Direct Fired vs. Indirect Fired Heaters

Image displaying direct and indirect heaters with "vs" betweenConstruction heaters have many different fuel sources and designs to better serve the general contractors across the country. CHI Companies carries natural gas/propane heaters, water/steam heaters, electric heaters, boiler ground thaw heaters, and diesel heaters. The two major different designs of natural gas/propane heaters are categorized as direct-fired and indirect-fired.Direct-Fired Heater: A direct-fired heater has the airflow go directly over the flame. Direct-fired heaters are usually smaller in size and can be placed either indoors or outdoors.Indirect-Fired Heater: An indirect-fired heater has an heat exchanger that air passes over. Indirect-fired heaters are larger in size and will always be placed outdoor. The combustion byproducts are expelled through a stack on top of the unit. On average. 20% of the heat generated by an indirect-heater is expelled up the stack with the combustion byproducts. Example: An indirect-fired heater classified as 1,000,000 BTUH will only provide 800,000 BTUH into the project area.Operating Costs: Direct-fired heaters are approximately half the daily rental cost of an indirect-fired heater. Fuel consumption of a direct-fired heater is identical to an indirect-fired heater, but the indirect-fired heater provides less heat. Therefore, the fuel operating expense is 20% higher with an indirect-fired heater.Portability: Direct-fired heaters can be placed anywhere within a building creating even temperatures throughout the project. Indirect-fired heaters need to be placed outdoors to expel the combustion byproducts and will need ductwork to move the heat throughout the project.Moisture: Direct-fired heaters will increase the moisture content into a project. On average, a direct-fired heater will increase relative humidity by 10% without drawing outside air and 3% while drawing outside air. An indirect-heater will not increase the relative humidity of the building. All moisture is expelled out the combustion stack.Carbon Monoxide: Direct-fired heaters will introduce less than 1 ppm of carbon monoxide into a building, while an indirect-fired heater will not introduce any.
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