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Direct Fire vs Indirect Fire Air-Make Up Units

If you have issues with negative pressure in your manufacturing facility or are considering installing a paint booth, you may need to consider an air make- up unit. You can have a better idea if you may want to consider an air make-up unit for your paint booth by reviewing this guide here. With the majority of paint booths a direct fire air makeup is used. However, understanding the differences between direct and indirect fire air make-up units will help you to know which may be right for your particular application.

How a Direct Fired Air Make-up Unit Works

A direct fire air make-up unit heats the air by passing the air directly over a flame. This method of heating make-up air has a lot of benefits and a few drawbacks as well.

Direct Fire Air Make-up Pros

  • Cost – a direct fired air make-up unit will cost much less than an equivalently sized indirect fire air make-up. The main reason for this is a matter of design, an indirect fire air make-up will have a heat exchanger included which adds to its overall cost.
  • Efficiency – Since air is directly heated by flame a direct fire air make-up unit will be more efficient and waste less energy in achieving a desired temperature rise
  • Weight – A direct fire air make-up unit will have less material in the air make-up which means it will weigh less. This can be helpful if you have a need for a roof mounted solution and weight is a concern.

Direct Fire Air Make-up Unit Cons

  • Products of combustion are passed directly into the air make-up stream – This is primarily an issue in a very small subset of applications. Particularly applications where any contamination of a facilities air are harmful due to the sterile needs for the process or potential for dangerous outcomes due to the facilities work that is being performed. To best know restrictions using direct fire air make-up units you should coordinate with your local fire marshal as well as  NFPA 33  and other guides .
  • Minimal adjustment ability – Due to the turn down ratio of the burner and safeties built into the direct fire air make up you can typically only adjust the speed of a direct fire air make up from 100% down to about 40% .

Direct Fire Air Make-up Unit Common Uses

  • General manufacturing facilities air replacement needs
  • Paint Booth Air Replacement needs

How an Indirect Fire Air Make-up Unit Works

With an indirect fired air make-up unit the flame never directly contacts the air that enters into the building or paint booth. Instead heat is created in an isolated chamber which then via a heat exchanger elevates the temperature of the air that will be supplied to the paint booth or facility.

Indirect Fire Air Make-up Pros

  • No byproducts of combustion are in the makeup air – BY isolating the flame from the make-up air, there is no introduction of flame byproducts in the air stream. This can be important in applications where air contamination would be problematic or in certain applications where combustion by products will cause safety or contamination issues (see above for additional information on safety considerations).
  • Doesn’t require large volume of air being used to be able to be used – A direct fire air make-up unit will usually a limited ability to reduce its capacity to accommodate a reduced need in make-up air. For many direct fired air make-up units if the unit is asked to operate at 20% capacity or less it will not effectively operate an indirect fired air make-up will typically be able to operate at lower percentages of its full capacity.

Indirect Air Make-up Cons

  • Expense – As mentioned the need for a heat exchanger will raise the overall cost of an indirect fired air make-up unit considerably
  • Weight – A indirect fired air make-up will always way more than an equivalent direct fire air make-up unit.
  • Efficiency – An indirect fired air make-up will typically range in efficiency from 76-80%. This will result in higher operating costs for the unit.  For an idea of the cost savings that come from improvements in efficiency you can check out this article here.
  • Additional Maintenance Needs – Due to having a heat exchanger an indirect fire air make up unit can often require additional maintenance to monitor the state of the heat exchanger and to ensure no cracks have occurred in the heat exchanger.

Typical Indirect Fire Air-Makeup Uses

  • In processes where the quality of air introduced into a facility are of top priority due to the process performed in the facility. This can be applicable to pharmaceutical manufacturing, laboratory environments, and chemical manufacturing facilities.
  • In areas where a direct fire air make-up unit is not approved.


The large majority of the time you will commonly be evaluating a direct fire air make-up unit for your project. Its lower cost, higher efficiency, and lighter weight make it the most attractive option typically unless there are safety or very strict air purity requirements. If you are evaluating the need for an air makeup unit we offer free onsite consultation to ensure a proper air make-up is selected for your application.

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