HVAC and KNX
KNX and heating control go hand in hand, although heating requires a completely new knowledge compared to lighting and AV control that is often not (yet) available at the electrical installer.
Course KNX – HVAC Specialist
To prepare yourself for complex KNX - HVAC projects, we offer you the KNX - HVAC Specialist course. This course is available online in Dutch and includes an English PDF for reference.
The course consists of 9 chapters with a total of 45 lessons.
During the course you can ask specific questions about the material to our experienced KNX Tutor.
Learn all about Proportional – Integral (PI) controls, Heat transfer, Sensors and the difference between 'steering' and 'control'.
Read more about the structure and content of the course: https://www.knxcontrol.nl/course/knx-hvac-specialist
KNX and HVAC, the background:
There are a number of ways to get the room temperature in KNX without having to install a special temperature sensor in each room. For example, most KNX switches have a built-in temperature sensor.
Although not all of these switches indicate the temperature in the room, this is not really an issue as we are seeing a trend, especially with underfloor heating (UFH), where the temperature is only displayed in a central place. This is because it very rarely needs to be changed, and any change will take at least several hours to take effect.
Other devices such as PIRs and CO2 sensors also often have built-in temperature sensors, but these are usually located in places that are not ideal for reading a temperature display, such as in hallways and hallways, walk-in closets, and other areas where there is no switch. Such places are not usually used as primary living areas, but the temperature of these sensors is sufficient for regulation.
The use of KNX temperature sensors offers the possibility to compensate for the value they provide in order to better read the center of the room. There is also the option to record an average temperature if the room is large.
As with temperature measurement, there are a number of devices that can control the temperature. Some switches can do this, and thermostats, special heating controllers or even some actuators can send out the heating demand.
KNX offers three standard forms of heating control:
• PI (Proportional Integral).
• PWM (Pulse-Width Modulation).
• On / off with hysteresis.
Proportional Integral (PI) is a control algorithm that calculates the magnitude of the error between the set point and the current temperature, and compares it to an average time function to give a 1-byte output suitable for a motorized drive or variable speed fan.
Simply put, the greater the difference between room temperature and the set point, the greater the output. This is then reduced as the room warms up. Once the target temperature is reached, the output is reset to prevent over modulation due to the time it takes to reach steady state.
If the error returns, the output responds with small increments to maintain the set point. When parameterising the thermostat, the type of heating is set, since the control must take into account the reaction power and the heat output.
When using PI, the control needs to know the power of the heating element to calculate the correct response times. In most cases, the predefined default settings are more than adequate, but manual settings can be performed.
Pulse width modulation
Pulse Width Modulation (from Pulse Width Modulation or PWM) uses an on/off signal based on the PI value above. PWM converts the 1-byte PI output to a 1-bit PWM output, allowing it to be used on systems with an on/off valve. This is most common with UFH systems that use valves.
In exactly the same way as PI, a small error results in a small change that is reflected over a period of time. So if the PI output is 10% and the PWM cycle is 10 minutes, then the output will be on for 1 minute and off for 9 minutes.
On/off is the simplest form of operation and is therefore only suitable for very simple systems.
On/off works as expected; if the temperature is above the set point, the output is turned off, and if the temperature is below the set point, it is turned on.
To prevent the output from fluctuating between on and off at the set point, most controllers have a hysteresis parameter. This works by setting an upper and lower offset, normally 1 degree, from the set point. Once the upper limit has been exceeded, the output will not turn on until the temperature has fallen below the lower limit. While necessary, it will lead to more temperature overshoots and undershoots, so this control type should be used with care.
KNX heating modes
There are four modes used in KNX for heating and cooling control:
These modes change the set temperature by a value of one byte (often called the RTR object). With some older devices, you will find that each mode is activated via a one-bit trigger and the device remains in the last activated mode.
On some controllers, there is an additional object for a higher priority mode setting, which overrides normal operation. Setting the mode to Auto (0) disables this override and returns control to the default RTR object, which can be especially useful when you put the house into standby mode.
These modes are self-explanatory: comfort is used during normal occupancy of the house, while night is used when the occupants of the building are asleep. In Night mode, the temperature can be lowered slightly, allowing the house to return efficiently to Comfort mode. Standby (or Away) mode is used when the building is unoccupied; the temperature is reduced, but can still return to Comfort mode relatively quickly when the residents return. Frost mode is used to protect the building when it is empty for a long time. It keeps the house at a lower but even temperature to prevent pipes from freezing.
