Spa Heater Union Split Nut to connect hot tub heater to PVC tailpiece connectors. 1 per pack. Two halves fit on stainless steel 2.25" heater housings and fasten with 2 included SS screws. Fits all spa heaters with 2-1/4 inch outside diameter housing, and 2-3/4" flanges. Sold as a single nut, with 2 nut halves with 2 screws. Photo shows both sides. HU-2FBTSN, rpl1-20-208s, HU20FBTSN, threads measure 3" inside diameter, collar portion is 2.3" ID, 400-5161, 415-5161WILL NOT FIT 3 INCH HEATERS. WILL NOT FIT 1-1/2" HEATERS.