AutomatedBuildings.com Column - Cutting Corners In Control Systems Design & Installation

In the estimating and pre-sales phases of this business, it’s a world of “wishful thinking”. “Yeah, we can get it done for this price”, or “I’ll substitute this device in my estimate that’s half the cost of what should be used, and if we get the job we’ll somehow make it work”. However when the project is booked and reaches the design and ultimately the installation stages, we need to do things right, margin be damned!

In the spirit of making a profit, we’re always looking for ways to cut costs, hopefully not cutting corners at the same time. But sometimes we take these risks, whether we’re aware of the potential consequences or not. Taking shortcuts can be a deliberate act, or perhaps one borne out of ignorance and inexperience. We’ve all been told that we should learn from our mistakes, and thus I present you with a small offering of corner-cutting pitfalls that I’ve been burned on at some point in my career.

Design Shortcuts

Ignoring Ambient Ratings: So you want to throw that digital controller in the rooftop unit controls compartment, huh? Not before you check to be sure that your controller is rated for the climate you’re in! Intense heat and extreme cold can wreak havoc with electronics, and some controllers are better off being put inside a suitable enclosure and installed indoors, below the roof line in the ceiling space if possible. Course anyone servicing the building will have a harder time finding the dang controller, but not if it’s well-documented on your control drawing. Sure, it’s easy and on the surface it makes perfect sense to mount the controller in the rooftop unit, but sometimes it’s not the best decision, and somewhere down the road you could be paying for it.

Ignoring Amp Draw Ratings: The classic example is powering multiple VAV box controllers from a single transformer. A common practice, and one that’s quite acceptable as long as the rules are followed. Rule number one: size the transformer for the total amp draw of all the devices being powered by the transformer, and include a safety factor. You want to maintain a Class II power-limited circuit so that you can “free-air” the low-v side of the transformer out to the VAV boxes. How many VAV boxes does that mean? Add up the VA ratings of the controllers, as well as the primary air damper actuators, control valves (if applicable), and relays, and multiply by 1.25. If you’re over the VA of the transformer, better drop a VAV box or two. Play it safe, or you’ll be sorry!

Ignoring Damper Sizes and Damper Motor Torque Ratings: It’s easy to be looking at a set of mechanical plans and figuring a single damper motor for every damper shown. You need to be mindful of the square foot area of each and every damper, lest ye be blowing your actuator budget right from the get-go of the project! There are rules of thumb, and specification sections devoted to how to size control dampers, and how to apply actuation to them. Generally speaking, a good rule of thumb for dampers is that no single damper shall exceed 15 square feet. If the dampered area is greater than this, your damper supplier can give you individual damper sections in the shapes and sizes to make up the entire area, and you’ll control each section with its own damper motor. And no skimping on the actuator either…get the motor that has the torque to turn that shaft!

Skimping on the Freezestats: OMG…this one kills me! Shoulda learned the first time I saw it. Was about ten years ago. I took over management of a large high rise project with huge air handlers. When I got on site for the first time, I was aghast over how many freezestats each air handler was equipped with: twelve! I realized then that a freezestat is only as good as it’s capillary length, and with typical freezestats having a 20 foot capillary, there was a definite limit as to how much freeze protection a single freezestat could offer a hot or chilled water coil. Generally speaking, for every square foot of coil surface, there should be one lineal foot of capillary.

Fast forward a few years, different company, different role. Now an estimator and sales engineer, I’m looking at my first project of size, and there are several air handlers that I need to account for. I prepare my estimate, come up with a price, and several weeks later we have the job. Upon turning over the project to our Operations Group, I’m appalled to learn that I underestimated the number of freezestats required for the job. In my haste to generate the estimate, I figured one per air handler, when in fact each air handler needed three. Not a terrible miscalculation, if it were a single unit, however we’re talkin’ eight air handlers, so I mis-budgeted by sixteen freezestats, device cost, labor, and all! Ouch…no wonder we got the job.

Installation Shortcuts

Improper Wire Sizing: Obviously a big concern for power wiring, less so for control wiring but still an issue. You may know to use a certain gauge for a specific amp draw, say you’re powering a dozen VAV box controllers from a common transformer (ring a bell?). Hey, just add up the current draws of everything to be powered and add in your safety factor. Select the conductor size based on that, right? Well, yes and no. If you have a long run from the transformer to the controllers, you’ll need to account for voltage drop. The longer the run, the more of a voltage drop between transformer and load. Larger conductors minimize voltage drop, and so you may be forced to go to the next size up, even though your project hasn’t the budget for it. I remember the time I powered (from a single point of origin) a start/stop relay in each of two dozen rooftop units scattered about the roof of a large manufacturing plant. Problem was, some of the relays weren’t pulling in. We found that the voltage drop was too severe toward the end of the line. We were faced with two options, pull larger wire, or try using relays that drew less power. Fortunately the relay changeout solution worked, and we were spared the expense of pulling new cable. Yet had we used properly sized conductors to begin with, even with the added cost, we’d have avoided a whole lot of extra work, and the additionally incurred expenses.

No Conduit in Wall Cavity: The requirement for conduit varies widely, from project to project, from town to town, and from state to state. Over here in my neck of the woods, conduit is required, or at least specified the majority of the time, for the “stub-ups” from thermostat locations up into the ceiling space. From there it can be run “open-air” if conditions are met. That aside, we run into retrofit situations where this hasn’t been done. Or in new construction, the walls go up before we’ve had the chance to install the stubs (oops!). Try fishing a wire down a wall to a sensor location…do this a dozen times, and you’ll understand why the conduit stub-up is an oft-specified requirement.

Ignoring Communication Wiring Guidelines: Conventional wisdom dictates that network cabling be run from controller to controller, in a “daisy-chain” fashion. Common practice should follow the conventional wisdom. In other words, no branching out from one point into more than one segment. Tee-taps and star configurations are not allowed! Will controllers communicate if done this way? Probably. But you need to think about this from a troubleshooting and serviceability standpoint. I’ve seen techs spend weeks trying to diagnose comm issues, and that’s when it’s done right. Just imagine the time spent when it’s not!

Combining Communication & Line Voltage Wiring in Same Raceway: Hey, why run another pipe for the low-v network cable if we can use this one with some 120v wiring in it? The communication cable is shielded, right? What could go wrong? Famous last words. Seriously, while it may be seemingly justifiable for short distances, seeing as if it becomes an issue, it would theoretically be easy to diagnose and correct, why then do it at all? No, do it right the first time: expend the extra labor and incur the added material cost, and there will be no issue, and nothing to correct at a later time.

Tip of the Month: Ask someone who’s been in the business awhile, what comes to mind when they think about cutting corners in control systems design and installation. If need be, give them some time to think about it before they get back to you. Do this over time with different people and you’ll have yourself a nice little collection of “what not to do”, without having to learn it the hard way!

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