Aquatemp Products Heater Uni-Nut®
It is a patented design to end leaking split nuts on heaters. The Uni-Nut® is a one-piece nut that can be tightened by hand and changed in the field by removing a color-coded retainer. The black retainer is generally used on 1-5/8" tube while blue retainers are used on 5.5kW heaters. Red retainers usually denote heaters of other wattages. Although there may sometimes be exceptions, the Uni-Nut® is generally not a replacement for split nuts due to different flange sizes