What is Emitter Regulation and How Does It Work in Practice
Equipotential regulation – what it is and how it works in practice
If you've ever dealt with the replacement or modernization of a boiler control system, you've almost certainly come across the term equipotential regulation. Many customers ask us what it actually means, why it's better than a standard room thermostat, and when it's really worth it. In this article, we'll cover it thoroughly and in depth – without unnecessary simplification, but in a way that makes sense even to someone who isn't a trained technician.
Equipotential regulation is not a new invention. It is based on a principle we've known for decades, but it wasn't until modern electronic controllers that it could be brought to a form that is reliable, adjustable, and accessible even for the average homeowner. The principle is elegant: the boiler should heat as intensively as the outside temperature requires – no more, no less.
Basic principle: why outside temperature and not room temperature?
A standard room thermostat works simply: when the room temperature drops below the set point, the thermostat turns on the boiler. When the temperature reaches the desired level, the boiler turns off. It seems logical – and for a simple application, it works. The problem is so-called response delay.
A building has thermal inertia. It takes time for the boiler to heat the water, for the hot water to travel through the piping to the radiators, for the radiators to transfer heat to the room, and for the thermostat's thermometer to register the change. During this time, it's still freezing outside. The result is cyclical fluctuations: the room slightly cools down, the boiler runs at full power, the room overheats, the boiler turns off – and the cycle repeats. Each such short on/off cycle (technically cycling) unnecessarily strains the boiler and increases gas consumption.
Equipotential regulation solves this problem differently: instead of waiting for a signal from the room, it monitors the outside temperature and proactively sets the heating water temperature to match current conditions. If it's –10 °C outside, the boiler will maintain a higher water temperature (e.g., 70 °C). If it's +5 °C outside, water heated to only 45 °C is sufficient. This way, the room is kept at a uniform temperature without large fluctuations.
The graph clearly shows the difference: a room thermostat creates a sawtooth temperature pattern in the room, while equipotential regulation maintains almost a flat curve. This is not only more comfortable – it is also energy-efficient, because the boiler operates more smoothly and efficiently.
Equipotential curve – the heart of the whole system
A key concept in equipotential regulation is the so-called equipotential curve (sometimes also called the heating curve). It represents the relationship between outside temperature and the required heating water temperature. This relationship is not linear, but typically slightly curved – because a building's heat losses are not purely proportional to the temperature difference (it also depends on wind speed, solar radiation, etc.).
The equipotential curve is set on the controller using one or two parameters:
- Curve slope: determines by how many degrees the heating water temperature increases when the outside temperature drops by 1 °C. A steeper curve = a stronger reaction to outside temperature. It is set depending on the specific type of heating system and the building's heat losses.
- Curve shift (parallel shift): shifts the entire curve up or down without changing the slope. It is used for fine-tuning if the entire heating is consistently too weak or too strong.
We describe how to correctly set the equipotential curve in detail in the article How to set the equipotential curve on the calorMATIC or Thermolink controller.
From the graph, an important difference is visible: radiator heating requires a steeper curve (the water must be much hotter in the cold), while floor heating operates with a flatter curve, as the water temperature is generally lower (usually 30–45 °C even in extreme cold).
Typical values of the heating curve
| Outdoor temperature | Water temperature (radiators 70/50) | Water temperature (floor heating 40/30) |
|---|---|---|
| −15 °C | 75 °C | 42 °C |
| −10 °C | 67 °C | 39 °C |
| −5 °C | 58 °C | 36 °C |
| 0 °C | 50 °C | 33 °C |
| +5 °C | 42 °C | 30 °C |
| +10 °C | 35 °C | minimum limit |
These values are illustrative – the actual setting depends on the specific building, its heat losses, radiator size and other factors. In practice, the curve is always adjusted according to the actual behavior of the house.
How it works technically – sensor and controller
Hydronic heating control consists of two main components: an outdoor temperature sensor (probe) and the controller itself. The sensor is mounted on the north side of the house's façade, at a height of about 2 to 3 meters above the ground – so that it is not exposed to direct sunlight, wind or heat from windows and ventilation openings. Proper placement of the sensor is very important – incorrect placement is one of the most common causes of incorrect operation of the hydronic heating control.
The controller calculates the required heating water temperature based on the signal from the outdoor probe and sends this value to the boiler as the desired temperature. Modern boilers (and also older Vaillant or Protherm models with eBUS interface) are able to directly accept this desired temperature and modulate the burner's output smoothly – this is much more efficient than simple on/off switching.
Communication between the controller and the boiler
Older controllers communicate with the boiler mostly via two-wire eBUS bus (for Vaillant) or OpenTherm. This interface allows bidirectional communication: the controller not only sets the desired water temperature for the boiler, but it can also read current values from the boiler – for example, the actual water temperature, burner status, possible error codes. This is a fundamental difference compared to old simple thermostats, which only closed or interrupted an electrical circuit.
More about how these devices are connected can be found in the article Wired vs. wireless connection of old controllers to a gas boiler.
