Technical content reviewed November 12, 2026 · Tillman & Associates
Where these duct heaters fit
The DF/DI catalog describes electric heaters for comfort-heating duties such as makeup-air heating, preheating and terminal reheat. They add heat to an air stream supplied by the air-handling system.
Start with the purpose of heating and the required inlet/outlet conditions. For a replacement, include the existing heater label and duct dimensions; available electrical power and current airflow may differ from the original design.
Flanged versus insert construction
DFF and DFT are flanged arrangements. DIF and DIT are insert arrangements. DFF/DIF use finned tubular elements, while the tubular Incoloy construction is described for the non-finned option.
The attachment method affects how the heater joins the duct and how it can be removed for service. Provide the duct opening, available insertion space, airflow direction and access-panel location rather than selecting from the family letters alone.
Air conditions and element choice
The catalog discusses non-finned Incoloy elements for high humidity or slightly corrosive contaminants, on special order. That does not establish suitability for every wet or chemically contaminated stream. Describe the actual air composition, condensation and carryover.
These comfort-heating families should not be treated as hazardous-location process heaters. If explosive vapors or dust may be present, provide the classification and ask Tillman to review a separately certified heater family.
Airflow, controls and the quote request
The catalog specifies horizontal-duct installation for the finned family, with inlet air no warmer than 77°F (25°C). Required output must also fit the duct area and minimum airflow. State SCFM or actual CFM explicitly and provide operating conditions; departures need factory review.
The catalog lists control options including SCR control, temperature controls and fan/airflow interlocks. Send the required control signal and sequence, electrical service and enclosure conditions. Tillman can coordinate the heater and control scope for review without treating a preliminary request as a completed selection.
What to send Tillman
- Application and required heat output or inlet/outlet temperatures
- Minimum/maximum airflow, units and operating conditions
- Duct dimensions, connection method, orientation and access space
- Voltage, phase, frequency and available control signal
- Humidity, contaminants, condensation and location classification
- Enclosure, interlock, disconnect and quantity requirements
Common questions
What is the difference between DFF and DIF?
DFF is flanged; DIF is an insert arrangement. Both use finned tubular elements.
Does Incoloy construction make a heater explosion-proof?
No. Element material does not establish hazardous-location approval.
Is kW enough information to select a duct heater?
No. Airflow, duct size, temperatures, electrical service and application conditions are also needed.