Table of Contents
Advertisement
Furnace Limit and Blower Controls
All furnaces are equipped with a thermostatic high temperature limit. 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" timing of 60 seconds should give the
best result. After the burner shuts down, the circulating air blower will continue to
operate until the fan "off" delay has been satisfied. Factory setting for this delay is 4
minutes. 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 timing 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 Timings, Limit Temp-Settings & Speed Settings
High Limit Thermostat Setting (deg. F.):
Fan "On" Setting:
Fan "Off" Setting:
Standard Fan Speed for 105 MBTUH Input Rate:
In addition to the high 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.
DIP SWITCH 2 SECTION STATE
1
2
3
OFF
OFF
ON
OFF
OFF
ON
ON
ON
OFF
ON
OFF
ON
o
130
(highboy / lowboy models)
o
150
(lowboy rear flue models)
o
190
(horizontal / counterflow model)
ML, Blue wire to "Heat" Terminal
BLOWER DELAY TIMES
4
ON - SEC
OFF - MIN
30
60
120
240
OFF
2
OFF
4
ON
6
ON
8
23
60 second delay
4 minute delay
Hide quick links:
Advertisement
Table of Contents
