What is the difference between direct and indirect warm air heater?

Answer 1

The direct air heater takes the space air or fresh air and mixes it with hot gasses from a flame source such as natural gas flame or oil fired flame and returns / redirects it to the space to be heated. The warm air contains all the flame byproducts and toxic gasses along with. This type of air handling system is used in industrial environment where the human beings are not exposed to such warm air.

In the indirect system the hot flame gasses circulate through a metallic high efficiency heat exchanger which isolates the flame gasses (aka flue gasses) from that of the air to be warmed by passing from the other side of the heat exchanger. This type of air is safe for human exposure as the flame products and toxic COx, NOx and SOx are isolated. usually all the domestic air heaters are of indirect type.