If you have low water pressure in your home and still have old galvanized steel pipes, chances are that they're corroded and clogged with rust and other minerals. Professional replacement is expensive, costing thousands of dollars.
But often you can do the job yourself, or at least solve the worst of the problem. The elbows and horizontal pipes in the basement or lowest level are usually the main culprits. But replacing all the plumbing is really a big job. You could save thousands and make the project more “bite size” by trying an incremental approach. That means starting with replacing an old water meter, and then moving on to the next likely suspects.
Start by opening the outdoor hose bib closest to the water meter. If you have low flow there, look at the water meter. If the meter fittings are smaller than the pipes going into and out of it, the meter itself is reducing flow. Contact your water utility to see if it offers a 1-in. meter. If so, have the utility shut off the water so you can install larger shutoff valves and the new meter (you may have to pay a fee to have it shut off). Next, replace the first 15 ft. of old galvanized pipe coming out of the meter with 1-in. copper or PEX. Many times, the increased volume from the larger pipes is enough to compensate for restriction in pipes farther down the run.
If the water pressure is still too low, start replacing the old stop valves to the sinks and toilets. The stop valves and galvanized elbows build up crud just like the rest of the pipes in the house. Turn off the main water valve and unscrew the stop valve. Dig out any buildup you see inside the galvanized fitting. Then install the new stop valve. Attach one end of a new flexible supply tube to the valve and route the other end into a bucket. Then open the valve and flush out any loose crud left from the cleaning procedure. When the water runs clear, turn it off and attach the supply tube to the faucet or toilet. Repeat the procedure at each fixture that has low pressure.
If the water flow is still wimpy, you have a big decision to make—replace all the horizontal sections yourself or call a plumber. This is actually not a huge project if your pipes are in a basement with an open or a suspended ceiling; you can probably handle it yourself. Replace them with PEX or copper, your choice. But before you cut the old pipes, heed this warning. Never remove the 90-degree elbows or tees attached to the bottom of the vertical pipes. The threaded portion of a pipe is always the thinnest and breaks easily when it’s old and rusty. If you break the threads on the vertical pipe while you’re trying to remove the elbow, you'll be in real trouble. So keep those fittings intact with the vertical pipes and remove only the horizontal sections. Even then, the threaded area on the horizontal pipes may collapse and crack as soon you crank on them. Buy a “nipple extraction” tool, or know where you can get one in a hurry if you run into that problem.
If you've changed out all the horizontal pipes and still have low pressure, you'll have to bite the bullet and run new vertical lines. This is the toughest part because you’ll often have to cut access holes in your upper room walls and floors for fishing the new water lines. Forget about removing the old pipes—just abandon them in the walls and run new lines with PEX. Pros prefer PEX because it's easier to fish and can bend around corners. That's especially important where vertical water lines turn to run horizontally along floor joists. Drill new oversize holes (to accommodate PEX expansion and contraction) and bundle the hot and cold PEX lines together with tape so you can make a single pull. Separate them at the top and make your crimp connections.