Higher/Lower Negative Pressures - International comfort products N9MP1 Service Manual

Service Manual
appearance. ONLY FACTORY PROVIDED or AU-
THORIZED SUBSTITUTES ARE ACCEPTABLE.
Furnaces installed at altitudes of 4,000' above sea level or
higher may require replacing the standard pressure switch
with a high altitude pressure switch. The different pressure
switch settings allow continued SAFE RELIABLE high alti-
tude operation.
Under normal operating conditions, sufficient negative
pressure will be created to close the pressure switch, and
keep it closed to keep furnace operating. Under abnormal
conditions, however, such as a restricted vent pipe, or a
leak in one of the heat exchangers, sufficient negative pres-
sure will not be created. This will result in the switch failing
to close or failing to remain closed during furnace operation.
When servicing a unit whose pressure switch will not close,
or remain closed during operation, the operating pressure
of that furnace should be checked and compared to
Lower (Lesser) Negative Pressures
Lower than normal negative pressures measured at the
Combustion Blower may be caused by:
1. Restriction on the Outlet side of the combustion blow-
er. (I.E. Blocked Flue, Vent too long, Heat Exchanger
leak, etc.)
Higher (Greater) Negative Pressures
Higher than normal negative pressures measured at the
Combustion Blower may be caused by:
1. Restriction on the Inlet side of the combustion blower.
(I.E. Plugged Heat Exchanger, air inlet orifice too
small)
Figure 14
Typical Upflow Installation
Aluminum or non−rusting shield recommended. (See
Vent Termination Shielding for dimensions).
*8" Min.
20' Max.
in same
atmospheric
zone
Vent Pipes MUST be
supported Horizontally
and Vertically
*
Increase minimum from 8" to 18" for cold climates (sustained temperatures below
0 ° F).
18
Coupling on ends of
exhaust pipe. Total
Inlet Pipe (not
pipe & coupling out-
used on Single
side structure = 8"
Pipe model)
*8" Min.
20' Max.
in same atmospheric
zone
25−23−33
Single Stage Multi Position Furnace
approximate operating pressures listed in this manual and
the switch setting(s) listed above for the model family you
are servicing.
It is important to remember, that greater negative pressures
are created by the furnace when "HOT" (I.E. upon initial
start−up) than when "COLD" (I.E. after furnaces has been
in operation for a few minutes). Because of this, furnace
pressure should ONLY be checked when "HOT" to insure
accurate readings.
The tables list approximate operating pressures. They are
included in this manual to provide you with a "Barometer"
to gauge your pressures against. The pressures you obtain
in the field will differ slightly from these figures based upon
vent length, gas pressure, operating temperature, etc.
Major discrepancies in pressures, will normally cause
problems with pressure switch operation. These Major dis-
crepancies should be investigated as follows:
2. Leak (lack of restriction) on the Inlet side of the com-
bustion blower.
Figure 15 Typical Downflow Installation
See Vent Termination
Shielding in Vent Sec-
tion.
*8" Min.
20' Max.
Inlet Pipe
in same
(not used on
Single Pipe atmospheric zone
model)
Vent Pipes MUST be
supported Horizontally
and Vertically
* Increase minimum from 8" to 18" for cold climates (sustained temperatures
below 0°F).
Coupling on inside
and outside of wall
to restrain vent pipe
8" Min.
*8" Min.
20' Max.
in same
atmospheric zone
25−23−33a
440 08 2011 00