Relative and absolute control
There are two ways to set the temperature for the above modes. These are relative and absolute.
Relative uses the comfort temperature as the master and uses a fallback figure to calculate and set the values for Night and Standby. This allows the user to set all the above modes from one temperature. However, the Frost mode has a set temperature that is not adjusted when the comfort temperature is changed.
Absolute allows the user to set a specific temperature for each mode. This provides an easier-to-understand scenario, but in reality results in a more complicated user interface, as each temperature has to be changed, sometimes as many as eight times – once each for the four modes, for heating and for cooling.
Whichever option is chosen, a dead band should be considered. Without a dead band, there would be a constant battle between heating and cooling for any slight overshoot by either. For example, if the temperature is set to twenty-two degrees without a dead band, the heating will come up to temperature and the cooling will be activated as soon as the room temperature rises above this set point.
If there is no dead band, there will be a constant battle between the heating and cooling system.
In cooling mode, using relative control, one value is given for the dead zone and one for the compensation for Standby and Night. However, when using absolute control, the deadband is a specific value for the comfort mode setpoint.
The easiest way for an end user to control the set temperature is through timers. Often these are built into thermostats, but there are a number of ways to easily adjust the varying mode times. Depending on the installation, a dedicated heating timer may be required, and DIN rail timers are available that allow setting times via a small screen, the bus, or both.
There are also a number of small touchscreens that will act as a dedicated heating/cooling timer to give the end user a central place of control. Indeed, in many cases there will already be a device in the installation that provides a high level of logic and visualization, to which the heating/cooling timers can be added.
In addition, you can also place timers in various visualization packages.
Two-stage heating and cooling
Most thermostats allow you to add an extra stage of heating and cooling or both. Two-stage heating provides a second heat source when it takes time for the primary heat source to fill the room. This can be especially useful when the occupants of the building return to Comfort mode after their absence. Although rarely used in the UK, there is an additional option for a second cooling mode for similar situations in warmer climates.
If the primary heating source, such as underfloor heating, needs a moment to reach comfort heat, radiators can be used as a secondary source.
As with the changeover between heating and cooling, there must be compensation before the extra stage of heating or cooling can be activated. This respect can be set in the parameters as soon as the extra stage is activated. The difference is usually set around two degrees so that if there is a difference of two degrees, the source of additional heating or cooling is activated.
Underfloor heating / cooling
UFH (Under Floor Heating) is becoming the most popular heating source in new construction as it has been proven to be one of the most comfortable and efficient heating systems. To achieve maximum comfort and get the most out of your system, the correct controls must be fitted and this can be achieved in a simple way with KNX.
As discussed earlier, a number of KNX thermostats are available. After you have selected the thermostat and set the parameters for the type of control you want and how the modes should work, the thermostat sends a request to the heating source. In this article I will discuss the process of using UFH as that resource.
If as a KNX professional you only take care of the control side of the application, there must be a clear understanding of where the boundary lies between your responsibilities and those of the heating engineer. In an ideal world, the UFH system should be fully commissioned and proven before the KNX integrator gets involved in any kind of control. But as we all know, this is often not the case, and knowing the basics of how the UFH system works can help troubleshoot system commissioning issues.
There are two main types of UFH namely electric and hydraulic.
With hydraulics, hot (or cold) water flows through pipes and gives off heat to the room, the heat content being dependent on the depth of the pipes. To maintain an even heat distribution, the pipes must be spiraled as shown below.
Depending on the size of the floor, it is not uncommon for a room to have more than one coil. For example, a large open living and dining room requires two coils. Likewise, two small rooms together, such as an en-suite shower room and a walk-in wardrobe, can be heated with just one coil.
Each coil is connected to a manifold and each zone on the manifold is controlled by an electro-thermal actuator. This is then connected to a KNX distributor controller.
Depending on the controller chosen, there is often the option of connecting two zones to one channel. This is especially useful if two coils are needed for a larger room. When using hydraulic underfloor heating, boiler control must also be taken into account.
There are two types of electric underfloor heating. The most common is when an electric heating element is integrated into a mat and installed under the floor. This usually occurs in bathrooms and retrofit applications. The second type, which is less common, is a cable with a large element that is attached to the reinforcement grid of the floor before pouring concrete. All element tails are returned to a central point and controlled with an appropriately sized relay, although it should be borne in mind that these will have large loads. While electric UFH is very responsive, it can be expensive to run.
