8.4.4 Cycling issues
Cycling issues
Issue Diagnosis Fix
Rapid
Cycling
Improper values entered via
keypad.
Check load maximum temps are above target
temps, by ½ 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.
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 the heat
exchanger.
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 for the
respective boiler model.
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.
Appliance over-fired. Clock gas meter/check gas pressure with
manometer/ check CO
2
level.
Appliance Oversized. Check load calculation vs. minimum boiler
output.
Improperly set or defective
operating/safety controls.
Check operation with ohmmeter/voltmeter.
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