Thermostat and Weather-Compensated Boiler Control

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What Is Weather-Compensated (Equitherm) Control and How Does It Work

Weather-compensated (equitherm) control is now standard on almost every modern condensing boiler installation and is one of the most effective ways to cut gas consumption without sacrificing thermal comfort. The principle is surprisingly simple – instead of regulating the heating water temperature "blindly" or just based on whether a room feels cold or warm, the boiler tracks the current outdoor temperature and, using a pre-set curve, automatically calculates what water temperature to send to the radiators or underfloor heating.

The key component of the whole system is the outdoor sensor (outside temperature sensor), which is mounted on the north or north-west side of the house facade, away from direct sunlight and away from heat sources (for example, well away from the chimney, an air-conditioning unit, or a window). The sensor continuously measures the actual outdoor air temperature and sends this value to the boiler's control electronics, or to a separate controller. The electronics compare the measured value with the stored weather compensation curve and set the flow water temperature to match the current weather conditions exactly.

The logic is simple: the colder it is outside, the more heat the house loses, so it needs hotter heating water to keep the rooms comfortable. Conversely, when it warms up outside, the system automatically lowers the water temperature, because the house is losing less heat and a lower circuit temperature is enough to heat it. The result is that the boiler doesn't have to keep "jumping" to full output and then switching off again (so-called cycling), but instead runs smoothly with a lower average return water temperature – exactly what a condensing boiler needs to condense the flue gases efficiently and achieve its rated high efficiency.

Protherm – Outdoor Temperature Sensor (wired) for boilers with eBus bus – a key component of weather-compensated control, compatible with the Thermolink B/P, MiPro and MiSet controllers. Price from €33.54.

Without an outdoor sensor, weather compensation simply doesn't work – the boiler would have nothing to base its calculation on. That's why, for any installation where you want to use this feature, the sensor is a necessary (and fairly inexpensive) investment, which typically pays for itself in gas savings within the first heating season.

Types of Thermostats and Heating Control Methods

Before we look at how to actually set the weather compensation curve, it's worth clarifying what types of thermostats and control commonly appear on the market. Not every house, and not every budget, needs the top control class – what matters is choosing a solution that matches your heating system.

Simple Room Thermostat (On/Off)

The most basic type of thermostat measures the temperature in just one room (usually the living room or hallway) and works on an on/off basis – when the temperature drops below the set point, the thermostat sends the boiler a "turn on" signal, and when it rises above the set point, the boiler switches off. It's a cheap and simple solution, but it causes bigger temperature swings and frequent boiler cycling, which shortens its service life and reduces efficiency, especially for condensing boilers.

Programmable Weekly Thermostat

A step up is the programmable thermostat, which lets you set different temperatures for different times of day and different days of the week – for example, a lower temperature overnight and during working hours when nobody is home, and a higher temperature in the morning and evening. This solution already delivers real savings, because the boiler doesn't heat unnecessarily to full temperature when it isn't needed, though it still works with room temperature only, not outdoor conditions.

Weather Compensation with an Outdoor Sensor and Room Thermostat

The most advanced and most energy-efficient solution combines an outdoor sensor with a room thermostat (or several room units for zoned heating). The outdoor sensor sets the basic, continuously varying heating water temperature according to the weather compensation curve, while the room thermostat acts as a fine correction – if a room is extra warm from sunshine or a fireplace, for instance, the thermostat can slightly lower the water temperature, and vice versa. This combination gives the smoothest heating profile, the lowest return water temperatures for the condensing boiler, and therefore the highest real-world efficiency.

Vaillant VRT 50 – an eBus room thermostat with weather compensation support, wall-mounted in the reference room (usually the living room) and working together with the outdoor sensor to control the water temperature optimally. Price from €71.64.

The reference room where the room thermostat is installed should be one where you spend the most time and where there are no extreme temperature swings (not the bathroom, not a room with a fireplace, not a draughty hallway by the front door) – otherwise the control will be skewed by local influences, and the rest of the house may end up too cold or overheated.

