure fluctuates between 1.0 and 2.4 bar depending on mood. Yet all it takes is knowing what each parameter does and why it should be set differently.

In this article, we'll look at all the key parameters of a condensing boiler with instantaneous DHW heating – heating water temperature, domestic hot water temperature, operating pressure, weather compensation, and other settings – from a practical point of view with specific values, examples and an explanation of the physical relationships. This is not just theory: every piece of advice is based on real field experience.

Why Boiler Settings Are So Important for Energy Savings

A condensing boiler has one key advantage over a conventional boiler: it can also make use of the heat contained in the flue gases in the form of water vapor. When the flue gases condense, latent heat is released – with natural gas this amounts to as much as 11% of the total energy content of the fuel. However, condensation only occurs when the temperature of the return water (the water flowing back from the radiators into the boiler) is sufficiently low – in practice below 57 °C, ideally below 50 °C.

And here's the catch: if you set the boiler water to 75–80 °C, the return water typically runs at around 60–65 °C, the boiler doesn't condense, and it essentially works the same as an old conventional boiler. Efficiency drops from the declared 107–109% to 88–92%. Over a heating season, that difference can easily show up on the gas bill as an extra 150–300 €, depending on the size of the house.

The chart clearly shows that with return water below 50 °C, we're in the zone of maximum efficiency (107–109%), while above 57 °C the boiler stops condensing and efficiency drops toward the values of a conventional boiler. That's why setting the heating water temperature correctly is so critical.

Heating Water Temperature: How to Set It Correctly

What Is the Flow Temperature and Why Should It Be as Low as Possible

The boiler's flow temperature (often referred to as the "boiler temperature" or "circuit temperature") is the temperature to which the boiler heats the water before releasing it into the radiators or underfloor heating. Most boilers allow settings in the range of 20–80 °C (some models even up to 85 °C).

The basic rule of a condensing boiler is: the lower the flow temperature, the lower the return temperature, the deeper the condensation, the greater the savings. To achieve genuine condensation, a flow temperature of no more than 55–60 °C is recommended, with the return running at around 40–50 °C.

But you can go even lower. Modern low-temperature radiators, and especially underfloor heating, operate with temperatures of 30–45/25–35 °C (flow/return). This is an ideal state for a condensing boiler – it condenses to the maximum throughout the entire operating period.

Problems arise with old cast-iron radiators designed for 90/70 °C. In such cases, a higher temperature is physically necessary for the radiator to release enough heat. In these situations, we either increase output (larger or more radiators), or accept partial condensation with a flow temperature of around 60–65 °C – which is still better than 80 °C.

Recommended Settings by Type of Heating System

Underfloor heating: flow temperature 35–45 °C, return 25–35 °C – maximum condensation throughout the season
New panel radiators (type 21, 22, 33): flow temperature 50–60 °C, return 40–50 °C – very good condensation
Older steel radiators: flow temperature 55–65 °C, return 45–55 °C – good condensation
Old cast-iron radiators (90/70): flow temperature 65–75 °C, return 55–65 °C – partial or no condensation
Mixed system (radiators + underfloor): solved via a mixing valve, each circuit has its own temperature

Weather Compensation: Automatic Temperature Adjustment to Outdoor Conditions

The vast majority of modern condensing boilers – for example the Vaillant VUW 26CS/1-5 ecoTEC plus IoniDetect or Protherm Panther Condens 20/26 KKV – support weather-compensated control using an external outdoor temperature sensor.

The principle of weather compensation is simple and clever: at -15 °C outside, you need water at 70 °C to keep the house warm; at 0 °C, 55 °C is enough; at +10 °C outside, maybe 40 °C is enough. The boiler automatically adjusts the temperature according to the outdoor temperature based on a set curve (the weather compensation curve). The result: the boiler doesn't heat unnecessarily hard during transitional periods, doesn't waste energy, and stays in condensing mode for most of the season.

With weather compensation, choosing the correct curve slope is crucial (labeled with numbers 0.5–3.5 on Protherm boilers, or "Steilheit" / slope 1–3 on Vaillant). A steep curve is suitable for an older system with a greater thermal dependence on outdoor temperature, while a flat curve suits well-insulated homes with underfloor heating. An incorrectly set curve means the boiler heats unnecessarily hard in relatively mild weather, or conversely, the house is cold during heavier frost.

