Sizing a direct-fired make-up air burner can seem complicated, but the process becomes much easier when the right variables are evaluated together. Burner sizing isn’t simply about choosing a burner that can produce enough BTUs. Airflow, temperature rise, burner length, firing rate, pressure drop, profile design, and equipment configuration all influence the final selection.
For OEM engineers and equipment designers, understanding how these factors work together can help create make-up air systems that deliver reliable combustion performance across changing operating conditions.
Midco International’s HMA burner line provides OEMs with flexibility for designing direct-fired make-up air equipment around specific airflow, capacity, and physical configuration requirements.
What Determines the Size of a Direct-Fired Make-Up Air Burner?
The starting point for burner sizing is determining the required heating load.
In a make-up air system, required heat input is primarily influenced by two factors: the amount of air moving through the system and the required temperature rise.
For example, a system bringing large volumes of cold outdoor air into a building requires significantly more heating capacity than a lower-airflow system operating under milder conditions.
Once airflow and temperature rise are established, engineers can estimate the required BTU-per-hour input and begin evaluating an appropriate burner configuration.
Start With Airflow and Temperature Rise
Airflow is typically expressed in standard cubic feet per minute (SCFM), while temperature rise represents the difference between entering and desired leaving air temperature.
Midco’s HMA documentation provides a calculation using blower SCFM, desired temperature rise, and an efficiency factor to estimate the required BTU-per-hour burner input.
This provides an important starting point, but it doesn’t complete the burner selection process.
The required heating capacity must then be matched with an appropriate burner length and firing rate.
How Does Burner Length Affect Capacity?
Direct-fired burners can provide different firing rates per foot of burner length.
This means engineers may sometimes have more than one burner configuration capable of providing the required total heat input.
A shorter burner operating at a higher firing rate may theoretically produce similar capacity to a longer burner operating at a lower firing rate. However, those configurations aren’t necessarily interchangeable.
Firing rate per foot can influence pressure requirements, airflow conditions, flame characteristics, emissions performance, and profile design.
Instead of simply choosing the shortest burner capable of producing the required BTUs, engineers should evaluate how the burner will perform as part of the complete make-up air system.
Why Firing Rate Per Foot Matters
The firing rate per foot describes how much heat input is being produced by each foot of burner.
Selecting the appropriate rate requires more than a capacity calculation.
Air velocity across the burner, differential pressure, desired combustion characteristics, available space, and equipment design all need to be considered.
This is one reason modular burner systems can provide valuable flexibility for OEM equipment manufacturers.
Midco’s HMA 2 and HMA 2A burners use modular sections that can be arranged in multiple configurations, allowing equipment designers to develop burner layouts around the capacity and physical requirements of the application.
Profile Design Is Part of Burner Sizing
Once burner length and configuration have been determined, profile design becomes another critical consideration.
The burner area and net profile opening must work together to create the airflow conditions required for proper burner performance.
Midco’s HMA technical information provides profile setup guidance that connects differential pressure, airflow velocity, and opening size.
This relationship matters because the way air moves across a direct-fired burner directly affects combustion.
A profile opening that isn’t properly designed can influence flame shape, combustion quality, emissions performance, and burner stability.
For this reason, profile design shouldn’t be treated as a separate sheet-metal decision. It is part of the overall combustion system.
Pressure Drop Matters
Differential pressure across the burner is another important factor in direct-fired burner selection and setup.
The target pressure relationship helps establish the airflow conditions required for stable combustion.
Changing burner length, profile opening, or airflow can affect this relationship. That means pressure drop should be considered during the initial design process rather than after the equipment has already been built.
Following the manufacturer’s technical data for the selected burner configuration helps engineers establish appropriate operating conditions.
Pull-Through vs. Push-Through Systems
Equipment configuration can also influence burner sizing and integration.
In a pull-through system, the blower is located downstream of the burner. As air is heated, it expands before reaching the blower, which needs to be considered when evaluating system airflow.
Push-through systems place the blower upstream and can introduce different considerations related to airflow distribution, mixing, and temperature uniformity.
Understanding the complete airflow path is important when determining burner configuration and profile design.
Don’t Overlook Gas Inlet Requirements
Gas supply and burner inlet configuration must also support the required firing rate.
As burner assemblies become longer or capacities increase, gas inlet requirements may change. Certain applications may require different inlet arrangements to provide proper gas distribution throughout the burner assembly.
Engineers should review burner documentation for the selected configuration rather than assuming the same inlet arrangement will work for every burner length.
Physical Space Still Matters
A burner can be correct mathematically and still be wrong for the equipment.
Cabinet dimensions, burner orientation, available profile area, access for maintenance, gas train location, ignition components, flame supervision, and other physical requirements need to be considered during equipment design.
This is where modular direct-fired burner designs can provide an advantage.
The ability to configure burner sections around equipment constraints gives OEM engineers more options when balancing heating capacity with available cabinet space.
A Better Approach to Direct-Fired Burner Sizing
Instead of asking only, “How many BTUs do I need?” engineers should look at burner sizing as a complete system.
The process should consider:
- Required airflow in SCFM
- Desired temperature rise
- Required BTU-per-hour input
- Appropriate firing rate per foot
- Required burner length
- Differential pressure across the burner
- Burner and profile opening area
- Push-through or pull-through configuration
- Gas inlet and supply requirements
- Available physical space and burner arrangement
When these factors are evaluated together, burner sizing becomes a repeatable engineering process rather than guesswork.
Direct-Fired Make-Up Air Burner Solutions From Midco International
Midco International’s HMA burner line is designed to give OEM engineers and equipment manufacturers flexibility when developing direct-fired make-up air systems.
The HMA 2 and HMA 2A modular burner platforms can support multiple configurations, allowing engineers to match burner capacity and layout to specific equipment requirements.
Proper burner selection ultimately depends on more than maximum BTU capacity. Airflow, temperature rise, burner length, firing rate, profile design, differential pressure, gas supply, and equipment configuration must all work together.
By approaching sizing as a complete system, OEMs can design direct-fired make-up air equipment for more consistent combustion, dependable operation, and real-world performance.