With over 100 years of experience designing and manufacturing custom industrial process air heaters, Stelter & Brinck knows that choosing between direct fired and indirect fired air heaters is important to your operation’s efficiency, product quality, and bottom line.
Whether you’re drying grain, curing composites, processing food, or heating sensitive chemicals, the right heater delivers reliable heat exactly when you need it.
How Direct Fired Air Heaters Work
In a direct fired air heater, the burner fires straight into the process air stream. The combustion gases mix directly with the air you’re heating. This delivers maximum heat transfer with virtually no waste.
How does a Direct Fired Air Heater Work?
Key benefits of direct fired heaters (Stelter & Brinck models):
- Highest combustion efficiency.
- Lower purchase price and operating costs.
- Compact designs: packaged units, duct-insert (AHDX/AHDR), and gun-style for high mass flows or fuel oil.
- Low-NOx and ultra-low-NOx options available.
- Ideal for rugged industrial duty.
Best for applications where combustion by-products won’t affect the end product:
Clay/sand drying, ceramics, laminate/composite curing, coating/print drying, cement production, fertilizer.
How Indirect Fired Air Heaters Work
In an indirect fired air heater, the burner fires into a separate combustion chamber and heat exchanger. Process air never touches the flame or exhaust gases—it’s heated as it passes over the exchanger surfaces. Clean, contaminate-free hot air is the result.
How does an Indirect Fired Air Heater Work?
Key benefits of indirect fired heaters (Stelter & Brinck models):
- Clean process air – perfect for chemically sensitive products.
- Up to 90% thermal efficiency.
- Recirculating, non-recirculating, straight-through, and USDA Dairy Compliant (IAH-R-USDA) designs.
- Stainless steel options and high-pressure configurations.
- Outlet temperatures to 900°F standard (higher on request).
- Low-emissions models available.
Best for applications for an Indirect Fired Air Heater:
food/pharma/grain drying, dairy processing, chemical production, tobacco, hydrated lime, print drying, and any process that demands zero contamination.
Chart of Indirect vs Direct Fired Air Heater Features
| Feature | Direct Fired | Indirect Fired |
| Combustion Efficiency | Nearly 100% | 80–90% (S&B up to 90%) |
| Operating Cost | Lower | Higher (due to heat exchanger) |
| Initial Purchase Cost | Lower | Higher |
| Air Quality | Contains combustion by-products (low-emissions possible) | Completely clean & separate |
| Max Outlet Temp (standard) | High (2000°F+) | Up to 1200°F+ |
| Maintenance | Lower | Slightly higher (exchanger) |
Quick Decision Guide – Which Heater is Right for Your Application?
Choose Direct Fired if:
- Your process tolerates combustion gases (most mineral, metal, and composite processes).
- You want maximum efficiency and lowest operating cost.
- You need a simple, rugged, easy-to-install solution.
Choose Indirect Fired if:
- Product purity is critical (food, pharma, chemicals, grain, dairy).
- You cannot allow any contamination or odors.
- You need USDA-compliant or stainless-steel construction.
Still unsure? That’s exactly why Stelter & Brinck exists. Our engineers review your airflow, temperature rise, fuel type, space constraints, and emissions requirements to custom-build the perfect heater—direct or indirect.
Ready to Heat Smarter?
Contact the Stelter & Brinck team today at (513) 367-9300 or visit stelterbrinck.com/contact-us. We’ll help you compare options side-by-side with real data for your exact process—no obligation.
Questions? Comment below or reach out—we answer every inquiry personally.