Domestic Hot Water Temperature: Comfort vs. Safety vs. Savings

Why Simply Setting the Lowest Possible Temperature Isn't Enough

With domestic hot water (DHW), the situation is more complex than with heating. Here we run into hygiene requirements: the bacterium Legionella pneumophila, the causative agent of Legionnaires' disease, survives in the range of 25–45 °C and dies at temperatures above 55 °C (within a few minutes at 60 °C, practically instantly at 70 °C).

With a condensing boiler using instantaneous heating (no storage tank), this risk is significantly lower – the water doesn't accumulate, it passes through the heat exchanger immediately, and doesn't sit in a tank for hours or days. Nevertheless, the DHW flow temperature should still be set to a reasonable level.

Recommended DHW temperature settings for instantaneous heating:

Minimum safe temperature: 45 °C (for family houses with direct consumption, no stagnation risk)
Standard setting: 50–55 °C – a good compromise between comfort, hygiene and savings
Maximum recommended: 60 °C – higher energy consumption, risk of scalding, limescale buildup
With a DHW storage tank: at least 60 °C, with a weekly thermal disinfection cycle at 70 °C

A practical example: a customer had DHW set to 65 °C – his reasoning was that "the water is always pleasantly warm." After calculation: heating water from 10 °C to 65 °C versus to 50 °C is a difference of 55 vs. 40 °C temperature rise, which at the same flow rate means 37% higher energy consumption for DHW. For a family with average water consumption, this comes to an extra 30–50 € a year – and yet that excessive temperature still gets mixed down to 38–42 °C at the tap anyway.

Instantaneous Heating and Temperature – Physics in the Customer's Shower

Instantaneous DHW heating in a condensing boiler has one characteristic that must be taken into account: the outlet temperature depends on the flow rate. The boiler heats water at a constant output – if it flows faster, it heats less; if it flows slower, it heats more. Therefore, setting the desired DHW temperature on the boiler actually sets a target temperature that the controller tries to maintain by modulating the burner.

In boilers such as the Protherm Gepard Condens 18/25 MKV or the Vaillant VUW 236/5-3 ecoTEC pro, burner modulation is electronically controlled – the boiler can smoothly vary output typically within the range of 20–100% of nominal output. That's why precise temperature setting isn't so critical, but it must be within a reasonable range for the control to work well (not too low, where the boiler would be forced to throttle down heavily, nor too high).

Heating System Operating Pressure: Correct Values and Their Maintenance

What Is Operating Pressure and How Is It Measured

Operating pressure is the pressure of the water in the closed heating circuit. It is measured with a pressure gauge directly on the boiler (mechanical or digital display). It consists of the static pressure of the cold system plus a dynamic component during operation.

Correct values:

Cold system (boiler off): 1.0 – 1.5 bar
Hot system in operation: 1.5 – 2.0 bar (at a temperature of 60–70 °C)
Maximum allowed pressure: 3.0 bar (above this value the safety valve opens)
Minimum operating pressure: 0.8 bar (below this value many boilers report a fault and shut down)

A typical reason customers call me in January that "the boiler stopped working": the pressure has dropped below 0.8 bar. This is the result of small leaks from the check valve, bleed valves, or a natural pressure decline over several months of operation. The solution is simple – top up water using the valve on the boiler or the filling valve until the pressure reaches 1.2–1.3 bar in the cold system.

Expansion Vessel: The Silent Player in Pressure Problems

If your pressure regularly rises above 2.5 bar when the system heats up and then drops below 1.0 bar when it cools down, you almost certainly have a problem with the expansion vessel. This vessel compensates for changes in water volume caused by thermal expansion – when water heats up it expands, the expansion vessel absorbs this "excess" volume, and returns it when the system cools down.

The expansion vessel contains a membrane and an air cushion, typically under a pressure of 0.75–1.0 bar. If the membrane bursts or the air escapes, the vessel no longer performs its function and the pressure oscillates. Check: if water (not air) leaks from the expansion vessel's valve, the membrane is damaged and the vessel needs replacing. Replacement costs 30–80 € depending on size, plus roughly another hour of the installer's labor.

The correct air pressure in the expansion vessel should be 0.1–0.2 bar lower than the static pressure of the empty cold system. For a two-story house with a static pressure of 1.2 bar, we set the air in the expansion vessel to 1.0–1.1 bar (measured with the circuit disconnected and drained).

