Service manual

A DJM3.2 MANUAL
especially at higher flow.
Series 20/M420, M520 and M620 (high turndown)
As these valves have 2 low fire adjustments the following procedure should be followed carefully.
Once the high fire pressure and combustion has been set up at point “B” above, turn the manual
firing pot (fully CCW) to achieve low fire. Adjust the low fire RPM Pot Low Fire until the low fire
speed on the combustion fan motor is 1250 RPM ±10 RPM (20.8 Hz) for 10:1 set up. (1195 RPM
±5 (19.91 Hz) for 15:1 turndown.) At this point the DC voltage reading at the modulating gas valve
terminals should be 0 VDC.
Under the cover on the Maxitrol series 20 valve is an adjustment screw (brass or silver) that has a
lock ring which uses a special tool (two very small screws) to loosen it, holding it in place.
1. There is a lock ring that needs to be loosened first. Turn it fully CCW, now turn the slot screw
in the centre of the lock ring (spring activated low fire adjustment) full CCW.
Caution: Do not over torque this screw at the top or bottom of its adjustment or it will
break the “c-clip” inside the valve, thus allowing the spring to ride free.
2. Adjust the low fire bypass screw at the bottom side of the valve body until the O
2
reading is
between 16.0% to 16.5%. (16.5 to 17.5% oxygen for 15:1 turndown.) At these settings the
turndown will be very close to 10:1. The maximum CO at low fire is 80 PPM.
3. Adjust the Service Pot (manual firing Pot) up very slowly until the DC volts at the modulating
gas valve terminals are about 2.25 VDC. If the voltage rises over 2.25 VDC while adjusting
the manual pot, turn it down and start again from less than 1.5 VDC until 2.25 VDC is
reached.
4. Check the O
2
reading again at this point. If the O
2
has increased, adjust the spring activated
low fire adjustment on top of the valve until the O
2
reading is same as the low fire reading at 0
VDC. You are correcting for valve hysteresis with this adjustment.
5. CURVE MATCHING PROCEDURE
Switch to high fire setting (approximately 3450 RPM or 57.5 Hz) for 1 minute. Adjust the Service
Pot (manual firing Pot) down very slowly until the combustion motor speed is about 3060
RPM (51 Hz). Let the burner stabilize for 2 minutes, then re-check O
2
reading. If O
2
has
dropped to less then the original high fire O
2
reading, adjust the curve matching pot (pot 1 near
terminal G on the top of the DJM3.2). Adjusting CCW increases the O
2
reading by reducing the
voltage to the Maxitrol modulating gas valve. The curve matching pot only changes the
voltage to the gas valve, it does not affect fan speed.
6. If the curve matching pot is adjusted to the fully CCW position and the O
2
reading is still not
within 10% of the original high fire O
2
reading, add a 180-ohm 2 watt resistor across the
modulating gas valve terminals; repeat above test until proper curve matching is achieved. (A
150-ohm, 2-watt resistor may have to be used if O
2
level still does not rise to within 10% of original
high fire O
2
reading.)
Switch burner to low fire again and re-check low fire, both at 0 VDC and 2.25 VDC. (An
addition of a resistor above may have changed the low fire setting, if it was needed.)
7. Lock the locking ring on the spring activated low fire adjustment screw. Failure to lock it may
cause the screw to vibrate loose allowing low fire gas pressure to increase. This can create
sooting of burner parts and the heat exchanger. If the locking ring is missing, drop one drop of
Loctite thread locker 222 onto the brass screw threads.
Series 50/M550, M650 AND ES350 (high turndown)
These valves have only 1 low fire adjustment (spring activated low fire).
8. The combustion set up is similar to the above except there is no low fire bypass adjustment
on the bottom of these valves. Therefore simply set up low fire by turning the manual firing
Pot Reset Ratio to the low fire position - 1250 RPM ±10 RPM 20.8 Hz. (15:1 turndown set at
Page 31 Revised: 3/11/99