make up air unit understand the differences

Exploring Make Up Air Unit Gas Heaters: Understanding the Differences Between Direct and Indirect Fired Systems

Outside air in some climates require heating in the winter. Direct gas-fired and indirect gas-fired heaters are the two most popular means of heating incoming outside air. The purpose of the make up air units is to provide fresh air to buildings to compensate for the exhaust of the kitchen hood. The International Mechanical Code (IMC 508.1.1) requires that the temperature difference between the incoming makeup air and room air does not exceed 10⁰F., except where the added heating and cooling load of the makeup air does not exceed the capacity of the HVAC system.

The main difference between the two gas heating systems lies in how the heating process is accomplished. Here are the differences between direct gas-fired make up air units and indirect gas-fired make up air units:

Direct Gas-Fired Make Up Air

In a direct gas-fired make up air unit, the combustion process occurs inside the unit. The incoming fresh air is drawn and heated directly over the burner flame, which results in a more efficient heating process. The heated discharge from the unit is distributed to the building through a duct system and diffusers. Direct gas-fired systems are often used in facilities that require high air exchange rates, such as commercial kitchens. There are strict regulations on dilution levels for the products of combustion. That is why they are well suited for kitchen exhaust systems. Systems are required to be approved and certified by the American Gas Association (AGA).

Indirect Gas-Fired Make Up Air

In an indirect gas-fired make up air unit, combustion occurs in a separate combustion chamber. The incoming fresh air is heated indirectly by a heat exchanger within the combustion chamber. A burner heats this exchanger, and the air passes over it without the air coming into contact with the flame. The heated air is then distributed to the kitchen through a duct system. The main advantage of indirect gas-fired systems is that they provide a more controlled heating process, which can result in higher indoor air quality.  

In summary, direct gas-fired make up air units provide a more efficient heating process, while indirect gas-fired make up air units provide a more controlled heating process. The choice between the two approaches depends on local regulations, and the facility’s specific needs, including air exchange rates, temperature and humidity control, and indoor air quality requirements.

If you are interested in further information on make up air units, the experts who manufacture these units can guide you on the proper selections needed to meet your requirement.


Our recommended next article would be: How not to ruin a good thing, Make up Air 101

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