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Undersize plumbing-supply lines

Q: My 1920s house has poor water pressure. Turn one tap on and you can forget about using the others. The supply lines in the house are 1/2-in. copper. But the city supply line is also 1/2 in. To increase the pressure, should I increase the size of the city supply line, increase the size of my lines, or both?

Clifford Janitsch, Toronto, Ontario, ca

A: Rex Cauldwell, a master plumber and electrician in Copper Hill, Virginia, replies: Half-inch supply pipe will never give you enough pressure or volume, as you’ve already found out. You can put in a pressure-booster pump, but if you are going to all that trouble and expense, you might as well do the job right and just change the lines.

If you have a big house, I advise replacing the 1/2-in. city supply line with a 1-in. line. If you do install a 1-in. supply line, reduce it to 3/4 in. a couple of feet past the main shutoff valve, which should be a full-flow valve, inside the house, and run the 3/4-in. line to the bathroom and kitchen plumbing.And don’t forget to install an anti-back-flow valve immediately after the main shutoff valve.

From Fine Homebuilding 136, pp. 24 January 1, 2001