BACNET Wack or Metasys Got Me HHHHHHHHHH

HHHHHHHHHHHHHHHHHHHHHH

Oh hi. There’s a slightly modified screenshot of a page from our building’s Johnson Controls Metasys UI while it was trying to roast our morning show hosts and stage guys to death and I was trying to get it to kindly, you know, not.

This is for the studio air handing unit. It just blasts cold air into the studio and does not have heating features built in. Various people have asked me at times why the heating doesn’t work. It just now occurs to me I could show them this graphic – no heating coils, reheat, or connection to the gas furnace next to the air handler exist for a now obsolete reason: this studio used to originally be lit by a multitude of terrifying hot lights to the point that heating would not be necessary while the studio is in use. Here’s the air handler viewed from outside:

Chungoid
Chungoid with mad engineer in tall goth boots for scale
Supply fan – 15 horsepower!

A couple years ago I used to have trouble with the programming on this drive, but that’s long since resolved. The issue I was having was that it was just constantly getting a command to run at 00.00 Hz, which meant no blowy. A quick glance at Metasys showed me that it was calling for 100% chilled water flow to the coil and it was nice and cold inside the air handler, so I just hit battle short on the bypass there to get it going and cool the studio while I investigated. After the fan came up at 60.00 Hz (absolutely fearsome) it sucked one of the doors closed as I walked past it and tore my pants halfway down the side. Nice.

Anyway… Back to looking at the controls. The system cycled once like that, reaching the setpoint temperature and overshooting it slightly, then slamming shut the chilled water valve and letting the studio overheat again. When it had actually cooled the place down I went back up the roof and took the drive out of override, but now it stopped the fans too. Hmm.

This time I set Metasys in override. On each of those control parameters (return fan output, supply fan output, mixed air damper, and chilled water valve output) I could override them to nonzero values and the command would take effect! The only thing that was missing was any semblance of thermostatic control whatsoever. The system was just any degree between on or off I wanted… but once set like that, the studio temp would just drift up or down… I just about froze Tina and Courtney who were sitting right under vents. Aaaaaaaaaa! A call was placed at 8 am for the one and only controls tech in the area, who came out and spent several hours on the hellbeast.

Nyoooooooom. Note the movable blower inlet vanes, an artifact of a former control system that didn’t use VFDs.

After a while of scratching his head over the way our BACNET system was configured and how it was just spouting communication errors, the controls tech spotted one of these thermostats on the wall of the mechanical room with its display dark. Curious, we thought… I grabbed a meter and tested between the wires in it and found about 3.6 volts AC where there should be 24.

Sorry, I didn’t get a picture of the inside of it but it’s basically just like any normal thermostat, just with a BACNET connection in addition to the normal R, RC, C, Y, G, W, and Y wires. It basically just lets Metasys schedule occupancy and setpoints and monitor the status and temp. The menu system is maddening.

So I walked over to the electrical panel for the air handler it controlled and saw the breaker appeared to be on. I turned the breaker off and back on and the thermostat lit up and the blower started. 26.1 V AC appeared between R and C.

I agree with Scrappy.

At this point I went back into the studio, wound up the timer switch on the wall that serves as the manual zone occupancy override, and after a couple minutes, the blowers started and the water valve opened until the discharge air temp met the setpoint. (Yes… It’s even adjustable!)

So uhhhhh, what the heck? My only thought is that when that wall thermostat was getting 3.6v (why that and not zero???) it was just getting enough power to make the microcontroller or RS-485 line driver in there wake up and periodically yeet glitches into the BACNET void

The controls tech explained to me that it works like Token Ring. If the token falls out of a powered down thermostat in an empty mech room, does it make a sound? Really this all reminds me of something the Space Toilet used to do wherein you could mess up its canbus network and make the amplifiers blink on and off like holiday lights from hell.

Was it you? Did one of you take the shiny token back to line your nest?

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