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B. Trouble shooting problems where a soft lockout code is displayed. When a soft lockout occurs, the boiler will
shut down and the display will alternate between the number “9” and the letter “b” followed by a two digit
service code. The boiler will automatically restart once the condition that caused the lockout is corrected.
CODE CONDITION POSSIBLE CAUSES
b 08 Pressure switch circuit open.
• Blockage in intake or vent system.
• Vent and/or intake system not constructed in accordance
with Part VII.
• Blocked or leaking pressure switch tubing.
• Heat exchanger or burner blockage.
• Terminals exposed to high winds.
• Blockage in condensate trap above vent.
b 18
MCBA supply sensor detected temperatures in excess
of 200°F.
• Heating load at time of error was far below the minimum
firing rate of the boiler.
• Defective primary pump or no flow in primary loop (Piping
Method 1).
• Control system miswired so that boiler operation is
permitted when no zones are calling.
b 19
MCBA return sensor detected temperatures in excess of
200°F.
• See possible causes for “b18”.
• Flow through boiler reversed.
• Sensor wiring reversed.
b 24
MCBA is reading a return sensor temperature higher
than the supply sensor temperature. Condition must be
present for at least 75s for this error code to appear.
• Flow through boiler reversed. Verify correct piping and
pump orientation.
• No boiler water flow. Verify that system is purged of air
and that appropriate valves are open.
• Sensor wiring reversed.
• Supply or return sensor defective.
b 25 Supply water temperature has risen too quickly.
• See possible causes for “b18”.
• Inadequate boiler water flow. Verify that pump is
operating and that pump and piping are sized per Part IX
of this manual.
b 26
Boiler safety limit, or external limit wired across terminals
3&4, is open.
• See possible causes for “b18”.
• Defective supply sensor.
b 30 Temperature rise between supply and return is too high.
• Inadequate boiler water flow. Verify that pump is
operating and that pump and piping are sized per Part IX
of this manual.
b 61 Pressure switch circuit closed with fan off.
• Blockage in pressure switch hose.
• Pressure switch wires shorted together.
• Defective pressure switch.
• Loose or miswired fan speed harness (if “b61” error code
is observed while fan is running).
b 65 Fan is not achieving set point speed.
• Loose or incorrect fan speed control connection.
• Defective fan.
Table 14.3: Soft Lockout Codes Displayed