Direct Versus Indirect Gas-Fired Make-Up Air Units

Direct Versus Indirect Gas-Fired Make-Up Air Units

Gas-fired make-up air units provide fresh outdoor air to your building to replace air that has been exhausted out through your air system. Depending on the needs of your facility, you may wish to install a direct gas-fired unit or an indirect one. Here is a look at the differences between these units to give you an idea of which one may be the best option for your project.

Direct Gas-Fired Make-Up Air Units

Direct gas-fired units recirculate the exhaust of its gas burner through the make-up air stream, distributing it through the building. Direct gas-fired make-up air units are ideal for facilities that do not require less than 20 percent make-up air or are not concerned about potential space contamination.

Direct gas-fired units provide higher energy efficiency than indirect units as well as steam heating coils, heat pumps, high-efficiency hot-water boilers, and even electrical-resistance heaters. They are more cost-effective to install than their indirect counterparts and offer better freeze protection. As water vapor is a byproduct of combustion, direct gas-fired units can provide humidification, too.

Indirect Gas-Fired Make-Up Air Units

Unlike direct gas-fired units, indirect gas-fired make-up air units have sealed combustion chambers that exhaust out through a flue to the outside environment. Therefore, none of the exhaust is discharged into the make-up air supply of the building. This makes indirect units a good option for facilities that cannot tolerate more than 20 percent make-up air.

Indirect gas-fired units offer higher control of the quality of the air it moves through a facility. As its exhaust is discharged outside the facility, it does not add humidity to the air from water vapor, nor does it add carbon monoxide or carbon dioxide. This makes indirect gas-fired make-up units better options for buildings where people will be sleeping or where air quality is a top priority.

Ideal Applications for Direct and Indirect Gas-Fired Make-Up Air Units

Well-designed and fabricated direct gas-fired make-up air units are good options for buildings that have high airflows, large open spaces, and no humidity concerns. Common applications include airports, commercial kitchens, auditoriums, and laboratories.

Indirect units are better options for buildings where air quality and human comfort are concerns or facilities with small rooms with low airflow. Common applications include hospitals, hotels, and schools.

TMS Johnson partners with the most reputable make-up air unit manufacturers in the industry to provide our customers with innovative, high-quality systems that will serve their facilities for years to come. Contact us today to learn more about these technologies and which option is right for your project.


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