Modern WiFi and Smart Thermostats

The latest generation of thermostats adds internet connectivity and control via a mobile app to weather compensation. This lets you change the temperature from anywhere, receive fault alerts, track consumption history, and some models even "learn" your habits and predict on their own when pre-heating needs to start so it's warm at home exactly when you arrive. This functionality is a nice extra benefit, but the basis of the savings is still a correctly set weather compensation curve – without it, even the most expensive smart thermostat won't reach its full potential.

Other Parts of the Control System – Sensors and Probes

Besides the outdoor sensor and room thermostat, a comprehensive control system usually includes several other sensors that give the boiler information about the state of the whole system. One of the most commonly used is the NTC temperature sensor for the domestic hot water cylinder. This sensor measures the current water temperature in the storage tank (cylinder) and tells the boiler's control unit whether the tank needs reheating or the temperature is already sufficient. This means the boiler doesn't overheat the tank beyond the set value, and it can also intelligently switch between heating the space-heating circuit and heating domestic hot water (DHW) so that household comfort is maintained with minimum energy consumption.

NTC Sensor Kit for Cylinder 10 kΩ – a temperature sensor for the hot water cylinder, an important part of the control system working together with the boiler's control unit. Price from €20.00.

These sensors are typically spare parts – if the original sensor fails (the most common symptom is that the boiler stops heating hot water to the correct temperature, or displays a fault), it is replaced with the same type having the same resistance value (for example 10 kΩ). When buying a replacement part, it's important to check compatibility with the specific boiler brand and model, since different manufacturers use slightly different values and connectors.

How to Set the Weather Compensation Curve – Slope and Offset

At the heart of the whole weather compensation system is the so-called weather compensation (heating) curve – a graph showing the relationship between outdoor temperature (X-axis) and the heating water temperature the boiler should supply to the system (Y-axis). This curve is not universal – every house is different (different insulation levels, different types of heat emitters, different orientation to the compass points), so the curve has to be set "to measure" for the specific building. This is done either directly on the boiler's display or through a room/weather-compensation controller, usually by entering two parameters: the curve slope and, optionally, its parallel offset.

The curve slope determines how sharply the heating water temperature should change in response to a change in outdoor temperature. The steeper the curve, the greater the difference between the water temperature during mild frost and during severe frost. A steeper curve is generally needed by:

older, poorly insulated houses with higher heat losses,
systems with classic radiators (they need a higher water temperature to release enough heat through a smaller heat-exchange surface),
houses with high ceilings or a larger proportion of glazed surfaces.

Conversely, a flatter curve is suitable for:

new builds and well-insulated houses with low heat losses,
systems with underfloor heating, which work with a low water temperature thanks to the large heat-exchange surface of the floor,
houses where too high a water temperature would cause overheating and needless energy waste.

The parallel offset (sometimes also called "offset" or "base temperature") then shifts the whole curve up or down without changing its slope – it's used for fine-tuning if, across the whole range of outdoor temperatures, the house is systematically colder or warmer than it should be.

Example of three weather compensation curves with different slopes – the steeper the curve, the higher the water temperature at the same frost level

The exact figures are always fine-tuned experimentally during the first heating season – you set an approximate slope based on the type of house and system, then observe whether the house stays warm enough on frosty days (without needing to push the room thermostat to maximum) and whether, conversely, the house doesn't overheat unnecessarily in milder weather. Most modern controllers let you adjust the curve slope in steps (for example 0.2 or 0.5), so fine-tuning is a matter of a few small adjustments over a couple of weeks.

Radiators vs. Underfloor Heating – Different Curve Settings

The type of heat emitter fundamentally affects what curve makes sense to set. Classic radiators have a relatively small heat-exchange surface, so they need a higher water temperature to release the required amount of heat into the room – typically in the range of 55 to 70 °C at the design (lowest) outdoor temperature. Underfloor heating, by contrast, has a huge heat-exchange surface (the whole floor of the room), so it only needs water at around 30 to 40 °C for the same heating output. If you sent radiator-level water temperature into underfloor heating, you would risk not only overheating and discomfort but, with some systems, damage to the floor covering or the pipework itself.

