Thursday, May 3, 2018

Specifying Control Logic

The controls on VAV boxes have changed quite a bit since their conversion from pneumatic in the early 80’s. Back then, we often referred to overly complex control sequences as ‘Klingon Spaceship controls.’ One would think when DDC controls arrived, that things would be much simpler. Of course, this isn’t what happened.

Instead, we got a proverbial ‘black box’ with “Trust me” written on its side in disappearing ink. Simply broken down, there are two types of controls for VAV boxes, line and block. A line controller is essentially a programmable device with its code written in a ‘line-by-line’ manner. A block controller’s code is written in pre-programmed blocks that work together, hopefully. Technically a ‘line-by-line’ program is broken into functional blocks too. So the question is: What functions are the blocks performing?

It’s difficult to get control suppliers to program controls to do what we expect them to do. We (Krueger) have discussed it a lot with Specifying Engineers and some of our reps. There is a lot of ‘push-back’ from control contractors who want to reuse what they can, and charge the maximum amount for inventing what they feel are new, ‘custom’ sequences. Specifying engineers have told us that they feel the control contractors really don’t understand what the specified products can or should do.

ASHRAE Guideline 36: High Performance Sequences of Operation for HVAC Systems is going to be available this June, and it should provide a basis for writing specifications for VAV systems. I’ll be cutting and pasting from this document to try to put together a guide controls specification for a VAV Series fan box with a sensible cooling coil. Stay tuned for updates!

Authored by: Dan Int-Hout, Chief Engineer Krueger