IBC VFC 15-150 Installation And Operating Instructions Manual page 60

5.3.3 Cycling Problems
SYMPTOM
RAPID CYCLING
5-8
VFC 15-150, VFC 45-225 MODULATING GAS BOILERS
DIAGNOSIS
Improper values
entered via keypad.
Excess Condensate in
venting.
Obstruction in
condensate trap.
Improper vent length
or improper slope to
vent.
Incorrect settings or
defective thermostat.
Air in system or
marginal water flow.
Slow combustion air
blower.
Dirty burner/heat
exchanger.
Insufficient water
flow due to improper
piping.
Insufficient water flow
due to undersized
pump.
Insufficient water flow
due to restrictions in
water pipe.
Insufficient radiation.
Unit over-fired.
Unit Oversized.
Improperly set or
defective operating/
safety controls.
REMEDY
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)
Check venting slopes on horizontal runs. Look
for sags.
Inspect and clean condensate trap.
Check venting. Compare vent length and
diameter to Table 4: Maximum Venting.
Check operation. Refer to manufacturer's
instructions. Check setting with ammeter.
Bleed/purge system as required. Confirm
adequate pump size and temp rise in HX
Check that CO2 level is OK per Tabel 8.
Check pressure drop.
Refer to recommended piping in Section 1.6
Check manufacturer's rating charts/check
temperature differential across heat exchanger.
Check temperature differential across zone/heat
exchanger.
Check actual amount of radiation per zone and
refer to manufacturer's rating tables.
Clock gas meter/check gas pressure with
manometer/ check CO2 level.
Check load calculation vs. min. boiler output.
Check operation with ohmmeter/voltmeter.
INSTALLATION AND OPERATION INSTRUCTIONS