Babel Buster Network Gateways: Big Features. Small Price.
Systems & Sequences
Part 1 of 2
Recently I was asked to present on the subject of controls specifications and sequences of operation. After hours of research and review of specification and sequence documents for projects that I’m currently involved in, as well as drawing upon past experiences, I came up with a presentation that I thought, in retrospect, worked very well for the intended audience that day, which consisted primarily of mechanical contractors and specifying engineers, many of whom were on the younger side of life and likely fairly new to the industry. Anyway, I thought it would make good column fodder if I were to recycle the bullet point presentation into prose. And so here it is!
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.
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.
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.
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.
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
VAV Air Handling Units
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.
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
Terminal Units (VAV & Fan-powered
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.
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 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
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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