What type of fitting is recommended when changing the size of a main in a hydronic heating system?

In a hydronic heating system, when changing the size of a main line, an eccentric fitting that is flat on top is recommended. This type of fitting helps maintain a consistent flow of fluid and prevents air pockets from forming in the system. The flat top allows for efficient transition between different pipe diameters while ensuring that the flow of water remains even.

Eccentric fittings are particularly advantageous in horizontal piping because they help prevent the accumulation of air, which can occur if there is a rise in the line. This is crucial in hydronic systems where air trapped in the lines can lead to reduced efficiency and potential damage over time. Therefore, using an eccentric fitting is the best choice to facilitate effective system design and operation while accommodating a change in pipe size.