Modine Manufacturing HD Installation And Service Manual - page 10
10
Input Adjustments
The gas pressure regulator (part of the combination gas control)
is adjusted at the factory for average gas conditions. It is
important that gas be supplied to the heater in accordance with
the input rating stamped on the serial plate. Actual input should
be checked and necessary adjustments made after the heater is
installed. Over-firing, a result of too high an input, reduces the
life of the unit, and increases maintenance. Under no circumstances
should the input exceed that shown on the rating plate.
Input can be determined by the meter-timing method provided
other gas equipment connected to the meter is off during the
test. If this is not possible, use the pressure method.
Important – Inlet pressure and manifold pressure must be
checked with unit in operation when making final adjustments.
(A) Meter Timing Method
1. Shut off all other gas-burning equipment, including other
pilot lights served by the gas meter.
2. Start the heater and determine the number of seconds it
takes to consume 1 cu. ft. of gas. Two basic formulas are
useful:
F1 = 3600 C/T
F2 = F1/C
where
F1 = input to heater, Btuh.
F2 = input to heater, cu. ft. per hr.
C = heating value of gas, Btu per cu. ft.
T = time to consume 1 cu. ft. of gas in sec.
The heating value of gas may be determined from the local
utility or gas dealer.
These are representative values:
GAS
Btu per cu. ft.
Natural
1000-1150
Propane
2500
3. If the seconds for 1 cu. ft. are more (input less) than shown in
Table 5 for model being tested, locate the combination gas
control and pressure regulator adjustment screw. Remove the
cap screw from the pressure regulator and take one clockwise
turn at a time on the adjustment screw until the correct time is
obtained. If the seconds are less (input greater) than indicated
in the table, follow the same procedure in a counter-
clockwise direction.
If the correct number of seconds cannot be obtained check
orifice size. Correct orifices can be obtained from Modine
Manufacturing Company, Buena Vista, Virginia. When
requesting orifices, state type of gas, heating value, and its
specific gravity. Also give model number of unit.
For example, if the input to the heater is 100,000 Btuh and the
heating value of the gas is 1000 Btu per cu. ft., then, by the
second formula, the input is 100 cu. ft. per hr. Table 4 indicates
the time for one revolution of various size meter dials with
various input rates. If a 1 cu. ft. meter dial is used, we proceed
down the cu. ft. column to 100 cu. ft. per hr. and then
horizontally to the left to determine a time of 36 seconds for one
revolution of the dial. Similarly, if the 1/2 cu. ft. dial is used, we
determine a time of 18 seconds for one revolution at the
required input. After proper firing rate has been achieved,
replace regulator cap screw.
Table 4
Meter-timing Gas
(Time required for one revolution is charted for various size
meter dials and various rates of gas input in cu. ft. per hour. To
convert to Btuh, multiply by the heating value of the gas used.)
(B) Pressure Method
The pressure method determines input by measuring the
pressure of the gas in the manifold in inches of water.
1.
Determine correct manifold pressure from Table 5.
2.
Locate combination gas control.
3.
Move gas control knob (or lever) to OFF.
4.
Remove the 1/8" pipe plug in outlet pressure tap in
combination gas control and attach water manometer or “U”
tube which is at least 12" high.
5.
Follow lighting instructions and turn thermostat up to get
unit to fire.
6.
If pressure as indicated by “U” tube is less than 1/2" W.C.
higher or lower than indicated in Table 5, adjust regulator
as described under “Meter-Timing Method,” Step 3.
Figure 9
Dials of Typical Gas Meter
Time for 1
Input, Cu. Ft. per Hour, when meter dial size is:
Revolution,
Sec.
1/2 cu. ft.
1 cu. ft.
2 cu. ft.
5 cu. ft.
10
180
360
720
1800
12
150
300
600
1500
14
129
257
514
1286
16
112
225
450
1125
18
100
200
400
1000
20
90
180
360
900
22
82
164
327
818
24
75
150
300
750
26
69
138
277
692
28
64
129
257
643
30
60
120
240
600
35
51
103
206
514
40
45
90
180
450
45
40
80
160
400
50
36
72
144
360
55
33
65
131
327
60
30
60
120
300
70
26
51
103
257
80
22
45
90
225
90
20
40
80
200
100
18
36
72
180
120
15
30
60
150
CAUTION
Check the gas inlet pressure at the unit upstream of the
combination gas control. The inlet pressure should be 6-7"
W.C. on natural gas or 12-14" W.C. on propane. If inlet
pressure is too high, install an additional pressure regulator
upstream of the combination gas control.
!
CHECKING INPUT RATE