Calculating Btu
While choosing a heater size isn't an exact science, some relatively simple math skills can give a contractor a good idea of the amount of Btu - or British thermal units - needed. Begin by determining the cubic area of the space you plan to heat - multiply length, width and height.

Next, subtract the minimum outdoor temperature from the maximum indoor temperature sought. For instance, if the mercury falls to 30 below zero outside and the area should be heated to as much as 45 degrees inside, the difference is 75.

The final number needed - heat loss - can be a little trickier to determine. It depends on the level of insulation, the number of doors and windows and any other covering that may be present. Calculated as a U-value, or the sum of heat lost through the combined materials of the site, every job likely will be different. Begin by determining the R-values of the sheltering materials - the level of thermal resistance or conductance for each material type on site. For instance, the R-value of 3/4-inch plywood is 0.47 and the R-value for 3.5-inch fiberglass batting is 11. The total R-value for an area constructed of these materials would be 11.47. Next, determine the U-value, which is the inverse of the R-value and represents the total heat lost. In this case, the U-value equals .087 (1 divided by 11.47).

Multiplying each of those three calculations together - cubic area, temperature difference and heat loss value - will provide the minimum number of Btu needed to achieve the desired temperature. In the given example, the U-value of .087 multiplied by the temperature difference of 75 determines that 6.525 Btu are being lost through each square foot of the structure. A 10,000-square-foot building would have a total heat loss of 65,250 Btu, so a heater that size or larger would be necessary to reach the desired 45 degrees. While doors and windows can be worked into the equation based on each item's size and composition, it complicates the math more than is necessary for getting a generalized estimate of what's needed. Instead, work an additional 20 percent into the equation to account for heat loss through any ventilation system, door or window. For assistance determining heat loss, consult with a dealer, equipment manufacturer or rental center once information regarding the application is available.

While calculating the amount of heat needed, take time to consider the amount of ductwork that will be required if an indirect-fired heater is used. The closer the heater can be placed to a jobsite means the less ductwork that will be required and the less heat that will be lost. Since ductwork typically isn't insulated, a considerable amount of heat could be lost when trying to run significant lengths.