Unlike the cooling fluid, oil cannot be allowed to boil. This presented a particular problem with DB 601-series engines, because oil is sprayed against the bottom of the pistons, resulting in a considerable amount of heat being transferred to the oil as opposed to the coolant. The He 100's oil cooling system was conceptually similar to the water cooling system in that vapor was generated using the heat of the oil and condensed back to liquid by surface cooling through the skins of the airframe. A heat exchanger was used to cool the oil by boiling ethyl alcohol. The oil itself was simply piped to and from this exchanger, which was apparently located in the aft fuselage. The alcohol vapor was piped into the fixed portions of the horizontal and vertical stabilizers and into a double-skinned portion of the upper-aft fuselage behind the cockpit. This fuselage "turtle deck" panel was the only double-skinned portion of the aircraft's cooling system. The use of a double-skinned panel was possible here because the inside of panel was accessible in the event of repair. Condensed alcohol was collected by a series of bellows pumps and returned to a single header tank that fed the heat exchanger. Some sources speculate that a small air intake located at the bottom front of the engine cowl was used for an auxiliary oil cooler. No such cooler was fitted, nor was there room for one at that point. This small inlet served simply to admit cool air into what was a very hot portion of the engine bay. Immediately above this vent were the two steam separators, and immediately behind it were the hot coolant pipes coming from the separators.