SERVICING
34
SUPPLY
AIR
RETURN
AIR
Checking Temperature Rise
3. Subtract the return air temperature from the supply air
temperature to determine the air temperature rise. Allow
adequate time for thermometer readings to stabilize.
4. Adjust temperature rise by adjusting the circulator blower
speed. Increase blower speed to reduce temperature rise.
Decrease blower speed to increase temperature rise.
Refer to
Circulator Blower Speed
section in the Product
Design section of this manual for speed changing details.
Temperature rise is related to the BTUH output of the
furnace and the amount of air (CFM) circulated over the
heat exchanger. Measure motor current draw to determine
that the motor is not overloaded during adjustments.
S-300 CHECKING PRIMARY LIMIT CONTROL
All use a nonadjustable, automatic reset, bi-metal type limit
control. Refer to the following drawing for the location of the
primary limit.
Location of Primary Limit
Primary Limit Control Location
(80% Upflow Furnace Shown, Counterflow Similar)
Style 1 drawing illustrates the Primary Limit used on the 80%
furnaces.
Style 1
WARNING
HIGH
VOLTAGE
D
I
S
CONNECT
ALL
POWE
R
BEFO
R
E
S
E
R
VICING
O
R
CHANGING
ANY
ELECT
R
ICAL
WI
R
ING.
M
ULTIPLE
POWE
R
S
OU
R
CE
S
MAY
BE
P
R
E
S
ENT.
F
AILU
R
E
TO
DO
S
O
MAY
CAU
S
E
P
R
OPE
R
TY
DAMAGE,
PE
RS
ONAL
IN
J
U
R
Y
O
R
DEATH.
1. Remove burner compartment door to gain access to
the primary limit.
2. Remove low voltage wires at limit control terminals.
3. With an ohmmeter, test between these two terminals
as shown in the following drawing. The ohmmeter
should read continuous unless heat exchanger tem-
perature is above limit control setting. If not as above,
replace the control.
COLOR
IDENTIFYING
TAB
Testing Primary Limit Control
