Service manual

A DJM3.2 MANUAL
Water and Ice From Combustion
These heat exchangers are very efficient. Water is one of the major products of combustion and as
such must be controlled. The units are equipped with a drain. It is however best to allow water to
leave the unit in the flue gases in vapor form. Combustion that is set at maximum efficiency will
produce excess moisture. Often it is desirable to have 1% to 2% higher excess oxygen readings then
those normally listed to assist in keeping the flue gases drier. Also note that extended chimneys can
contribute to condensation problems (especially if they are not insulated or at least double walled). Water
in the heat exchanger can lead to premature failure due to thermal stress or freezing. Another
source of excess condensation is when a unit designed with capacity to heat 100% outside air is
operating with very little outside air, therefore operating at a low fire or cycling. This allows the cool
flue gases from low fire to cool and condense in the chimney.
Burner Pulsing, Backfiring, Exploding, Noisy
An improper air/fuel mixture usually causes this condition. Do not attempt to do any combustion
analysis until the inlet fuel pressure has been checked with the unit operating at high fire. (This should
include other loads on the same gas system being on.) If the inlet pressure falls too low then the velocity of
the air over the flame will be too great and tear the flame. If the burner is designed and set up for
proper fuel pressure and it does not have it, it will react exactly like a burner with too much
combustion air. If the inlet fuel pressure is correct at high fire then the combustion set up should be
checked.
If an explosion occurs after the burners flame has definitely extinguished, and the burner is in the
process of being re-lit, it is likely due to a leaking gas valve. It is recommended that both the main
and pilot valve be replaced if you cannot prove the source of a leak.
Fire Without Combustion Fan
If there is a flame operating in the burner section without the combustion fan in operation, check to
see if there is power across the DJM3.2 terminal 3 and neutral. Record the result. Next check to see
if there is power on DJM3.2 terminals 2 and neutral. Record the result. Next, check to see if there is
power to DJM3.2 terminals T1 and T2. Record the result. If there is no power to any of the above
terminals and if the unit has not had any wire changed, there should not be any power to the pilot or
main gas solenoid valve. Very gently turn off the manual valve on the pilot line. Check to see if the
fire has gone out. If it has, then the pilot solenoid is leaking and should be changed. If there was no
difference, gently turn off the firing valve for the main manifold. If this stops the fire, this would prove
the main solenoid-blocking valve was leaking.
If there is no power to any of the above terminals and if all wiring is as per the original diagram, there
should not be any power to the pilot or main gas blocking valve. Very gently turn off the manual valve
for the pilot gas. Check to see if the flame has gone out. If it has then the pilot solenoid is likely
leaking. If the flame is still on gently turn off the main firing valve. If this stops the flame then the main
valve has been leaking. At this time turn off the line valve to the unit and repair the problem.
Replacement of Combustion Fan Motors
Motors being used for combustion fan application are applied to a device (DJM2 or DJM3) that
reduces the motor speed. Speed reduction of a motor results in additional heat being generated
within the motor as well as reduced cooling capability from the motors internal cooling fan. If a motor
that is not designed for this duty is installed on the DJM, it may result in failure of the new motor
and/or the DJM. For this reason we recommend combustion fan motors only be replaced with motors
that meet all the specification requirements of the original motor. Also note improper burning above.
Set Up When Replacing DJM3.2 in Field
1. The two pots on the back of the board are factory adjusted and should not be field adjusted, they
are for the tachometer calibration. Only on the odd occasion should the low fire light off pot be
adjusted, that being if the burner rumbles on during every light off (even when warm).
2. There are two possible burners that the DJM3.2 can be applied to. Check the burner in your unit
and if necessary cut the jumper before mounting the control. (See Burner Selection, Page 28.)
Page 42 Revised: 3/11/99