Water pressure drops in house when 2 faucets are one at the same time

Question

I bought a house 2 years ago that was built in 1958. Had galvanized plumbing when I moved in and it was almost completely sealed off with rust. I had all the plumbing from the main line replaced a week after moving in. My water pressure is great with just one item on but drops drastically with more than 1 item one. I bought a water pressure gauge and installed in on the bib where the main line comes in from the street and my pressure is 78 PSI, so I know that I have great pressure. They did not install a PRV when they put in the Pex pipe throughout the house. Is there something else I can test, should I test on my cold water hook up for my washer which is the furthest away from my main line? Is there amything I can do to have a more steady pressure?

Thanks!

Answer 1

I presume you have a restricted main feed line from the supplier, if you have replaced all the piping in the house [and that was done sensibly in terms of pipe sizes, not running the whole house through a 1/2" PEX supply, for example.] So you "have great pressure" while nothing is running, but pressure drops significantly when you have flow.

The only approach other than having the main line replaced (generally cost-prohibitive) that I know of is to add a large pressure tank (as used on a well system) inside the house, that can (temporarily) help to maintain pressure with stored water inside the house, past the restriction in the feed, and then be slowly refilled though the restricted main line. Unless you spend a lot for a very large tank or tanks, that will be a limited effect, but it will help for short-term dual-uses.

So, two people washing hands at the same time, likely good. Filling the tub and running the clothes washer, will soon run out of stored water and be back to what the supply supplies.

Edit to add: I suppose it's probably worth checking that all your shutoff valves on the main feed (both yours and the suppliers) are fully open, as a partially-closed valve would cause an "easily solvable" restriction. That may require giving the supplier a call to check theirs.

Answer 2

Here is what I had the installing plumber do when I built my house 23 years ago. The main supply line to my house from the street was 3/4" copper as per the area code. The plumber ran all 3/4" copper with only 3/4"x3/4"x1/2" or 3/4"x1/2"x1/2" reducing tees to access each outlet and no 1/2" tees what so ever. The only real pressure drop at any faucet is when someone opens 2 unobstructed faucets such as the hot and cold faucets in the laundry tubs or when filling the washer using warm water. (hot and cold at the same time) Even when one of the 2 above senarios happens you can still utilize the shower, toilet or wash sink but you just have a reduced flow. Any of my neighbors that had all 1/2" copper piping installed have the same problem that you describe. My 2 cents. My water pressure with no flow is similar to yours at 75psi with no PRV.