Page 1
INSTALLATION
INSTRUCTIONS
ML180DF
507326-02
11/2017
Supersedes 05/2016
THIS MANUAL MUST BE LEFT WITH THE 
HOMEOWNER FOR FUTURE REFERENCE
 This is a safety alert symbol and should never be 
ignored. When you see this symbol on labels or in man-
uals, be alert to the potential for personal injury or death
 CAUTION
As with any mechanical equipment, contact with sharp 
sheet metal edges can result in personal injury. Take 
care while handling this equipment and wear gloves and 
protective clothing.
 WARNING
Improper installation, adjustment, alteration, service 
or maintenance can cause property damage, personal 
injury or loss of life. Installation and service must  be 
performed by a licensed professional HVAC installer or 
equivalent, service agency, or the gas supplier.
© 2017 Lennox Industries Inc. 
Dallas, Texas  USA
MERIT® SERIES GAS FURNACE
DOWNFLOW AIRDISCHARGE
Contents
Unit Dimensions .............................................................2
ML180DF Gas Furnace .................................................. 3
Shipping and Packing List ..............................................3
Safety Information ..........................................................3
Use of Furnace as Construction Heater .........................4
General ...........................................................................4
Combustion, Dilution & Ventilation Air ............................5
Setting Equipment  .........................................................9
Filters ............................................................................10
Duct System ................................................................. 11
Venting.......................................................................... 11
Gas Piping ....................................................................16
Electrical .......................................................................18
Integrated Control .........................................................21
Unit Start Up .................................................................22
Gas Pressure Adjustment .............................................23
Proper Combustion.......................................................23
High Altitude .................................................................23
Other Unit Adjustments.................................................24
Service..........................................................................26
Repait Parts ..................................................................28
AIR FLOW