Indirect Fired Heaters. Indirect fired heaters are just like your home oil or gas burning furnace with a chimney. With propane or natural gas heating units, the flame is contained in a burn chamber which heats a “heat ex-changer”. The cooler air passes over and around the heat ex-changer, thus heating the air.

Direct fired air heaters are typically less expensive and more efficient than indirect fired units, as direct-firing units converts nearly 100% of the combustion gases used to hot air. ... “Indirect Fired” means that the air is heated “indirectly”; the burner is fired into a heat ex- changer

Indirect fired heaters offer some positive points, like:

• 100% clean, dry air – The air being circulated never comes in direct contact with the flame.
• Built in thermostats – Almost all indirect fired heaters use a thermostat
• Can operate in tightly sealed spaces – Indirect fired heaters do not release carbon dioxide into the space.

Some negative aspects about indirect fired heaters are:

• Higher Cost – The rental cost is nearly quadruple that of direct fired
• Lower Efficiency – Fuel efficiency is usually close to 80%
• Electricity Required – Indirect fired heaters always need an electrical outlet
• Requires exhaust ventilation – You must either vent the exhaust outside or duct the hot air inside. This makes installing and removing indirect heaters cumbersome because of the set up and tear down of duct work.
• Hard to transport – Indirect fired heater units are much larger and less portable.

Direct and indirect fired heaters are used for specific applications.  Generally, direct fired heaters are used on construction sites, outdoor applications, or warehouses. On the other hand, a work space that is tightly sealed and lacks a working heating system is an ideal setting for a indirect heater.

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