5-9
TROUBLESHOOTING
SL 80-399 MODULATING GAS BOILER
SL 26-260 G3, SL 40-399 G3 MODULATING GAS BOILERS
5.3.3 Cycling Problems
SYMPTOM
DIAGNOSIS
REMEDY
RAPID CYCLING
Improper values entered
via keypad.
Check load maximum temps are above target temps,
by 1/2 of the selected boiler differential. Ensure boiler
differential is OK (16-30°F is generally adequate)
Excess Condensate in
venting.
Check venting slopes on horizontal runs. Look for
sags.
Obstruction in
condensate trap.
Inspect and clean condensate trap.
Improper vent length or
improper slope to vent.
Check venting. Compare vent length and diameter to
Table 4A or Table 4B: Maximum Venting.
Incorrect settings or
defective thermostat.
Check operation. Refer to manufacturer’s instructions.
Check setting with ammeter.
Air in system or marginal
water flow.
Bleed/purge system as required. Confirm adequate
pump size and temp rise in HX.
Slow combustion air
blower.
Check that CO
2
level is within specification.
Dirty burner/heat
exchanger.
Check pressure drop.
Low water flow due to
improper piping.
Refer to recommended piping in
Section 1.6
Low water flow due to
undersized pump.
Check manufacturer’s rating charts/check
temperature differential across heat exchanger.
Low water flow due to
restrictions in water
pipe.
Check temperature differential across zone/heat
exchanger.
Low radiation.
Check actual amount of radiation per zone and refer
to manufacturer’s rating tables.
Unit over-fired.
Clock gas meter/check gas pressure with manometer/
check CO
2
level.
Unit Oversized.
Check load calculation vs. min. boiler output.
Improperly set or
defective operating/
safety controls.
Check operation with ohmmeter/voltmeter.
