A mixing valve assembly constructed of plastic parts in which end pieces that form valve bodies receive the ends of conduits of a mixing chamber. Ends of the conduits are received in recesses of the end pieces, are secured by an adhesive or solvent, oriented by mating non-circular portions, and sealed by both the adhesive or solvent and an O-ring between each conduit and the connected end piece. In addition, the parts can be further secured and sealed by an ultrasonic weld.