Low Water Pressure in the House: Six Causes, Ranked by How Common They Are
First, narrow it down
The single most useful question is whether the pressure is bad everywhere or at one fixture. One weak faucet is a blocked aerator or a failing cartridge, and it is a ten-minute job. Weak pressure at every tap in the building is a system problem, and that is what this post is about.
The second question is whether it dropped suddenly or faded over years. Sudden means something broke or something closed. Gradual means something is slowly filling up.
Cause one: a partly closed valve
It sounds too simple, but it is genuinely the most common cause of a sudden drop. The main shut-off, or the valve at the meter, gets knocked or was never reopened fully after other work. Check both are turned all the way open before you assume anything expensive.
Cause two: a failing pressure reducing valve
Most houses on a municipal supply have a pressure reducing valve where the water comes in. They have a working life, and when the internals fail the valve can drift closed, choking the whole house. Symptoms fit perfectly: everything is weak, everywhere, and it happened over a few weeks rather than a few years.
Cause three: corroded galvanized pipe
If your house was built before the 1960s and still has original galvanized steel supply pipe, the inside of that pipe has been closing up with rust its entire life. A pipe with a nominal three-quarter inch bore can be down to the diameter of a pencil. Pressure at the street is fine; what reaches the shower is not.
This is the gradual version, measured in decades, and no valve adjustment fixes it. Repiping is the answer, and it also solves the brown water that usually comes with it.
Cause four: a leak on the service line
Water escaping underground before it reaches the house takes volume with it. If pressure is down and your bill is up, those two facts are the same fact. Go and read the meter with everything in the house switched off; if the dial is moving, you have your answer.
Cause five: scale from hard water
Hard water leaves mineral deposits in the water heater, in the aerators, in the shower heads and, over long enough, on the pipe wall. If the pressure is noticeably worse on hot than on cold, look at the heater first, because that is where scale collects fastest.
Cause six: the municipal supply itself
Occasionally the pressure at the street really has changed, often after work on the main. Your neighbours are the fastest test available. If theirs dropped at the same time, the problem is not inside your walls.
What a plumber will actually do
A pressure gauge on an outside hose bib takes thirty seconds and tells us whether the supply is fine and the house is restricting it, or the supply itself is weak. That one reading eliminates half the list above immediately, which is why it is the first thing we do rather than the last.
If any of this describes what is happening at your address right now, call (888) 809-6756. We answer 24 hours a day across South Carolina and will tell you what to expect before anyone comes out.