Compared to dehumidification, heat drying greatly increases evaporation rates and speeds the drying process. Until recently, all heat drying systems employed indirect fired furnaces. Now direct fired systems have entered the marketplace claiming: “25% better fuel efficiency over indirect heating systems. 100% of the BTUs are delivered directly into the structure. There is no wasted energy – you utilize the full power of the state-of-the-art propane furnace on every job.”
What is the difference between direct and indirect systems? An indirect system employs a heat exchanger to separate combustion byproducts from the air being heated. Heat from the combustion air is transferred through the heat exchanger to the fresh air. This keeps the fresh air clean. The heat depleted combustion air goes up the exhaust stack along with combustion byproducts such as carbon dioxide, water vapor, and small amounts of carbon monoxide and other noxious compounds. Some heat is exhausted and this is considered “wasted” energy. Indirect furnaces typically have an efficiency of about 80% and can burn fuel oil, diesel fuel, propane, or natural gas. Most commonly used for water damage restoration is diesel.
Direct fired furnaces have no heat exchanger to separate combustion air from the heated air. Fresh air is blown through an open flame where it gains heat energy. Along with the heat energy water vapor, carbon dioxide, and small amounts of noxious compounds are also added to the fresh air and these combustion byproducts go right into the structure being dried along with the heat. Direct fired systems must use clean burning fuels, usually propane for water damage restoration. These systems do capture all the heat energy and are about 94% efficient. (Some heat is lost to chemical energy.) The combustion process creates about one gallon of water for every 100,000 BTUs of fuel burned. A 500,000 BTU direct fired system will add almost five gallons of water to the buildings atmosphere every hour which most certainly has a negative impact on the grain load.
While in theory a direct fired system is more efficient than an indirect fired system, the indirect fired system actually costs considerably less to operate per delivered BTU of heat energy. How can this be? The propane used by the direct system costs 45% more per BTU than the diesel fuel used by most indirect systems. This cost disadvantage more than offsets the efficiency advantage. On top of that, the water vapor added to the air by the direct system impedes the drying process consequently it must dry at a higher temperature or take more time to dry; both of these options will cause an increase in fuel consumption.
Bottom Line: the direct fired system is more cost efficient and provides cleaner air than the direct fired system.