In houses combining both systems (for example radiators upstairs and underfloor heating on the ground floor), a mixing unit with its own control for the underfloor circuit is essential – it further reduces the water temperature coming from the boiler down to a safe level. In that case, the boiler's weather compensation curve is set according to the more demanding circuit (usually the radiators), while the underfloor heating has its own separate control. You'll find a more detailed explanation in the separate topics on radiators and underfloor heating combined with a condensing boiler.

The Most Common Mistakes When Setting Up Weather Compensation

Even though the principle of weather compensation is simple, the same handful of mistakes keep showing up in practice when it's being set up, and they significantly reduce its benefit:

Curve set too steep. If the curve is set steeper than the house actually needs, the boiler sends unnecessarily hot water into the system every time it gets colder. The house heats up quickly, but this causes overheating, higher gas consumption, and lower condensing efficiency (hotter return water condenses the water vapour in the flue gases less effectively).
Curve set too flat. The opposite extreme – the curve is set flatter than the house's real needs, so during heavier frosts the boiler can't deliver enough heat. The result is complaints of cold precisely on the coldest days, when comfort should be at its highest.
Setting it once and forgetting about it. The curve set during installation is only an estimate. Many owners never fine-tune it after the first season, even once they have real experience of how the house behaves at different temperatures – yet that correction is exactly what can bring further noticeable savings.
Incorrect placement of the outdoor sensor. A sensor mounted on a sunny facade, near a window, a door, or close to the boiler's flue outlet measures a distorted temperature, and the whole control system then works with bad input data no matter how well the curve is set in theory.
Conflict between weather compensation and the room thermostat. If the room thermostat is set to a target temperature that's too high for what the weather compensation curve can deliver at that outdoor temperature, the system ends up constantly "playing catch-up," and the main benefit of smooth control is lost.
Forgetting to switch seasonally on mixed systems. In houses with both radiators and underfloor heating, correctly setting both circuits separately is sometimes overlooked, leading either to the underfloor heating being underheated or to a risk of overheating.

The good news is that none of these mistakes are permanent – you simply need to revisit the setting, or call in a service technician who can fine-tune the curve based on how the house has actually behaved during a heating season already lived through.

The four types of control discussed in this article.

The procedure for setting up weather compensation.

Different weather compensation curve settings depending on the heat emitter.

Frequently Asked Questions

Do I also need a room thermostat with weather compensation? It's not a strict requirement – the boiler can work with just an outdoor sensor and no room thermostat, but combining both gives more precise and more comfortable control, because the room thermostat also accounts for internal heat gains (sunshine, appliances, the number of people in the room) that the outdoor sensor alone cannot see.
How can I tell that my weather compensation curve is set incorrectly? A typical sign of a curve set too steep is that the radiators are very hot even in milder weather and the rooms feel stuffy, while gas consumption is higher than you'd expect. A sign of a curve set too flat, conversely, is a persistent feeling of cold precisely during the coldest frosts, even though the boiler is running continuously.
Can weather compensation be retrofitted to an older boiler? In most cases, yes, as long as the boiler has electronics that support connecting an outdoor sensor (this is standard on all condensing boilers made in the last 10-15 years). You just need to add the sensor itself, plus a compatible controller if needed – the wiring is usually run to the boiler's installation location.
Is it true that weather compensation saves gas even without changing the room temperature? Yes – the main saving doesn't come from reduced comfort, but from the boiler running with a lower average return water temperature, which significantly increases efficiency in condensing boilers (more water vapour condenses out of the flue gases, and its latent heat is used for heating). So you can genuinely save at the same room temperature.
Where exactly should the outdoor sensor be placed? Ideally on the north or north-west facade, at a height of roughly 2 - 2.5 m above ground, away from direct sunlight during the day, and away from windows, doors, the boiler's flue outlet, or an air-conditioning unit, so it measures the true outdoor temperature rather than a locally distorted value.
Is it worth investing in a smart WiFi thermostat if I already have standard weather compensation? If your weather compensation curve is working reliably, a WiFi thermostat will mainly bring extra comfort (remote control, consumption overviews, alerts), not a fundamentally bigger energy saving – that comes mainly from correctly setting the curve, not from the internet connection itself.

Video: How Weather Compensation Works

Source: Skupina ČEZ, YouTube

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