Service manual

A DJM3.2 MANUAL
selected discharge temperatures we chose above is 85 - 56 = 29º band width. Referring to the
chart found in the "Set-Points" section and looking at the "Discharge Set-Point" of 60°F, note that
the minimum is 54ºF and the maximum is 77ºF. The band width is 23º.
Our calculations above noted we need a bandwidth of 29º. Therefore peruse down the band width
column until you find a width close to 29º with a minimum discharge close to 56ºF and a
maximum discharge close to 85ºF. This would be found under the 70ºF discharge set-point area
with Pot Reset Ratio set at 3. The minimum discharge shown is 57ºF and the maximum is 90ºF.
As we have the mix box temperature set at 58ºF, we will want the minimum temperature to be just
below that setting. Otherwise as the dampers modulate open to give cooling to the space, the heat
will be on, thus warming up the discharge air. To get the 56º-discharge set-point we desired, note
our theoretical selection gives us 57ºF with the set-point dial at 70ºF. If we turn the set-point dial
down 1º to 69ºF we will then have the desired 56º low discharge and when the room thermostat
calls the discharge temperature will rise to 89ºF. If you have a meter handy it will be best to set
these points by reading the values on the "Calculated Set-Point Readout" resistor with the room
thermostat calling and not calling. Adjustments can be made using the set-point knob and pot #5
to obtain your desired low and high discharge temperatures. (With the above set up, after the
dampers close back to minimum position, and as the outside temperature continues to fall thus
forcing the mix box temperature below the desired 58ºF, the heat will come on to maintain the
discharge temperature at 56º. This will temper the discharge air into the space thus not allowing
air that is too cold to blow onto the patrons.)
NOTE: With the above theoretical settings, the set-point will read 69º but the control
will never try to operate with a 69º discharge. It will operate with a 56º
discharge when the room thermostat is not calling and with an 89º discharge
when the room thermostat calls.
During the "Night" mode when the room thermostat calls, the discharge temperature will go to
120º if the unit heating capacity will allow it. There is a high limit function built into the DJM3 to
stop the night heat from going above the 120º discharge.
Page 20 Revised: 3/11/99
C1
Damper
Motor
λ λ
H/L
FAN SW
HOT
NEUTRAL
4 2
λ λ
HS
IGN.
CONTROL
PV
MV
1
3
Q
U
M
OR
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YS
G
S
SP
V
SPEED
SENSOR
PV PV
X
Y
Z
+
-
Room Thermostat
T1
T2
CB
CB
120 V HOT
24 V SUPPLY POWER
DM DM
SA C
FS
LL
COMBUSTION MOTOR