Indirect-fired heaters are heaters that have a combustion chamber surrounded by another chamber that fresh air enters, absorbs the heat, and discharges usually through a port. Home furnaces are indirect-fired. The burnt air is exhausted through a pipe instead of a chimney. The delivered air is clean and breathable. Indirect-fired heaters are the best solution for heating areas such as tents, temporary housing, construction sites, bridge painting projects, etc. The units can be stationed away from the heate are, with the heated air delivered via flexible ducting.
Indirect-fired heaters are available in a wide variety of sizes, BTU ratings, etc. When purchasing consider the specifications of air flow (CFM or cubic feet per minute).
Be aware that indirect-fired heaters are rated by BTUs which are calculated by the amount of heat burnt fuel emits. Actual deliverd BTUs will be less, because 20-30% of the BTUs are lost in the exhaust. Generally, people purchase larger units than what may be minimally needed. FYI a 2,000 square foot home furnace is often about 200K BTU.