Bringing heat: what kind of portable heater should you choose?

When it's too cold on the job, some tools, materials--and workers--just don't perform. Ask Ken Polhamus of Great Plains Drywall, who has seen his share of frozen Illinois winters. "We'll rock if it's 20 below zero, as long as we've got heat and it's warm enough to work;' he says. "The temperature doesn't affect the rock--it's more of a labor effect. It costs more to get the job done when it's cold."

Fortunately, when you need to bring heat to a jobsite, either by buying or by renting portable heaters, there are plenty of options. The hardest part is deciding which type of heater, and fuel, suits your needs.

Portable heaters designed to condition large areas include fan-forced hot air or convection heaters; infrared and electrical-resistance units that radiate heat directly at people and objects; and radiant ground heaters for thawing the earth itself. All of these units operate much like the fixed residential heating systems they're modeled on. Most can be used indoors or out, although their effectiveness diminishes rapidly, especially outdoors, depending on ambient air conditions and how close or far the work activity is to the heat source.

Heaters are rated on the amount of BTUs, or British thermal units, they produce per hour. Air temperature is always a factor, but even a small portable heater rated at 25,000 BTUs can adequately warm an average room in cold weather, while very large (yet still "portable") space heaters that produce up to a million or more BTUs are available for heating whole buildings.

Because jobsites, like air temperatures, can vary from day to day, the size of heater you'll need on a given day will also vary. Whether that space is partially or completely enclosed, or insulated or not, are other considerations that must be factored in. Obviously, a dried-in frame building with tarps over the window openings and no insulation will require a greater amount of continuous heat than a more closed-in, insulated space, and providing adequate heat for, say, framing outdoors in winter is a challenge of a different magnitude.

Most portables do not offer variable heat output, but there are rules of thumb for estimating heater size. A rough calculation is to multiply the total cubic feet of space to be heated by a factor of five to determine the minimum heat output needed (5,000 cubic feet X 5 = 25,000 BTUs). A second method, used for estimating electric heaters in enclosed spaces, is to provide 10 kilowatts (10,000 watts) of heat output for each 1,000 square feet of floor space (10 watts per square foot.); because 10 kW equals approximately 34,000 BTUs, or 3.4 BTUs per watt, you can do the math to choose a unit rated in BTUs.

A WIDE RANGE OF CHOICES

The compact wheeled portable heaters typically used in residential construction range from 25,000 to around 200,000 BTUs and cost from about $125 up to $2,000, depending on size, features, and quality. Because many contractors use them infrequently, they are a popular tool rental item. But for contractors who work through cold winter weather and require large-scale, dependable heat sources, there are heavier weapons that they can turn to.

Indirect-fired heaters are designed to be set up outside a building where they can safely produce vast amounts of heat that is then transferred to a liquid or airstream and piped indoors. Universal Portable Heaters produces a line of indirect forced-air heaters from 150,000 to 570,000 BTUs, priced from $2,000 to $6,000. One of the largest portable indirect heaters available is the Sahara Air Heater from Ground Heaters. This trailer-mounted, hydronic "hell on wheels" pumps 180-degree liquid to any number of box-sized remote heat exchangers located inside the building. Maximum heat output is a blazing 1,250,000 BTUs. A well-equipped unit with four heat exchangers, adaptable for LP, natural gas, or diesel fuel, has a manufacturer's suggested retail price of $63,640.

The Sahara produces "a very dry heat, so there's very little moisture introduced into the workspace," says Jennifer Thompson of Ground Heaters. "Other types of heaters put tremendous amounts of moisture into the building, and they have associated ventilation problems. Hydronic heat is much better for drywalling, painting, wood flooring and trim, and other types of finish work."

True to its name, the company also makes ground heaters, which use heated propylene glycol pumped through tubing--much like radiant floor heating--to thaw frozen earth for pouring concrete and other site work. These systems can defrost up to one foot of frozen earth a day, according to the manufacturer, and typically cost from $23,000 to $58,000.

Several fuel options are available for portable heaters. Units fueled with natural gas are nearly 100 percent efficient, producing virtually no unburned combustion residue. However, not all jobsites offer piped-in gas connections. Bottled gas or liquid propane (LP) fuel is similar, but it often contains additives and contaminants, so the fuel does not burn as cleanly or efficiently. Also, National Fire Protection Association code regulations prohibit using any bottled gas tank larger than one pound inside a residence. Kerosene--and diesel-fuel heaters are generally less expensive to own and operate, but the fuel gives off more odor and can produce a greasy carbon airborne combustion residue when the flame is not properly adjusted.