Pure hydronic heating control vs. hydronic heating with room correction
It is important to distinguish between two approaches that you will encounter with real products:
1. Pure hydronic heating control
The controller monitors only the outdoor temperature and adjusts the heating water temperature accordingly. No room sensor. Advantage: simplicity, robustness, no dependence on what is happening in a specific room. Disadvantage: if internal heat gains (sunlight, cooking, many people) enter the heated room, the control will not detect this and the house may become slightly overheated.
2. Hydronic heating control with room correction
In addition to the outdoor sensor, a room sensor (or a room controller with a temperature sensor) is added. The controller combines both inputs: the outdoor temperature determines the base, but the room temperature corrects the result. If the room is warmer than the desired 21 °C (for example due to sunlight), the controller slightly reduces the water temperature. This combined approach is the most effective in practice and saves the most energy.
Such combinations can be found in the Vaillant calorMATIC 430 and Vaillant calorMATIC 430f series – these controllers operate with an outdoor sensor and also have a built-in room sensor directly in the body of the controller placed in a living room. The combination of both values gives a significantly more accurate result than outdoor temperature alone.
Hydronic heating control in practice – real examples from field experience
Over the years of practice, we have seen dozens of houses where the hydronic heating control either worked excellently or was incorrectly set and caused problems. Here are a few typical scenarios:
Scenario 1: Old house, large radiators, harsh winters
The customer had an old panel apartment with large cast iron radiators, a gas boiler Vaillant atmoTEC and an original ON/OFF thermostat. He complained that the apartment was sometimes too hot and sometimes too cold, and gas consumption was high. We installed a hydronic heating control Vaillant calorMATIC 392 with an outdoor probe. After setting the curve with a slope value of 1.4 (for older radiators in panel houses), gas consumption decreased by an estimated 15 % and the customer no longer had to wear a t-shirt in December.
Scenario 2: New construction with floor heating
In a new construction with a condensing boiler and floor heating, the customer installed a simple room thermostat. Problem: floor heating has a huge thermal inertia – it can take up to 2–3 hours from the boiler being turned on until the room feels warm. A simple thermostat was not at all prepared for this, and the rooms alternately overheated and cooled down. An equithermal regulation with a flat curve (slope 0.6) and room correction solved the problem – the boiler runs smoothly, the floor is constantly comfortably warm, and there are no sudden jumps.
Scenario 3: A cabin with intermittent heating
The customer had a cabin that he visited only on weekends. He thought that equithermal regulation was exactly what he needed. In reality, for intermittent heating (when you let the house cool down and want to heat it up quickly), equithermal regulation is less advantageous – its strong point is smooth temperature maintenance, not rapid heating. For cabins and garden houses, a combined approach with a weekly schedule and a frost protection function is more suitable.
Scenario 4: Poorly placed outdoor sensor
The installer placed the outdoor sensor on the southwest side of the house above the kitchen window. Result: in the afternoon, when the sun shone directly on the sensor (and warm air escaped from the kitchen), the sensor indicated 25 °C even in January. The boiler limited heating, and the house cooled down. Moving the sensor to the north-facing façade immediately solved the problem.
Wireless vs. wired equithermal regulators
Regulators calorMATIC from Vaillant come in two versions – wired and wireless (the designation „f" in the name = funk, meaning wireless):
- Vaillant calorMATIC 360f – wireless version, communicates with the boiler via radio (868 MHz), suitable where running a cable is problematic (thick walls, historical buildings)
- Vaillant calorMATIC 392f – wireless version of calorMATIC 392 with similar functions
- Vaillant calorMATIC 430f – wireless version of the premium 430, with room correction and full programmability
Note: the outdoor temperature sensor is connected by cable directly to the boiler even in wireless models (not to the regulator). This is because the sensor requires stable power and reliable measurement without interference. The wireless part refers only to communication between the regulator (in the room) and the boiler.
A detailed comparison of these models can be found in the article Vaillant calorMATIC vs. Protherm Thermolink – comparison of older regulators.
Energy savings – how much do you really save?
This is the question customers ask most often. The answer depends on many factors, but from practice we can say the following:
- Compared to a simple ON/OFF thermostat: a typical saving of 10–20 % of annual gas consumption is quoted. In houses with poor thermal insulation, the saving can be higher (because the boiler cycles less and operates more efficiently).
- With a condensing boiler, the effect is greater: a condensing boiler operates most efficiently at lower water temperatures. Equithermal regulation naturally keeps the water temperature as low as possible, maximizing condensation and thus efficiency. A boiler that condenses 80 % of the season is significantly cheaper to operate than one that condenses only 30 % of the season.
- The investment in the regulator itself: calorMATIC regulators range in price from about 80 to 200 EUR (depending on the model). With a saving of 100–200 EUR per year on gas, the investment is paid back in 1–2 years.
It is, however, important to be realistic: equithermal regulation is not a miracle device that solves all problems. If the house has poorly sealed windows, uncorrected piping or an oversized boiler, the regulator alone will not dramatically reduce consumption. It is one of the tools for comprehensive optimization.
