In cold weather, portable heaters are a convenient way to heat temporary shelters for framing and siding crews working on unheated buildings. More important, portable heaters help control temperatures during crucial construction phases. Freshly placed concrete, for example, must be protected when average air temperatures drop below 40°F for more than three consecutive days (see "Cold-Weather Concreting," 1/95). In moderate cold, insulating blankets and temporary shelters will usually provide enough cold-weather protection; but in severe cold, using portable heaters is often the best way to maintain proper temperatures. Combustion heaters. Fuel-fired, or combustion, heaters are popular on construction sites because they are economical and feature a wide variety of options. Typical fuels include liquefied petroleum gas (LPG), natural gas, and liquid fuels such
