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7. Frequently the cause for safety lockouts relates to gas pressures that are too high, and
or combustion air dampers not set correctly. Cycle pilot several times to make sure that
the combustion air and pilot gas pressure is sufficient to provide instant ignition, and a
stable flame. Check to make sure the pilot flame signal is steady. Pilot regulator
adjustment may be necessary if the adjustment of the needle valve does yield the
required flame signal (if applicable).
8. Install a manometer at the test port to check burner manifold pressure.
9. Once pilot has been established (if applicable), the main manual valve is turned on.
Reset the controller and the ignition sequence will recycle. Once the main automatic
gas valve is energized the burner will light, and the control valve will modulate to
thermostat setpoint. Check to make sure the main flame signal is steady.
10. If the burner does not light within a few seconds after pre-purge shut the burner off and
repeat the previous steps.
11. Check the combustion air setting. The duct furnace has been test fired in the factory for
firing rate and combustion. Field conditions may require adjustments to be made.
These checks should be done by a qualified service man.
a) When the main flame is established, check the gas manifold pressure at the test
points nearest the burner, downstream of the gas control valve. Check the nameplate
for the correct pressure and adjust the appliance regulator if necessary.
b) Check the CO and O
2
reading at maximum and minimum inputs shown on the rating
label. O
2
readings typically range from 4-10%. CO readings should not exceed 400
PPM at any time. Adjust the air/gas ratios with the control linkages making fine
adjustments before measuring again.
c) Ensure proper overfire draft at test port in burner section is set per nameplate.
d) Visually observe the flame and insure that the flame does not impinge on the heat
exchanger surface and that there are no hot spots on the combustion chamber.
e) Check all gas piping again for leaks using a soap bubble solution.
f) When the duct furnace installation is complete, recheck the amperage draw of each
motor.
g) Insure all safety controls are operative, i.e., flame relay, high limit, air proving
switches, etc.
h) Reset all operating controls back to proper set-points for normal running conditions.
3.3 High Altitude Operation
The rated output of gas-burning appliances decreases with higher altitudes. For IFD series
duct furnace installations at altitudes above 2,000 feet (610 m), the furnace shall be
derated 4 percent for each 1,000 feet (305 m) of altitude above sea level. Factory testing
and rating plate information is recorded on sea level conditions. High altitude ratings may
be obtained by a change in manifold pressure when the appliance is equipped with a gas
pressure regulator. Appliances must be suitably marked to indicate their altitude adjusted
input rating.
