Smart control of condensing boilers: regulation and smart thermostats

We are still preparing products.

But you can look at other categories.

Smart control of a condensing boiler: regulation and smart thermostats

A condensing boiler today is a technically advanced appliance that can operate with an efficiency of over 100 % (in terms of calorific value) – but only if you give it the chance to do so. And that's where a topic comes in that many customers only address secondarily: regulation and control. In practice, I often see situations where a house has an excellent condensing boiler installed, but it's treated like an old gas boiler from the 1990s – you turn a knob, set the water temperature to 70 °C and let it run. The result? The boiler works, but condensation is minimal, actual savings are a third of the potential, and the customer wonders why the payback period keeps getting longer.

Proper regulation of a condensing boiler is not a luxury or a marketing add-on – it is a basic prerequisite for the entire system to work as the manufacturer promises. In this article, we'll look at what regulation actually does, the difference between a simple thermostat and a modern smart solution, why the weather-compensation curve matters, and how to set up the whole system correctly so it saves you tens of euros every month.

Why regulation is crucial for a condensing boiler

To understand why regulation is more important for a condensing boiler than for old non-condensing boilers, we need to go back for a moment to the basic principle of condensation. A condensing boiler achieves high efficiency by cooling the flue gases enough that the water vapour in them condenses and releases additional latent heat. Condensation only occurs when the return water temperature is sufficiently low – usually below 57 °C, ideally below 50 °C.

The lower the average water temperature in the system, the more the boiler condenses and the higher its actual efficiency. And this is where regulation comes in: its task is to ensure that the boiler delivers exactly as much heat as the house needs at any given moment, with the lowest possible water temperature. If the regulation "doesn't know" what is happening outside or inside, the boiler works unnecessarily at high output, the water temperature rises unnecessarily, and condensation decreases.

The diagram makes it clear: when the return temperature drops to 30–35 °C (typical for low-temperature underfloor heating), the boiler operates at an efficiency of around 107–109 %. At a return temperature of 70 °C (an old radiator system without regulation), efficiency approaches 90 % – which is still a decent figure, but the whole point of condensation is lost. It is precisely intelligent regulation that keeps the temperature at the lowest level that still ensures thermal comfort.

Types of condensing boiler regulation: from the simplest to smart

1. Simple room thermostat (on/off)

The simplest form of regulation – the thermostat measures the room temperature and switches the boiler off when the set value is reached, and back on when it drops. The boiler works at a fixed temperature that you set manually. This approach has several fundamental drawbacks: the boiler switches on and off cyclically (so-called cycling), which increases burner wear and reduces comfort. Moreover, the water temperature is in no way linked to outdoor conditions – whether it's freezing outside or spring-like, the boiler has no idea.

Such a solution is sufficient today only in the simplest cases – for example, in a holiday property where precise regulation is not a priority.

2. Programmable (weekly) thermostat

One step further: the thermostat has a clock and allows you to set different temperatures for different times and days of the week. For example, on weekdays 21 °C from 6:00 to 8:00 in the morning, a setback to 17 °C during the day, 21 °C again from 16:00 to 22:00 in the evening, and 16 °C at night. Such regulation can save 10–20 % of costs compared to constant operation without a setback.

The drawbacks remain: it's still just on/off control of the boiler, with no link to the outdoor temperature and no adaptability to changed conditions (you came home earlier, it suddenly got warmer outside).

3. Weather-compensated (outdoor-temperature-dependent) control

This is the first genuinely intelligent type of regulation designed specifically for condensing boilers. A weather-compensation controller measures the outdoor temperature and continuously adjusts the boiler's water temperature accordingly (the so-called heating curve or weather-compensation curve). The principle is simple: the colder it is outside, the higher the water temperature the boiler maintains, and vice versa.

As a result, the boiler works with smooth output modulation, and the water temperature in the system is always only as high as necessary for the given outdoor cold. The result: a higher share of condensation, lower gas consumption, and more comfortable regulation – the house never overheats or underheats.

Correctly setting the slope of the weather-compensation curve is a key task for the installer. A curve that's too flat = the house doesn't heat up enough during severe frosts. A curve that's too steep = the house overheats in milder weather and consumes more gas. The setting is done empirically during the first heating season – usually 2–3 corrections are enough to optimize it.

4. Smart thermostat connected to the boiler

Modern smart thermostats go even further. They combine weather compensation, scheduling, smartphone control, adaptive functions (the boiler learns how long it takes to heat the house), and in some cases geofencing (the boiler lets the house cool down when you leave and heats it up before you arrive). They connect to the boiler either via digital buses (OpenTherm, EMS bus) or via Wi-Fi/cloud.

