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I-EEDU (02-21) 150492-A
•
The combustion air proving switch is a pressure-sensitive switch that monitors air pressure to ensure that proper
combustion airflow is available . The switch is a single-pole normally-open device that closes when a decreasing
pressure is sensed in the outlet duct of the flue gas collection box .
•
At startup when the heater is cold, the sensing pressure is at the most negative level . As the heater and flue
system warm up, the sensing pressure becomes less negative . After the system has reached equilibrium (about
20 minutes), the sensing pressure levels off .
•
If a restriction, excessive flue length, or turns cause the sensing pressure to become less than the switch setpoint,
the pressure switch will function to shut off the main burners . The main burners will remain off until the system
has cooled and/or the flue system resistance is reduced .
lists the approximate water column negative
pressure readings and switch setpoints for sea level operating conditions .
Table 19. Pressure Switch Sea Level Operating Conditions
Operating Condition
Pressure Reading (IN WC)
Startup cold
−1 .00
Equilibrium
−0 .60
Setpoint OFF
−0 .48
Setpoint ON
−0 .65
Limit Switch
•
A non-adjustable high limit switch mounted at the outlet air side of the heat exchanger acts to shut off the gas
supply in the event of air handler motor failure, lack of correct airflow (refer to
restriction due to filters and/or duct design .
•
Depending on accessibility, the limit switch may be serviced from either outside the unit or inside through the
access panel in the discharge duct (see
Figure 28
) . When units are installed side by side, the service access
panel must be in either the top or bottom of the ductwork for limit switch service (refer to
sections) .
Figure 28. Limit Switch Access
Fan Control
•
The fan control is a field-installed option that is required with makeup air gas control options .
•
The fan control provides delay of fan operation preventing circulation of cold air and fan operation as long as the
unit is hot . The fan control provides additional safety by keeping the fan in operation in the event that the gas valve
fails to close when the thermostat is satisfied .
•
To ensure that the fan can continue to operate, the power supply to the heater MUST NOT be interrupted except
when servicing the heater .
INSIDE
OUTSIDE
