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Direct Gas-Fired Make-Up Air Unit

Direct gas-fired make-up air units consist of nearly 100% efficient burners suited for applications that demand 100% outside air.

This unit eliminates the need for a heat exchanger due to the burner directly heating outside air being streamed in. They are configured for propane and gas applications and are suitable for outdoor and indoor installations.

Maintenance is significantly reduced due to the burner construction, which consists of cast aluminum manifolds with stainless steel mixing.

As the application requires, they provide filtered and heated make-up air 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 review the generalities of direct gas-fired air units, their components, benefits, drawbacks, and typical applications.

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Key Takeaways

Direct Gas-Fired Make-up Air Unit Diagram

direct gas-fired make-up air unit diagram


Table of Contents


When fresh outdoor air needs to be circulated into a building for ventilation, a make-up air unit is required.

The object is to maintain ambient air quality and balance indoor air pressure that meets the 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.

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.

Authorized MUA Dealers

We are the authorized distributors for the leading American manufacturers of makeup air units. We’ll be pleased to provide the ideal make up air unit for your application as authorized representatives.

For a fast quote or if you have inquiries about commercial or industrial MUA, get in touch with us.

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:


Make up air unit housing is constructed of aluminum or galvanized steel.

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. When the unit is not in operation in the cold months, 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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