OpenTherm vs. proprietary communication protocol: what it means in practice

This is a technical topic, but customers should know about it. A classic thermostat communicates with the boiler simply: contact open = boiler off, contact closed = boiler on (on/off). A smart thermostat can do more – but only if the boiler "understands" its language.

OpenTherm is an open standard for digital communication between a thermostat and a boiler. Via a two-wire connection, the thermostat and boiler exchange much more information: the boiler's current output, water temperature, return temperature, error codes, a request for a specific water temperature (not just on/off). This allows the thermostat to tell the boiler: "Give me water at 43 °C" – and the boiler fulfils this exactly through burner modulation. The result is much finer and more efficient regulation without cycling.

Bosch uses its own protocol, the EMS bus (Energy Management System), which is even more advanced than OpenTherm and transmits more data. Bosch thermostats – such as the Bosch Easycontrol CT 200 – communicate with Bosch boilers directly via the EMS bus and allow genuinely deep integration: you can see consumption, history, operating parameters, and change the boiler's settings directly from the app without having to go into the boiler's service menu.

Practical consequence: if you're buying a Bosch boiler and want a smart thermostat, it's easiest to stay within the Bosch ecosystem. If you want to use a thermostat from another manufacturer, check whether it supports OpenTherm and whether the specific Bosch boiler also supports it (not all models do).

Bosch Easycontrol CT 200: a concrete example of a smart thermostat in practice

When customers ask for a specific recommendation for Bosch Condens boilers, I most often recommend the Bosch Easycontrol CT 200. It's a smart thermostat that connects to the internet via Wi-Fi and communicates with the boiler via EMS bus. What does this mean specifically in everyday life?

Smartphone control: wherever you are, you can see the current temperature in the house and change it. Real-life examples: you're heading home earlier from work – turn on the heating from your car. You're away for the weekend – lower the temperature to a minimum.
Weekly schedule: set your schedule directly in the app, clearly, without deciphering a thermostat manual.
Adaptive control: within a few weeks, the thermostat "learns" the thermal inertia of your house and starts pre-heating the boiler in advance so the house is warm at the right time – not only once the thermostat detects a temperature drop.
Boiler optimization: via EMS bus, the thermostat can set the boiler's exact required water temperature, not just turn it on/off. The boiler modulates its output as needed, cycles less, and works more efficiently.
Weather link: the thermostat takes the current weather forecast into account when regulating – for example, if sunny weather is coming in two hours, the boiler won't unnecessarily preheat the house.

The CT 200 is compatible with all newer Bosch Condens boilers, including models such as the Bosch Condens GC2300iW 24 P or the Bosch Condens GC8700iW 30 P. Installation is simple: two EMS bus wires, a Wi-Fi connection, downloading the app – and that's it. An installer will usually manage this in half an hour on top of installing the boiler itself.

Regulation when combining heating and domestic hot water preparation

Boilers with instantaneous domestic hot water heating, such as the Bosch Condens GC2300iW 22/25 C, have one important characteristic in terms of regulation: domestic hot water (DHW) preparation always has priority over heating. When you open a tap, the boiler immediately switches its full output to preparing hot water and temporarily interrupts heating. This is fine as long as the regulation manages it correctly – modern thermostats and control units take this into account and don't treat a brief interruption of heating as a fault.

With storage tank systems (indirect cylinders), regulation is slightly different: the tank is heated to the set temperature (usually 55–60 °C) at set times, and outside those times the boiler is available only for heating. Correctly programming the tank heating times – typically in the morning before getting up and in the evening before bathing – is important here, so you always have hot water available without the boiler heating unnecessarily during the day.

Zone control: room-by-room heating

A more advanced step is dividing the house into heating zones, where each zone has its own thermostat and its own valve actuator (a zone valve or thermostatic head). The boiler can thus maintain 22 °C in the children's room, 18 °C in the bedroom, and only 10 °C in the garage – all at the same time and independently.

However, for a condensing boiler to work correctly in a zoned system, one thing is important: minimum flow. A condensing boiler needs a certain amount of water constantly flowing through it, otherwise it overheats and triggers a fault. When all the zone valves close at once, the boiler has nowhere to dissipate its heat. The solution is either a hydraulic separator, a bypass valve, or ensuring that at least one zone always remains open (usually common areas).

In family houses with underfloor heating and several housing units or floors, this solution significantly increases both comfort and savings. In simple flats with four radiators, however, it adds unnecessary complexity – there, a single thermostat with weather compensation and thermostatic radiator valves is enough.

