8.3.4 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 controls.
Check operation with ohmmeter/voltmeter.
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