Considerations When Controlling UFH
There are three considerations to make when arranging underfloor heating:
1) Maintaining air temperature The most common way to control underfloor heating, maintained air temperature, takes the demand for each room from the KNX thermostat.
2) Maintained floor temperature often used in bathrooms and other tiled areas, this ensures a minimum comfort temperature at all times. To achieve this, a floor probe is required, either used as an input on the room controller with a weighting in favor of the floor, or as a separate loop.
3) Overheat Shutdown This is used with electric UFH or as a safety device for certain floor types, such as delicate wood finishes. The easiest way to achieve this is to install a floor probe with a separate thermostat. This provides a clear definition between the default thermostat and the 'over temperature' override.
In all cases it is recommended to use a PI command which will be converted to a PWM in most manifold controllers. This prevents under- and overshoot of the set room temperature.
There are a few other things to consider when dealing with UFH regulation.
• Monitoring the water temperature
With hydraulic heating, it is important to monitor the temperature of the water used to heat the floor. This can be managed on the manifold or on the pipe itself. Depending on the specification, there may be a variable temperature valve that needs to be controlled.
• Check the water flow
If you are having problems with the heating control, but you are confident that the KNX aspect of the installation is running smoothly, there are some general checks you can make. For example, it is important to ensure that the water is not pumped too strongly through the valve heads. If it's too fast, the water will cause the system to work inefficiently. Too slow, and the floor will rarely fully warm up. Depending on the pipe length, the flow rate must be set to ensure optimum heat transfer to the floor.
• Check labels and take photos and thermal images.
The distributor output circuits should also be checked as they are often mislabeled. This can lead to one room being warmer than another, or in the worst case scenario, a room not being heated at all. It is best to take pictures of the installation before laying the floor. It may also be a good idea to use thermal imaging technology to determine if the system is working efficiently.
Copper pipes, water pressure and boilers are largely reserved for the plumber. With vents and gauges, drain valves and secondary flows, it's a mystery at first glance and best avoided.
It is not for nothing that specialists are deployed to install the boilers. We need heat (or at least control it), and most of the time we provide a potential free contact to the plumber and that is our 'demand' signal. However, this does not necessarily give the customer a particularly efficient system, and in certain cases, such as underfloor heating, the system cannot react quickly or adapt to changing seasons or bad weather.
In general, if we can influence the choice of boiler, we can offer the customer a more energy-efficient system if the boiler:
a) Respond to a 0-10V question.
b) Have the ability to fit a weather compensation kit.
c) Or better yet, stick to the OpenTherm standard.
By using a 0-10V signal, the boiler can modulate its burners to vary the supply temperature, leading to a higher efficiency. With condensing boilers, we, or rather the plumber, must ensure that the return temperature is low enough to allow the boiler to run as efficiently as possible in condensation mode. That topic deserves its own article. The 0-10V means that the boiler does not produce excess heat when it is not needed.
Demand-driven control allows the boiler to modulate its burners, resulting in higher efficiency.
Another variable that can be a problem is a sudden drop in the outside temperature. By installing a weather compensation kit, the boiler can adjust the supply temperature upwards in accordance with these drops outside, bringing more heat into the system to compensate for the heat loss the building will experience during a cold spell.
If neither is done at the boiler, we can provide similar functionality via KNX components, but this requires a bit of logic and we may not be taking full advantage of the boiler's capabilities.
OpenTherm (OT) is interesting and is KNX compatible. The OpenTherm protocol, which is applied internationally, allows a room thermostat or other device such as a KNX heating actuator to modulate the boiler based on the demand of the room or the system as a whole. In addition, boilers can be interrogated for errors, burn times, flow temperature settings and a wide range of operating parameters that can be of value when used in conjunction with a VPN (Virtual Private Network) on the data network, as it can be useful for maintenance.
Products such as the Theben KNX-OT-Box bring these messages back to the KNX backbone and indeed offer the possibility to send weather compensation information directly from the bus to the boiler.
Logic modules are inexpensive, but PI (Proportional Integral) and PID (Proportional Integral Derivative) curves are not inherently embedded in the cheaper units. Obtaining such functionality costs a little more, so the easiest route to boiler efficiency is normally to let the boiler modulate the flow temperature. There are products like the Loxone Miniserver that give you a formidable programming environment to take control of the tech room, but it requires some programming skill and a clear understanding of the tech room design.
Source: KNX Control & knxtoday