Direct fired heaters burn gas to produce heat. When gas burns, it produces water as a by-product. Methane produces 2 cubic feet of water vapor for every cubic foot of gas burned while propane produces 4 cubic feet of vapor for every cubic foot.
That means a typical 1 million Btu/hour burner produces 90 pounds of water vapour every hour. That means a ton of water is produced every day for natural gas and two tons for propane. If your exhaust system isn’t big enough to ensure that water ends up outside, the water will collect in your building and cause all the problems associated with excessively damp conditions. These include wall and floor buckling, mold and mildew damage as well as corrosion on metal elements.
The exhaust system must match the CFMs of the make up air system. If your direct fired burner utilizes a 50, 000 CFM make-up air unit, the exhaust system must be a 50, 000 CFM system in order to ensure that all the water vapour is vented from the building.