Above is a photo and cross section view of the plastic heat exchanger base for a HRS. It has vertical drain water channels. The copper heat exchanger core (not shown) fits into the channelled base. If the base was made with tapered walls, the shape of the copper heat exchanger core would have to be re-engineered to fit the base. This is a situation where the shape of the plastic part is critical for good performance. This is an "extreme shape". There is 5 square feet (.46 square meters) of male vertical walls. In addition, the drain hole has a very deep draw ratio with an o-ring surface at the bottom.