June 2011
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Terminal Units Primer
Select the appropriate digital controller to fit the application

Steven R Calabrese
Steven R. Calabrese
Control Engineering Corp.

Contributing Editor

The term Terminal Units (no pun intended!) is a catch-all phrase used to describe unitary HVAC equipment, equipment primarily designed and built to serve a single zone of temperature control. The term can be broadened to include other types of unitary equipment, however what we’re really talking about here are VAV and fan-powered boxes, including all of their variations.

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Control Solutions, Inc

The purpose of this column is to provide insight to specifying and properly selecting digital controls for the various types of terminal units. The content herein will concentrate on controller point counts and types, and also what types of input/output devices are required for each variation.

VAV Box – Cooling Only

So first things first. The basic VAV box consists of a damper, and not much else. The unit is pressure-independent, meaning that the instantaneous CFM setpoint is maintained regardless of fluctuations in pressure. For this to happen, the VAV box controller needs to be able to read the CFM through its damper. This is done rather indirectly by measuring velocity pressure through the unit, and converting this to CFM. A pressure-independent VAV box is equipped with a mechanical flow measuring apparatus (ring or cross with holes in it), and this device connects to a pressure transducer, most often built in to the controller. The controller monitors velocity pressure, and internally calculates CFM. The CFM setpoint is then a function of the deviation in temperature from zone setpoint. Got it? Okay, in simpler terms, the hotter it is in the space, the more cool air gets dumped in to the space via the VAV box!

In the above discussion, we’ve uncovered a number of input/output points required. Specifically, we need an input for velocity pressure, an input for zone temperature, and an output for the damper actuator. Typically the velocity sensor will be “hardwired” into the controller’s printed circuit board, and thus won’t consume an available physical input on the controller, so you don’t need to add that one to your point count. On the other hand, the zone temperature sensor will consume at least one input point (for temperature), and perhaps another input point for a setpoint adjustment. As for the actuator, these days it’s more common for the actuator to be an integral part of the digital controller, however it’ll still likely consume a physical analog output, factory-wired and tested.

So the simple cooling-only VAV box will require a digital controller with at least one input and one analog output. If the zone sensor is to be equipped with a setpoint adjustment, then the controller will need two inputs.

VAV Box with Electric Reheat

Moving on, we now add staged heat to the simple VAV box. What does this do to the requirements of the digital controller? Glad you asked! It adds the need for more outputs, specifically the binary (two-state) variety. So in addition to the analog (modulating) output you need for the damper actuator, you also need a binary output for each stage of heat to be controlled. Most VAV box electric heating coils will have one or two stages of available heat. Seldom any more than that is needed, but watch out…you will come across a VAV box heater with three available stages of control. If you have the point count on your controller, then no sweat. If not, then you may have to get creative (more on this later).

VAV Box with Hot Water Reheat

Now, instead of having staged heat, we have proportional heat, in the form of a hot water coil and a modulating control valve. The valve therefore requires an analog output from the controller, yet now you scale back on the number of binary outputs that you need. Adding the requirement for a second analog output (remember, the damper actuator needs one as well) is not a problem, as any manufacturer’s product line will certainly be able to accommodate the control of a terminal unit with proportional hot water heat. Also, with the addition of the means to heat at the unit, and really we should have discussed this in the last section (oh well!), it’s customary to provide a discharge air temperature sensor on the leaving side of the terminal unit. For cooling-only VAVs, we typically know the temperature of the air being delivered from the leaving side of the unit, as it’s what’s going in to the unit (55 degrees, give or take). For VAVs with reheat, it’s of value to know what the temperature of the air is downstream of the (electric or hot water) reheat coil. At the very least, it’s a good verification tool for the functionality of the heating coil.

Fan-powered Box with Electric Heat

Reliable Controls Now we add a fan into the mix, and call it a fan-powered box (real creative equipment description!). To control the fan, we need an additional binary output. Remember for a terminal unit with electric heat, we need a binary output for each stage of heat. Add the requirement for an additional output for the fan motor, and you have yourself a need for a whole lot of outputs. Shouldn’t be a problem, however again you just need to be weary of the number of electric heating stages. Anything out of the ordinary will require some consideration. For instance, say you have three stages of electric heat. Along with the fan output, your requirement for binary outs stands at four. But your controller selection only has three. What to do??? Well, providing that there is no other controller in the product family that can accommodate all required outputs, you’ll need to get creative (told ya there’d be more on this!). You can’t ignore that last stage of heat; the heater was sized for the load, and ignoring it would get you into trouble on those days where it’s really needed. You can double up on your last output stage, in other words, have it control the second and third stage of heat simultaneously. Not the worst of scenarios, but not the best either. You can one-up this by installing a time delay relay on the third stage, so that when the controller calls for the second stage of heat, the second stage of the electric heater is instantly invoked, yet the third stage doesn’t activate until the time delay relay ”times out”. Now, if the call for second stage heating is satisfied before the timer elapses, then the second stage drops out, no harm no foul. If the call for second stage heating persists, the timer will time out, and the third stage of electric heat will be activated. Now, when the call for second stage heating is satisfied, both the second and the third stage of electric heat simultaneously drop out. Not a bad way to handle this dilemma, however be sure to get it approved beforehand.

Fan-powered Box with Hot Water Heat

Finally we get to the last terminal unit of our discussion. After that last section, this one should be easy, you would think. No staged heating, so no extra binary outputs required. Only an additional analog output needed, for control of the modulating hot water valve, right? Well traditionally, yeah. But along came the development of the variable speed fan motor, and with it came the need to control it. What this means is that, along with the need for a binary output for start/stop of the fan motor (or perhaps supplanting the need for it, depending upon the application), there’s a need for an additional analog output, for controlling the speed of the fan motor. So you end up requiring a total of three analog outputs: one for the damper motor, one for the hot water valve, and one for the fan motor. Can a controller in your chosen family of products handle it? Most likely, yet you still need to be aware of this, for various reasons, not the least of which is, properly pricing and bidding the project at hand.

Factory vs. Field Mounting

Often the consulting engineer will specify factory mounting of the terminal unit controls. For cooling-only VAVs, this amounts to the controls contractor shipping the digital controllers and actuators to the factory, along with an accurate and easy-to-understand wiring diagram. Not much to show on the drawing really, until you start adding other items, such as fans and heaters. The more appurtenances, the more complicated the wiring diagram becomes, and the more of a margin for error, either on the part of the wiring diagram creator, or on the part of the factory labor force. For projects of which there are many, many terminal units, factory mounting becomes a good option, especially when we’re talking fan-powered boxes with variable speed fan motors and electric heaters. However, you need to be certain that the wiring diagram is free from errors and omissions, lest ye end up on the jobsite having to rewire the controls for a hundred units!

Tip of the Month: There is much debate on when/whether it makes sense to ship controls to the VAV box manufacturer for factory mounting. Certainly on a project with a half-dozen cooling-only VAVs, it makes little sense to go the route of factory mounting. Get it done in the field, enough said. However if you’re talking dozens and dozens of fan-powered boxes with electric heat, it makes good sense to have the factory get it done for you. I have no hard and fast rule on how to decide, other than to say it doesn’t always make good economical sense to blindly call out for factory mounting. Take each project as it comes, analyze the situation, weigh the pros and cons, and make your best, well-informed decision!


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