What’s the Difference Between Direct Fired Heaters and Indirect Fired Heaters?

Understanding the pros and cons of each system will help you choose the best portable or commercial heater for your application.

Whether you’re heating a construction site in winter, a warehouse, an event arena or an office, retail or healthcare space, you have a decision to make when it comes to choosing a heat source: a direct fired or indirect fired heater.

Learning the difference between direct and indirect heating systems will help you choose the right heater for your application. Both have distinct advantages and disadvantages.

Direct fired heaters

In a direct fired heater, also called a direct air heater or direct industrial air heater, fresh air come comes into direct contact with an open flame. The fuel — typically propane or natural gas — goes into the burner and combusts when exposed to a spark and oxygen, similar to how a gas grill works. Direct heating units are commonly used on construction sites and in large warehouses and outdoor arenas, places where there’s plenty of ventilation.


  • Highly efficient: In a direct fired heater, 100% of the fuel burned is used to supply warmth. That efficiency lowers fuel costs. Most models have a built-in thermostat for precise heat output.
  • More power in a smaller unit: A direct fired burner can create the same amount of heat than a larger indirect heater.
  • Portable and convenient: Direct fired heaters are easier to transport than indirect heaters and don’t require setting up ductwork. Some units don’t need electricity to operate, making them ideal for remote worksites.
  • Easy to maintain: Fewer parts mean less opportunity for something to break. Direct fired heaters also have a reputation for easy maintenance and longevity.
  • Less expensive: Direct fired heaters are simpler units and typically cost less than indirect fired heaters.


  • Require ventilation: Direct heating creates carbon monoxide, a colorless, odorless gas that can be fatal if inhaled. Direct fired heaters are used primarily in larger, well-ventilated spaces. They can’t be used with ductwork.
  • Require fresh air: Direct fired heaters need fresh air and good air circulation to function effectively.
  • Create moister air: Most direct fired heaters operate on either natural gas or propane, fuels that produce some water during combustion. This is an important consideration for worksites where minimizing moisture is critical.

Indirect heaters

In an indirect fired heater, the flame is contained in a burn chamber, which warms a heat exchanger. Fresh air is pulled through an intake vent and warmed as it moves across the heat exchanger.

Indirect fired heaters typically run on oil, diesel, natural gas, propane or dual fuel. When used to heat enclosed spaces, the heating unit remains outside and the warm air is pumped inside through ducts.

These heaters are popular for heating outdoor construction sites, hospital buildings and large retail spaces.


  • Creates cleaner air: Combustion in an indirect fired heater happens inside the heat exchanger, and unwanted fumes are exhausted through a chimney or flue. The heated air doesn’t contain carbon monoxide, so these units are safe to use in small spaces with the proper ductwork.
  • Creates drier air: Indirect fired construction heaters deliver less humid air. Excess humidity can create drying and curing problems or, in extreme cases, lead to mold development. Indirect fired heaters can be beneficial for curing concrete in colder temperatures.


  • Less portable and less convenient: An indirect fired heater is larger than a direct heater capable of heating a space of the same size and requires ductwork.
  • Less efficient: Only about 80% of the combustion gases produced are used to heat the air.
  • Less precise temperature control: An indirect heater is generally not as effective as a direct heater at delivering a precise target temperature.
  • More expensive: These heaters typically cost more than direct fired heaters.

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