Direct Gas-Fired vs. Indirect-Fired Make-Up Air
Which one fits your plant? Start with the airstream.The first question is simple: should combustion products stay out of the supply air, or can they enter it? The answer points you to direct-fired or indirect-fired make-up air. Here is how each works, where each fits, and when direct-fired is the wrong choice.
How Direct-Fired Works
In a direct gas-fired unit, the burner sits in the outdoor-air airstream. The flame heats the air directly, so combustion products, including CO₂ and water vapor, enter the supply air. Standard units serve high outdoor-air exhaust replacement; dilution needs continuous outdoor air. Nearly all of the fuel's heat goes into the airstream: there is no heat exchanger and no flue.
Standard non-recirculating units are listed for 100% outside air and must not recirculate room air across the burner. Separately listed recirculating designs (often called 80/20) exist, with outdoor-air minimums and listing safeguards.
How Indirect-Fired Works
In an indirect-fired unit, combustion happens inside a heat exchanger and the flue gases vent outdoors. Supply air is heated on the other side of the exchanger, so combustion products do not enter the airstream.
That favors recirculation, mixed air, process cleanliness and humidity control. The tradeoffs are flue loss and a higher first cost.
When to Use Each
| Direct-fired fits | Indirect-fired fits |
| High outdoor-air exhaust replacement in plants, warehouses and hangars, where combustion byproducts and moisture are acceptable and the local code official allows it | Recirculating or mixed-air duty, spaces that cannot accept combustion products or added humidity, and places where local code requires separated combustion |
Efficiency Comparison
Where combustion products are acceptable in the supply air, direct-fired units typically show higher thermal efficiency, about 92–95%, because the flame sits in the airstream and no heat is lost through a heat exchanger or flue. Conventional non-condensing indirect-fired units typically fall around 80–85% thermal efficiency, with some heat lost up the flue; condensing indirect models can exceed that range.
These are typical thermal-efficiency figures that vary by model and test basis, not whole-building or seasonal savings guarantees. Compare like-for-like published ratings for the units under consideration.
When Direct-Fired Is the Wrong Choice
Skip direct-fired when:
- Codes prohibit it, including in sleeping quarters.
- Process or indoor air quality rules reject combustion products or moisture in the supply air.
- Ignitable vapor, dust or solvents may be in the unit airstream, or the space is a classified hazardous location. (Solvents elsewhere in the building are not, by themselves, a reason.)
- A standard non-recirculating unit is proposed for recirculation without the correct listing.
- Outdoor-air or airflow interlocks are missing.
Listing and the local code official govern.
Low-NOx Rules: Check California and Utah Early
California and Utah are important markets to check early for low-NOx requirements on gas-fired heating equipment, including many make-up air applications. Limits are not one blanket statewide rule that treats every direct- and indirect-fired make-up air unit the same: California regulates by air district and equipment category, and Utah's numeric boiler limits differ from its broader low-NOx burner-replacement rule. Confirm the district or state rule that applies to the unit type and size before selecting burners.
A low-NOx burner does not remove combustion products or moisture from the supply air on a direct-fired unit. The airstream question still governs.
Real-World Examples
Indirect-fired: food processing. A food-processing plant chose indirect-fired make-up air because of concern about a direct flame in the airstream and CO₂ in the supply air. That was the customer's selection concern, not a claim that product was exposed to open flame or that contamination had been proven. An indirect heat exchanger keeps combustion products out of the supply air, which is useful for contamination-sensitive food and process applications. It is not an industry mandate to use only indirect-fired equipment.
Indirect-fired: copper manufacturing. A copper manufacturer required indirect-fired heating to keep combustion-added CO₂ and water vapor out of the supply air. If moisture condenses on copper surfaces, dissolved CO₂ can form carbonic acid and contribute to corrosion. The concern is condensed moisture carrying dissolved CO₂, not CO₂ itself condensing, and not acid forming in dry air.
Direct-fired: typical applications. Where large exhaust loads need 100% outdoor-air replacement and combustion products are acceptable, direct-fired make-up air is a common fit. Examples include welding and fabrication shops replacing process exhaust, and paint-shop make-up air when the application listing and local code allow it and contaminated return air is not drawn across the burner. Hazardous or solvent-laden airstreams still require the correct listing and the cautions above.
The Bessamaire Vertical 80/20
Bessamaire offers vertical direct-fired and indirect-fired make-up air units built as a listed recirculating (80/20) design, not a field conversion of a standard non-recirculating unit. Outside air and return air mix at the bottom of the unit, then travel up through the burner and out to the blower. See the JH Just Heat Series and HC Heating and Cooling units, or read how a 40,000 CFM vertical JHV 80/20 heats a distribution warehouse.
Bottom Line
Lead with the airstream: does combustion stay out of the supply air, or enter it? Then confirm the outdoor-air fraction, recirculation, process limits, local code and any low-NOx district or state rules. Bessamaire's team can help you work through it for your building.
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