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 requirement 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.

Indirect Fired Heaters

In an indirect fired heater the burner is fired into a heat exchanger. Air is heated by passing over the heat exchanger, allowing the combustion by-products to remain within the heat exchanger which is then exhausted through a flue. An everyday example of an indirect fired heater would be a gas furnace with a chimney. Titan Air offers indirect fired make-up air and indirect fired air turnover equipment. Cooling and energy recovery functions may also be added to this equipment.

Indirect Fired Benefits

• No products of combustion are introduced into the workspace – As opposed to direct fired heaters, indirect fired heaters are the preferred choice in areas such as office spaces, event arenas, specific manufacturing facilities, schools and pharmaceutical applications.
• Reheating Ability – Unlike direct fired heaters, 100% of the air may be recirculated through an indirect fired heater as no combustion by-products are introduced into the airstream by the heating process.
• Greater Flexibility with Cooling Coils – As the air being heated does not directly pass through an open flame, an indirect fired burner can be used downstream of a DX cooling coil. There is no risk of any leaking refrigerant passing through an open flame and creating a toxic gas.

Indirect Fired Disadvantages

• Lower Efficiency – Due to heat loss through the flue and inefficiencies in the heat exchanger, indirect fired heaters are approximately 80% efficient. This results in higher operating costs compared to a direct fired heater.
• Cost – Higher equipment costs due to the inclusion of the heat exchanger and higher price of the air handler compared to direct fired heaters.
• Complexity – Due to the addition of a heat exchanger, flue and condensate drain, installation and maintenance are more time consuming than a direct fired heater with the same heating potential.
• Larger Equipment Size – Due to the heat exchanger required, equipment size is larger than a direct fired unit with the same BTU output.
• Lower Flexibility – As heat exchangers are available from manufacturers in only specific sizes you will have to choose the “best fit” in relation to your BTU requirements.
  • Indirect fired heaters also have lower turndown ratios than direct fired heaters, resulting in less ability to vary the heat output, unless additional heat exchangers are added to improve turndown.
  • Temperature control of an indirect fired heater is not as precise as a direct fired heater as it often “under or overshoots” the desired temperature by a few degrees. This is due to the heater reaching a set cutoff temperature and then shutting down the burner until it reaches a minimum temperature before firing the burner again. This may be unacceptable for applications requiring tight temperature ranges.

This general overview should give you a basic understanding of the similarities and differences of these two types of heaters. There are several other factors to consider in choosing the best heating equipment for your project. It is wise to speak with an experienced, knowledgeable professional to ensure you are making the best possible choice.