Indoor Separated Combustion (DFS)

The indoor separated combustion duct furnace was designed as a building's primary room heating source and is certified for, non residential, commercial applications only. Separated combustion duct furnaces are specifically designed for buildings with hostile atmospheric conditions such as high humidity, contaminated indoor air or rooms with negative pressure. Available in 11 power exhausted model sizes, the unit covers a wide variety of applications. Input ranges from 75,000 to 400,000 btu/hr for either natural or propane gas.

Airflow direction can be specified for location downstream from the dehumidifier and has a drain pan that allows connection to a condensate drain line. There are many other options available including various types of gas valve controls to suit specific requirements.
Unit of Measure

Standard Features

N/A  - 115V control step down transformer with 24V gas controls  - 20 gauge aluminized steel cabinet  - 409 stainless steel drip pan  - 409 stainless steel heat exchanger and burners  - 80% thermally efficient  - C.S.A. certification for use in the US and Canada  - Certified to 3.0" W.C. external static pressure  - Differential pressure switch for proof of combustion air  - ETL certification  - High limit safety control  - Left side (when looking into the discharge) access to burner    and gas controls with slide-out burner drawer  - Natural gas input  - Power exhauster motor  - Power exhauster relay for pre-purge and post-purge  - Separate electrical, venting and gas control access with fully    gasketed doors  - Separate line voltage and low voltage terminal strips  - Side or bottom gas connection access  - Single stage intermittent pilot with continuous retry control    system for operation on natural gas.  - Two-stage gas valve

Optional Features

N/A  - Adjustable differential air flow proving system  - Building management compatible gas controls for    modulation control using a 0-10 VDC or 4-20 mA input  - Control relay - double pole, double throw  - Fan delay timer  - Gas control step down transformers for 208/230V/1Ph and    208/230/460/575V/3 Ph  - High and/or low gas pressure switches  - Manual reset high limit switch  - Mechanical modulation and electronic modulation controls    for natural gas  - Multiple furnace electronic amplifier for controlling up to 4    duct furnaces with one discharge air sensor  - Right side access to burner and gas controls  - Single stage intermittent pilot with retry control system for    operation on propane gas  - Supply air fire stat  - Timed freeze protection  - Two-stage, mechanical modulation, and electronic    modulation controls for propane gas

Gas Connection Pipe Size

N/A 3/4 in

Approximate Unit Net Weight

N/A 275 lb

Approximate Unit Shipping Weight

N/A 385 lb

Btu/Hr Input

N/A 300,000

Btu/Hr Output

N/A 240,000

20°F Air Temperature Rise

N/A 11111 CFM

25°F Air Temperature Rise

N/A 8889 CFM

30°F Air Temperature Rise

N/A 7407 CFM

35°F Air Temperature Rise

N/A 6349 CFM

40°F Air Temperature Rise

N/A 5556 CFM

45°F Air Temperature Rise

N/A 4938 CFM

50°F Air Temperature Rise

N/A 4444 CFM

55°F Air Temperature Rise

N/A 4040 CFM

60°F Air Temperature Rise

N/A 3704 CFM

Air Temperature and External Static Pressure Limits

N/A The maximum allowable discharge air temperature is 150ºF. The maximum allowable air temperature rise for Low Air Temperature Rise Outdoor Units is 60ºF. The maximum allowable air temperature rise for High Air Temperature Rise Outdoor Units is 100ºF. All duct furnaces are designed for a maximum allowable static pressure of 3.0" W.C. on the heat exchanger.


  1. Ratings are shown for elevations up to 2000 feet. For higher elevations, the input rating should be reduced at the rate of 4% per 1000 feet elevation above sea level. For Canada, in elevations between 2000 and 4500 feet, the unit must be derated to 90% of the rating listed above.
  2. Units approved for use in California by CEC.
  3. High air temperature rise separated combustion/outdoor units include an air distribution baffle and restrictor change when compared to the low air temperature rise separated combustion/outdoor units. Field conversion of a high air temperature rise to a low air temperature rise unit (or the opposite) requires a factory supplied conversion kit. Many times a portion of the total discharge air, from the dehumidifier into the gas duct furnace, must be diverted around the duct furnace with a field supplied duct controlled with a hand damper to achieve the desired air flow though the gas duct furnace.

115/60/1 Supply Voltage

N/A 2.40 A

Total Unit Amp Draw

N/A The total unit amp draw is a combination of the control step down transformer and the power exhauster motor. The control step down transformer includes damper actuators, ignition controllers, gas valves, control relays, amplifiers, and motor starters.
Diagram1 N/A
Indoor Gas Duct Furnace Schematic


N/A 35.60 in


N/A 25.85 in


N/A 24.48 in


N/A 37.04 in


N/A 32.61 in


N/A 26.43 in


N/A 19.21 in

H (duct width)

N/A 26.99 in

J (duct height)

N/A 22.90 in
K2 N/A 5.86 in


N/A 6.18 in


N/A 9.60 in
  • 1 Dimensions are subject to change without notice
  • 2 Nominal vent pipe size is 4" (Models 75-175) and 6" (Models 200-400). Exhaust pipe installed over collar. Combustion air pipe installed inside collar.