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B.
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 VI.
• 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
ring rate of the boiler
• Defective primary pump or no ow 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 ow. 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 ow. Verify that pump is
operating and that pump and piping are sized per Part
VIII 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 ow. Verify that pump is
operating and that pump and piping are sized per Part
VIII 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