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Direct Fired vs. Indirect Fired Heat Drying Systems

Heat drying systems have earned a place in the water damage restoration industry because they greatly increase evaporation rates and speed the drying process.  Until recently, all heat drying systems employed indirect fired furnaces with heat exchangers to keep combustion byproducts separate from the heated air. Now direct fired systems have entered the marketplace. There are a number of factors to consider when evaluating direct vs. indirect fired heat drying systems for water damage restoration.

 

System Cost

While direct fired systems cost less, it is because they are simple systems with no heat exchangers to keep combustion byproducts out of the heated air.

 

Water Vapor

With no heat exchangers, direct fired systems inject moisture into the building being dried. At 400,000 BTU per hour, over 30 pounds of water are added to the building’s atmosphere each hour which impedes the drying process.  Direct fired systems overcome the added moisture by drying at higher temperatures or taking more time to dry. Consequently they burn additional fuel in the process and negate claimed efficiency advantages.

 

Fuel

Mobile direct fired systems are limited to burning propane while indirect fired systems can burn either propane or diesel fuel.

Cost per BTU. Propane is much more expensive than diesel. Diesel fuel contains 129,500 BTU/gallon (Diesel 2)  and costs approximately $3.00 per gallon while propane contains 84,300 BTU/gallon and costs $3.20 per gallon. 

 

Fuel Management

Four 100# propane tanks (500,000 BTU) would last a direct fired system about 17 hours whereas the diesel indirect system with 150 gallon fuel tank (240,00 BTU) would have a run time of over three days. For larger jobs, the direct propane system requires a propane fuel supply company to drop a “pig”. This limits mobility as the system is tethered to the tank. Diesel tanks can be refilled at the local gas station, via a transfer fuel tank in the back of a pickup truck, or by a fuel company truck. 

Availability. Diesel is universally available 24/7.  Propane supply points are limited.

Restrictions. Some cities prohibit or place significant restrictions on the use of propane. Some tunnels and bridges prohibit the transport of propane. 

 

Conclusion:

Diesel fueled indirect fired systems produce clean hot air free of added water vapor and carbon dioxide, cost less to operate, dry faster and/or at lower temperatures, and give the restorer both flexibility and convenience.