Practical recommendations for choosing and setting up regulation

New boiler = new regulation

From experience I know: a customer buys a new condensing boiler but keeps the old wired thermostat from the 1990s on it because "it still works." The technician connects it, the boiler runs – but operates with on/off control at a fixed water temperature of 70 °C. The savings are significantly lower than the potential of a condensing boiler. An investment in a smart thermostat (€200–400 including installation) pays back in 1.5–3 years for a family house with an annual gas consumption of 15,000–25,000 kWh. This is a calculation I always make for customers.

Thermostatic radiator valves are not a substitute for a thermostat

Thermostatic valves on radiators regulate the flow of water through each radiator individually – but they don't communicate with the boiler. Without a room thermostat, the boiler works continuously at the set temperature, and the thermostatic valves only limit how much of that heat each room receives. This isn't bad, but it isn't optimal either. The correct solution is a combination: a room thermostat (or smart thermostat) controls the boiler, while thermostatic valves fine-tune individual rooms.

Setting the weather-compensation curve for Bosch Condens boilers

If you operate a Bosch Condens GC2300iW boiler (for example the GC2300iW 15 P model for smaller flats or houses) without an external smart thermostat, but still want at least basic weather compensation, most models have a built-in controller with the option to connect an outdoor sensor. Setup procedure:

Install the outdoor temperature sensor on the north side of the house (not in direct sunlight, not above a window).
Connect the sensor to the appropriate boiler terminals (see the manual).
Activate weather compensation in the boiler menu and set the basic curve slope (typically 1.2–1.5 for radiators, 0.5–0.8 for underfloor heating).
After 2–3 weeks, observe whether the house overheats or underheats at extreme temperatures, and adjust the slope.

The importance of temperature setback (night or reduced mode)

Temperature setback means lowering the set temperature (typically by 3–5 °C) during night hours or when you're not at home. For a condensing boiler, an important rule applies: the setback should not be too deep or too long, because reheating a cold house puts extra load on the boiler and may temporarily increase consumption more than was saved during the setback. Practical recommendation: a setback of a maximum of 4–5 °C, lasting 6–8 hours. In a well-insulated house (low-energy standard), a setback makes less sense, since the house cools down slowly even after the boiler is switched off.

The most common mistakes in installing and setting up regulation

Over the years of practice, I've seen these recurring mistakes:

Outdoor sensor on the sunny side – the controller "thinks" it's warm outside even though it's freezing. The house underheats, and the customer wonders why.
Room thermostat near a heat source (a radiator, kitchen, sunlight through a window) – the thermostat switches off the boiler before the room is actually warm.
Maximum water temperature set too high in the boiler when using a smart thermostat with modulation – the thermostat requests 40 °C, but the boiler has a minimum temperature set at 60 °C. The regulation misses its purpose.
Neglecting software updates for the smart thermostat – manufacturers release updates that improve control algorithms and add features. A thermostat that hasn't been updated for a year may work less efficiently.
Summer mode not enabled – in summer, the boiler still keeps the heating water on standby even though it's not needed. Properly set summer mode turns off heating and lets the boiler run only for hot water preparation.

Regulation and energy class: what the ErP directive says

Since 2017, the European ErP directive (Energy Related Products) has been in effect, which bans the sale of non-condensing boilers and also classifies heating systems according to energy efficiency. Importantly, the quality of regulation also factors into the overall energy class of the system:

On/off thermostat without scheduling: control class I (basic)
Programmable thermostat: class II
Programmable thermostat with room temperature sensor: class III
Controller with OpenTherm/EMS communication, adaptive control, geofencing: class V–VIII

A higher controller class raises the overall energy class of the heating system – which has practical implications, for example, for subsidy schemes (where the energy efficiency of the whole installation is assessed) or for a building's energy performance certificate.

Integration with smart homes

For customers who have or plan to install a smart home system, integrating the boiler and thermostat is important. The Bosch Easycontrol CT 200 is compatible with Amazon Alexa and Google Assistant platforms – you can say: "OK Google, set the heating to 21 degrees" and the thermostat will carry out the command. Some more advanced systems (such as KNX or Home Assistant) allow the thermostat to be connected with other devices as well – for example, when a window is opened, the thermostat automatically lowers the temperature in that room.

For most family houses and flats, however, these integrations are not necessary, and a standalone smart thermostat with its own app is sufficient. Comprehensive integrations are relevant for new builds with full automation.

For a more detailed comparison of the boiler types for which you'll be choosing regulation, we also recommend related articles in the Knowledge Centre: Wall-mounted vs. floor-standing condensing boiler: which is more suitable, Condensing boiler with instantaneous water heating vs. with a storage tank: difference and choice, and What condensing boiler output do I need for my house – regulation and boiler output are closely related, and correctly setting one without the other is not complete.

