When designing a direct-fired make-up air system, burner selection is only one part of the equation. Airflow arrangement can have an equally important impact on combustion performance, temperature uniformity, equipment layout, and overall system operation.

One of the key decisions OEM engineers and equipment manufacturers need to make is whether to use a push-through or pull-through configuration. Both arrangements can perform effectively when designed correctly, but each creates different airflow and mixing characteristics.

Understanding these differences can help engineers select the right configuration for rooftop make-up air units, process heating equipment, spray booths, and other commercial and industrial applications.

What Is a Pull-Through Make-Up Air System?

In a pull-through arrangement, the burner is positioned upstream of the blower. Outdoor or process air passes across the burner, is heated, and then travels through the blower before being discharged from the equipment.

One of the primary advantages of a pull-through configuration is improved air mixing.

Because heated air passes through the blower, the fan can help mix the air and reduce temperature stratification before it leaves the unit. This can help provide a more uniform discharge temperature.

For this reason, pull-through designs are commonly found in rooftop make-up air equipment and other applications where consistent supply air temperature is important.

What Are the Advantages of Pull-Through Design?

Temperature uniformity is one of the biggest reasons OEMs may consider a pull-through configuration.

Air leaving a direct-fired burner may contain temperature variations across the airflow path. Passing that heated air through the blower provides another opportunity for mixing before the air reaches the building or process.

Pull-through designs can therefore reduce the amount of downstream space required specifically for mixing.

Engineers also need to account for the fact that heated air is less dense than colder incoming air. This change in air density can influence blower operation and should be considered as part of the complete system design.

What Is a Push-Through Make-Up Air System?

A push-through arrangement reverses the component order.

The blower is positioned upstream of the burner, pushing air through the profile and across the burner before the heated air exits the equipment.

Push-through configurations are commonly used in certain process heating applications, including spray booth systems and other equipment where the physical layout or process requirements make upstream blower placement practical.

Because the blower is located before the burner, it isn’t available downstream to provide additional mixing after combustion.

As a result, temperature stratification may require additional consideration.

Why Might a Push-Through System Need a Mixing Plenum?

Direct-fired burners introduce heat directly into the moving airstream. Depending on airflow conditions and equipment geometry, the air temperature immediately downstream of the burner may not be completely uniform.

In a pull-through system, the downstream blower can help mix this air.

A push-through system doesn’t have that advantage. For applications requiring a highly uniform discharge temperature, engineers may need to provide sufficient downstream mixing distance or incorporate a mixing plenum into the equipment design.

This doesn’t make push-through systems less effective. It simply means mixing needs to be considered differently during the design process.

How Does Blower Placement Affect Burner Performance?

Whether the blower is upstream or downstream, airflow across the burner needs to remain within the operating conditions specified for the burner.

Burner performance depends heavily on the relationship between airflow, differential pressure, profile design, gas input, and burner configuration.

Poor airflow distribution can lead to unstable flame characteristics, inconsistent combustion, temperature stratification, or increased emissions.

This is why the burner should never be selected independently of the equipment’s airflow path.

Why Is Burner Profile Design Important?

The profile opening helps establish the airflow conditions across a direct-fired burner.

Midco International’s HMA burner documentation provides guidance for burner placement, profile sizing, differential pressure, airflow, and required clearances.

The burner should be properly positioned within the profile, and the opening needs to be sized for the intended airflow and burner operating conditions.

Improper profile geometry can change the way air moves across the burner and negatively affect combustion performance.

For OEM engineers, profile design should therefore be considered part of the burner system rather than simply part of the cabinet.

Maintain Uniform Airflow Across the Burner

Direct-fired burners perform best when airflow is relatively uniform across the burner assembly.

Turbulence, obstructions, poorly positioned components, abrupt transitions, or uneven airflow entering the profile can affect burner operation.

Engineers should evaluate what happens to the air before it reaches the burner, across the burner itself, and after combustion.

The goal is to create predictable airflow conditions that support stable combustion throughout the required operating range.

Differential Pressure Must Be Measured Correctly

Differential pressure across the burner is another important part of direct-fired system setup.

Static pressure probes need to be positioned appropriately so measurements accurately represent the pressure relationship across the burner and profile.

Incorrect probe placement can result in misleading readings, making burner setup and troubleshooting more difficult.

Proper measurement allows engineers and technicians to confirm that airflow conditions match the requirements established for the burner configuration.

Push-Through vs. Pull-Through: Which Is Better?

There isn’t one configuration that is best for every direct-fired make-up air application.

Pull-through systems can be a strong choice when temperature uniformity and downstream mixing are priorities. This helps explain their use in many rooftop and commercial make-up air applications.

Push-through systems can be well suited to process heating equipment and applications where cabinet layout or operating requirements favor placing the blower upstream of the burner.

The better configuration depends on factors such as equipment layout, airflow, temperature uniformity requirements, available mixing space, process requirements, and overall system design.

Design the Burner, Blower, and Profile as One System

One of the most important principles in direct-fired make-up air design is that the burner doesn’t operate independently.

Burner selection, blower placement, airflow, profile geometry, differential pressure, gas input, mixing space, and equipment dimensions all influence performance.

Evaluating these components as a complete system helps OEMs design equipment for stable combustion, consistent temperature control, and dependable field operation.

Direct-Fired Make-Up Air Burners From Midco International

Midco International provides burner solutions for OEMs and equipment manufacturers designing commercial and industrial direct-fired heating systems.

Midco’s HMA burner line provides flexibility for direct-fired make-up air applications, including systems using push-through and pull-through airflow arrangements when properly designed and configured.

Choosing between push-through and pull-through shouldn’t be based solely on cabinet convenience. Engineers should evaluate airflow behavior, mixing requirements, burner profile design, pressure conditions, and the needs of the final application.

When the burner, blower, profile, and airflow path are engineered together, OEMs can create direct-fired systems designed for reliable combustion, consistent temperature performance, and real-world operating conditions.