For a controller to open a window when the room air temperature exceeds 25°C, select “Room air temperature (°C)” from the first drop-down list, “is greater than” from the second drop-down list, and select “<numerical value>” from the third drop-down list, then type the value “25” into the third drop down list.
The dialog will then look like this:
Click on the Create Formula button to see the formula “ta>25” appear in the Formula Profile box. This formula is the mathematical expression of the control condition, and it will be transferred to the selected row of the daily profile if the OK button is clicked. Note that if IP units were selected the units shown in the drop-down lists would be shown as IP units.
To create a formula to trigger a control action when the room temperature drops below 15°C, for example switching on an auxiliary heater defined as an internal gain, the second drop down box would need to be changed to “is less than” and then “<numerical value>” chosen, with the value 15 input into the third drop down list. (It’s worth emphasising that a formula such as this is not required for the main room heating control, which is managed in a straightforward way with the room heating set point.)
The dialog will then look like this:
The Create Formula button will now show “(ta<15)” in the Formula Profile box. Clicking on Graph will display the formula graphically, as follows:
The graph shows a value of 1 (ON) below 15 °C and a value of 0 (OFF) above 15 °C. Use the X-scale limits to set the minimum and maximum values displayed along the x-axis. Note that in cases where the formula contains more than one variable, the Graph facility plots the formula as a function of one variable only, but a value must be entered for the other variables which are initially set to 0.
