I posted this in the Ventless Fireplace thread but thougt it should be here as well as it is educatonal in nature. It concerns commercial applications.
I became aware of gas **Direct-Fired** air handling units when I worked as an engineer for a compliance lab before becoming a HI. The gas guys in our lab would test these units for compliance. They are built to very stringent standards with regard to burn efficiency and combustion products released into the heated space. **These particular units are intended for commercial applications only**. I found an introductory course that will give anyone interested a good overview of the issues with this type of heating unit and a decent understanding of their application.
This is from the introduction:
*This course is intended to provide the student with an understanding of the operation and application of heating systems that utilize Direct Gas-Fired Burners to provide comfort heating of building spaces. Typical applications are shown. Symptoms of problems that may be addressed using this type of equipment are described. Basic system configurations are discussed, with descriptions of options that are available. Basic operation of units, with a discussion of fuels, air quality requirements, operational limits, and installation requirements are presented. Industry Construction Standards and Codes are listed. Insurance standards are also explained. Safety issues are addressed.
*You can read the entire document here: