I-OH (01-20) PN120390R7
23
Fan and Limit Controls
•
The unit is provided with a fan and limit control assembly consisting of a specially-designed control-mounting
bracket with a fan control, a circulating air high limit control, and a super high limit control attached.
•
The main functions of the fan/blower control are: 1) delay fan or blower operation to prevent the circulation of cold
air at startup and 2) continue fan or blower operation as long as the unit temperature is above the minimum setting
(120°F). The fan/blower control also provides additional safety control by operating the fan or blower in the event
that a malfunction causes the oil burner to continue to fire when the thermostat is satisfied.
•
The circulating air high limit switch has a nonadjustable setting of 145°F for unit size 095 and 160°F for unit sizes
140 and 190. The limit control automatically cycles when the internal temperature exceeds these setpoints. For
the heater to operate properly and safely, the cause for the temperature exceeding the high limit setpoint must
be determined and corrected.
•
The super high limit switch provides redundant safety control and is calibrated to open at a much higher temperature
(350°F) than the standard automatic-reset limit switch. An interruption of the circuit by the super high limit switch
indicates a major failure caused by a malfunction of the primary safety controls or by incorrect wiring. Before
attempting to restart the heater, the cause must be corrected and the fan and limit assembly replaced.
•
To access the fan and limit control assembly, remove the plate shown in
Figure 16. Access and Identification of Fan and Limit Controls
Primary Control System
•
The primary control, mounted on a 4 × 4 outlet box on the burner, is responsible for starting the burner, for monitoring
a safety operating cycle, for shutting off the burner at the end of the call for heat, and for locking out
on safety
if
there is a flame failure beyond the safety timing.
•
The ignition system is comprised of an ignition transformer and two electrodes that deliver a concentrated spark
across a fixed gap to ignite oil droplets in the nozzle spray.
•
A cadmium sulfide cell responds to light intensity to sense the presence or absence of flame. If the spark is
inadequate, the cadmium sulfide cell will not sense the presence of flame and the primary control system will lock out.
Blocked Vent Switch (CSA Models Only)
•
The blocked vent switch is a manually-reset, temperature sensitive switch designed to shut down the burner when
there is a positive pressure in the combustion chamber.
•
The blocked vent switch is mounted on a bracket on the control side of the heater.
•
Before attempting to restart a heater that has been shut down by the blocked vent switch, find and remove the
blockage causing the positive pressure. After correcting the problem, reset the switch.
Pre-Startup Checklist
Check clearances:
ensure that clearances are in accordance with
.
Check hangers:
ensure that all hangers are adequately anchored and that all unions or threaded fittings are
snug and do not rotate under conditions of vibration. Heater must be level.
Remove plate to access fan and limit control assembly
Circulating Air
High Limit
Fan
Control
Fan and Limit
Control Assembly
Super
Limit
