Service manual

A DJM3.2 MANUAL
The Optional Reset Inputs
These may be controlled from room temperature, ambient temperature, voltage, or current input. The
voltage and current inputs will only reset the discharge air calculated set-point (SPC) in the up direction.
The room and ambient temperature resets will reset SPC in both directions. To use any resets, the
DJM3.2 must be properly wired and configured. The resets permit electronically changing the
“calculated set-point” if external reset devices are wired correctly and the appropriate dipswitches are
configured correctly. The “calculated or operating discharge air set-point” is the mathematical addition
of the master set-point and one optional reset signal.
Night Heat
In addition the control can be activated for night heat if configured and wired correctly. (See below for
details.) Night heat operates best on units with return air. Night heat is not as compatible with units
designed to operate as 100% “make-up air” because at night the unit will have to open the outside air
dampers and heat outside air, which is not truly cost effective. If there is a modulating or single stage
room thermostat connected to terminals “X, V, Y, and Z” the night heat function will be activated when
terminal “K” is powered.
The only exception to “discharge air control” is when the DJM3.2 is wired to provide space temperature
control at night (OPTIONAL). During this night time operation the discharge sensor acts as a
modulating high limit and prevents discharge air temperature from exceeding 120ºF.
The DJM3.2 has proportional and integral (PI) control. On initial start-up, (each time the DJM3.2 is
powered or switched to “day” mode) it will quickly raise the discharge temperature within 12ºF of the
calculated discharge air set-point. When this point is attained, small output corrections to the gas valve
are gradually made until the discharge air temperature is within ±2ºF of the “calculated” set-point.
There should be very little temperature swing once the burner has settled into its modulating range.
When the heat load is very light (often less then 5%) and the temperature rise at low fire exceeds
“calculated set-point” it becomes necessary to cycle the burner on and off to avoid overheating. When
the burner is cycling between low fire and off there will be some temperature swing.
XV. INDUCED VOLTAGE ON REMOTE CONTROL WIRING
Following are a number of discharge temperature reset methods. When any of these are ordered at
the time the unit is manufactured the control is calibrated at the factory and this calibration will satisfy
most applications, unless resistance from long wiring runs or induced voltage on the remote wiring
cause it to be out of calibration. (To check for induced voltages, disconnect all of the field wiring in question
from the DJM3.2 while the unit is left operating, and check for AC volts across each questionable wire and
ground.)
XVI. NIGHT HEAT THERMOSTATS
Modulating Room Thermostat For Night Heat
• If used for night heat, the DJM3.2 must have a room thermostat connected to it. There are two
options, modulating or single stage.
• If it is a modulating room thermostat it will be wired to the DJM3.2 terminals “V, X, Y”, and “Z”. If
the night time room temperature drops 1.5ºF below the night room set-point, the heat will be
brought on, and kept on, until the room temperature rises 1.5ºF over the room set-point. During
this time, the burner will fire at maximum rate until the room temperature is satisfied or the
discharge temperature is becoming too hot. The internal modulating high limit will act to prevent
the discharge temperature from exceeding 120ºF.
• In night time operation the supply air fan will start about 30 seconds after the heat starts. The
supply air fan will stop about 60 seconds after the heat goes off. After the heat call is finished, the
combustion fan will run for about one minute in the cool down mode (to cool the heat exchanger).
• If there is a make/break room thermostat operation, detail is noted below this next section.
Depending on the position of dipswitch 3 (room reset), and the power to FS, HS, etc. (see chart on Page
8), the room thermostat can be made to operate in the following manner:
Page 11 Revised: 3/11/99