How much smart regulation saves: indicative figures

Customers always ask for specific figures. Savings depend on many factors, but based on available measurements and practical experience, the following indicative overview applies:

Type of regulation	Indicative savings vs. no regulation	Solution cost (indicative)
On/off thermostat (basic)	5–10 %	€20–60
Programmable thermostat	10–20 %	€50–120
Weather compensation (outdoor sensor)	15–25 %	€80–200
Smart thermostat with OpenTherm/EMS	20–35 %	€150–400
Smart thermostat + zone control	25–40 %	€400–1200

These percentages are relative – their absolute value depends on the size of the house, current gas prices, and the starting situation. For an average annual heating gas consumption of 15,000 kWh and a price of €0.08/kWh (annual cost of €1,200), a 25 % saving means a real €300 per year. A smart thermostat costing €250 pays for itself in less than a year.

Frequently Asked Questions (FAQ)

Do I have to replace the thermostat when buying a new condensing boiler?

It's not mandatory, but it is strongly recommended. An old on/off thermostat will work with a new condensing boiler, but it won't allow it to operate with modulation and weather compensation. As a result, the boiler falls far short of the savings it could achieve. Investing in a quality smart thermostat is a logical and beneficial step when replacing a boiler.

What is OpenTherm and do I need it?

OpenTherm is a standard for digital communication between a thermostat and a boiler, allowing much finer control of the boiler's output compared to classic on/off switching. It's very beneficial for condensing boilers – it eliminates cycling, improves efficiency, and extends burner life. Bosch boilers use their own similar protocol, EMS bus. If you're buying a smart thermostat for a Bosch boiler, check compatibility with EMS bus.

Can I install the thermostat myself, without a technician connecting it?

A simple wired thermostat with two wires can be installed by a handy homeowner following the manual – no gas installer's licence is needed for that. A smart Wi-Fi thermostat is similarly easy to connect electrically, but its parametrization (setting the weather-compensation curve, boiler water temperature limits, DHW integration) should ideally be done by a service technician who understands the whole installation. Incorrectly set parameters can lead to undersized heating or unnecessary waste.

Is a smart thermostat safe – what if the internet goes down?

Good smart thermostats (including the Bosch Easycontrol CT 200) also work without an internet connection – local control via buttons and a programmed weekly schedule run offline too. The internet is only needed for remote control via the app and for some advanced features (updates, weather forecast). If your router goes down, the boiler will continue heating according to the set schedule.

Is weather compensation worth it if I have underfloor heating?

Yes, with underfloor heating, weather compensation is even more important than with radiators. An underfloor system has high thermal inertia – it reacts slowly, and overheating or underheating shows up with a delay of several hours. Weather compensation, which continuously adjusts the water temperature according to the outdoor temperature, prevents fluctuations and keeps the house within a comfortable temperature range without overheating. In addition, underfloor systems work with low water temperatures (30–45 °C), which is ideal for a condensing boiler.

How long does it take for a smart thermostat to "learn" my house?

Modern adaptive thermostats (including the CT 200) collect data during the first 2–4 weeks of operation. They gradually learn how quickly the house responds to heating, how long it takes to heat up from the setback temperature, and how the house behaves under various outdoor conditions. After this "learning period," regulation works significantly more precisely and comfortably. Customers usually notice this effect towards the end of the second or the beginning of the third week of operation.

Conclusion: regulation is not an extra cost, but the foundation of the system

A condensing boiler without proper regulation is like a powerful car with a non-functioning cruise control on the motorway – you'll still reach your destination, but you'll use far more fuel than necessary. Regulation is not an add-on; it is the foundation without which the whole investment in a modern boiler loses much of its point.

When choosing regulation, follow a few rules: at minimum, weather compensation with an outdoor sensor; ideally, a smart thermostat with digital communication with the boiler (OpenTherm or EMS bus); and for larger houses with multiple rooms, consider zone control. If you have a Bosch Condens boiler, combining it with the Bosch Easycontrol CT 200 is a natural choice – both devices are designed to work together as efficiently as possible.

For a comprehensive view of the whole area of condensing boilers, we also recommend other articles in the Knowledge Centre: How to choose a condensing boiler: what to focus on before buying, Servicing and maintenance of a condensing boiler: what and how often to check, and Common condensing boiler faults and how to recognize them. Proper regulation not only saves money but also extends the life of the entire boiler by preventing unnecessary wear from cycling.

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.