Two- Stage Multi Position Furnace
Service Manual
13
440 08 2002 02
Table 3
APPROXIMATE OPERATING PRESSURES (INCHES OF W.C.)
@Blower/@Transition
Model
Vent Length
(High Fire)
(Low Fire)
50 Mbtu &
Short -- (5 Ft. No Elbows)
--1.80/--2.60
--1.20/--1.90
50 Mbtu &
75 Mbtu
Long -- (40 Ft. + 5 90
°
Elbows)
--1.30/--2.30
--1.00/--1.80
100 Mbtu
Short -- (5 Ft. No Elbows)
--1.80/--2.60
--1.20/--1.90
100 Mbtu
Long -- (40 Ft. + 5 90
°
Elbows)
--1.70/--2.50
--1.00/--1.80
Short -- (5 Ft. No Elbows)
--1.80/--2.60
--1.30/--2.30
125 Mbtu
Long -- (40 Ft. + 5 90
°
Elbows)
--1.70/2.50
--1.20/--2.20
Always check current “
Technical Support Manual
” for
updated information.
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.)
2. Leak (lack of restriction) on the
Inlet
side of the com-
bustion blower.
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)
I
nlet Pipe
Vent Pipes
MUST
be
supported Horizontally
and Vertically
*8
²
Min.
20
¢
Max.
in same
atmospheric zone
Figure 14
*8
²
Min.
20
¢
Max.
in same atmospheric zone
Coupling on ends of exhaust
pipe. Total pipe & coupling
outside structure = 8
²
Typical Vent/Combustion Air Piping Installation
Aluminum or non--rusting shield recommended.
(See Vent Termination Shielding for dimensions).
*
Increase minimum from 8
²
to 18
²
for cold climates (sustained temperatures below 0
°
F).
DISCHARG
E
AIR
25--23--33
I
nlet Pipe
UPFLOW
*8
²
Min.
20
¢
Max.
in same
atmospheric
zone
Vent Pipes
MUST
be supported
Horizontally and
Vertically
*
Increase minimum from 8
²
to 18
²
for cold climates (sustained temperatures
below 0
°
F).
See Vent Termination Shielding
in Vent Section.
*8
²
Min.
20
¢
Max.
in same
atmospheric zone
8
²
Min.
Coupling on inside and
outside of wall to
restrain vent pipe
25--23--33a
DOWNFLOW
15. VENT/COMBUSTION AIR PIPING
Vent and combustion air piping are an extremely important
part of the total furnace installation. Improperly installed or
inadequately sized vent and/or combustion air piping can be
the source of many perceived furnace problems.
For example, most problems associated with pressure
switch operation can normally be traced to short comings in
the vent and/or combustion air piping. Anytime these type
problems arise, a thorough inspection of the vent and/or
combustion air piping should be conducted.
ALL MODELS
require a vent (exhaust) pipe to carry flue
products to the outside of the structure.
Direct VENT
(ONLY) models require a combustion air inlet
to bring in
all air for combustion from outside
the struc-
ture.