Comparing Direct Fired and Indirect Fired Make Up Air Systems: Understanding the Differences and Benefits
May 19, 2023
Fresh air is vital for maintaining a comfortable and healthy indoor environment. When it comes to make up air systems, two popular options emerge: direct fired and indirect fired systems. Both systems are used to provide fresh tempered outdoor air to a building to replace the air that is exhausted by the kitchen hoods. However, there are some key differences between the two types of systems that may make one more suitable for a particular application than the other.
Direct Fired Systems
Direct fired make up air systems use a gas burner to produce an open flame into the air stream to temper the incoming air before it enters the kitchen.
Advantages:
1. Higher efficiency: Direct fired systems have higher thermal efficiency, and provide more heat to a building with the same amount of fuel, since there is no heat transfer loss to a heat exchanger.
2. Lower capital cost: These systems typically have lower initial costs because they don't require a separate heat exchanger.
3. Simpler design and installation: Direct fired systems are easier to design and install as they don't need a separate heat exchanger or associated ductwork.
4. Faster response time: They have a faster response time since there is no delay associated with heat transfer through a heat exchanger.
Disadvantages:
1. Emissions: Direct fired systems can produce emissions such as nitrogen oxides (NOx) and carbon monoxide (CO), which may be regulated in certain areas.
2. Air quality: These systems can introduce combustion by-products, including carbon dioxide (CO2), water vapor, and combustion particulates into the building, potentially impacting indoor air quality.
Indirect Fired systems
Indirect fired make up air systems use a heat exchanger to transfer heat from a separate combustion chamber to the air stream to help temper the incoming fresh air before it enters the kitchen.
Advantages:
1. Emission-free indoor air: Indirect fired systems don't introduce combustion by-products into the building, ensuring cleaner indoor air quality.
2. No combustion-related emissions: They don't produce emissions like NOx or CO within the building.
Disadvantages:
1. Lower efficiency: Indirect fired systems have slightly lower thermal efficiency due to heat transfer losses in the heat exchanger.
2. Higher capital cost: Due to the separate heat exchanger required in an indirect system, the initial costs are generally higher when compared to direct fired systems.
In summary, direct fired make up air systems offer higher efficiency, lower capital cost, simpler design, and faster response time, but they can produce emissions and can affect indoor air quality. Indirect fired make up air systems provide emission-free indoor air but have lower efficiency, and higher capital costs. While both models offer different pros and cons, when deciding which system works best for your project, more often than not it will ultimately come down to your specific application needs as well as your local regulations regarding emissions and indoor air quality.