Here’s a graph of setpoint versus outside air temp and the basic middle school math that drives it. That took me way longer than it should have…!
Here’s a general formula in control basic. Most of these variables can stay as local variables so they appear grey in the control basic editor. The SETPOINT is a variable which we’ll show on graphics displays, use in the programs and PID loops so it is set up in the VARS table and therefore shows as blue in the editor at Tab1. Similarly the outside air temperature is defined in the inputs table at Tab2 and also shows as blue in the editor.
Set the outside air temperature to MANUAL mode at Tab3 and watch the setpoint follow the formula, it checks out OK at both the max and the min outside temperature.
…and good thing I did that test, we need to clip the setpoint at the min and max values, added that in line 140.
10 REM **** OUTDOOR AIR RESET FOR HOT WATER SUPPLY TEMP *******
20 REM CHANGE THESE NUMBERS TO SUIT YOUR PROJECT
30 REM X1,Y1 ARE THE OUTSIDE TEMP AND SETPOINT RESPECTIVELY AT OAT=12C
40 REM X2,Y2 ARE THE OUTSIDE TEMP AND SETPOINT RESPECTIVELY AT OAT=20C
50 XONE = 12
60 YONE = 80
70 XTWO = 21
80 YTWO = 20
90 RISE = YTWO - YONE
100 RUN = XTWO - XONE
110 SLOPE = RISE / RUN
120 BINT = YONE - SLOPE * XONE
130 SETPOINT = SLOPE * OUTSIDE + BINT
140 SETPOINT = MIN( YONE , MAX( SETPOINT , YTWO ) )
