Blog – Construction Drying

What’s the Difference: Indirect Fired Heat VS Direct Fired Heat

Keeping construction sites dry and warm is not just a convenience. It’s a necessity to maintain project deadlines, prevent mold growth and promote worker productivity. Among the solutions available for construction drying services are indirect-fired heat and direct-fired heat. Knowing the difference will help you determine the best one for your application.

Indirect-Fired Heat

Indirect-fired heaters are similar to oil- or gas-burning furnaces with chimneys in homes. In such units, a flame in a burn chamber warms the heat exchanger. The device draws cool air into two different chambers. The air passes over and around the heat exchanger and gets warm. The air that the heater discharges is clean, making this type of heater good for applications that require sanitary conditions, such as health care facilities, food processing plants, and areas with chemically sensitive products.


  • The air emitted is 100 percent clean and dry.
  • The air circulating in the unit does not contact the flame.
  • The heater re-circulates air, which saves fuel.
  • You may use ductwork to deliver hot air into different areas of a project site without the risk of fuel-hazardous byproducts, such as carbon dioxide.
  • Because the heaters don’t release harmful gases, you can use them in tightly sealed spaces.
  • Units have built-in thermostats for temperature control.
  • The heaters are good for frost protection and accelerating drying processes.


  • Indirect-fired heat involves multiple working parts, making it less efficient than direct-fired heaters.
  • Rental prices tend to be more expensive compared to direct-fired heater rental costs.
  • Indirect-fired heaters use about 80 percent of the combustion gases to heat air.
  • The heaters are larger than direct-fired heaters, making them more difficult to transport.
  • The heater uses oil or gas, as well as electricity.
  • While the heater produces clean air, it requires exhaust ventilation using a flexible tube that vents the fumes outside.

Direct-Fired Heat

Direct-fired heaters are similar to barbeque grills or gas stovetops, as they heat the air that passes through an open flame. Internal components within the heater control the warmed air, making it an efficient heating solution. This type of heating application is good for temporary applications, for areas with limited insulation, or those that need large volumes of warm air, such as construction sites or warehouses.


  • Direct-fired heaters use 100 percent of the combustion gases to heat air, making them more efficient than indirect-fired heaters. 
  • Some heaters can operate in areas with temperatures as low as -30°F.
  • You may use ducts to carry hot air throughout a site or facility without worrying about fumes or exhaust.
  • This type of heater is generally less expensive to purchase or rent than indirect-fired heaters. They are also less expensive to maintain.
  • The heaters are made of simpler components that are easier to transport.
  • Small heaters produce high amounts of heat (BTUs).
  • Some direct-fired heaters do not need electricity to operate.
  • Most models have built-in thermostats.
  • The heaters feature redundant safety features that allow you to operate them with minimal supervision.


  • Direct-fired heaters are not appropriate for use in tightly sealed spaces or near flammable materials because of the exhaust and fumes produced. The heated space must have some type of air exchange or an exterior window or door that you can open.
  • The warm air produced is not always dry.
  • You should not use direct-fired heat in spaces where materials will be affected by combustion byproducts.

When it comes to construction drying services, the manner in which you heat or dry space matters. Using the right equipment protects construction workers and their future inhabitants prevent project delays and promotes a building’s health. To learn which is best for your site, get in touch with Polygon to schedule a consultation.

[Dean Jarvey via CC License 2.0]

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