Temperature Control – Heating Curve
“Heating Curve” is a popular term within the Building Management Systems industry. You will find at least one heating curve within most BMS control systems out there, they can be applied in different ways, but they are mostly used to calculate a required temperature for a system or circuit.
A heating curve is something which calculates an output based on the measured input and some user defined settings.
- The software relies on always knowing the measured input.
- The software relies on some basic settings being correctly configured.
- The calculated output can be varied further with additional information.
Examples where this type of BMS software strategy could be used:
- Boiler flow set point.
- VT heating flow set point.
- Frost valve set point.
There are other examples as well, but you will notice from above that they are often “set points”. This is because the software calculates an output, or in most cases, a set point.
For this post we are going to look at using a heating curve to calculate a VT heating flow set point, also known as a “weather compensation curve”.
The flow temperature set point is scheduled against the outside air temperature in this case. This means that the outside air temperature is acting as the measured input and the calculated flow set point is the output.
Settings for the controls software
In order for the calculation to work you must set some parameters within the software. These parameters may be labelled differently depending on the software / equipment that you are using, but they often look something like this:
- Input low
- Input high
- Output low
- Output high
- Maximum out
- Minimum out
The input points are referring to the measured input you are monitoring. In this example our input is the outside air temperature so the input settings should be set accordingly. Usually, a low of 0°C and a high of 20°C will be fine, this typically covers the range for which we want to control against.
The output points are settings which scale the output based on the measured input. Often we set these to a low value of 80°C and a high value of 20°C. This means that we have set up a simple heating curve that will provide a set point of 80°C when the outside temperature is 0°C and a set point of 20°C when the outside air is 20°C.
The scale will work linearly between these values as shown in the following chart. Notice how the output “low” is higher than the output “high” value, this is because we are setting the output value based on the input, so the output low value refers to what you want the output to be when the input is at it’s low point.
So, with an outside temperature of 5°C we will provide a set point of 65°C with the above settings.
Example using Priva Controls
When working with Priva Controls the predefined modules give you everything you need for this type of control, you simply select the option and then set it up with the required figures.
Because the Priva Controls are programmed using pre-defined modules it already knows that you are using the heating curve for weather compensation, that is why the settings are already labelled up for you.
The min and max limits will override anything set in the Y1 and Y2 (output) settings, so if you set a curve to work between 20°C and 80°C but had a maximum flow temperature of 50°C then the curve will stop increasing after the outside air temperature drops below 10°C.
Example using the Niagara4 software with Tridium Controls
The software looks slightly different when setting it up on a Tridium system when working within the Niagara workbench. Unlike the Priva control software, Niagara works with objects placed on a wire sheet. To achieve the same results, we use a block called a “reset” and then configure the settings exactly the same.
This can now be taken further!
Simply calculating the VT flow set point based on the outside air temperature is not always the best form of control. What if we are putting 65°C into a space that is already at 23°C?
The solution for this is to add “room temperature compensation”.
Room temperature compensation takes the calculated heating flow set point further by adding a layer of feedback into the calculation. To do this you must:
- Set the heating curve correctly to start with.
- Measure the room temperature as well as the outside air temperature.
- Set a required room temperature set point.
- Add in a compensation factor to the calculation.
Room Compensation using Priva Controls
You may have noticed an additional tab on the first screen shot of the Priva software? This tab is labelled “compensation” and it is there because I had already selected this as an option for the control module I was using.
This tab provides the engineer with all the settings required to set up room temperature compensation.
It already knows the calculated flow temperature based on the heating curve you have previously set. It then monitors the actual room temperature and compares it to the required set point.
In the above example we have the following:
- Basic heating curve asking for 50°C set point.
- Room temperature reading of 20°C.
- Room temperature set point of 22°C.
- Compensation factor of 5 for both increase and decrease.
The control system now uses these settings to offset the heating curve to provide a flow temperature that reflects the space conditions.
The Calculation
Set point – Room temp = difference = 2°C
Factor inc * difference = 10°C
Calculated flow value + Compensation = calculated flow value after compensation = heating flow set point.
See how the feedback has now offset the required flow temperature because the space is below set point.
Should the set point be lower than the space temperature the same would apply but in reverse.
Space Temperature 22°C
Space Temp Set Point 20°C
Room Compensation using Niagara 4 software
The same logic is applied when creating this type of control within the Niagara workbench, it is just a little bit more manual and involves adding a few more blocks to the wire sheet.
Another important thing to note here is that the room temperature compensation now adds to the heating slope without a min or max to the calculation. For example, if the room temperature were to drop to 12°C the calculation would add 50°C onto the required temperature, this would then output a VT flow temperature set point of 100°C.
Obviously this is not achievable and would be incorrect, for this reason you have to add an additional limiting object to maintain your maximum and minimum flow temperature.
Adding the limit output object above and setting the values now prevents the heating flow set point from reaching unrealistic values whilst also allowing the system to provide room temperature compensation.
Additional Information
For more information about physical points within a BMS check out this post!
Interested in becoming a BMS Engineer?
Check out the following link to the BCIA website for more information.
