Home > Information > Direct Fired Vs. Indirect Fired

Direct Fired Heaters

Propane direct fired heaters have 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.

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 package compared to an indirect fired burner. This results in an overall smaller equipment footprint in most cases.

  • Flexibility – 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, 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

Combustion By-Products – 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.

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.

Indirect Fired Benefits

  • No products of combustion (Carbon Monoxide) are introduced into the workspace

  • Air recirculation – 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.

    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 – 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.