Direct Fired vs. Indirect Fired Heaters

In planning a new HVAC project, one of the main criteria after determining your specific heating requirements is determining your heat source. The two most common sources are direct and indirect fired heaters.

Direct fired heaters are both more efficient and less expensive than indirect fired heaters. However, there are some cases where an indirect fired heater is the better choice. Having a thorough understanding of all of the differences between these two heater types will ensure you choose the optimal solution for each project.

Direct Fired Heaters

A natural gas or propane direct fired heater has an open flame that provides a safe way to heat industrial and commercial areas by maintaining a proper air-to-fuel ratio. In a direct fired heater, the gas is fed directly to the burner while the airstream provides the needed oxygen for combustion. Air is forced through the burner baffle where it mixes with the gas. The burner is installed to fire with, and parallel to, the airflow. The direct fired air handlers that Titan Air manufactures can also have cooling and energy recovery functions added to the equipment.

Direct Fired Benefits

• Efficiency – As nearly 100% of the fuel is being converted to heat, fuel consumption and operating costs are reduced. These heaters are 100% combustion efficient with an overall thermal efficiency of 92% (8% heat loss due to water formation during combustion).
• Smaller Size – A direct fired burner can produce more heat in a smaller envelope compared to an indirect fired burner. This results in an overall smaller equipment footprint in most cases.

• Flexibility – Direct fired burners can be constructed to essentially any required size in order to achieve the required BTU rating, so you will not have to choose between either an undersized or oversized heater.

-Additionally, direct fired heaters have much higher turndown ratios than indirect fired heaters, resulting in a greater ability to vary the heat output.

• Simplicity – As a heat exchanger and flue are not required, installation is made easier and maintenance costs will be decreased.

• Equipment life expectancy is longer than indirect fired heaters.

• Temperature control is more precise compared to indirect fired heaters. 

Direct Fired Disadvantages

• Ventilation Requirements – Although these heaters operate within code prescribed safety limits, due to combustion products entering the airstream, proper ventilation is required to avoid buildup of gases such as carbon monoxide.

• Combustion By-Products – Depending on a building’s intended use, direct fired heaters may not be allowed.

• Reheating Concerns – Due to the fresh air required to maintain proper combustion, if building air is to be recirculated, a minimum of 20% outside air must be introduced.

• Less Flexibility with Cooling Coils – If the refrigerant used in a DX (direct expansion) cooling coil leaks and then passes through an open flame, a toxic gas may be created. Because of this, a direct fired burner cannot be used downstream of a DX coil.

Published by: Titan Air Blog