The uses of mobile furnaces are increasingly recognized as an important part of the restorative drying process.

Whether diesel or propane-fired, indirect-fired mobile furnaces provide steady, dry heat. Indirect-fired furnaces have the additional advantage of not introducing any combustion by-products or additional moisture to the drying environment. Here are a few suggestions for getting the most out of your mobile furnace.

Provide adequate airflow

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

A mobile furnace will perform best when it operates 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, try to find 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, and this can reduce the efficiency of the furnace, causing excessive fuel consumption, oily build-up on the burner nozzle and sooty exhaust.

Other factors can influence burn efficiency, such as fuel quality and it’s a good idea to check the furnace for proper operation every time you use the heater.

Use a remote thermostat

A remote thermostat allows you to precisely 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 your run time between refueling. 

Use negative air pressure

Establishing negative air pressure in the target area can help prevent moisture, odors and contaminants from spreading to unaffected areas through cracks, crevices and openings in the structure walls. It can also accelerate drying.

An area has negative air pressure when air is removed from the space, resulting in a lower air pressure inside than outside.smoke stick

To create negative air pressure in the affected area, follow these steps:

1. Close or block any large openings between the affected area and other parts of the structure.

2. Select an opening between the affected area and the outdoors to use as an exhaust, preferably on a side opposite of the location where you are ducting in the heated air. A window or crawlspace vent hole usually will do, provided it does not restrict airflow.

3. Place one or more airmover(s) in the selected opening and direct air out of the space.

Take note of the CFM produced by your heater. As long as you exhaust more CFM from the space than the heater is bringing it, you will be creating negative air pressure within the target area.

You can use a smoke pencil or smoke stick to confirm the presence of negative pressure. Light the smoke stick inside the target area and place it near a crack or crevice on an outside wall. If negative pressure has been established, smoke from the pencil will drift away from the crevice and into the negatively pressurized room. If the smoke drifts in to the crevice, you haven’t established adequate air pressure in the space.

To provide temporary heat in a living space, the heating system should be set up to provide positive pressure. This is usually accomplished by ducting the heater into the space to be heated — just make sure the doors and windows are closed. Use the remote thermostat to help provide steady, comfortable heat for the occupants.

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 chair, RIA restoration council member, and many other industry roles. You can contact him at