"Automated Logic" Sizing error, need sequence of operation help.

[plainjs]

I'm just now going through training on AUTOMATED LOGIC control system, system has been up and running for 4 months now.

Problem.
System designed for full redundency consisting of two x 5Ton Carrier splits. System controlled by Automated Logic BMS system. Lead/Lag was specified to swap every 24 hours, Lag is designated a higher Zone Temp Setpoint to allow it to restart if problem with Lead unit, but was basically needed only for back up.


Initially the system worked fine, compressors would cycle at about twice per hour, 20 mins on 8-9 off based on a 72 degree setpoint.
Load has increased and now one unit basically runs continuously for 24 hours with lag unit only providing support to maintain setpoint.

As the load is not going away I am working on additional A/C unit to be installed and leave this system as back up, but in the meantime I am working these units into the ground. death
I would like both units to work together to maintain setpoint (redundancy is out of the window now) but 5 tons is not quite enough and 10 is way too much. My thoughts were to lower the SAS and widen deadband but fear this will result in Lead unit short cycling due to proximity of the load to units. I am having difficulty explaining myself to my control company as I'm not up to speed on the jargon and they are not really familiar with basic HVAC principles.

[Tesla]

Hi Plainjs
Welcome to the forum.
You could lower the SAS (Supply Air Setpoint) to compensate for increased load, but. The control system should control the SAS according to load measured by space temp or return temp error. If you are concerned about the splits, the control system could be programmed to change lead lag on each cycle or period of cycles. It's a bit like chiller sets, it is very differcult and costly to control individual units loading/staging. So as the lead does not have to work so hard lower the lag setpoint temp. When you learn a bit more about controls you might consider doing controls work and sharing you knowledge of AC with the control techs. It's much cleaner and easier work for when we get older.

[plainjs]

Thanks for response, played with it a little on Saturday and ended up RAISING setpoint a little and increasing hysterisis, Lead compressor now gets a break every couple hours, am thinking about switching Lead/Lag every cycle (or perhaps every couple of hours). I find the Control side quite interesting and am thinking it could be a good gig, certainly beats troubleshooting on a hot roof!