035-15241-003 Rev. A (201)
Unitary Products Group
9
2.
If two duct systems are used as could be the case with a
coil-blower or a self-contained A/C unit, the furnace and
A/C unit should be controlled by a single combination
heating and cooling thermostat which will prevent the fur-
nace and A/C unit from operating simultaneously.
RETURN AIR AND FILTERS
RETURN AIR TEMPERATURE
This furnace design is to be operated in normal household
temperatures. The continuous return air temperature must
not be below 60°F or above 85°F.
RETURN AIR CONNECTION - UPFLOW
The return air may be brought in through the bottom of the
furnace or through one or both sides of the furnace casing.
The furnace casing may be cut out so that side return air con-
nections may be used. The furnace is supplied with flanges
on the top of the furnace to which the return air duct may be
fastened. The casing top is embossed to indicate where to
bend the flanges. If a side return is to be used, cut out the
side of the casing 14" high by 16 1/4" wide using the lances in
the casing side as a guide.
DO NOT CUT THE OPENING
LARGER THAN 14"x 16 1/4".
It is not permissible to cut out
the back of the furnace.
The return air ducts to the furnace must have a total cross
sectional area of not less than two square inches per 1000
BTUH of furnace input rating for heating operation. If air con-
ditioning is to be installed with the furnace, or if it may be
added at a later time, larger return air ducts may be required,
depending on the capacity of the air conditioner and the air-
flow required. For applications requiring more than 1800
CFM, it will be necessary to use the bottom return, both side
returns or one side plus the bottom return.
RETURN AIR CONNECTION - DOWNFLOW
(50-125 MBH MODELS ONLY)
The return air connection to the furnace must be attached to
the top of the furnace. It is not permissible to cut out the side
of the furnace casing for side return ducts. The furnace is
supplied with flanges on the top of the furnace to which the
return air duct may be fastened. The casing top is embossed
to indicate where to bend the flanges.
The return air ducts to the furnace must have a total cross
sectional area of not less than two square inches per 1000
BTUH of furnace input rating for heating operation. If air con-
ditioning is to be installed with the furnace, or if it may be
added at a later time, larger return air ducts may be required,
depending on the capacity of the air conditioner and the air-
flow required. The return air opening in the top of the furnace
is large enough for the largest capacity air conditioner for
which the furnace blower is rated.
RETURN AIR CONNECTION - HORIZONTAL
The return air duct must be connected to the inlet end of the
furnace. It is not permissible to cut out the side of the furnace
casing for side return ducts. The furnace is supplied with
flanges on both ends of the furnace to which the supply and
return air ducts may be fastened. The casing is embossed on
both ends to indicate where to bend the flanges.
The return air ducts to the furnace must have a total cross
sectional area of not less than two square inches per 1000
BTUH of furnace input rating for heating operation. If air con-
ditioning is to be installed with the furnace, or if it may be
added at a later time, larger return air ducts may be required,
depending on the capacity of the air conditioner and the air-
flow required. The return air opening in the top of the furnace
is large enough for the largest capacity air conditioner for
which the furnace blower is rated.
FILTERS
Air filters must be used with this furnace. Failure to do so will
cause dirt to accumulate on the furnace blower motor, blower
wheel, heat exchanger and air conditioning coil, resulting in
reduced system efficiency, erratic control performance and
possible equipment damage.
Air velocity must not exceed 300 feet per minute through low
velocity disposable filters. Air velocity must not exceed 650
feet per minute through high velocity cleanable permanent fil-
ters. Use of a filter that is too small will cause static pressure
in the duct system to be too high, which will have an adverse
effect on heating and cooling operation.
If a separate heating and separate cooling ther-
mostat is used, a manually operated electrical in-
terlock switch must be installed to prevent
simultaneous operation of both systems and to
avoid a possible hazardous condition due to
overheating of the conditioned space.
When side return air ducts are used, a solid metal
block-off panel must be used to block the bottom
opening in the furnace. Failure to do so could
cause flue gases to be drawn into the living
space, which could result in asphyxiation.
When the furnace is installed so that supply ducts
carry air circulated by the furnace to areas out-
side the space containing the furnace, the return
air must also be handled by a duct(s) sealed to
the furnace casing and terminating outside the
space containing the furnace. Failure to do so
can result in asphyxiation.