DJM2.4 MANUAL
19 Oct 00 R2
1. Once the high fire pressure and combustion have been set up at point “B” above, turn the
manual firing pot (fully CCW) to achieve low fire
2. Adjust the low fire RPM pot 8 (MINSP) until the low fire speed on the combustion fan
motor is 1250 RPM ±10 RPM (20.8Hz) for 10:1 set up. [1150 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.
3. Under the cover on the Maxitrol series 20 valve is an adjustment screw (brass or silver)
that has a lock ring. Holding it in place, this lock ring uses a special tool (two very small
holes) to loosen it.
4. The lock ring 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.
5. 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.
6. Adjust the manual firing pot 4 SER 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.
7. 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).
8. CURVE MATCHING PROCEDURE
Switch to high fire setting (approximately 3450 RPM or 57.5 Hz) for 1 minute. Adjust the
manual firing pot 4 SER down very slowly until the combustion motor speed is about 3060
RPM (51 Hz). Let the burner stabilize for 2 minutes and re-check the O
2
reading. If the O
2
has dropped to less then the original high fire O
2
reading, adjust the curve matching pot
(labelled “CURVE”, second from the right on the bottom row, pot 7). Adjusting the CW
increases the O
2
reading. As the curve matching pot is adjusted the voltage to the gas
valve changes.
9. If the curve matching pot is adjusted to the fully CW 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 and 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).
10. 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
To Next Page
Loading...
