What you mean use a valve 1 or 2 sizes smaller than line size?

I think they are just referring to nominal pipe size - so that 1 size smaller would mean 1-1/2" pipe and 2 sizes smaller would mean 1-1/4" pipe.

This is typically done with control valves in order to save money - but it is a balance between money savings and pressure drop/flow rate through the smaller valve.

You don't say what type of valve.

In general it would be possible because

A smaller valve will afford better flow control over a wider range. (compare the percentage opened required for equivalent "orifice" areas required for a flows between big and small valves)

Although a smaller pipe (1 1/4") may be big enough to carry the flow the bigger pipe (2") may be used to reduce friction losses.

Valve sizing is needed for proper flow control.

You must have the design criteria for water use to size the valve properly.

http://cgproducts.johnsoncontrols.com//MET_PDF/347VB.PDF

This PDF will help you get some idea what is needed.

There is no typical valve size!!!!!!!!!!

philrot,

The link you kindly provided filled in all the blanks. Thank you and Hendricks and Guest who know their stuff.