The Difference between Direct and Indirect Fired Heaters

25 Nov
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Figuring out what kind of heat source you are going to use is the first step in planning out any Heating, Ventilation and Air Conditioning project (HVAC). The two primary types are direct and indirect fired heaters.

Direct Fired Heaters

A direct fired heater is called such because the heating fuel is funneled directly into the burner and combusts when combined with a spark and oxygen. Direct heaters are usually powered by natural gas or propane. The heat source is burned in an open flame but in a safe and effective manner. Heaters feature systems to channel the air-fuel mixture to ensure correct amounts of both. Direct fired heaters can heat huge industrial and commercial spaces.

Benefits of Direct Fired Heaters

Direct fired heaters have several benefits which make them very popular. One of the greatest advantages of using a direct fired heater is efficiency. Nearly 100 percent of the fuel is burnt in a way that is productive in creating heat. This means that you are not wasting fuel by having it not burn or not burn correctly. You can also control the temperature to a greater degree than with indirect heaters

Another benefit is easy maintenance, as these heaters feature a simple design that is less prone to breakages. Any error or breakdown can be resolved quickly, cheaply and without an extraordinary amount of skill or labor. Compared to indirect heaters, direct fired heaters tend to last longer, meaning you will have a reliable heating system for years to come.

Direct fired heaters also can be constructed to whatever size you will require in order to create the heat that you need. In many cases, this means a physically smaller heater that can better suit your specific needs to a greater degree. You can build the exact heater that is right for your specific purposes.

The downside to Direct Fired Heaters

Unfortunately, direct heaters are not perfect. Because they burn fuel directly in the form of an open flame, direct fired heaters require a proper ventilation system to dispose of exhaust fumes. The resulting gas from the combustion of fuel is toxic and needs to be redirected out of the workspace the heater is being used in.

This also means that certain workspaces will not be able to use direct fired heaters. The exhaust and byproducts may not be suitable or allowed in certain environments and workplaces. Be sure to make sure you can operate a direct fired heater on whatever premises you intend to use it on before making your decision.

Not only must exhaust be removed from the workspace, but outside air is required for the air-fuel mixture. Without proper air circulation within an enclosed space, fresh air often needs to be pumped inside. 

While direct fired heaters may be more versatile in their heating capabilities, they cannot be used everywhere. 

Indirect Fired Heaters

An indirect fired heater performs its combustion cycle in what is called a heat exchanger. As the heat exchanger grows hot, the air is drawn in around it and then warmed by coming in contact with the heat exchanger. The combustion byproducts remain in the heat exchanger for some time before leaving through a flue.

Benefits of an Indirect Fired Heater

Indirect fired heaters have several advantages that make them ideal for certain situations. The largest benefit is that they do not produce any combustion byproducts that are released back into the space they are heating. This makes them ideal for closed spaces and when working with material that is sensitive to contamination, like food. 

By recirculating air, indirect fired heaters save on fuel and do not usually require as much when compared to direct fired heaters. 

At no point does the air that is being heated actually make contact with the flame. While direct fired heaters are still incredibly safe, this can be seen as an additional safety feature that indirect fired heaters have. All of the air that is emitted from the heater is also fully clean. 

Disadvantages to Indirect Fired Heaters

While they might not use as much fuel, indirect fired heaters are not as efficient as direct fired heaters. They usually only operate at about 80 percent efficiency, meaning a good deal of the fuel that is burned does not go directly towards providing heat.

Indirect fired heaters are oftentimes more expensive than direct fired heaters because of their more complicated design. Not only does this raise initial costs, but maintenance will be much higher should anything breakdown. 

Because they are more complicated, getting the right heater for your needs may mean that the right-sized indirect fired heater will take up more space than its direct fired counterpart that produces the same amount of heat. They have specific sizes that cannot be as easily modified as direct fired heaters.

Another disadvantage of indirect fired heaters is that they do not feature as precise temperature control settings when compared with direct fired heaters. While still equipped with thermostats, these heaters tend to generate heat within a few degrees of the intended target temperature. If you need precise and constant heat, an indirect fired heater will not be suitable.


Both of these types of heaters can do a fantastic job of heating whatever sized space you need them to heat. Direct fired heaters feature a fluidity that makes them perfectly adaptable to your specific environment. Indirect fired heaters come in a variety of sizes, but those sizes are set by the manufacturer and cannot be tailored to your needs.

Because an indirect heater does not produce or secrete any type of exhaust back into the space in which they operate, they can be used in tightly sealed environments. A proper ventilation system is an absolute necessity for any direct fired heating system unless it is operating outdoors.

Whichever heating system is right for you and your business, Preferred Climate Solutions has you covered. We offer a variety of heaters and can work with you to better understand your HVAC needs. Give us a call today at 888-466-7180, and we will help you figure out exactly what kind of heating you require. We serve the entirety of Texas and offer 24/7 customer support. Take care of your business environment, and it will take care of your business. 

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