Specifications

89Environmental Automation
Thermostat
Programmable electronic thermostats in
homes and similar, but more sophisticated
systems for commercial buildings keep
track of time and days of the week,
allowing for temperature proles that
increase comfort and save energy. In
a home, for example, a programmable
thermostat performs slow, energy-
ecient heating in the morning in
anticipation of occupants getting up
to start their day. A slow-heating cycle
is accomplished by the thermostat
slowly ramping up the set-temperature
point over a period of an hour, so the
heater does not constantly run due to
a large step function setting change.
Similarly, a temperature drop during the
middle of the day, when the home will
be unoccupied, can be programmed,
followed by a second warming in the
evening for the occupant’s return.
On weekends, the morning cycle
can be delayed and the mid-day
temperature dip can be removed.
In commercial buildings, individual
rooms can be programmed with similar
anticipatory temperature proles.
Most home thermostats use custom
ASICs for the bulk of their functions—
real-time clock, temperature sensing,
µP, communication. But if individual
components such as these are needed
outside of the ASIC for safety, redundancy,
or accuracy, use the DS7505 thermostat,
the MAX6682 thermistor-to-digital
converter, or the MAX1472 ASK
transmitter for wireless communication
from the thermostat to the furnace.
Wireless communication between the
thermostat and the HVAC system is
common in homes and some businesses,
but the limited range of most of these
systems curtails their use in large
facilities. For more details, see the
Wireless Systems section in this chapter.
Damper
Dampers are air valves similar in
concept to those found in replace
chimneys. They can be controlled by
simple mechanical levers, relying on
a person to operate, or they can be
controlled by electric motors, vacuum
actuators, or compressed air actuators.
They direct air into dierent paths in
response to commands from the HVAC
controller portion of the BAS. The
dampers can respond as needed to
achieve individual room temperature
proles that had been programmed
into the system. This complex system,
to be fully eective, needs room
temperature sensing, thermostat
communication with the heating system,
and a damper actuator drive with
position sensing. This position sensing
is fed back into the BAS to conrm that
the damper responded correctly.
Maxim has a variety of position
sensing and signal conditioning
solutions including proximity sensors
(MAX44000), variable-reluctance
sensor interfaces (MAX9924), and Hall-
eect sensor interfaces (MAX9621).
Air Handling Unit (AHU), Rooftop Unit
(RTU), Makeup Air Unit (MAU)
An AHU, RTU, or MAU conditions and
circulates air as part of the HVAC system,
which can include heating, cooling,
humidication/dehumidication, mixing,
fans or blowers, and ltering. AHUs
are typically found inside buildings
and have large capacities. Large AHUs
usually contain a squirrel cage blower
driven by a 3-phase AC induction motor
to move the air. VFDs are available
for higher eciency units. Very large
units can have multiple blowers.
Filters, dampers, mixing chambers,
and heating and cooling elements
are the other key components. The
heating and cooling elements of large
AHUs are coils. These coils have ns,
which create a large surface area. A
central boiler or chiller plant supplies
the coils with hot water, steam, or
chilled water. Through the ns in the
coils, this hot or cold water or steam
is eectively released to warm or cool
the air. Mixing chambers use dampers
that alter the mix of fresh outside air
with return air from the system. If the
temperature of the outside air is close
to the desired room temperature,
the damper can automatically alter
the mix of outside and return air
to achieve the desired heating and
cooling. By doing this, the central
boiler or chiller plant does not need
to provide steam or cold water, which
lightens the load on these systems.
RTUs are smaller self-contained units
with complete internal refrigeration
and heating capabilities. They have
Rooftop unit part of HVAC system for large commercial building.