Are You ‘Pushing on a Rope’ in Your Controls-System Designs?

Back in my college days, I was sitting in my engineering-mechanics class, and a very wise professor gave us young “wannabe” engineers a bit of advice. He said, “Remember this: If you want to go far in your career, you need to remember you don’t push on a rope; you pull on a rope.” At the time I didn’t have a clue what he was talking about, but over the years, as I have observed how some people go about designing building controls, I now understand exactly what he meant. In fact, the longer I am in this business, the more amazed I am at the logic used to achieve a desired result.

Why do we use airflow (cfm) control to manage space static pressure? Why do we use terminal unit cfm control to maintain space-temperature control?  Why do we reference outside static pressure for a building at the roof when we are trying to control the front doors? J. Christopher Larry recently wrote an article for HPAC Engineering reminding engineers to use the KISS (“Keep it Simple, Stupid”) principle as it applies to controls, and I am sure there must have been other similar articles. However, many engineers who are designing control systems evidently are either not reading these articles or not heeding them.

I have had the opportunity to experience and learn the mechanical equipment and control business from a few different perspectives. First it was as a sales engineer for a major controls manufacturer. After receiving my PE license I went to work for a mechanical/electrical consulting firm as a mechanical design engineer. Finally, I arrived where I am today, as the service manager of an independent controls contractor that provides commercial and industrial service work on control systems, variable-frequency drives (VFDs), and most HVAC equipment. Each step of the way, I had the opportunity to get a different perspective of how controls are viewed and applied to HVAC equipment. Along the way, I have developed a few opinions on how HVAC controls are applied—and should be applied—to mechanical equipment.

A building owner sits down with an architect and engineer in the early design phases of a project and informs them that certain areas need to be held at a slightly positive or negative pressure in relationship to an adjoining area. The engineer takes this design criteria and set about designing an elaborate air-exchange rate into a room based on cfm. He uses airflow measuring stations to measure the amount of air moving in and out of the space and then, through some algorithm, attempts to determine and control what the space differential static pressure will be. Although this sounds very high-tech and creative, it is complicated, usually not very dependable, and maintenance-intensive. If the goal is to maintain a pressure differential between two spaces, then we should measure pressure differential between the spaces and control the equipment designed for the space accordingly. Using cfm measurement makes no sense to me.

On variable-air-volume (VAV) box controls, I have seen control algorithms that control the cfm between the maximum and minimum settings based on offset of the space temperature from room set point.  Does it work? Yes, somewhat, but we are relying on the weakest link in the whole VAV box configuration. The flow probe in the VAV box is generally an inexpensive part, and most differential-pressure transmitters on VAV box controllers are generally on the lower end of dependability because the manufacturers have to keep the cost of the controller down to be competitive. So why don’t we just control the VAV damper based on temperature, and let the flow controller limit the travel of the box damper between the maximum and minimum airflows? To me this just makes more sense.

When it comes to building static-pressure control, what are we really trying to control? The theory in building mechanical design is to maintain a slightly positive building pressure to control infiltration. But where do you measure it? Depending on whether it is extremely cold or hot outside, and if the building has any height to it, you have this little phenomena called stack effect that is wreaking havoc on the building’s pressure gradients.

Then you throw in wind velocity and direction and it is anybody’s guess as to the perfect location or locations of the outside static pressure tip(s). More often than not control contractors and engineers select the roof to reference the outside pressure, and the lobby for the interior reference point. There are so many things wrong with this relationship and the measurement of it that most of the time you are set up to fail. The reality of building-pressure control is not infiltration, but whether or not the doors on the main level open and close properly. This is the main focus that the facility engineers really care about. If the building is too positive and the doors stand open, a security issue is created. If the doors are hard to open and slam closed as they are used, a safety issue is created. Neither of these scenarios is acceptable. We have found that if we measure the differential pressure between the lobby areas and outside relatively close to the main doors, we have a much better chance of controlling how these doors work under most conditions.

Keep it simple. Think things through and analyze your own building-controls systems. Make sure you are not pushing on ropes you should be pulling on.

Mickey Schroeder, PE, is HVAC service manager at Control Services Inc., Omaha, Neb. The company installs DDC systems and services pneumatic, electronic, and DDC control systems, variable-frequency drives, and HVAC equipment. Schroeder received his bachelor’s in mechanical engineering from the University of Nebraska in 1977. He has nearly 40 years of experience in the HVAC industry. He can be reached at [email protected]