DJM2.4 MANUAL
31 Oct 00 R2
is suddenly lowered the burner may delay before cycling off. If this effect is undesirable, decrease
the Integral effect as set on pot 3. This is done by rotating the pot CCW to full proportional.
NOTE: If the unit is controlled by a zero to ten VDC signal such as from a C-TRAC or
METASYS it is important to set the BMSS and BMSZ pots correctly also. Refer
to “DJM2 controlled by a C-TRAC”, Page 12.
Burner Stays on Low Fire
If a DJM2.4 is staying on low fire for an extended period of time and overheating the space it may
be the CONT pot is giving too much integral authority. If so reduce the integral.
Combustion Fan Motor Surging in Speed
If a combustion fan motor is surging in its speed after the motor has been in operation for an
extended period of time, it is likely that the “triac” is overloaded and is running hotter then
design. When this occurs, the surging is caused by the triac shutting down due to heat. The
surging occurs at lower modulated speeds of the combustion fan and pulses about twice a
minute. Be careful to not incorrectly assume this is a problem as it may also be possible the fan is
changing speed in relation to the demand from the DJM2.4 control system. The surging described
above will not occur until the triac has operated for a few minutes at low speed and built up
considerable heat.
Modulating Valve Will Not Respond to Signal to Open
Items that may contribute to this are:
Ice in valve (in bottom or above diaphragm).
Burnt out coil.
Sticky substance or debris in valve.
Modulating Valve Sticks When Returning to Low Fire
If the modulating valve sticks partly open as the combustion fan slows down, the combustion
process will be very poor. The products of combustion may smell badly or in rare instances even
form carbon. The usual symptom of a sticking valve is when voltage is low to it and the manifold
pressure is higher then it should be, a solid rap on the valve corrects the problem. Replace valve.
Sooting Burner/Heat Exchanger
Soot is created during improper combustion. Do not try to correct combustion set up with a badly
sooted burner or heat exchanger, clean it first. One cause of sooting is on units with horizontal
flues. The wind blows into the outlet harder then the combustion fan blows. Correct this by
installing a vertical chimney (double walled in colder climates). The second item to check on high
turndown units is loose lock ring on series 20 valves. Described in Section XIV, sub section Low
Fire Set-up – refer to the type of valve in your unit. The third item is combustion set up is correct
but the modulating gas valve sticks partially open when returning to low fire. This can usually be
determined by monitoring gas pressure and analyzing combustion while at low fire, then rapping
the modulating valve if above was not correct. If the valve was stuck then it releases and low fire
returns to the desired settings. (Refer to Combustion Set Up on page 16.)
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