In order to know all that Phil has mentioned (and all of it is true), you are required to have, and be proud of, a few gray hairs.
The Building Performance Team
James V. Dirkes II, P.E., BEMP , LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Philip Haves
Sent: Wednesday, May 16, 2012 12:29 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Cc: ohchoen
Subject: Re: [EnergyPlus_Support] P or PI control
EnergyPlus assumes perfect control, i.e. set-points are met and followed instantaneously, subject only to capacity constraints. At the local loop level, a well-tuned PI (or PID) controller that controls equipment that responds quickly relative to the time-step will closely approximate this perfect control. So, in effect, EnergyPlus implements PI control for most HVAC control loops - exceptions include thermal storage tank control and zone temperature control. Most modern digital control systems in buildings are set up to implement PI control - how well these loops are tuned in practice is another matter.
Most of the pneumatic control systems found in older buildings implement proportional-only (P) control. The consequence of this is that the controlled variable is not controlled to a fixed set-point but varies across the 'proportional band', depending on the load. Consider a heating coil in an air handler that produces a 10 K temperature rise: if the supply air temperature set-point is 30 C and the proportional band is 5 K, the supply air temperature will be 30 C only if the load is zero, i.e. if the temperature of the air entering the coil is 30 C. If the entering air temperature is 15 C, the valve needs to open fully to meet the load. However, proportiona
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