International comfort products N9MP1 Service Manual page 29

Single Stage Multi Position Furnace
lected" for each particular installation to insure proper op-
eration.
The criteria for selecting the proper blower speeds IS NOT
"High for Cooling, Low for Heating". Although that may be
how it works out SOMETIMES, it can (in many cases) be
exactly the opposite. (I.E. a Lower speed for Cooling, and
a Higher speed for Heating)
The PROPER CRITERIA FOR SELECTING BLOWER
SPEEDS is as follows:
HEATING
A blower speed must be selected that will provide proper
temperature rise through the furnace. (See "checking tem-
perature rise" found elsewhere in this manual), The re-
quired CFM for a particular temperature rise can also be
calculated using the following formula:
Output BTU
Temp. Rise X 1.08 = CFM
EXAMPLE: using a 75 Mbtu furnace with an output of
67,500 Btuh (90% x 75000) and a desired temperature rise
of 50° F (range of 40−70° F allowable) and a measured ex-
ternal static pressure of 0.2" W.C. with a dry coil.
67,500 or 67,500
50 X 1.08 54
Checking the blower performance data for this model, (see
Table 12) indicates that @ 0.2" W.C. E.S.P. medium−high
speed will deliver 1249 CFM. Accordingly, medium speed
should be used in this example for the HEATING speed.
COOLING
A blower speed must be selected that will provide proper air
flow (Nominal 400 CFM per ton) for the size (capacity) air
conditioning coil being used at the external static pressure
of the Duct system (installation). This requires CHECKING
THE EXTERNAL STATIC PRESSURE, then consulting
the BLOWER PERFORMANCE DATA to determine the re-
quired speed tap.
Using a manometer or magnehelic, take a pressure reading
on the supply and return ductwork close to the furnace. The
supply is positive while the return is negative. To find ESP
add the supply and return readings together disregarding
positive/negative.
EXAMPLE: The supply measurement is found to be 0.3"
W.C. while the return is measured at −0.2" W.C. The ESP
would be 0.5" W.C.
EXAMPLE: A 24,000 BTU (2 TON) air conditioning system,
using the same 75,000 BTU furnace as in the previous ex-
ample. The external static pressure is measured and found
to be 0.4" W.C.
400 CFM (nominal) per TON required
400 x 2 = 800 CFM required
Checking the blower performance data (see Table 12) for
this model indicates that @ 0.4" W.C. ESP low speed will
deliver 788 CFM. Accordingly, low speed should be used in
this example for the COOLING speed.
440 08 2011 00
= 1250 CFM
Blower Performance Data
Table 12
75,0000 BTUH
Air Delivery in Cubic Feet per Minute (CFM)
(Furnace Rated @ 0.5" W.C. ESP)
TAP LOW
.10 778
.20 786
790
.30
.40 788
.50 781
.60 765
.70 743
SAMPLE ONLY
Table 13 Blower Speed Chart
Wire Color
Black
Orange*
Blue
Red
*Med−High speed may not be provided on all models.
CHANGING BLOWER SPEEDS
1. Refer to Furnace Wiring Diagram for location of the heating
and cooling speed taps located on the furnace control as
well as location of unused blower motor speed leads. Use
the chart (Table 13) to determine the blower motor speed
settings.
2. Change the heat or cool blower motor speed by removing
the motor lead from the "Heat" or "Cool" terminal and re-
place it with the desired motor speed lead from the "Unused
Motor Lead" location. Connect the wire previously removed
from the "Heat" or "Cool" terminal to the vacated "Unused
Motor Lead" terminal.
3. If the same speed must be used for both heating and cool-
ing, remove the undesired motor speed lead from the
"Heat" or "Cool" terminal and connect that lead to the open
terminal at "Unused Motor Lead" location. Attach a jumper
between the "Heat" and "Cool" terminals and the remaining
motor speed lead.
Note: For motors with (4) speed leads, it will be necessary to
tape off the terminal of the motor speed lead removed from
the "Heat" or "Cool" terminal with electrical tape since an
open terminal will not be available at the "Unused Motor
Lead" location.
"M1" & "M2" TERMINALS
There are two (2) terminals (marked "M1 and M2" on the
Furnace Control Board, which have no internal connection
to the control. Their purpose is to provide a place to connect
any "UNUSED" speed tap wires to keep them out of the way
and prevent them from shorting out against the furnace cas-
ing, or each other.
Service Manual
MED L HIGH
MED H
984 1263 1576
1003 1249 1532
1003 1244 1489
1001 1215 1432
982 1186 1371
962 1146 1308
923 1094 1229
Motor Speed
High
Med−High
Medium
Low
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