For which boilers is equithermal regulation suitable?
Equithermal regulation makes sense primarily for boilers that can modulate output – that is, change the intensity of combustion smoothly from minimum to maximum. Almost all modern condensing boilers from Vaillant (ecoTEC) or Protherm (Panther Condens) can modulate. Older atmospheric boilers (for example, older generation Vaillant atmoTEC) may have limited or no modulation – in this case, the regulator simply turns the boiler on and off to reach the desired water temperature, which is still better than a simple ON/OFF thermostat, but is not full modulation.
If you have an older boiler and are considering which regulation is suitable for it, we recommend reading the article How to choose a regulator for an older Vaillant or Protherm boiler.
Most common mistakes with equithermal regulation
- Poor placement of the outdoor sensor – sunny side, proximity to windows, heating elements or the boiler's exhaust vent. The sensor must measure the actual outdoor temperature, not a local anomaly.
- Too steep a curve – the house overheats, and the customer solves this by opening windows (which is obviously energy inefficient). The correct approach: reduce the slope of the curve.
- Too flat a curve – the house cools down during frost. The slope should be increased or the entire curve shifted upwards.
- Ignoring thermal inertia – the customer changes the settings every day, and the curve does not have time to „stabilize". Changes should be made gradually and evaluated after at least 24–48 hours.
- Equithermal regulation with convectors or electric heaters – equithermal regulation controls the water temperature in the circuit, not the electric system. A different solution is needed for combining different systems.
- Incorrectly set frost protection parameter – the regulator should have a minimum water temperature set so that the boiler does not stop heating during extreme frost (e.g., minimum water temperature of 20–25 °C).
Frequently asked questions (FAQ)
Do I need to replace the outdoor sensor if I only replace the thermostat?
No, not always. The Vaillant outdoor sensor (NTC sensor, resistance 10 kΩ at 25 °C) is typically compatible with multiple generations of thermostats. If the old sensor is in good condition and properly installed, it is usually sufficient to connect it to the new thermostat. Before replacing the thermostat, check the sensor compatibility – in case of doubt, the sensor is a cheap item and it is worth replacing it preventively.
Can I use the weather compensation without an outdoor sensor?
Some thermostats (including calorMATIC 430) can operate without an outdoor sensor in room thermostat mode. However, in this case, the weather compensation principle does not work – the thermostat functions only as a room thermostat without external correction. You lose the main advantage of weather compensation. The outdoor sensor is key in this technology.
How long does it take to properly set up the weather compensation curve?
Setting up the curve is an iterative process that can take the entire heating season. In the first season, you set approximate values, monitor the house's behavior, and gradually fine-tune. It is ideal to test during stable freezing weather (±1 °C throughout the day), not during variable weather. An experienced technician can set up the curve quite well on the first visit, but fine-tuning always takes time.
Is weather compensation suitable for heat pumps as well?
Yes, weather compensation is even more important for heat pumps than for gas boilers. A heat pump works most efficiently at the lowest possible heating water temperature – and weather compensation ensures that the water temperature is always only as high as necessary. However, specialized thermostats are used for heat pumps, not universal calorMATIC units.
Can I connect hot water heating (DHW) to weather compensation?
Yes – thermostats calorMATIC 392 and 430 have built-in DHW heating programs. The thermostat can control the priority heating of the storage tank during set times, during which heating is temporarily paused (so-called DHW priority). This is a standard feature that most customers use. It is important to correctly set the DHW heating times to match the actual times when hot water is needed.
What if weather compensation is not working – the house is still too cold or too warm?
The first step is always to check the placement of the outdoor sensor. The second step: check the set curve – most problems can be solved by adjusting the slope or shift. The third possibility is a faulty sensor itself – it can be easily measured with a multimeter (resistance at 0 °C should be approx. 32.5 kΩ, at 20 °C approx. 12 kΩ, at 40 °C approx. 5.2 kΩ). Further tips can be found in the article Common faults in old Vaillant and Protherm thermostats and how to fix them.
Conclusion: Is weather compensation worth it?
From the perspective of comfort and energy efficiency, the answer is clear: yes, weather compensation is better than a simple room thermostat in most common situations. This is especially true for homes with central heating, where the boiler runs continuously throughout the season and where you want smooth, comfortable thermal comfort without sudden changes.
The investment in a thermostat is relatively low – for example, Vaillant calorMATIC 392f is an affordable entry into weather compensation with wireless connection, while Vaillant calorMATIC 430f offers full features including room correction, weekly schedule, and DHW control.
If you are unsure which model is right for your boiler and home, we recommend reading the article Is it worth repairing an old thermostat or is a new model better? – there you will find a practical guide to making a decision. And if you want to know how the installation proceeds, see Installation and wiring of Vaillant calorMATIC step by step.
Weather compensation is not a complex technology. It is an elegant principle of "thinking ahead instead of reacting late" – and that is exactly what makes it one of the most effective tools in modern heating.
Do you have a question about this topic?
Can't decide or are you dealing with a specific situation in your home? Write to us – we are happy to help.