4
Offset Screw Adjustment
The gas valve Offset Screw (see Figures 1-1 & 2-1) is used to calibrate the CO
2
offset at minimum modulation vs.
maximum modulation. Making the offset more positive (+) increases the CO
2
concentration at minimum modulation
rate; making the offset more negative (-) decreases the CO
2
concentration at minimum modulation rate.
NOTE:
Calibration of the Offset Screw must only be performed with the burner operating at the minimum
modulation rate (see Table 2-2).
Adjustments to the
Offset Screw
may only be made by a qualified gas technician using a
calibrated combustion analyzer capable of measuring CO
2
and CO, and only with the burner
at the minimum modulation rate (see Table 2-2). Attempting to set the Offset Screw while the
burner is operating at a modulation rate other than the minimum will result in incorrect
combustion and may lead to burner damage or excessive CO.
Gas Line Pressure
The boiler gas valve is equipped with a line pressure test port; see Figure 1-1. Use the following procedure to
measure the gas line pressure to the boiler to ensure if falls within the range given in Table 2-1:
1.
Turn the supply of gas to the boiler off.
2.
A bleed screw is located on the inlet flange of the gas valve; open it approximately 1-1/2 turns. See
Line
Pressure Test Port
in Figure 1-1.
3.
Slide the gas pressure manometer tubing over the bleed screw fitting located on the inlet flange of the gas valve;
connect the other end of the tubing to the gas pressure manometer. Ensure both ends of the tubing make a tight
connection.
4.
Open the supply of gas to the boiler and check for gas leaks.
5.
Observe the line pressure under static conditions and compare it to Table 2-1. The pressure will be greatest under
static conditions.
6.
With all other gas appliances in the application running, operate the burner to the maximum firing rate (see Table
2-2) and compare the observed line pressure with Table 2-1. The pressure will be lowest during the maximum
flow of gas.
7.
Adjust the gas line pressure to ensure the parameters in Table 2-1 are attained under all conditions (see NOTICE
below). If possible, adjust the line pressure to the "Nominal/Desired" value listed in Table 2-1, while the unit is
operating at the maximum modulation rate, see Table 2-2.
8.
Continue observing the gas line pressure until the completion of the combustion analyses, in case adjustments
need to be made.
9.
Upon completion of the line pressure testing, turn the gas supply off and close the bleed screw. Turn gas on and
check for leaks.
The line pressure is a function of the gas supply and is affected solely by field provided
parameters such as line size and regulator settings. Under no circumstances can the boiler gas
valve influence or be used to adjust the gas line pressure.
Failure to properly close the
Line Pressure Test Port
bleed screw will cause severe leakage
of gas, resulting in a fire or explosion causing property damage, serious injury or death.
Table 2-1 Line Pressure and Combustion Parameters
Gas
Line Pressure (inches w.c.)
Offset Pressure
(inches w.c. / [Pa])
2
CO
2
(%)
1
Max. CO
(ppm)
Nominal/
Desired
Min.
Max.
Min.
Max.
Natural
7
4
10.5
-0.01 to 0 / [-3 to -1]
8.3
9.3
175
Propane
11
8
13
-0.04 to -0.03 / [-10 to -8]
9.6
10.6
175
Notes:
1
Combustion values listed are for burner operation at maximum modulation rate; when tested at minimum modulation
rate the CO
2
must be 0.5-1.0% lower than CO
2
at maximum modulation rate.
2
The Offset Pressure must be checked, and if necessary adjusted, in accordance with the procedure detailed in Figure 2-1.