Rain caps protect the opening of a vertical exhaust stack from debris and water. The flapper of the rain cap lays horizontally across the opening of the exhaust stack when the exhaust stack is not emitting exhaust gas.
Rain caps are simple mechanical devices comprising of two pieces - a mounting collar and a flapper. The flapper and mounting collar are connected together at the hinge point. On the opposite side of the flapper is the counter weight. This is a piece of metal that weights slightly less than the weight of the flapper or lid. The counter weight allows the flapper to pivot up with very little exhaust pressure coming from the exhaust stack. The mounting collar fits around the outside diameter of the exhaust stack and is tightened into place by the collar adjustment. The collar adjustment is typically a nut and bolt arrangement that when threaded together takes the clearance from the mounting collar.
The differences are the actual outside diameter of each type. Tubing's outside diameter is the actual physical dimension as measured.For example, an 8 inch tube has a physical outside diameter measurement of 8 inches. The dimension associated with a particular pipe size (or Nominal Pipe Size (NPS)) references the inside diameter. For example, an 8 inch pipe has a physical inside diameter of 8 inches, and an outside diameter of 8.5 inches for a schedule 40 pipe.
Since rain cap sizes are selected by the outside diameter of the tube/pipe mounted to, it is important to understand these differences especially if you are receiving information from an end user or field technician.
The process is easy and requires only basic tools.
Rain caps < 8 inch in size:
Rain caps 8 inch and larger in size:
Proper facing direction of the flapper: