60
Airflow
AIRFLOW
The importance of proper air flow over the heat exchanger cannot
be over emphasized.
One of the most common causes of heat exchanger failure is over-
heating due to low air flow. Airflow selections are in the dipswitch
section of this manual.
TEMPERATURE RISE CHECK
To determine if the air flow is correct, make a temperature rise
check.
1. Insert a thermometer in the supply air duct as close to the fur-
nace as possible yet out of a direct line from the heat ex-
changer. See Figure 43.
2. Insert a thermometer in the return air duct as close to the fur-
nace as possible.
3. Operate the furnace.
4. When the thermometer in the supply air duct stops rising (ap-
proximately five minutes), subtract the return air temperature
from the supply air temperature. The difference is the temper-
ature rise.
5. Compare the measured temperature rise to the approved tem-
perature rise range listed on the furnace name plate. See Fig-
ure 44.
If the measured temperature rise is above the approved range, ei-
ther the air flow is too low or the manifold pressure needs to be ad-
justed. More air must be moved by speeding up the blower, by
removing restrictions in the duct system, or by adding more supply
or return air duct. If the measured temperature rise is below the ap-
proved range, either the air flow is too much or the manifold pres-
sure needs to be adjusted. Use lower speed selection on the
variable speed blower.
IMPORTANT:
The measured temperature rise should be in the
middle of the range.
IMPORTANT:
Some high-efficiency filters have a greater than nor-
mal resistance to airflow. This can adversely affect furnace opera-
tion. BE SURE TO CHECK AIRFLOW if using any filter other than
factory-provided filter.
!
CAUTION
IT IS IMPORTANT THAT EACH DUCT SYSTEM BE SIZED
AND INSTALLED FOR THE SPECIFIC APPLICATION BY
PROPERLY APPLYING THE APPROPRIATE INDUSTRY AC-
CEPTED STANDARD. IF LESS THAN MINIMUM STAN-
DARDS ARE APPLIED, THE EQUIPMENT USER COULD
EXPECT TO EXPERIENCE HIGHER UTILITY BILLS, MAJOR
COMPONENT FAILURE, VARYING DEGREES OF AIR
NOISE OR OTHER UNSATISFACTORY ISSUES, OVER
WHICH THE MANUFACTURER HAS NO CONTROL.
FIGURE 43
TEMPERATURE RISE MEASUREMENT
R
HEEM
S
ALE
S
CO
M
P
ANY, IN
C
.
F
OR
T
S
MITH, A
R
KAN
S
A
S
M
OD
EL N
U
M
B
E
R
S
E
R
IAL N
U
M
B
E
R
MF
G
.
D
ATE:
CO
NT
RO
L
S
Y
S
TEM: XX
H
OUR
LY IN
PU
T
R
ATIN
G
MAXIM
U
M
Bt
u/hr
(
Kw
)
H
OUR
LY IN
PU
T
R
ATIN
G
MINIM
U
M
Bt
u/hr
(
Kw
)
FA
C
T
OR
Y EQ
U
I
PP
E
D
F
OR
F
OR
G
A
S
IN THE
U
.
S
., AT ELEVATI
O
N
S
A
BO
VE
2
,
000
FT.
(
6
10
M
)
,
D
E
R
ATE THE IN
PU
T 4%
P
E
R
1
,
000
FT.
(305
M
)
A
BO
VE
S
EA LEVEL THI
S
A
PP
LIAN
C
E EQ
U
I
PP
E
D
F
OR
ALTIT
UD
E
S
OR
IFI
C
E
LIMIT
S
ETTIN
G
A
U
XILIA
R
Y LIMIT
S
ETTIN
G
MAXIM
U
M
P
E
R
MI
SS
I
B
LE
G
A
S
SUPP
LY
PR
E
SSUR
E T
O
F
UR
NA
C
E
MINIM
U
M
G
A
S
SUPP
LY
PR
E
SSUR
E F
OR
PURPOS
E
S
O
F IN
PU
T A
D
J
US
TMENT
MANIF
O
L
D
PR
E
SSUR
E
AI
R
TEM
P
E
R
AT
UR
E
R
I
S
E/
HI
G
H FI
R
E
L
O
W FI
R
E
MAXIM
U
M EXTE
R
NAL
S
TATI
C
PR
E
SSUR
E
IN
C
HE
S
W.
C
.
(
k
P
a
)
NAT
UR
AL
G
A
S
IN. W.
C
.
(
k
P
a
)
10
.
5
(2
.6
1)
5
.
0
(1
.
2
4
)
3
.
5
(0
.
87)
L
P
G
A
S
OR
PROP
IN. W.
C
.
(
k
P
a
)
13
.
0
(3
.
23)
11
.
0
(2
.
73)
10
.
0
(2
.4
9)
A
SS
EM
B
LE
D
IN MEXI
CO
91
-
2217
6-
11
C
ATE
GOR
Y I F
ORC
E
D
AI
R
F
UR
NA
C
E F
OR
IN
DOOR
O
NLY, IN A
BU
IL
D
IN
G
CO
N
S
T
RUC
TE
D
O
N-
S
ITE.
ELE
C
T
R
I
C
115
V. 6
0
HZ.
1
P
H., MINIM
U
M
C
I
RCU
IT AM
P
A
C
ITY ,I
B
M
O
T
OR
FLA ,I
D
M
O
T
OR
FLA
MAXIM
U
M
O
VE
RCURR
ENT
PRO
TE
C
TI
O
N
D
EVI
C
E AM
P
E
RS
.
N
o
x EMI
SS
I
O
N LIMIT
(
NAN
OGR
AM
S
/ J
OU
LE
)
°F
(
°
C)
°F
(
°
C)
°F
(
°
C)
°F
(
°
C)
T
O
T
O
#
D
M
S
ENERGY
PERFORMANCE
VERIFIED
ST-A1194-100-00
802
VA
075
4
17
Z
S
A
XXXXXX
FIGURE 44
TYPICAL FURNACE NAME PLATE
!
WARNING
THE MEASURED TEMPERATURE RISE SHOULD BE AS
CLOSE TO THE MIDDLE OF THE STATED RANGE AS POSSI-
BLE. FOR EXAMPLE, IF THE RISE RANGE IS 40 TO 70°F
(4.5°-21°C), THE MIDDLE OF THE RISE RANGE IS 55°F
(12.8°C). IN ALL APPLICATIONS, THE INSTALLER MUST AD-
JUST THE TEMPERATURE RISE TO THIS “MIDDLE” POINT
AS CLOSELY AS POSSIBLE. ALSO, THE TEMPERATURE
RISE SHOULD NEVER BE ABOVE OR FALL BELOW THE
STATED RANGE. DOING SO COULD CAUSE DAMAGE TO
THE HEAT EXCHANGER OR INTERMITTENT OPERATION.
THIS COULD CAUSE INJURY OR DEATH AND WILL VOID
THE MANUFACTURER’S WARRANTY FOR THIS PRODUCT.
