Air Exchange and Heat Drying


Air exchange and heat drying employs outdoor air and supplemental heat to facilitate evaporation and humidity control.

The process is similar to those used by lumber mills during kiln drying. Very high temperatures are used to increase the vapor pressure associated with free and bound water in the structure. As moisture evaporates, wet air is exhausted to the outdoor environment to control indoor humidity.

Exhausted air is replaced by heated outdoor air. The heated outdoor air is relatively dry, and is promptly placed in contact with the wet surface to continue the cycle until materials reach their drying goal.

A major difference between kiln drying and structural drying is the restorer’s access to the number of affected surfaces. In a kiln, lumber can be accessed on all sides. Materials are separated to ensure that the hot air can effectively heat the entire material, thereby maximizing the internal vapor pressure associated with water in the material. Air is then evacuated from all sides, ensuring that all evaporated water vapor is removed.

In a wet structure, restorers often do not have access to all sides of affected materials unless invasive methods are employed. If all wet surfaces are made immediately available to the heated air, the method can work effectively. This requires the removal of several finish materials, the application of several inter-air drying systems, or access to subsurfaces by other means.

If any affected surface or subsurface is not in immediate contact with the hot and dry air, water vapor can collect. Humidity can increase and may eventually result in secondary damage. If surfaces (which are in direct contact with hot, dry air) are allowed to dry at a greater rate than subsurfaces, damage such as checking, splitting or warping can occur.

Use heat and outside air to better dry structures heat drying air exchange

Courtesy Dri-Eaz

When considering the use of heat and air exchange, restorers need to take into account the temperature limits set by occupants, sensitive contents, changing weather and psychrometric factors. Heat and air exchange can be used to dry structures in circumstances determined appropriate after each of these factors have been addressed.

Forced air heaters used in restoration are typically indirect fired furnaces. Indirect fired furnaces, also called mobile furnaces, are placed outside a structure and pump heat into the affected area either by blowers or liquid heat exchangers. Diesel or propane burners create the heat to increase the temperature inside the structure, but noxious fumes are vented outside.

Outside air acts as the dehumidifier. Forced air heaters and air exchange devices offer the ability to make outside air “thirsty” to remove moisture from the structure. Forced air heaters are most effective when the outside air is dry. Warm, dry air is ducted into the structure from the heater, gathers moisture from the structure, and is then vented out the opposite end of the structure with forced air.

Procedures for mobile furnaces

Indirect-fired furnaces have the advantage of not introducing any combustion by-products or additional moisture to the drying environment. To maximize efficiency and safety when using forced air heaters, observe the following procedures:

Provide adequate airflow

An indirect-fired furnace must always be operated outdoors and ducted into the space being heated. Pay close attention to the ducting. Avoid sharp bends and kinks that restrict airflow, and only use the length of ducting needed to reach the target area.

A mobile furnace performs best when operating at maximum airflow. Take care to allow for the free flow of air around the furnace when it is operating. This will help to ensure adequate air for combustion as well as for delivering heat to the target area.

When introducing heated air into a crawlspace or basement, direct the ducting through the widest opening available. Use a basement window or access panel if possible.

Adjust the burn

Mobile heaters are usually factory-set to provide the correct air/fuel ratio for atmospheric conditions found at sea level. At higher altitudes, the amount of oxygen in the air is decreased, which can reduce the efficiency of the furnace, causing excessive fuel consumption, oily build-up on the burner nozzle and sooty exhaust. Consult the owner’s manual on how to adjust the air/fuel ratio for higher altitudes. Adjustment is needed if the exhaust smoke is dark, black or white. The mobile furnace should produce a clear exhaust when operating with a proper air/fuel mixture.

Other factors can influence burn efficiency, such as fuel quality. Adopt a best practice of checking the furnace for proper operation every time the heater is used.

Use a remote thermostat

A remote thermostat allows precise control the temperature of the affected area. Maintaining a steady temperature can reduce fuel consumption while maintaining the ideal drying temperature inside the structure.

Of course, thermostatic control is critical for providing safe, comfortable, temporary heat in occupied areas. In many cases, using the thermostat will also significantly increase the run time between refueling.

Brandon Burton is the technical education manager for the Restoration Sciences Academy (RSA), a part of Legend Brands. He teaches IICRC-approved classes in the categories of Applied Structural Drying (ASD) and Water Damage Restoration (WRT). Burton has served the restoration community for more than 15 years as an IICRC-approved instructor, ANSI/IICRC S500 chapter chair, RIA restoration council member and in many other industry roles. You can contact him at [email protected].

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