DJM2.4 MANUAL
26 Oct 00 R2
Constant Heat Call Light, No Combustion Fan
If the heat call light is on and the pre-purge light off and there is no attempt to start or slow the
combustion fan down to ignition speed, check if DJM2.4 terminal VFB24 or VFB120 depending on
the gas valve has constant power on it. If the previous heat call did not have this signal removed
after it was complete, it can result in the lockup as described. This would usually only occur if
DJM2.4 terminal VFB24 OR VFB120 is powered by a relay contact that has stuck in a closed
position.
The second possible cause could be a false or grounded tach signal. Try removing “OS, YS, and
GS” wires. If the combustion fan doesn’t run, check the motor. If it now starts change the Hall
Effect speed sensor.
Failed Operation or Solid State Control Contacts by Others
There have been a number of new controls introduced into the marketplace recently that allow a
building to be controlled by a central computer (BMS or Building Management System). Many of
these devices use relays to switch external power devices on and off and their operation is fine.
However, some of the new devices use “opto couplers” or “solid state contacts”. These devices
will not interface properly with the DJM2.4 contacts “HS, or terminal VFB24/VFB120”. The solid
state contacts allow enough “voltage leak” through them when they are in an off (open) position
to cause the DJM2.4 to malfunction. Voltage leaks of 4 to 5 VACS are common. Only dry contacts
should be tied to the DJM2.4 terminals unless instructed to do otherwise.
Air Balancing (Refer to next item)
Cold Discharge Temperature in Cold Weather
At the time of installation and air balancing, the air balancer in warmer weather often adjusts the
unit’s airflow. If the unit is designed for “Make-up Air” (outside air being drawn into the building
and heated as it passes through the unit) then the quantity of air being delivered into the space
will vary considerably as the outside air temperature varies. This is due to air density, volumetric
expansion due to heat and the fact that the fan is located before the heat exchanger. If the air
balancer did not allow for the changes that will occur in air volume and density that will occur in
colder weather while he is doing an air balance in mild weather then the unit will appear to be
short of temperature rise on cold days.
The fan is a constant volume device and is located before the heat exchanger, so it is necessary
to recognize what occurs as air expands while being heated passing over the heat exchanger. The
change in air volume will be about a 20% increase as the air warms from minus 30ºF to plus 70ºF.
To better demonstrate with a rough example:
Assume a unit specified to deliver 10,000 CFM is located in an area that reaches -30°F
in the winter was being air balanced on a 75°F day to deliver 10,000 CFM with the
heat off. The unit’s design temperature rise is 100°F from -30 to 70°F. It should be
balanced to deliver about 8,200 CFM on the 75° day with the heat off. On a day that
is -30°, the fan will be 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 100° design to give a discharge temperature of 70°. (If the unit
had been balanced to deliver 10,000 CFM on the 75° day, then, on the -30° day noted
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