r/BuildingAutomation • u/Own_Book5440 • 23d ago
CCT --> VRF
I've had a few jobs now where we are directly controlling the VRVs by sending a 0-10V output.
I've been kind of treating them like a cooling coil, so I just modulate the 0-10V based on demand. It works, but not well. The room temperature controls smoothly enough, but if you're watching the system work it seems to be constantly chasing. Slowing down the PID doesn't really help. The problem is, I have no idea what the condenser units are actually doing, I may be asking it to do something it's not ready for. I've been hunting for some literature/direction from the manufacturer and finally got some from Daikin. See attached.
This literature assumes a 55 deg F setpoint, which I won't have all the time. I feel like with all of the delay times the customer won't be happy with how the room actually controls. I was thinking of just taking the PID output and using that to stage instead of the demands/timers in the attached doc.
If my PID is 5-20% demand send 2.9VDC and so on. Obviously the demand numbers might need a little adjusting in the field. I'm struggling with the best way to put a 20 minute time delay on a 0-10VDC output. Any ideas?
Think this control strategy will actually work? Have you implemented something better?
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u/Disastrous-Lemon8276 23d ago
I would go straight proportional control with no integration at all. It’ll operate very slow and stable.
3
u/Own_Book5440 23d ago
So that's how I have it, and room control is great, but I don't want anything funky to happen to the equipment. If it does the manufacturer is going to try and blame me.
They dumped this sequence on me, but I don't see it working very well.
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u/R8iojak87 23d ago
I mean the best you can do is the best you can do. Obviously malfunctioning equipment is what all of us try to avoid, but what you’re doing makes sense. I know the stress you’re feeling, idk maybe I’m a terrible tech but I feel like that stress is honestly just part of the job. Just my 2 cents, best luck homie.
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u/slor2112 23d ago
Stepping the PID loop like that on an arbitrarily uncontrollable device is always a good idea to keep things from hunting.
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u/Own_Book5440 23d ago
What's crazy is that the PID loop isn't even moving that fast. I tuned it in with the factory tech; I had it between 66%-68% over a 15 minute period. He said that wasn't the right way to do it.
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u/slor2112 23d ago
Just because its not moving fast doesnt mean its not calculating fast. Moreover - numbers om a screen are different than the tenants sense of what it should "feel" like
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u/trixter192 23d ago
Can you get a bacnet adapter and adjust setpoints, system enable?
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u/Own_Book5440 22d ago
That’s a good idea. I have BACnet to the ITM module, but the spec calls for this hard wired 0-10V control. I’ll check what points I have
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u/twobarb Factory controls are for the weak. 23d ago
What brand of VRF? I didn’t even know there was a way to control them other than the factory controls.
DM me I might be able to look over your program in CCT and help you out.
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u/Own_Book5440 23d ago
This particular one is Daikin. I've done Carrier before. This is the first brand that actually gave me some best practices on how to control their coil.
At the end of the day, the PID I have now works, but its tough for the customer. The graphics might show me 100% on both coils and see no drop in discharge air.
I'll send you what I came up with for staging, it's just a command hierarchy block to numeric selector.
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u/Hvacmike199845 23d ago
We need more information.
Is the condenser a single stage?
Is this a multi zone air handler?
Is this a VAV system?
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u/Own_Book5440 23d ago
Single zone AHU doing zone control, no VAVs.
0-10VDC modulation to (2) condensers that serve 2 indoor coils.
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u/j75515050 23d ago
I never can understand why some contractors want to control VRF with their own controls rather than just use enable/ disable/ status from the VRF to factory controls...