October 2014

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Operator Error

Overriding the BAS

Steven R Calabrese

Steven R. Calabrese
Control Engineering Corp.

Contributing Editor

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We in the Building Automation industry often run into situations in which we find the Building Automation Systems (BAS) that we install and/or service, to be in an overridden state. At least partially. Equipment turned off, or forced on, with no regard as to how the mechanical systems were designed to operate. Following are some of the more common scenarios we encounter…any of these ring a bell?

Who Turned off the Boiler?

We see it all the time. Especially in facilities that don’t particularly have a dedicated maintenance engineering staff. The boiler is supposed to operate year-round, because it serves the reheat coils of terminal units, perhaps serving interior zones, that need to function in the summertime as well as in the winter.

My latest run-in with this type of situation was a retail space in the Windy City. The store is served by VAV boxes with hot water reheat coils. The VAVs are in turn served by a VAV air handling unit, and a hot water boiler. The VAV air handler does what it does, for all occupied modes of operation. And that is to provide cool air at a constant pressure. The supply air temperature during summer operation is generally around 55 degrees, and this is to ensure that all zones get what they need in terms of cooling, and in terms of dehumidification as well.

The landlord is adamant about turning the hot water boiler off in the summer months, thinking logically that there is no need to operate a heating plant when it’s eighty degrees outside. Of course the problem is that the system was designed in that the boiler may very well need to operate in summer. Try explaining that to the landlord!

Anyway, so what happens on those 70+ degree days is that the perimeter zones call for cooling and all is well, seeing that those VAVs serving the perimeter zones get the 55-degree air that they need to satisfy the spaces. However at the interior zones, the spaces may tend to become too cool, as there is no ambient load to combat. With the boiler disabled, these zones cannot “reheat” the 55-degree air, and thus cannot achieve zone heating setpoints.

The maintenance personnel on duty fields the complaints of the retail staff, and goes to the BAS front-end to turn up the discharge air temperature setpoint. Now the interior spaces come up in temperature, and so do the perimeter spaces, up above the zone setpoints! Add to the problem that the warmer supply air does not have the ability to effectively maintain the indoor humidity levels. There you have it.

Overridden Economizer

First, a brief and oversimplified description of economizer operation for a typical built-up air handling unit. The economizer section is physically comprised of outside and return air dampers, that operate together in opposing directions, meaning that as the outside air damper is driven open, and return air damper is driven closed, and vise-versa. During periods when the outside air temperature is too hot and/or too humid to contribute to the cooling operation of the air handler, the outside air damper sits at minimum position. If however the outside air is suitable for “free cooling”, then the outside air and return air dampers are modulated to maintain a mixed air temperature of, say, 55 degrees. If the need for cooling cannot be satisfied by economizer operation alone, then the means of mechanical cooling (DX, chilled water) is employed as well.

On cooler days, economizer operation alone is typically enough to maintain the setpoints of the space(s) served by the air handling unit. There is a philosophy that, when the outside air temperature is above a certain point, the outside air can no longer provide assistance in satisfying the cooling load. So the operator goes ahead and overrides the economizer, closing the outside air damper to its minimum position.

The philosophy is unfortunately incorrect, as warmer air can most certainly help the cause, if the air is dry and not too warm. But again, try telling that to the old-school maintenance guy that doesn’t trust that ole computer sitting on the bench in his office. So the economizer gets overridden, and gets left in that state for all time. And along with it go any opportunities to save on energy via “free cooling”.

HOA Switch in Hand Position

This applies mostly to fans and pumps that are controlled by the BAS. The example that’s fresh in my mind is that of an industrial facility that has a dedicated motor control center. What this is, is a large electrical cabinet located in an electrical room, that houses the starters for motor-controlled equipment. The equipment sourced from this particular motor control center were mostly warehouse exhaust fans.

Most of the exhaust fans were controlled through the BAS. Meaning that most of the fan starters had hand-off-auto (HOA) switches embedded in the face of the starter compartment. For these fans, they were typically designed to operate on a time-of-day schedule, via the BAS. However, every last one of these fans was overridden at the HOA switch. Overridden to the Hand position. Apparently someone didn’t trust the scheduling function set up within the BAS. Either that or the schedule did not match the needs of the operation. Either way, the easy thing to do is to simply override the fans “on”, and perhaps override the fans “off” when the facility goes unoccupied. More often that not though, in these types of situations, is that the fans just end up running 24/7, with no regard to their need and to their energy usage.

VFD Placed into Full Bypass Mode

This one is a no-no. Which is why we don’t even care to install variable frequency drives (VFDs) with bypass contactors. The issue is that, if and when a VFD fails, if the VFD is equipped with a full voltage bypass, and the user does not have the wherewithal to understand the consequences of throwing the VFD into bypass, all heck can break loose!

Consider a VAV air handling unit. The VFD is doing its thing, varying fan speed in order to maintain a supply duct static pressure setpoint. Then bam, failure! What to do? If the VFD is equipped with bypass contactors, and the user, in a frenzy, throws the switch, the fan motor will suddenly take on full power, and the fan will strive to operate at full capacity. If the VAV boxes served by the air handler aren’t “forced” to full-open (and even if they are!), the pressure in the ductwork can build up to a dangerous level. At the very least, the fan will trip out on its high static pressure safety device. One would hope. Worst case is that the ductwork would build up so much pressure that it would literally come apart at the seams!

A workable solution? Well, VFDs are becoming more reliable and less expensive all the time. So why not install a “spare” VFD? Or at least have one on hand. No bypass, no worries. Just press the spare into duty, and you’re good to go. For smaller systems, this can be a very feasible solution. Sure beats the alternative!

Tip of the Month: Education is the key in all of the aforementioned scenarios. As well as “buy-in” on the concept of the automated control system. These days we’re mostly inclined to trust technology, however when it comes to the BAS, some are still skeptical. Proper training can help, but only if the end user is open to the concept, and has faith in the system.


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