Setting Burner Modulation and Minimum Output

Modern condensing boilers with instantaneous DHW heating – for example the Protherm Puma Condens 18/24 MKV – have burners with continuous output modulation. This means the boiler doesn't have to run constantly at full output (100%), but can operate at 30–40% output in mild weather.

Where most installers make a mistake: they set the boiler's minimum output unnecessarily high. The boiler ignites, quickly reaches the required temperature, and switches off. Then the temperature drops, the boiler ignites again, quickly heats up, and switches off. This cyclical on/off switching (so-called cycling) dramatically reduces efficiency and shortens the boiler's lifespan. With every start, the boiler must overcome inertia (heating the exchanger), and with every shutdown it loses a bit of heat through the flue.

Solution: set the minimum output as low as possible (usually 20–30% of nominal output), or extend the boiler's runtime by adjusting the hysteresis (the difference between the switch-on and switch-off temperatures). For example: the boiler switches on when the temperature drops 5 °C below the target, and switches off when it reaches it. The greater the hysteresis, the longer the run, and the fewer starts per hour.

Another parameter worth checking: the maximum output setting for heating (not for DHW). A 24 kW boiler in a house with a heat loss of 8 kW on the coldest day – if we leave the maximum output at 100%, the boiler will almost always run for less than a minute at full output. By limiting the maximum heating output to 50–60% of maximum (in this case to 12–14 kW), we achieve a longer and more even boiler run.

The diagram clearly shows that continuous modulation means an even load on the burner and heat exchanger without thermal shocks. The result is a longer lifespan, lower gas consumption, and more stable room temperatures.

Setting the DHW Temperature with Regard to Comfort and Pressure Conditions

Water Flow and Comfort with Instantaneous Heating

With instantaneous DHW heating, the physical law applies: Q = m × c × ΔT, where Q is the boiler's heat output, m is the mass flow rate of water, c is the specific heat capacity of water (4.18 kJ/kg·K), and ΔT is the temperature rise. A boiler with 24 kW output, at an incoming cold water temperature of 10 °C and a target DHW temperature of 45 °C (ΔT = 35 K), can heat:

m = Q / (c × ΔT) = 24,000 / (4.18 × 1000 × 35) ≈ 0.164 kg/s ≈ 9.8 l/min

If you lower the required DHW temperature from 55 to 45 °C (ΔT from 45 to 35 K), the flow rate the boiler can handle at the same output increases from around 7.7 l/min to 9.8 l/min. In practice, this means a stronger stream of water at the same boiler output – greater comfort while showering. Or put another way: a lower temperature setting is enough to achieve the same comfort.

A customer with a 20 kW Vaillant boiler complained that the shower flow was weak. The reason: DHW set to 60 °C, incoming water at 8 °C (winter), ΔT = 52 K, maximum flow through the boiler only 5.5 l/min. After setting it to 50 °C (ΔT = 42 K), the flow increased to 6.8 l/min and the customer was satisfied – without any hardware change.

Cold Water Pressure and Instantaneous Heating

For the correct operation of instantaneous DHW heating, the cold water supply pressure is also important. Most condensing boilers with instantaneous heating require a minimum inlet water pressure of 0.5–1.0 bar and a maximum pressure of 6–10 bar (depending on the manufacturer). If the local cold water pressure is high (3–6 bar), it is recommended to install a pressure reducing valve set to 2–3 bar after the water meter – this protects not only the boiler but also the entire plumbing system and fittings.

Excessively low cold water pressure (below 0.5 bar) causes the boiler's flow sensor to fail to register sufficient flow, and the boiler doesn't switch on DHW heating at all – "cold water instead of hot." This is one of the most common service complaints in older apartment buildings with outdated plumbing.

Summer and Winter Boiler Mode: Correct Switching

Condensing boilers with instantaneous DHW heating typically have three basic operating modes:

Winter mode (heating + DHW): the boiler provides both heating and DHW heating
Summer mode (DHW only): heating is switched off, the boiler only heats DHW
Manual / ECO / comfort DHW mode: some models offer DHW preheating or an anti-cycling function

Most owners forget to switch the boiler to summer mode in spring. The result: the heating circuits remain active (the pump keeps running), and the boiler may heat unnecessarily during a transitional period if there's a random temperature drop. In summer mode, the boiler is more efficient – the heating pump doesn't run, the boiler doesn't heat to a higher temperature than DHW requires (typically only 45–55 °C), so it condenses every time water is used.

