Two- Stage Multi Position Furnace
Service Manual
24
440 08 2002 02
Table 7
Blower Performance Data 75,000
BTUH (PSC Motor)
Air Delivery in Cubic Feet per Minute (CFM)
(Furnace Rated @ 0.5
²
W.C. ESP)
TAP
LOW
MED L
MED H
HIGH
.10
706
917
1163
1368
e
.20
677
875
1120
1319
ssu
re
.
.30
636
840
1076
1263
Pr
es
s
W.
C
.
.40
595
812
1031
1202
Stati
c
P
eso
fW
.50
546
766
987
1148
nal
St
nc
he
s
.60
490
702
889
1077
Exter
n In
.70
------
630
821
989
Ex
.80
------
550
750
914
.90
------
462
676
833
1.0
------
------
601
747
Always check current “Technical Support Manual”
EXTERNAL STATIC PRESSURE (ESP)
External Static Pressure can best be defined as the pres-
sure difference (drop) between the Positive Pressure (dis-
charge) and the Negative Pressure (intake) sides of the
blower. External Static Pressure is developed by the
blower as a result of resistance to airflow (Friction) in the
air distribution system EXTERNAL to the furnace cabinet.
(i.e pressure inside duct)
Resistance applied
externally
to the furnace (I.E. Duct
work, Coils, Humidifiers, Filters, Etc.) on either the Supply
or
Return side of the system, causes an INCREASE in Ex-
ternal Static Pressure, accompanied by a REDUCTION in
airflow.
ESP is affected by two (2) factors.
1. Resistance to Airflow as explained above.
2. Blower Speed. Changing to a higher or lower blower
speed tap will raise or lower the External Static Pres-
sure accordingly.
These effects MUST be understood and taken into consid-
eration when checking ESP/ Airflow to insure that the sys-
tem is operating within design conditions.
Operating a system with Insufficient or Excessive air flow
can cause a variety of different operating problems.
Among these are premature heating component and/or
compressor failures, reduced capacity, freezing evapora-
tor coils, etc.
System air flow should ALWAYS be verified upon comple-
tion of a new installation, or BEFORE a change--out, heat
exchanger replacement, or in the case of a compressor
failure to insure that the failure was not caused by improp-
er air flow.
Figure 34
Checking Static Pressure
Supply
Indoor
Section
Inclined
Manometer
Return
CHECKING EXTERNAL STATIC PRESSURE
The air flow through the unit can be determined by measur-
ing the external static pressure of the system, and consulting
the blower performance data for the particular model furnace
you have.
1. Set up to measure external static pressure at the sup-
ply and return duct connections (See Figure 34).
2. Drill holes in the ducts for pressure taps, pitot tubes, or
other accurate pressure sensing devices.
3. Connect these taps to a level inclined manometer or
Magnehelic gauge.
4. Ensure the coil and filter are clean, and all the registers
are open.
5. Determine the external static pressure with the blower
operating.
6. Refer to the Air Flow Data for your particular furnace to
find the actual airflow for the current speed tap (or dip
switch setting).
7. If the Actual airflow is either too high, or too low, the
blower speed tap (or dip switch setting) will need to be
changed.
8. Refer to Changing Blower Speeds on the pages that fol-
low for the proper procedure.
9. Select the speed tap (or dip switch setting) that ap-
pears to most closely provide the required air flow for
the system.
10. Recheck the external static pressure with the new
speed tap. External static pressure (and actual air-
flow) will both have changed (either higher, or lower),
depending upon speed tap selected. Recheck the ac-
tual airflow (at this “new” static pressure) to confirm
speed tap selection.
11. Repeat steps 9. and 10. (if necessary) until proper
Speed Tap (and airflow) has been obtained.