Where water pressure is poor in the distribution system, the most common cause is corroded galvanized steel. Galvanized plumbing can close down so that the inner diameter is reduced to 1/8” or less. The only solution is to replace the plumbing lines, typically with copper.
The diameter of the new pipe should never be smaller than the original piping. It is often wise to replace galvanized steel with larger diameter pipe on the main feeds to improve pressure further. A ¾” pipe is recommended from the entry point to the water heater, at least. On large or multi-family homes, this should be larger.
When galvanized pipe is present, and pressure is inadequate, a common practice is to replace the readily accessible plumbing across the basement ceiling or to replace only the hot supply and not the cold. While decreased diameter may exist in the risers, the horizontal pipes tend to deteriorate more quickly. The accessibility of the horizontal plumbing makes it less expensive to replace. Changing the easily accessible plumbing will help in the short term, but eventually replacement of all plumbing will have to take place.
Typically the most expensive cost with replacing plumbing is the labor. When changing concealed pipes, a large part of the labor cost is opening walls and ceiling to get to the pipes, and repairing the damaged surfaces when repairs are completed.
