Aftermarket Replacement for Cal Spa XL Exchanger This heater assembly is not an exchanger but an in-line flow thru replacement. This heater assembly is designed as a drop-in replacement. 2" x 15"s/s housing with 2" slip tailpieces, dual thermowell grooves w/cover, 1/8"npt pressure switch tap, and metal box. Pressure switch not included Note: The two thin wires with the male and female terminal ends are usually not used. It is okay to connect them together.