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10
Operation
(continued)
Additionally, a soft lockout occurs if the supply
temperature rises to fast (i.e. faster than the
degrees Fahrenheit per second limit). Output
reduced and recycle responses are inactive for
a stand-alone boiler without a header sensor.
Temperatures are field adjustable except
manual reset hard lockout cannot exceed
210°F (98.9°C).
2. High Limit Differential Temperature Limit
The control monitors temperature difference
between return and supply sensors. Default
response to temperature difference is as
follows.
• Differential exceeds 43°F (23.9°C) – output
(fan speed) reduced
• Differential exceeds 53°F (29.4°C) –
recycle
• Differential exceeds 63°F (35°C) –
shutdown; automatic restart after
temperature difference has decreased and
minimum off time has expired
3. Return Temperature Higher Than Supply
Temperature (Inversion Limit)
If return water temperature exceeds supply
water temperature for longer than a limit time
delay, the control shuts down the boiler and
delays restart. If the inverted temperature
is detected more than five times, the boiler
shuts down with a hard lockout. Condition is
caused by incorrect supply and return piping
connections.
4. Flue Temperature High Limit
The control monitors flue gas temperature
sensor located in vent outlet at rear of heat
exchanger. Response to flue temperature is as
follows:
• Flue exceeds 184°F (84.4°C) – output (fan
speed) is reduced
• Flue exceeds 194°F (90.0°C) – recycle
• Flue exceeds 204°F (95.6°C) – manual reset
hard lockout
Temperature is field adjustable except manual
reset hard lockout cannot exceed 204°F
(95.6°C).
5. Flow Switch
The flow switch shuts down the boiler when
there is insufficient water flow in the boiler
primary loop. When water flow is restored to a
boiler-specific minimum value (see Table 6-3),
the flow switch detects flow and automatically
restarts the boiler. The flow switch is required
and is factory provided.
106308-05 - 3/23
APEX
Installation, Operating, & Service Instructions
6. Ignition Failure
The control monitors ignition using a burner
mounted flame sensor. Response on ignition
failure is as follows:
• Size 425: retries five times, then soft lockout
for one hour
• Size 525-825: retries one time, then manual
reset hard lockout
7. Automatic Reset Limit Devices
If any below listed limit opens, the boiler shuts
down and provides an open limit indication.
The boiler will automatically restart once the
limit closes. An external limit control with its
own manual reset button requires pressing
external limit reset button after limit closes
even when connected to Auto Reset External
Limit terminals.
• Sump pressure switch – opens if inadequate
air flow is detected during operation
• Condensate float switch – opens if
condensate drain is blocked
• 24V LWCO (if used) – opens if low water
condition is detected
• Device(s) connected to Auto Reset External
Limit terminals
8. Manual Reset Limit Devices
If any below listed limit opens, the boiler
shuts down, provides an open limit indication,
and closes the Lockout Alarm contact. The
boiler will restart only after the limit closes
and the boiler control manual reset button is
depressed. During boiler start sequence, air
proving switch must prove open before blower
starts and closed after blower starts. If air
proving switch is not in the required position,
the control waits for a preset time period and
then shuts down with a manual reset hard
lockout.
• Thermal link – opens if rear of combustion
chamber overheats; one time use device
• Burner door thermostat – opens if burner
door overheats, manual reset button on
thermostat
• High and low gas pressure switches (if
used, size 525-825 only) – open if gas
pressure is outside of preset limits, manual
reset button on each switch
• Air proving switch – opens if inadequate air
flow is detected prior to ignition
• Device(s) connected to Man Reset External
Limit terminals
79
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