Specifications

IM 685-2 23
MOTOR RUNS, PILOT IGNITES...
3.1 Burner motor starts. After 30
(60 or 90) seconds the PILOT
LED comes on, the FLAME
LED comes on momentarily
and then goes out.
a. Check Table 6. The LED code may diagnose the problem.
b. The power is only momentarily proving itself to the flame safeguard. It must be proven at the end of the of the 10 second igni-
tion trial.
c. On a new start-up, this could indicate the gas lines have not been sufficiently purged of air.
d. Improper flame rod position.
e. Improper pilot air or gas adjustments.
f. Air leakage into the pilot burner at the porcelain bushing or through cracks in pilot burner.
g. Defective or improperly installed pressure regulator upstream of pilot gas cock that passes enough gas for pilot, but when
main valve opens, gas pressure drops drastically.
3.2 Pilot operates, the flame
safeguard does not lock out.
but the main flame does not
come on.
a. Check that the main manual gas cocks are open.
b. If the Main LED does not come on, check the voltage at terminals 9 to L2. If no voltage across 9 to L2, replace the RM7897A.
c. Check for defective or improperly installed pressure regulators and determine that their vents are not plugged.
d. Check for defective or improperly installed main gas valves, or open wires to the valve.
e. On diaphragm type gas valves, check for plugged or misadjusted bleed orifice or bleed line.
BURNER OPERATES; HOWEVER...
4.1 Combustion tests indicate
CO
2
and/or CO are not within
the expected range.
a. Measure gas manifold pressure at Port (1), Figure 16a, both at the maximum and minimum firing rate and correct if required.
See Table 7, Column (9) and (10).
b. Measure the burner air pressures at Port (4) and (5), Figure 16a and 16b. If readings are significantly different, from Table 7,
Columns (6), (7), and (8) determine why. Possible reasons include a clogged blower wheel, air leaks, or loosened compo-
nents that could cause variations.
c. Combustion test should be performed when the furnace is at operating temperature (typically after 10 to 15 min.)
d. Models 040 and larger have multiple valves in parallel. Determine that they are all functioning. While operating at maximum
rate use the manual operator to momentarily close the valves one at a time while observing the manifold pressure. If a reduc-
tion in manifold pressure does not occur the valve was not open before the test.
4.2 At maximum firing rate, the
burner runs rough.
a. Gas manifold pressure is too high and furnace is being fired above its rated capacity. See “Verify Input Rate” on page 17. Also
check CO
2
and CO levels.
b. The heat exchanger needs cleaning. Increased pressure drop through heat exchanger reduces airflow and affects
combustion.
c. Inspect gun assembly and blast tube for warpage or deterioration.
4.3 Flame is not symmetrical as
observed through rear
inspection window.
a. Too high airflow relative to gas flow. Check gas manifold pressure. Check CO
2
level.
b. Gun disc is not perpendicular to the blast tube, or gun disc is warped or otherwise out of alignment.
4.4 Nuisance tripping of the flame
safeguard.
a. Check Table 6. The LED code may diagnose the problem.
b. Check gas pressure situation. Marginal pressure during normal times can become low pressure during time of peak demand
and lead to trip-outs, etc. Pressures higher than that for which the gas train is designed can also cause problems. Line pres-
sure should not exceed 13.9' W.C. (½ psi) into the combination gas controls. Pressures higher than this require an additional
stepdown regulator to maintain the pressure below 13.9" W.C. even at "no flow" conditions. The preferred pressure to the
combination gas controls is 7.0 in. W.C. 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 reduced pressure during times of maximum demand.
d. Check the flame signal while modulating from minimum to maximum firing rate.
e. Check the ignition electrode gap and orientation. Check the porcelain for cracks or other defects.
f. Observe the flame signal DC volts when turning on the burner switch. Any indications before the ignition cycle could indicate a
short to ground. This could be an intermittent situation from moisture conditions. With line gas cock closed any movement
during the ignition attempt would indicate ignition interference.
g. Check supply voltage and if suspicion warrants arrange to have a recording voltmeter connected to the burner for a period
of time.
h. Marginal flame signal. Adjust flame rod position.
i. Check the ground path from FSG terminal G to the burner gun assembly. A wire runs from G to the ground screw on the left
side of the burner air box. The path continues through the variable orifice valve through the union to the burner gun assembly.
4.5 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 Table 6. The LED code may diagnose the problem.
b. Check the burner fan air proving switch and tube. As the burner air control valve opens further to provide more air for an
increased firing rate, the static pressure inside the air valve box is reduced. This is the pressure being sensed by the air prov-
ing 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.
c. 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 firing rate can occur. Also see 4.1d.
4.6 At the instant spark comes
on, the flame safeguard
drops out and restarts the
pre-purge cycle.
Ignition interference. Flame rod or its wire is sensing voltage from ignition. Also verify that ignition electrode spark gap is within
specifications.
4.7 When the flame safeguard is
powered it locks out and the
ALARM LED comes on.
a. Purge card missing or bad, terminals are energized that should not be at that stage, or there is an internal system fault.
Replace purge card or RM7897A as indicated.