Single Stage Multi Position Furnace
Service Manual
11
440 08 2001 02
To verify this, the cut--out (opening) point of the switch should
be checked (using a thermocouple type thermometer con-
nected to the face of the switch) as follows:
1. Operate furnace for several minutes.
2. Block return air grille(s) to furnace.
3. Observe temperature at which switch opens (burner
operation ceases).
4. Remove blockage from return grille(s).
5. Observe temperature at which switch closes (burner
operation resumes).
6. Compare readings with the limit setting listed in the
appropriate chart for the model you are servicing.
If switch is opening within the specified range, then it is sim-
ply doing its job, and the cause of the over--temperature must
be determined and corrected.
If, however, the switch is found to be opening prematurely,
then it should be replaced. When replacing ANY limit switch,
use ONLY a switch of EXACTLY the same temperature set-
ting. Use of a different temperature limit switch can create a
dangerous situation. Some of the main limit switches used
in this series are SIMILAR IN APPEARANCE. DIFFERENT
TEMPERATURE SETTINGS, HOWEVER, ARE USED for
different models. Be certain you have the correct control for
the model you are servicing.
ROLL OUT LIMIT
A “Normally Closed” switch (wired in series with the Main
Limit switch) mounted on the burner box.
This switch may be of the manual type, depending upon the
particular model and/or family. Different temperature
(OPENING) settings are also used on different models.
When replacing this switch, be absolutely certain the correct
one is used.
Typical Roll Out Limit Switch
Figure 12
CAUTION
NEVER use an automatic reset roll out switch to replace
a manual reset type roll out switch.
Doing so may cause potentially unsafe and/or intermit-
tent operation.
The roll out switch monitors the temperature inside the burn-
er box, and interrupts furnace (burner) operation when its
temperature indicates flame roll out has occurred.
If the roll out switch has opened, the cause must be deter-
mined. Some possible reasons for flame roll out include a re-
stricted primary or secondary heat exchanger or over fired
furnace.
MANUAL RESET SWITCH MODELS
Furnace models which are equipped with a Honeywell
ST9160 Fan timer/furnace control use a manual reset roll out
switch. Once the roll out switch has opened, burner opera-
tion will be prevented until the roll out switch is “Manually Re-
set” by pressing the red button located on the switch. While
the roll out switch is open, (Depending upon the particular
model) the combustion blower and/or circulating blower will
be energized continuously.
AUXILIARY LIMIT SWITCHES
All models are equipped with one (1) additional (AUXILIARY)
limit switch mounted on the blower deck. Its purpose is to
monitor return air temperature, and interrupt burner opera-
tion when a temperature is sensed which could result in the
filter surface(s) exceeding allowable temperatures. Depend-
ing upon the particular model, the combustion blower, and/or
circulating blower may be energized continuously while the
auxiliary limit switch remains open.
This control is an “Automatic” reset control which will reset
itself when the temperature drops to a safe level. See the
Tech. Service Data Sheet for the model you are servicing, to
obtain its specific auxiliary limit switch setting.
