While heated enclosures are probably the most effective means of protecting concrete during cold weather placing, there are some risks. Two types of heaters are used in cold weather concreting: direct fired and indirect fired. Indirect fired heaters have a heat exchanger in the firebox and are vented to blow exhaust gases outside. They discharge only clean, heated air into the enclosure. Direct-fired heaters heat intake air as it passes through the heat source. The heated air contains both carbon monoxide and carbon dioxide, requiring good ventilation. To lower health and placing risks, indirect-fired heaters are recommended when concrete is placed in heated enclosures.
Combustion heaters, whether fueled with kerosene, natural gas, or propane, must be used carefully to limit danger to workers. Never use heaters in enclosures where gasoline or other liquids having flammable vapors are stored. Check the electrical supply cord for proper grounding and use the heater only with the electrical voltage and frequency specified on the model plate. When using propane heaters, always use and store propane cylinders in an upright position. Keep the heater at least 6 inches from the propane supply and do not direct hot exhaust toward the propane supply.