Service manual
A DJM3.2 MANUAL
plenum and the supply fan, delayed on, cannot remove the heat fast enough after it starts. The
burner modulates off, as the discharge sensor is limited to 120ºF, 60 seconds later S/A goes off, and
then combustion fan goes off. Room thermostat is still calling.
Resolution
1. In night mode ensure that terminals “HS, K, and FS” are powered. This will make the supply fan
operate continuously at night.
2. Turn dipswitch 7 on. This will place the DJM3.2 into a constant combustion fan and constant
supply fan mode.
3. Install an automatic fan switch (AFS).
Air Balancing - Refer to next item
Discharge Temperature in Cold Weather
Installation and air balancing is often done during warmer weather than that experienced in the cold
of winter. If the air balancer did not allow for the changes that will occur in air volume in cold weather
then the unit will appear to be short of temperature rise. As the fan is a constant volume device and
as it is located before the heat exchanger, air will expand as it is heated. The amount of change will
be about 20% increase in air volume from -30ºF to +70ºF. Therefore, to give a rough sample,
assuming a 10,000-CFM unit located in an area that reached -30ºF in the winter was being air
balanced on a 75ºF day with the heat off. The unit design temperature rise is 100ºF. It should be
balanced to deliver about 8,200 CFM. On a day that is -30º, the fan is still delivering 8,200 CFM onto
the heat exchanger, but as the air expands over the exchanger there is 10,000 CFM coming off of
the unit. Temperature rise should meet the 100ºF design to give a discharge temperature of 70ºF. (If
the unit had been balanced to deliver 10,000 CFM on the 75ºF day, then on the -30ºF day noted above, the
temperature rise would only be about 80ºF to give a final discharge temperature of about 50º at about 11,800
CFM.)
Ignition Problems
Check the condition of the pilot assembly. Check for damaged or dirty ceramics, proper gasket, and
pilot set up. If you are still having problems lighting the burner, check the items itemized below.
If these are all OK, check the combustion settings. Either the manifold pressure is incorrect, orifices
plugged, the curve matching pot is set wrong, or the combustion air set incorrect. For further
information refer to the section for combustion set up.
If you have been having problems lighting the burner, check the following items:
1. If the combustion has been set up, the ignition should be checked. Whenever the total air slider
has been adjusted it affects how much air flows into the pilot air tube.
2. Occasionally some ignition control devices have the power to throw a spark through the casing
to the mounting screw near the spark connection, or through the back of the case. Mount the
ignition control off the wall on insulated spacers and do not install a mounting screw next to the
spark wire location.
3. Check the resistance of the flame and spark rod to the wire connector terminal on the rod. This
should not exceed 2-ohms. If it does, replace the assembly.
4. Ensure that the ignition control and its transformer secondary are both adequately grounded. If
there is not an adequate ground then there will be a weakened flame rod signal.
5. Some extreme cases of airborne electrical noise a special sheathing is available to shield heavy
ignition spark wire. Please consult factory. It may be necessary to install a ground wire from
burner ground terminal to chassis. Scrape the paint on chassis.
6. Spark gap should be set at 1/8 inch.
7. Check for cracked porcelains on the spark and flame rods. These have been found to be
cracked in the area where they pass through the mounting plate also. You cannot see into this
area and you can even have it feel tight. (If possible it may be best to check it with an ignition device.)
Page 38 Revised: 3/11/99