20
Gas Valve - The gas valves for the gas furnaces are a 100%
shut-off type and will fail safe if for some reason the gas is
turned off. The valve is a “step-open” type for M1G*-models
– which means it opens to a “low-fire” position, and after 14
seconds, “steps-open” to “high-fire.”
A small orifice is placed in the gas passage to delay the build
up of the working pressure. A minimum of two minutes pause
time is required between cycles to allow the orifice time to
exhaust all working gas from the diaphragm chamber and
obtain full dwell time on subsequent cycles. Factory set
nominal dwell time is approximately 14 seconds and is
not
field adjustable.
Testing Operating Pressure:
a.
Remove furnace door and set the gas valve in the OFF
position.
b.
Using a 3/16” Allen wrench, remove the plug from the right
side of the valve.
c.
Install the barbed adapter in the plug hole. Tighten by
hand at first, then with an open end wrench. Do not
overtighten!
d.
Install the hose and manometer to the barbed fitting. Be
sure to check all connections for leaks.
e.
Light the pilot by following the instructions on the furnace
and set the gas valve switch to the ON position.
f.
With the valve in the ON position, set the thermostat
above room temperature so the furnace will run throughout
the test procedure.
g.
With the furnace operating, check for valve staging. The
operating pressure should be approximately 3.5” WC for
Natural Gas, 10” WC for LP gas.
Pilot Assembly, Thermocouple, and Venturi Burner
The pilot lights the main burner. The thermocouple generates
around 30 MV unloaded, (unscrewed from valve) and around
15 MV loaded (screwed into valve). See gas valve and pilot
outage section for troubleshooting.
The pilot orifice is not changed when furnace is converted to
LP (see conversion section). If the flame is more than about
1” in height, it can be adjusted using the pilot adjustment
screw. Turn the screw clockwise to decrease the flow rate,
counterclockwise to increase it. Note: Do not confuse the pilot
adjustment screw with the pressure regulator, which in-
creases by turning clockwise and decreases by turning coun-
terclockwise.
To clean pilot or service thermocouple or burner:
1.
Remove burner.
2.
Remove pilot assembly screws and pilot shield.
3.
Remove pilot tube from pilot assembly.
4.
Inspect pilot assembly - tap out pilot orifice and clean or
replace if plugged.
5.
Replace the pilot tube, shield, assembly screws, and
burner. Re-start and test.
Figure 16. Burner Assembly with Venturi Assembly
Venturi
Assembly
Picture 7. Gasket
Picture 8. Burner Assembly
Picture 9. Pilot Assembly and Thermocouple
Pilot Assembly
Thermocouple
