Hello, I am having trouble figuring out the logic for a system I am working on. We have a T3-TB controlling exhaust/recirculate dampers on a bank of 3 large air compressors, and a pair of outdoor air intake dampers. Dry contacts in the compressors tell the T3 which compressors are operating, and a space sensor diverts the hot exhaust to the room or outside based on room temp.
It currently works reasonably well, but the compressors produce about 250,000 btu each into the 1000sq’ space, and massively overshoot.
If the compressors cycled in a predictable order it would be relatively simple to use a PID to stage them, but they have their own lead/lag rotation schedule, so the heat stage of each compressor will change, and I’m not sure how I would do that in control basic.
This is my current program:
10 REM ENABLE HEATING IF BELOW SETPOINT
20 IF TEMP2 < RMSP THEN RM1HT = 1 ELSE RM1HT = 0
30 REM DAMPERS FAIL OPEN TO RECIRCULATE
40 IF SRUN = 1 AND RM1HT = 0 THEN START SDMPR ELSE STOP SDMPR
50 IF MRUN = 1 AND RM1HT = 0 THEN START MDMPR ELSE STOP MDMPR
60 IF NRUN = 1 AND RM1HT = 0 THEN START NDMPR ELSE STOP MDMPR
70 REM DETERMINE NUMBER OF COMPRESSORS RUNNING
80 CMPNUM = SRUN + MRUN + NRUN
90 REM OPEN INTAKE 1 IF SYSTEM OPERATING
100 IF CMPNUM > 0 THEN START W1DMPR ELSE STOP W1DMPR
110 REM OPEN INTAKE 2 IF ROOM ABOVE SETPOINT AND MULTIPLE COMPRESSORS ON
120 IF CMPNUM > 1 AND RM1HT = 0 THEN START W2DMPR ELSE STOP W2DMPR
130 REM OPEN TRANSER DAMPER IF SYSTEM OPERATING
140 IF CMPNUM = 0 THEN START UDMPR ELSE STOP UDMPR
Thanks for any advice or recommendations with my syntax or anything, I’m just getting started with control basic.
Cody,
Although I don’t completely understand the system you have, it appears that you only open intake damper 2 if multiple compressors are running. Would it help to open intake damper 2 if the room is, let’s say 1° above the setpoint regardless of the number of compressors running ?
The system works OK when the space is above setpoint, and I think opening the other outdoor damper when only one compressor is running could cause issues on extremely cold days.
My issue is that the waste heat from the compressors acts as the primary heat source for the space during operating hours, but right now it’s set up as a single stage heat system. I could make each compressor its own heat stage, which would work if they are all running. The compressors might not all be running though, and the compressor that starts first is determined by their runtime hours. So I can’t just call the north compressor “heat stage 1” (for example), because it might be the only one not running that day. So I need to define the heat stages based on which compressors are running at that moment, and the number of heat stages will vary. I don’t know if that makes sense.
It looks like a typo at line 60 starting N damper but stopping M damper.
Peter’s comment is right. At least one damper should be floating or 0-100%. Then it can be partially opened. 250kbtu is too much for 1000 feet so you need the damper to be half open regardless of room temp if the comp is on. Not that the air is linear through the damper but for the sake of illustration, that would out you at 125kbtu.
Can any damper work with any compressor or do they have to be paired? Do the actuators require power to open and power to close? If so, use 2 outputs and then you can open the damper when the comp is running and only close the damper for a few seconds at a time with a 30 second delay between close points to give the room some time to warm up.
John D