The heater pipe coil is made of 2 layers with a specific gap in each layer, and the inner layer is made with a more tenuous design so that the hot air gap enters the outer layer, so that it can enter into the reverse fire 3. Basically, the design of the TOH is made like a steam boiler , because in principle so that many surfaces that can be exposed to fire / hot gas from the burner. So that the design of the flow flow gas fire into 3 passes. If it’s less than that, then a lot of energy / calories are wasted, making wasteful fuel.
For external design, there is no specific form that is a significant consideration, only based on the convenience of operation. For example the position of the control panel tends to be simple attached to the body oil heater, but some are stored in a separate support frame so as to avoid the heat of the oil heater body. For collecting tanks, the placement is more regulated higher by considering the specific gravity at the surface boundary.