5-9
TROUBLESHOOTING
VFC 15-150 - VFC 45-225 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 3: 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 Co2 level is oK 9.0% nat. gas
(10.2% for propane).
Dirty burner/heat
exchanger.
Check pressure drop.
Insufficient water
flow due to improper
piping.
refer to recommended piping in
Section 1.6
Insufficient water flow
due to undersized
pump.
Check manufacturer’s rating charts/check
temperature differential across heat exchanger.
Insufficient water flow
due to restrictions in
water pipe.
Check temperature differential across zone/heat
exchanger.
Insufficient 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 Co2 level.
Unit Oversized.
Check load calculation vs. min. boiler output.
Improperly set or
defective operating/
safety controls.
Check operation with ohmmeter/voltmeter.
Summary of Contents for VFC 15-150
Page 81: ...NOTES...
Page 82: ...NOTES...
