Service manual

A DJM3.2 MANUAL
cause occurs 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 problem, as it may also be possible the fan is changing speed in relation to the
demand from the DJM3.2 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.
Improper Burning
• Plugged 12 AO6 vent orifice (on units equipped with one on regulator).
• Modulating valve sticks.
• Incorrect burner setup.
Improper Burning After Changing Combustion Motor Or Fan
If the fan is located at a different spot on the motor shaft, it changes the airflow pattern and quantity.
The quantity is changed by the size of the gap changing around the fan inlet. This allows more or
less air to “leak” from the fan discharge side back to the fan inlet.
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, and a solid rap on the valve corrects the problem.
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 a loose lock ring on series 20 valves. Described in section XIV, subsection Low Fire Setup – 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 analysing 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 heat/cool units the combustion air is normally drawn by the combustion fan from the intake
screen located on the compressor floor area. As this floor area is cantilevered it can plug up with
blowing snow, especially if the unit is mounted on a low level roof curb. This can cause combustion
air to starve and result into sooting of the heat exchanger.
Smell From Flue (Products of Combustion)
Smells are usually created by improper combustion set up. This can be due to a sticking valve (note
above), low inlet gas pressure, or just a bad combustion set up. Note the comments below about
combustion set up.
Page 41 Revised: 3/11/99