Two- Stage Multi Position Furnace
Service Manual
3
440 08 2002 02
Component Locations for Four Position Furnaces
Figure 1
Circulating Air Blower
Rating Plate
Gas Valve/Ignition Module
D
oor Interlock Switch
Pressure Switches
Fan/Delay Control
Furnace Vent Pipe
(Vent Pipe Connection through
Side Panel on Some Models)
Combustion Air Blower
Vent Pipe Grommet
Primary Heat Exchanger
Secondary Heat Exchanger
dwg 25--23--29a
Diagnostic Light
Manual Gas Valve
Vent Drain Fitting
3
/
4
²²²²
OD Transition Box
Drain Hose
5
/
8
²²²²
OD Vent Pipe
Drain Hose
Plastic Transition Box
Air Intake Pipe
(Dual Certified or Direct
Vent furnaces)
Condensate Trap
D C Motor Control
(some models)
Coils Air Baffle
3. FURNACE THEORY OF OPERATION
The high efficiencies and lower profile (compared to previous
series) of this furnace have been obtained using design tech-
niques not typical of traditional furnace designs. A brief de-
scription of these new design techniques and the purpose
they serve follows.
1. Reducing the height of the furnace while maintaining
the high efficiency of pervious models required work-
ing the heat exchanger more efficiencly and yet mini-
mizing the overall size.
The design required to achieve these results is the “SER-
PENTINE” design, wherein the flue gasses must follow a
serpent shaped passage through the heat exchanger via
convection.
This “Serpentine” path is resistive to normal convective flow,
and requires that a partial vacuum be created at the outlet
of the heat exchanger to maintain the flow of flue products
through the heat exchanger.
2. The serpentine heat exchanger design does not lend
itself well to the ribbon type, or slotted port type burner
found in more traditional design furnaces for the follow-
ing reasons:
A.The secondary combustion airflows at right angles to the
burner flame, making it likely to “pull” the flame off a ribbon
or slotted port type burner.
B.The flame “height” of a ribbon or slotted port type burner
would make it difficult (if not impossible) to prevent im-
pingement of the flame on the heat exchanger surfaces
whole maintaining the low profile heat exchanger.
For these reasons, an “INSHOT” type burner is used in this
series. The inshot burner (also called a “jet” burner) fires a
flame straight out its end. This burner is designed to fire into
a tube style heat exchanger, making it an ideal application in
the tube--like passages of the serpentine heat exchanger.
3. In order to extract the maximum amount of heat pos-
sible from the flue gasses, a secondary heat exchang-
er (condenser) is connected to the outlet of the primary
heat exchanger. This condenser removes additional
heat from the flue gasses, causing their temperature to
drop below dew point, thus increasing operating effi-
ciency of the furnace, and the term “
Condensing Fur-
nace
”. This results in the forming of condensation (wa-
ter) which then must be routed to a drain.
4. The placement of the secondary heat exchanger at the
outlet of the primary heat exchanger creates additional
resistance to the flow of gasses.
5. To overcome the resistance to convective flow of the
Primary and Secondary heat exchangers requires the
use of an Induced Draft Combustion Blower Assembly.
