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F − Exhaust CO
2
(Carbon Dioxide) Content
Carbon Dioxide is a colorless and odorless gas produced
in small amounts by all furnaces during the combustion
process. When the unit is properly installed and operating
normally CO
2
content of the exhaust gas is within 8.010.0
percent for natural gas and 9.011.5 percent for L.P. gases.
If the unit appears to be operating normally at or beyond the
upper limit of the CO
2
range, the unit should be checked for
abnormally high CO (Carbon Monoxide) output which
might indicate other problems in the system.
One method of measuring the CO
2
content is to use the Ba
charach CO
2
test with a Fyrite CO
2
indicator. Other meth
ods of testing CO
2
are available. Closely follow the instruc
tions included with the test kit that you choose. A method
for connecting the CO
2
test kit to the GSR21 is outlined in
the next section.
G − Exhaust CO (Carbon Monoxide) Content
If the unit appears to be operating normally with the CO
2
lev
els at or near the upper limits listed in section F, the unit
should be checked for abnormally high CO content. When
the unit is properly installed and operating normally, the CO
content of the exhaust gas is less than 0.04 percent (400
PPM) regardless of the type of gas used.
High CO output may be fatal.
Do not allow unit to operate at CO output levels
above 0.04 percent.
The source of improper combustion must be lo
cated and corrected.
WARNING
Conditions Which May Cause Abnormally High CO:
1 − Partial blockage of exhaust pipe and intake pipe;
2 − Abnormally high exhaust back pressure and intake re
striction due to pipe length or routing;
3 − Dirty or worn air flapper. (Air flapper should be checked
and replaced if needed.)
H − Testing Exhaust CO
2
/CO Content
Use the following as a guide to test for abnormally high
CO
2
/CO content.
To Measure CO
2/
CO Content:
1 − Drill size R" or 11/32" in. hole on top of the exhaust out
let PVC elbow (inside unit cabinet) and tap 1/827 NPT
as shown in figure 67. This hole is used as the
CO
2
/CO test port.
2 − Install a hose barb into the test port. See figure 67.
CO
2
/CO SAMPLING PROCEDURE (GSR21 SHOWN)
TEST EQUIPMENT
DIFFERENTIAL
PRESSURE
SWITCH
1/8"
FLEXIBLE
HOSE
EXHAUST OUTLET
FIGURE 67
3 − Attach end of Fyrite sampling tube to hose barb on ex
haust outlet elbow.
4 − Set unit to highest setting and allow unit to run for 15
minutes.
5 − When CO
2
/CO testing is complete, turn off unit, re
move hose barb from exhaust outlet elbow and use a
1/8" pipe plug to close off the test port.
The exhaust vent pipe operates under positive pres
sure. Completely seal 1/8" NPT plug to prevent leak
age of combustion products into the living space.
WARNING
I − Heat Exchanger Temperatures
During operation, temperature at the top of the combustion
chamber and tailpipe is 1,000 to 1,200
°
F. At the tailpipe en
trance to the exhaust decoupler, temperature has dropped to
approximately 600
°
F. Temperature from the exhaust decoup
ler outlet to coil intake manifold is 350
°
F. At the coil exhaust
outlet manifold, temperature range is approximately 100 to
120
°
F. These are average temperatures and will vary with
blower speed input.
J − Exhaust Temperature Range
Exhaust temperature range should not exceed 135
°
F for any
of the G21/GSR21 series units. If it does, the high exhaust
temperature may cause the unit to trip out on limit.
Most units run with a maximum exhaust temperature of
110
°
F to 135
°
F for lower to higher unit inputs. Maximum
temperatures are listed in table 15.
MAXIMUM EXHAUST TEMPERATURES
GSR2110
0
G21/GSR2180
UNIT
LIMIT SETTING
TABLE 15
120
°
125
°
130
°
135
°
115
°
G21100
G21/GSR2160
G21/GSR2150
