Daikin DM80TN, Service Instructions Manual

SYSTEM OPERATION

11

must supply a means of filtering all of the return air. Filter(s) 

shall comply with UL900 or CAN/ULC-S111 Standards. If 

the furnace is installed without filters, and is not covered by 

the warranty.

Upflow furnaces with air delivery of less than 1800 CFM: 

Use one side return or one bottom return ductwork connection.

Upflow furnaces with air delivery of 1800 CFM or higher:

Use two side returns or one side return and one bottom return 

connection.

Counterflow Furnaces must have a return air duct connection 

attached to the end of the furnace (top of the furnace when 

installed vertically) whether the furnace is installed vertically of 

horizontally. Filters must be installed externally to the furnace 

cabinet, in the return air plenum or centrally located.
Guide dimples locate the side and bottom return cutout 

locations. Use a straight edge to scribe lines connecting the 

dimples. Cut out the opening on these lines. An undersized 

opening will cause reduced airflow. For bottom return 

connection, remove the bottom of the cabinet before setting 

the furnace on the raised platform or return air duct.
A closed return duct system must be used, with the return 

duct connected to the furnace. 

NOTE:

 Ductwork must never 

be attached to the back of the furnace.

 Supply and return 

connections to the furnace may be made with flexible joints 

to reduce noise transmission, if desired. If a central return is 

used, a connecting duct must be installed between the unit 

and the utility room wall so the blower will not interfere with 

combustion air or draft. The room, closet, or alcove must not 

be used as a return air chamber.
When the furnace is used in connection with a cooling unit, 

the furnace should be installed in parallel with or on the 

upstream side of the cooling unit to avoid condensation in 

the heating element. With a parallel flow arrangement, the 

dampers or other means used to control the flow of air must 

be adequate to prevent chilled air from entering the furnace 

and, if manually operated, must be equipped with means to 

prevent operation of either unit unless the damper is in the 

full heat or cool position.
When the furnace is heating, the temperature of the return 

air entering the furnace must be between 

55°F

 and 

100°F

T

O

AVOID

PERSONAL

INJURY

OR

DEATH

DUE

TO

ELECTRICAL

SHOCK

, 

TURN

 OFF 

POWER

TO

THE

FURNACE

BEFORE

CHANGING

SPEED

TAPS

.

WARNING

CIRCULATING AIR AND FILTERS

DUCTWORK - AIR FLOW

Duct systems and register sizes must be properly designed 

for the C.F.M. and external static pressure rating of the 

furnace.  Ductwork should be designed in accordance with 

the recommended methods of “Air Conditioning Contractors 

of America” manual D.
A duct system should be installed in accordance with 

Standards of the National Board of Fire Underwriters for 

the Installation of Air Conditioning, Warm Air Heating and 

Ventilating Systems, Pamphlets No. 90A and 90B.

A return air filter is not supplied with the furnace. The installer 

must supply a means of filtering all of the return air. Filter(s) 

shall comply with UL900 or CAN/ULC-S111 Standards. If 

the furnace is installed without filters, and is not covered by 

the warranty.

Upflow furnaces with air delivery of less than 1800 CFM: 

Use one side return or one bottom return ductwork connection.

Upflow furnaces with air delivery of 1800 CFM or higher:

Use two side returns or one side return and one bottom return 

connection.
Guide dimples locate the side and bottom return cutout 

locations. Use a straight edge to scribe lines connecting the 

dimples. Cut out the opening on these lines. An undersized 

opening will cause reduced airflow. For bottom return 

connection, remove the bottom of the cabinet before setting 

the furnace on the raised platform or return air duct.
A closed return duct system must be used, with the return 

duct connected to the furnace. 

NOTE:

 Ductwork must never 

be attached to the back of the furnace.

 Supply and return 

connections to the furnace may be made with flexible joints 

to reduce noise transmission, if desired. If a central return is 

used, a connecting duct must be installed between the unit 

and the utility room wall so the blower will not interfere with 

combustion air or draft. The room, closet, or alcove must not 

be used as a return air chamber.
When the furnace is used in connection with a cooling unit, 

the furnace should be installed in parallel with or on the 

upstream side of the cooling unit to avoid condensation in 

the heating element. With a parallel flow arrangement, the 

dampers or other means used to control the flow of air must 

be adequate to prevent chilled air from entering the furnace 

and, if manually operated, must be equipped with means to 

prevent operation of either unit unless the damper is in the 

full heat or cool position.
When the furnace is heating, the temperature of the return 

air entering the furnace must be between 

55°F

 and 

100°F

.