Your building represents a significant long term investment.  Selection of the proper heating and ventilation systems for your specific building is crucial to insure the comfort and energy efficient performance you deserve. Unfortunately, to meet budget constraints, mechanical systems are often cheapened and corners cut that the building owner may pay for over and over again. If you are storing a product that may be adversely affected by moisture or condensation, the difference between direct fired heating equipment and indirect fired  heating equipment could be a factor you need to seriously consider as you evaluate the  the best choice for your building application. Although most people have been led to believe that direct fired heating equipment efficiency is 92%; the real systematic energy efficiency can be much, much lower.  A research study project by Mr. Bin Wang at Indiana University Purdue University Indianapolis (IUPUI) released in July 2019 sheds new light on the real efficiency of HTHV heaters.
Energy Efficient ThermoRotation    Air Turnover Heating and Ventilation Systems for Warehouses and Pre-Engineered Industrial Buildings Thermo-Cycler Industries Inc.    P.O. Box 22,  111 E. Hamilton St. Union Mills, IN 46382    (866) 767-2990   2019 Thermo-Cycler Industries Inc. Pre-Engineering Energy Efficient Thermo Rotation   AirTurnover Heating, Cooling  and Ventilation Solutions for Warehouse and Industrial Buildings Since 1984!
More Resources:
Direct Fired Heaters Can Result in Excessive Moisture and Condensation By William A. Lotz, PE
When is Direct Fired Makeup Air needed?
Health Effects of Combustion Byproducts
Do direct fired industrial space heaters really destratify
Thermo-Cycler Advantages
Direct Fired Moisture Problems?
Systematic Energy Efficiency Study For Direct and Indirect Fired Heaters By: Bin Wang