Carrier 48MA - Manual (Page 22)

Roof mounted air conditioning equipment
usually has adequate vibration isolation of internal
components. However, light roof construction or
equipment location displaced from main roof
supports may dictate additional isolation to elimi­
nate vibration.
Special vibration isolating bases and curbs
designed for rooftop applications are available
from some vibration isolator manufacturers. This
equipment virtually eliminates vibration trans­
mission on critical applications. However, care
should be exercised when selecting this equipment
for use with a multizone. The design and installa­
tion of vibration rails on a Carrier 48MA/50ME
should ensure that the interfacing of the vibration
isolator and the curb maintain watertight integrity.
Thermostat Usage and Control
USAGE - The thermostats used with the 48MA/
SOME units are either a 2-step heat/l-step cool or
2-step heat/2-step cool. A single module can have
only one step of cooling, but can have 2 steps of
heating. When 2 or more modules are grouped
together, the 2-step heat/2-step cool thermostat
can be used. Modules are grouped together by the
installation of factory-supplied jumpers on the unit
zone control board.
The thermostats are automatic changeover with
a 3 F dead spot between heating and cooling.
There is a 1°F differential between the first and
second steps of heating or cooling. Two subbases
are available for use with the thermostats; one with
off-heat-auto.-cool
switch and one without
switches for tamper-proof installation. The tamper­
proof subbase has provisions for locking the
thermostat cover and temperature selectors.
CONTROL - The thermostat field wiring connec­
tions are made at the screw-type terminals on the
printed circuit board near the heating end of the
unit. This is commonly called the zone control
board. (See Fig. 24.)
Each module has the following thermostat
terminals: R (24-v power supply), Y (for cooling),
W1 (first step heating) and W2 (second step
heating). Pairs of modules are combined to form
nests: 1 and 2; 3 and 4; 5 and 6; etc. Each nest
forms National Electrical Code (NEC) Class II
circuit powered by its own 40 va transformer. Each
transformer is basically limited in capacity to
operate only the relays within its nest. Therefore,
contacts are provided in the relays to transfer the
signal to another module in an adjacent nest.
By correctly installing jumpers on zone control
board terminals, the contacts from a relay in one
nest power a relay in the adjacent nest using the
transformer of the adjacent nest. This technique is
known as multiplexing.
Same Nest Ganging - Figure 25 shows 2 modules
(1 and 2) of the same nest ganged together to form
a 2-module zone. Field jumpers are installed on
same lettered terminals to energize both control
relays simultaneously for cooling or first- and ^
second-stage heating relays or gas valves for
heating.
Figure 26 is similar to Fig. 25 except a 2-step
cooling thermostat independently powers the Y
connections for 2-step cooling in one zone. These 2
examples have not left the particular nest, so no
multiplexing has been done.
Adjacent Nest Ganging — Figure 27 shows 2
modules (2 and 3) of adjacent nests ganged
together in one zone so multiplexing is in effect.
Module 1 is in a zone of its own. When CR2 is
energized by the cooling thermostat, terminal 6 is
powered thru CR2 contacts 1 and 3 by the
transformer of the second nest. Then terminal 6 is
jumpered to 1 and CR3 is energized. Thus, on a
call for cooling, modules 2 and 3 are energized
simultaneously. The same principles and pro­
cedures are followed for the first and second steps
of heating.
Multiple Ganging — Figures 28 thru 31 are further
examples of ganging and multiplexing zones to
provide 2, 3 or 4 modules per zone. These typical
examples demonstrate the principles of multi­
plexing. The same procedures are followed for the ^
remainder of the zones on the unit. Many more ^
combinations are possible. Do not overload a
transformer by powering more than one relay of a
different nest in additon to the relays in its own
nest.
Multiplexing is done from top to bottom of the
zone control board . . . from module 2 to 3 and 4,
from 4 to 5 and 6, and from 6 to 7 and 8. The
contacts of one module are powered by the nest of
the next higher numbered module (see Fig. 24).
Return Air Systems
If the ceiling plenum on a top floor is used as a
return air plenum, the return air is heated from the
time it leaves the room and enters the unit. This is
due to roof load or if heat from lighting is added to
the plenum.
When considering the top floor, the roof load
does not raise the return air temperature signifi­
cantly and, therefore, its effect is considered
negligible when selecting a unit.
Return air light troffers, however, can add
considerable heat to the return air. Using a
48MA/50ME unit with a return air light troffer
system can impose various design problems since
the purpose of the system is to reduce the supply A
cfm to the space by reducing the space load. With ^
the 48MA/50ME this may result in a very low
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