A
DJM3
IOM-17
17 of 27 Mar 99 R3
High Fire Set Up
1.
Turn on the service switch (dipswitch 5).
2.
Set the Service Pot to high fire. Adjust the high fire input to the correct clocked rate. Adjust
combustion air slider or inlet damper until the high fire O
2
reading is between 3.5 and 4.2%.
Low Fire Set Up
Ensure that there is aluminium tubing connected from the top of the modulating gas valve vent
connection to the burner box. This is to guarantee that the varying burner box air pressure allows the
proper gas flow rate through the modulating gas valve. Also ensure that the small vent hole under the
oval shaped cover is sealed off with silicone or Tremco tape. When doing combustion analysis it is
important that the top of the valve be sealed by either reinstalling the cap after each adjustment, or
placing your finger fully over the top to seal the open top on the valve. The reason for this requirement
is that the valve is “top loaded” which means that the valve is having pressure from the combustion fan
added to the top of the valve’s diaphragm thus increasing the gas flow through the valve, especially at
higher flow.
Series 20/M420, M520 and M620 (High Turndown Burner)
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, turn the Service Pot fully counter‐
clockwise to achieve low fire. Adjust the Low Fire Pot until the low fire speed on the combustion fan
motor is 1250 RPM ±10 RPM (20.8 Hz) for 10:1 set up or 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 holes) to loosen it.
1.
The lock ring needs to be loosened first. Turn it fully CCW (up) now turn the slot screw in
the centre of the lock ring (spring activated low fire adjustment) full CCW (up).
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 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.
