Burner Operates; However . . .
4.1 Main flame light OFF is rough.
a. The furnace is being fired above its rated capacity. See
b . Heat exchanger needs cleaning . Increased pressure drop through heat exchanger
reduces airflow through burner and affects combustion.
c. At light off, the gas valve is opening too fast. On diaphragm type valves check
the bleed orifice adjustment. Some models use a bleed orifice on the pressure
regulator to smooth its opening. When replacing regulators, the bleed orifice must
be reinstalled on the new regulator .
d. The burner is improperly adjusted. Check gas pressure and orifice size or time
a gas meter to verify firing rate; check the CO
2
to verify the combustion air
adjustment .
e . Check the adjustment of the burner gun assembly . Particularly check the pilot
position within the gun on Models 200 and 250 .
f . Inspect the gun assembly and blast table for warpage or deterioration .
4 .2 Flame is not symmetrical as
observed through rear
a. Unproportionally high airflow. Check CO
2
.
b . The gun discs are not perpendicular to the blast tube, or the discs are warped or
otherwise out of alignment .
4.3 Nuisance tripping of the flame
safeguard .
a . Check
. The LED code may diagnose the problem .
b . Check gas pressure situation . Marginal pressure during normal times can become
low pressure during time of demand and lead to trip-outs, etc . Pressures higher
than that for which the gas train is designed can also cause problems . Line
pressure should not exceed 13 .9" W .C . (1/2 psi) into the standard gas train .
Pressures higher than this require an additional stepdown regulator to maintain
the pressure below 13.9" W.C. even at “no flow” conditions. A regulator that does
not shut off tight at “no flow” will allow a small amount of gas to leak past and
eventually the high pressure will build up on the downstream side, thus exceeding
the rating of the gas train components .
c . Undersized piping can also cause problems by delivering reducing pressure during
times of maximum demand .
d. On modulating burners check the pilot ignition and flame signal when operating at
both high fire and low fire.
e . Check that the pilot ignition electrode porcelain bushing is blocking air passage
into hole in pilot assembly . Check the pilot burner for cracks that could allow air
leakage into the pilot burner . On modulating burners, this leakage changes as
the burner air damper changes air pressure, and perhaps the pilot will produce a
strong microamp signal at low fire but not at high, etc.
f. Observe the flame signal DC volts when turning on the burner switch Any
movement of meter needle before the ignition cycle could indicate a short to
ground . This could be an intermittent situation from moisture conditions . With
pilot gas cock closed, any movement during the ignition attempt indicates ignition
interference .
g. Check for loose or cracked pilot gas tubes, air tubes, and fittings that could vary
leakage from time to time as vibration might move them around .
h . Check supply voltage and if suspicion warrants arrange to have a recording
voltmeter connected to the burner for a period of time .
i. Marginal flame signal. Adjust pilot air and gas and flame rod position.
j. If the pilot air pickup tube is kinked or flattened from bending, pilot air is reduced.
When this happens, the pilot is unstable and nuisance tripping results . Replace the
pilot air pickup tube .
4 .4 Modulating burners: Pilot lights,
main flame comes on at low fire, but
as actuator attempts to reposition
for an increased firing rate the flame
goes out . Then the sequence is
repeated .
a . Check the burner fan air proving switch and tube . As the burner air control
damper opens further to provide more air for an increased firing rate, the static
pressure inside the fan scroll is reduced . This is the pressure being sensed by
the air proving switch, and if it falls below its set point the burner will drop out .
The adjustment screw is located next to the wiring box cover . Turn screw CCW to
reduce set point .
b. Use a manometer to determine if the gas pressure at the orifice is dropping prior to
the flame going out. If gas pressure is dropping, check for a plugged vent on a gas
pressure regulator or something that restricts the gas flow in the line so only a low
flat rate can occur.
IM 684-6 • ROOFTOP SYSTEMS 28 www .DaikinApplied .com
Troubleshooting