In winter mode, at low outdoor temperatures below -5 °C, it's normal for the boiler to temporarily interrupt heating and focus output on heating water when DHW is switched on, then return to heating. This process is normal and controlled by the boiler's electronics – don't interfere by manually switching settings.

Practical Step-by-Step Setup

If you want to truly optimize your boiler, I recommend the following procedure:

Check the cold system pressure – it should be 1.0–1.5 bar. If not, add or drain water.
Set the heating temperature – start at 60 °C and observe whether the house is warm enough during the coldest weather. If so, lower it by 5 °C and observe again.
Activate weather compensation – install an external sensor (if not already present), set the compensation curve to a mid-range value, and fine-tune during the season.
Set the DHW temperature to 50–55 °C – check shower comfort, and adjust by ±3 °C if needed.
Check the maximum heating output – if the boiler cycles (short 1–3 minute runs, dozens of starts a day), reduce the maximum heating output by 20–30%.
Switch to summer mode in spring (end of March to April depending on weather).
Check the air pressure in the expansion vessel once a year and have the heat exchanger cleaned.

This isn't a theoretical exercise – it's the procedure I follow for every custom optimization job, and the result is typically a 10–20% gas saving compared to the original settings, sometimes more.

Mistakes That Cost Money: The Most Common Incorrect Settings from Practice

Over years of practice, I've seen the same mistakes over and over. Here are the costliest ones:

Boiler water temperature at 80 °C all winter – the boiler doesn't condense, losing 15–20% of potential savings
DHW temperature at 65 °C even in summer – unnecessary consumption with every draw, risk of scalding
Weather compensation disabled despite an existing sensor – the boiler heats constantly at the same temperature regardless of outdoor frost or thaw
System pressure at 0.8 bar – the boiler shuts down with a fault, the customer calls a service technician, when all that was needed was to top up water
Pressure at 2.8 bar – the safety valve drips – a clogged expansion vessel, costing 50 € plus labor; if left unresolved, it can damage the boiler
Boiler left in winter mode all year – in summer it unnecessarily runs the heating pump and heats even when it only needs to heat water
Excessive room thermostat hysteresis (3–5 °C) – the temperature in the house fluctuates, comfort is low, consumption is high

Setup Differences Between Models: Protherm vs. Vaillant

With Protherm boilers (Gepard, Panther, Puma series), settings are usually accessible via the installer menu (pressing and holding two buttons simultaneously, code typically P.01 and higher). The weather compensation curve is set with the "HC1 curve" parameter or similar, and heating output with the "max output heating" parameter.

With Vaillant ecoTEC models, settings are accessible via the diagnostic menu (d.0x for operational diagnostics, C.0x for service parameters). The weather compensation curve is set using the "Steilheit" (slope) value and "Fußpunkt" (base temperature). More modern models with senoAtik control (such as the Vaillant ecoTEC plus IoniDetect) also have intelligent self-adaptive functions that gradually calibrate the weather compensation curve themselves.

Important note: accessing the service (installer) menu and changing parameters is a job for a professional or at least an experienced owner who knows what they're changing. Incorrect settings can lead to malfunction or improper gas combustion. Basic temperature and pressure settings from the user menu (accessible without a code) can be changed by anyone.

The Effect of Water Hardness on Temperature Settings

This is an aspect rarely considered in practice, but it has a direct impact on the recommended DHW temperature setting. Limescale (CaCO₃) begins to build up intensively at temperatures above 50–55 °C. In areas with hard water (most of Slovakia, hardness of 2–3 mmol/l and above), every increase in DHW temperature above 50 °C significantly accelerates scaling of the heat exchanger.

A limescale layer just 1 mm thick increases the thermal resistance of the heat exchanger and reduces its output by 5–10%. At 3 mm, this can be 15–20%. The boiler must compensate for the reduced heat transfer with higher burner output – meaning it consumes more gas. In addition, there's a risk of overheating the heat exchanger and its premature damage.

Practical advice: in areas with hard water, keep the DHW temperature at 48–52 °C and have the heat exchanger descaled every 2–3 years (chemical decarbonization). The Protherm Puma Condens 18/24 MKV has a stainless steel plate heat exchanger inside, which tolerates decarbonization well. An alternative is a water softener before the boiler inlet, though that's an additional investment of 200–600 €.

