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.
With a direct fired system the airflow goes directly over the flame. An indirect fired air heater has a heat exchanger that the air passes over. Indirect units provides heat without contamination from combustion by-products.
Direct heating means heating an object directly by a heater but indirect heating means heating an object by another object which is being heated by a heater.
In a directly heated cathode, the filament is the cathode and emits the electrons. In an indirectly heated cathode, the filament or heater heats a separate metal cathode electrode which emits the electrons.
With an indirect water heating system and cylinder, the water that circulates around the boiler, pipes and radiators is kept totally separate from the stored water in the cylinder that is used for bathing. From the outside the indirect cylinder looks identical to the direct cylinder save from two additional pipe connections on the side of the cylinder. Inside the cylinder a coil heat exchanger is installed, which allows hot water from the boiler to flow into the upper connection and leave at the lower connection to return to the boiler for re-heating. The hot water in the coil indirectly heats the water in the cylinder. A Direct Cylinder is heated by a source located within the cylinder - such as an electrical immersion heater. Because the Immersion Heater is heating the water 'directly' the cylinder is called a 'Direct Cylinder'. In a direct cylinder there are no other external heat sources.
A desuper heater is for low and medium pressure and temperature application as required in process plants. Attemperator covers all the range depending on the steaming parameters of the boiler. In functioning, they are both the same i.e. they remove the superheat of the steam with help of water either by direct contact with spray or by indirect contact in a heat exchanger.
a direct vent water heater is more fuel efficient (cost's less to operate). aside from that there isn't much difference, aside from not needing a double wall flue through the roof on a standard gas water heater.
Heater produsces - heat recovery finds heat and reuses it
Lighting, both direct (lightstrip) and indirect (sun in the window), all motors including pumps/filters, room temperature
== == It is accomplished by adding an indirect fired water heater and controls.
The difference between a mobile gas water heater and a standard gas water heater is that a MH unit will use an outside air intake for combustion. That is the most important difference.
the Frost Fighter IHS #700 Indirect Space Heater ahs 700000 BTUs.
There really isn't except that a commercial model may have more failsafe features.