Cold temperatures bring opportunity. All kinds of customers need portable sources of heat - for work in unfinished buildings and on outside construction job sites, for sporting and special events and for horticultural and agricultural operations.

You want to work on your boat this winter - inside your garage in Duluth? Well, sir, what you need is one of our space heaters.

Portable heaters warm people, building interiors and construction materials, and protect plants and animals, too - instant heat at the flick of a switch. Why shiver?

Output of heat is measured in British thermal units generated per hour. (One Btu is the amount of heat required to raise the temperature of one pound of water from 60 to 61 degrees Fahrenheit at a constant pressure of one atmosphere).

The type and size of heating unit needed depends on the area to be warmed and the outside temperature.

Heaters may burn propane, kerosene, No. 1 fuel oil or natural gas, depending on the model. Electrical models also are available, but they are not practical for heating large areas.

There are four basic types of portable heating units: forced-air, convection, infrared and radiant.

Bill Patterson, National Riverside Co., Upland, Calif., explains that forced air units have fans or blowers to circulate warm air. They are preferred for large enclosed areas. Models are available in sizes from 5,000 to more than 7 million Btu per hour. Propane and natural gas are the most popular choices of contractors; both burn efficiently and produce less odor than fuel oil and kerosene. Limited availability and rising costs of oil and kerosene also are factors. Heaters operating with these two fuels also require more maintenance than propane equipment. Substituting diesel fuel for kerosene - which contractors often do - causes maintenance problems.

Forced-air heaters usually are the first choice for heating enclosed areas.

Convection heaters draw in cold air at the bottom and heat it as it passes through the unit and out the top. Convection equipment cannot circulate warmed air and heat rises immediately to the ceiling of a structure. Most convection heaters are fueled by propane. For their cost, convection heaters produce the most Btu output. They do not require electricity and are simple and inexpensive to maintain.

Radiant heaters are designed to send out waves of heat, much like old-fashioned wood and coal-burning stoves. Radiant units are popular space heaters used to warm people and objects within reach of their direct radiation, rather than heating air in an enclosure. Heat is radiated 360 degrees around the unit. Propane and natural gas models are most widely used.

Infrared heaters produce "line-of-sight," localized heat. They radiate heat in only one direction. They're often used in large, open areas to warm nearby people or objects. Propane models are most popular. Smaller units are mounted on 20-pound tanks.

Heater designs are suited to different applications. "Forced air heaters warm large areas the quickest," says Clare Reid, DESA International, Bowling Green, Ky. "Convection and infrared heaters do not require electricity. Infrared models direct heat to people standing near the heater."

Fuels have advantages and disadvantages.

"Propane and natural gas are the cleanest-burning," says Patterson. "But propane requires handling of tanks, and natural gas heaters are not as portable as other types. Kerosene heaters produce an odor."

Most construction-class portable heaters are described as direct fired - that is, combustion of fuel occurs inside the heating unit.

"The inherent design of a direct-fired heater causes it to emit byproducts of combustion," says Larry Kokkonen, Patron Products, Scarborough, Ontario, Canada. "Therefore, enclosed areas must have access to fresh air."

An option is placing the heating unit outside a structure and venting the heated air inside, a procedure often used to heat tents.

"When evaluating heaters, be sure products are certified by a third-party agency," says Sarah Patterson, Pelsue Co., Englewood, Colo. "Look for UL and CSA certification of electrical components and AGA or CGA for design safety."

Portable heating extends the work season for contractors. Utility company crews and industrial plant and commercial building maintenance personnel are frequent renters of portable heaters. Homeowners need temporary heat in garages and workrooms.

Portable heaters are used by contractors of all types, including home builders, carpenters, drywall and paper hangers, painters, plumbers, electricians, glazers, masons, carpet installers, heating and air conditioning crews and concrete contractors.

Greenhouse operators frequently need extra heaters to protect fragile plants, says Kokkonen.

Industrial plant and warehouse operations, auto body shops, farmers and ranchers, nurseries and party and special event planners need portable heat in winter months.

Homeowners rent heaters to warm garages or workshops, during remodeling and repair projects and for private parties.

"People who do ice fishing and contractors installing and splicing fiber-optic cable are good prospects," suggests Sarah Patterson.

Bill Patterson and Kokkonen recommend promoting heaters ahead of the season. "Have 'early-bird' specials, encouraging customers to make a commitment at a special rate for rentals when cold weather arrives," says Patterson.

Rental store personnel must be prepared to assist customers in selecting the correct portable heater for the purpose. Many renters seek guidance about how much heat they will need, what type of heater will best fit their requirements and how much fuel will be needed during the rental.

National Riverside has this simple formula for calculating Btu needed: multiply the cubic feet of the area by the number of degrees the temperature needs to be increased and then multiply that number by .133.

Contractors and other professional customers typically rent heaters in the 100,000- to 200,000-Btu categories, but there are heaters that can deliver a million Btu.

"It is important that customers understand what types of heaters can be used in confined spaces and what types cannot," says Sarah Patterson.

City codes may apply to the use of portable heating units. Regulations vary from city to city. Rental store staff members must be familiar with local requirements, be able to explain them to customers and know what government agency to call for details.

Learn everything possible about each customer's project. You may rent various heater accessories, such as ducts for carrying air to tents, or sell cold-weather clothing and gloves.

The best add-on profit potential is in fuel sales. Bill Patterson estimates that a heater can burn fuel costing 200 times its rental rate during the course of a single season. "A rental center can position itself as the area's 'Temporary Heating Specialist' by renting, selling, stocking parts and servicing portable heaters," he says.

Offering pre-season tune-up specials for customers who own heaters also leads to future business.