Frequently Asked Questions (FAQ)

What temperature should I set the boiler water to for the best savings?

It depends on the type of heating system. For underfloor heating, set 35–42 °C; for new panel radiators, 50–58 °C; for old cast-iron radiators, 65–72 °C. The lower the temperature, the more the boiler condenses and the higher its efficiency. If you have radiators sized for 90/70 °C and don't renovate your heating system, the boiler will condense only minimally – this is a physical limitation, not a boiler defect.

What should the DHW temperature be with instantaneous heating?

The optimal setting is 50–55 °C. Lower than that reduces hygienic safety (below 45 °C the risk of Legionella increases, although with instantaneous heating this risk is significantly lower than with a storage tank). Higher than that unnecessarily increases energy consumption, limescale buildup, and the risk of scalding. Remember that you're setting the maximum outlet temperature on the boiler – with a higher flow rate, the actual water temperature at the tap will be lower, and you mix it with cold water anyway to a pleasant 38–42 °C.

What pressure should a condensing boiler have during normal operation?

In a cold system (boiler off or shortly after starting), the pressure should be 1.0–1.5 bar. During full operation with a hot system, the pressure naturally increases to 1.5–2.0 bar due to thermal expansion of the water. If the pressure rises above 2.5 bar during operation, check the expansion vessel. If it drops below 0.8 bar, top up water using the filling valve on the boiler to 1.2 bar.

What is weather compensation and does it really save money?

Weather compensation automatically adjusts the boiler water temperature to the outdoor temperature. The boiler heats harder in frost and less in mild weather – without any need for manual intervention. In real operation, it saves 5–15% of gas compared to a constant setting, because during transitional periods (spring, autumn) the boiler doesn't heat unnecessarily hard and spends more time working at lower temperatures, i.e. in condensing mode. The condition is installing an external temperature sensor on the north side of the building (not on the sunny side, not near a ventilated window).

Why does the boiler keep switching on and off (cycling)? Is it a fault?

Frequent short on/off cycles (cycling) are usually not a fault but an incorrect setting. The boiler's output is much greater than the house's current heat demand. Solution: reduce the maximum heating output in the service menu to 50–70% of maximum, or increase the temperature controller's hysteresis. If the boiler cycles even on the coldest winter day, the output is oversized – which is common, since boilers are historically sized with a margin. Cycling increases gas consumption and shortens the lifespan of the burner and heat exchanger.

Can I set the boiler myself, or does it have to be done by an installer?

Basic user settings – heating temperature, DHW temperature, switching between winter/summer mode, topping up water pressure – can be done yourself without any risk. These parameters are accessible directly from the boiler's main control panel. The service (installer) menu, where the weather compensation curve, maximum output, hysteresis, and other technical parameters are set, should only be accessed by a professional or a truly technically skilled owner who understands the boiler's settings – an incorrect change here can cause improper combustion or malfunctions. If you're not sure, call for a service visit – the investment of one service hour in optimizing settings usually pays for itself within the first heating season.

Conclusion: Small Settings, Big Savings

A condensing boiler with instantaneous DHW heating is a technically sophisticated device capable of working at an efficiency of 107–109% – but only if all parameters are set correctly. Three basic rules are key: keep the heating water temperature as low as possible (while maintaining thermal comfort), set DHW to 50–55 °C, and regularly check the system's operating pressure. Add to that weather compensation, proper burner modulation, and switching between summer/winter mode – and you have a boiler set up for maximum efficiency.

If you're planning to buy a new boiler and are interested in comparing specific models, read the article "Protherm vs. Vaillant Condensing Boilers with DHW: Comparison of Models and Parameters." For those deciding on output size, we recommend the article "What Output Condensing Boiler with DHW Do I Need for My House." And if you're interested in how instantaneous DHW heating works in depth, take a look at the article "Instantaneous DHW Heating in a Condensing Boiler: How It Works and What Affects Comfort."

All products in the condensing boilers with instantaneous DHW heating category on atria.sk are from proven manufacturers with a long-standing tradition and available service support in Slovakia. Getting the setup right from the start pays off – and this article gives you all the information you need to get it right.

Have a Question on This Topic?

Can't decide, or dealing with a specific situation in your household? Write to us - we're happy to help.