Direct Gas-Fired Make-Up Air Unit
Direct gas-fired make up air units consist of burners that are nearly 100% efficient (source) and are suited for applications that demand 100% outside air. With this unit, the need for a heat exchanger is eliminated due to the burner directly heating outside air being streamed in.
These units are configured for propane and gas applications and suitable for both outdoor and indoor installation. Maintenance is significantly reduced in the field due to the burner construction, which consists of cast aluminum manifold with stainless steel mixing.
They provide filtered and heated make-up air, as required by the application, to maintain balanced air pressure. They are applicable to work for constant volume or in tandem with other ventilation equipment for a wide range of heating/cooling outputs and volumes.
This article will go over generalities of direct gas fired air units, its components, benefits, drawbacks and typical applications.
If you have further questions regarding any type of make up air unit you can contact a reputable engineer to assist you.
Direct Gas-Fired Make-up Air Unit Diagram
Table of Contents
When fresh outdoor air needs to be circulated into a building for ventilation purposes then a make up air unit is required. The object is to maintain ambient air quality and balance indoor air pressure that meets building code while keeping occupants safe.
The main factor that differentiates a direct gas fired make up air unit from other heating MAU is that it directly heats outside air by the combustion of natural gas or propane with an air burner.
The byproducts of combustion, while safe, remain in the heated air. This is why these types of units are ideal for industrial and commercial applications.
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How Direct Gas Fired MAU heats 100% Outside Air
When outside air enters the unit it is fed through the filter and then the damper. A gas burner is installed directly in the air stream, which is made of 100% outside air. The combustion products are released into the air stream and pushed into the structure.
Advantages of Direct Gas-Fired Make-Up Unit
Direct-Fired Make Up Air Unit Components
Although MUA units appear simple, the control system is sophisticated. Unit sizes typically start at 1,500 cfm (cubic feet per minute) and up to 120,000 cfm.
Make up air unit components include:
Air Intake Hood - Damper
Intake hoods are necessary due to weather concerns. Bird screens should also be provided inside the hood before the damper.
Dampers serve several purposes. In the cold months when the unit is not in operation dampers prevent warm air from leaking out. They also prevent cold air from entering a warm structure.
The filter cabinet consists of typical 30/30 filters. In order to increase filter area as well as reduce static pressure drop a V-bank configuration is implemented.
A burner is typically an aluminum or cast iron pipe. It is recommended that the burner be placed in the outside air stream in order to impede potentially hazardous combustion byproducts located inside the structure.
Centrifugal fans are commonly using double width/double inlet (DWDI) with backward incline or forward-curved configuration. The fans and bearings are rated lower due to temperatures being less than 100°F.
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