The Model VM131 Straight Steel Coupling is made from central vacuum grade 2-inch inside diameter steel tubing. Local building codes in some areas may require that steel tubing must be used for the central vacuum’s exhaust system. The exhaust system routes the exhaust air from the central vacuum power unit to outside the residence. This fitting provides a straight connection for steel tubing. AirVac steel tubing and fittings can be connected using Pliobond® Brand Steel Tubing Adhesive.