Packaged Rooftop Units

Sequence Items

Talking here about small-tonnage, constant-volume rooftop units designed for single-zone applications. Sequence items include, for starters, heating and cooling of the particular space (and how that’s accomplished), and occupied/unoccupied modes of operation. For comfort control (heating and cooling), a thermostat is typically provided that controls the rooftop unit modes of operation. If the unit is not to be in continuous operation 24/7, then the thermostat will likely be specified to be programmable, so as to be able to set an occupied/unoccupied schedule. If intended to be a part of a Building Automation System (BAS), the thermostat may be replaced with a digital controller that sits inside of the rooftop unit controls compartment, and a temperature sensor is located in the space served. Or if it’s an available option, the unit may come with its own microprocessor-based controls, and a digital communications interface.

In the occupied modes, the supply fan runs continuously, and the occupied heating and cooling setpoints are maintained. The outside air damper is opened to its minimum position, and stays that way for all occupied modes, unless there is an opportunity for economizer operation, in which case the damper may be positioned anywhere from its minimum position, to fully open.

In the unoccupied modes, the unit shuts down and the outside air damper closes. The supply fan cycles as the unoccupied heating and cooling setpoints are enforced. The outside air damper should stay closed during periods of unoccupied calls for heating or cooling, as there are theoretically no occupants for which to satisfy ventilation air requirements.

Do’s & Don’t’s

For specification purposes, do indicate, first and foremost, the type of control, whether it be a stand-alone thermostat (programmable or non-programmable), factory-furnished microprocessor-based control with a communications option, or separately provided DDC control (ala temperature controls contractor). And do specify duct-mounted smoke detectors if/when required. Don’t belabor the inner-workings of the package (there’s an O&M manual for that!). And don’t specify a freezestat for units with direct expansion cooling and gas heating; you simply don’t need it.

Fan Coil Units

As with the previously discussed topic, we’re talking again about unitary equipment designed to operate at a constant volume of supply air and serve a single zone of temperature control. Sequence items again include heating and cooling of the particular space, and occupied/unoccupied modes of operation. We’ll restrict the conversation to fan-coil units with hot and/or chilled water coils. A four-pipe setup would be a unit with two coils, one for hot water, and one for chilled water, each with a pair of pipes connected (hence the “four-pipe” designation). A two-pipe unit would be one that has only a single coil, that pulls “double-duty” as the heating and cooling coil.

For the four-pipe unit, the coils would each be equipped with proportional control valves. For the two-pipe unit, the “dual-temp” coil, as it is sometimes referred to, is equipped with a proportional control valve, which modulates on calls for cooling if chilled water is available, and on calls for heating if hot water is available. The mode of operation is typically determined by an “aquastat” that is strapped onto the pipe locally at the unit, that determines whether there is hot or chilled water available. In applications in which there is a requirement for outside air to be drawn in, the coil may be fitted with face/bypass dampers. Operation is based on outside air temperature. If the OA temperature is above freezing, then typically the face damper is fully open to the coil, and the valve is modulated to maintain temperature. If the OA temperature is below freezing, then the (hot water) valve is fully open to allow full flow through the coil, and the face/bypass dampers are modulated to maintain temperature.

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Sequence Items

VAV air handling units have basically three functions in life: to maintain supply air temperature setpoint, to maintain supply air pressure setpoint, and to maintain general space pressure setpoint. Beyond that you have your specialized sequences, of which I won’t discuss here but I’ll at least list some of them as follows: minimum outside air damper control, economizer control, Demand Controlled Ventilation, discharge air temperature and/or pressure reset control, humidification & dehumidification, etc. And, don’t forget your occupied and unoccupied modes of operation.

Do’s & Don’t’s

I have more do’s than don’t’s here on this subject. Do specify failure modes, in no uncertain terms. Do describe the intended operation of the return fan. For instance, is it directly controlled to maintain space pressure? Or to maintain a CFM setpoint? Or perhaps to track the supply fan? Whichever it is, it needs to be described. Do specify smoke evacuation modes as they apply, and do describe those specialized sequences, as they play an important part in the overall control of the unit. For the don’t’s, only one to list as far as this writing goes, although I’m sure you can think of many others not included here: don’t underestimate freeze protection, whether it’s specifying a coil circulating pump, or having the appropriate amount of freezestat capillary to cover the area of the coil. Freeze protection is a big concern, for those climates that actually have winter seasons!

Terminal Units (VAV & Fan-powered Boxes)

Pressure-independent control is the norm, and is defined as “resetting the CFM setpoint based on the deviation in space temperature from setpoint”. Simply put, the hotter it is in the space served, the greater the CFM setpoint that is attempting to be maintained out of the box, this being done by modulation of the primary air damper. For VAV boxes with no heat, the damper is modulated to a minimum position as the cooling needs of the space are satisfied. For VAV boxes with reheat coils, the heat is engaged when the space temperature falls below setpoint, and the primary air damper typically jumps to a heating position.

For parallel fan-powered boxes, the standard sequence of control is intermittent fan operation (on for heat call only) and variable air volume. For series boxes, it’s constant fan (on for all occupied modes of operation) and constant air volume, and variable temperature. Finally, if there is perimeter baseboard zoned with the terminal units (VAV or fan-powered boxes), then sequence the perimeter heat with the terminal unit heat. I’ll let you be the judge as to which method of heat is the first to be engaged, as there are seemingly two schools of thought on this.

Do’s & Don’t’s

Do specify factory or field installation of the terminal unit controllers. When it’s just a few, it may make more sense to let the installing electrical (controls) contractor handle this. However, when there are dozens and dozens of units, it’s typically more cost-effective to have the controllers shipped for factory mounting and wiring. And do specify discharge air temperature sensors if the units have heating coils. Do describe unoccupied mode operation, as it pertains to both the terminal units and the VAV air handler serving them. And when describing the basic modes of operation, do use accepted terms (i.e. pressure-independent). Along the same lines, don’t belabor industry-accepted modes of operation, simply for the sake of adding meat to the sequence! And finally, don’t specify more than one space temperature sensor per terminal unit (you can specify a single sensor to control more than one unit, however be careful with this; make certain that it’s do-able with the particular controller that’s being specified.).

 

Tip of the Month: Be wary of specifications and sequences of operation that contain archaic phrasing and obsolete control methods. Often these documents are rehashed over and over again, to fit each new project, and in the process they carry over defunct clauses and old school ideas, often unintentionally, but there nonetheless. When in doubt, rather than dismissing an item and replacing the line of thought with a “newer” design concept, try to get a direct line into the consulting engineer and get their opinion on it, for there may be a very good reason for what they’re specifying!

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