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Furnace Limit and Blower Controls
All furnaces are equipped with a combined thermostatic high temperature limit and
blower (fan) control. The high temperature limit is set such that it does not permit a
supply (discharge) air temperature above 200ºF.
The thermostatic fan control should be set so the greatest fuel utilization efficiency of
the furnace is obtained. Generally, a blower “ON” setting of 130ºF should give the best
result. After the burner shuts down, the circulating air blower will continue to operate
until the air temperature inside the furnace falls below the lower setting on the fan
control.
This delay on blower shutdown extracts residual heat from the furnace heat exchanger
that would ordinarily be lost to the outdoor atmosphere. If a longer, or shorter, cooldown
period is desired, the fan control differential setting may be reset to lengthen, or shorten,
the cycle as desired. Refer to Table 2 for the standard fan and high limit control settings.
Table 2: Standard Furnace Fan / Limit Control and Speed Settings
High Limit Thermostat Setting (deg. F.): 230 (highboy / lowboy models)
190 (horizontal / counterflow model)
Fan “On” Setting (deg. F.): 130
Fan “Off” Setting (deg. F.): 100
Standard Fan Speed for 105 MBTUH Input Rate: Low, Orange Wire to “L” Terminal
In addition to the combined fan and limit control, the horizontal / counterflow furnace is
equipped with an auxiliary high limit temperature control located in the blower
compartment, refer to the electrical diagram in Appendix B. The purpose of this control
is to prevent overheating of the blower motor and air filter(s), if airflow is severely
reduced.
If the duct system becomes partially, or totally, blocked or the furnace air filter(s)
become excessively dirty causing the blower to fail to circulate enough air, the
thermostatic high temperature limit (or, if equipped, the auxiliary high temperature limit)
may activate to prevent very high discharge air temperatures from occurring.
Should airflow be restricted, the furnace might cycle on and off too frequently or become
inoperative. To correct this condition, verify that all supply and return dampers and
registers in the air distribution system are open. If this fails to resolve the problem, turn
off the electrical power to the furnace. Remove the blower compartment access door to
examine the air filter(s) and blower for blockages or a loose blower wheel. (On the
highboy and horizontal / counterflow furnaces, air filters should be mounted external to
the furnace casing.) If possible, correct the condition by cleaning or replacing the air
filter(s), freeing the blower wheel of any foreign materials, or securing the blower wheel
to the fan motor shaft. Always replace the blower compartment access door when
service is finished. Restore electrical power to the furnace.