: Steve, the difference between volume and pressure is a commonly misunderstood concept. Think for a moment about bicycle tires and automobile tires. That skinny little bike tire may hold 90 psi, and your car tire only 32 psi. Which hold the most volume? Water is similar. If you had a 1/4" pipe with 100 psi squirting water into a swimming pool, it would take days to fill the pool. But, if you had a 6" pipe with 5 psi, the pool would fill much faster. As e-plumber and Sylvan Tieger, two knowledgeable pros, have said, get a pressure reducer to regulate the pressure because partly closing a valve only reduces the volume, not the pressure.
Thank you Gary. I try to help without demeaning replies by others. The poster in this thread actually stated the the main valve was closed partially to try and regulate water pressure in the system. You gave a prime example why it won't help.
In my post, I stated that partially closing the main valve will cause no harm, (damaging the valve, pitting the piping, causing pressure to build up, etc.) presumably the valve will be shut for a short time to test if it will help their situation, the person will most likely open the valve wide soon after or call a licensed plumber when they realized there wasn't enough volume passing through the piping to use two fixtures at once. Another reply to this original post went on to explain about this, that and the other thing, I'm surprised there wasn't a law suit mentioned.