Water Pressure is Tricky to Understand

Does This House Have Good Water Pressure?

A common question that my clients ask during home inspections is: “Does this house have good water pressure?”

What they actually want to know is whether the flow from the showerhead will be strong enough for a quality shower, and whether someone flushing a toilet will interrupt that shower. Questions like these can’t be answered by examining water pressure on its own. Instead, we have to look at the combination of water pressure and functional flow.

Dylan Chalk photo of water pressure reading

Water pressure gauge showing slightly elevated pressure

  • Water pressure is the amount of force from the water main into your home. Water pressure is measured in pounds per square inch (PSI), and normal, good waterpressure is typically between 30 and 80 PSI.
  • Functional flow is the volume of water flowing through your pipes and arriving at individual fixtures. A house can have satisfactory water pressure and still have lousy functional flow if, for example, pipes are occluded with rust, pipes were not sized properly or other restrictions are blocking the flow of water.

Perfect Pressure

You don’t want your home to have water pressure that is too low, but water pressure can also be too high. Piping systems are designed to have no more than 80 PSI. When you exceed this pressure it can cause problems. High pressure will rarely cause pipes to burst, but it stresses the weak links in your piping system such as rubber hoses and gaskets, making them vulnerable to leaks and failure.  Ideal water pressure is between 40-80psi.

Achieving good water pressure

You can correct high water pressure by installing a pressure reducing valve. This is a bell-shaped device that reduces the water pressure.

If the house has low pressure, you first want to determine if the house is on a public water supply system or a private well system. Most public systems are required to deliver a minimum of 30 PSI to your house, so inadequate pressure on public water systems is rare.

  • If the house is on a public water supply and the utility cannot improve your pressure, the solution involves installing a pressure tank and a pump. This gives your supply piping system a pressurized boost.
  • If the house is on a private well, poor pressure could indicate a problem with the captive storage tank and/or the pump and you should have the well system serviced by a qualified well expert.

    Dylan Chalk photo of a steel pipe occluded with rust

    Steel pipe occluded with rust

Flaws in the Flow

Poor functional flow is a common problem in old houses with galvanized steel pipe. This type of pipe was commonly installed until the late 1960s and early 1970s. This pipe was manufactured with a coating of galvanization that was designed to prevent corrosion of the steel pipes. When this galvanization wears off, the pipes occlude with rust. The result is a restricted piping system that will not deliver adequate water to the fixtures even with all the pressure in the world. To fix the problem, you’ll need to replace the pipes.

Another common cause of poor functional flow is unprofessional water piping systems. Good plumbers know how to size the pipes correctly so that adequate water is delivered to each fixture. An amateur mistake is running too many fixtures off of pipes that have too small a diameter. The result is inadequate water supply to fixtures or poor functional flow. This can be difficult to repair without piping replacement.

Other factors affecting functional flow

Just because a fixture has poor flow, don’t assume anything about the pipes yet. Other factors in the piping system can result in poor flow. Sometimes an angle stop (one of those shut offs below the sink) may be partly closed. Fixture aerators (the little screens inside the faucets) can become restricted. The main water shutoff to the house could be party closed or restricted. Supply connector hoses could be kinked or restricted. These problems can be easily checked and repaired by a plumber.

Dylan Chalk photo testing functional flow

You can test functional flow by running water in the sink and shower, then flush to see if the flow diminishes

Field Test

To determine whether a home has enough functional flow, go to the bathroom and turn on the sink and the shower. Wait for a satisfactory water temperature in the shower and then flush the toilet. See if the flow diminishes. You can go to other fixtures and run water too, but at some point every system will diminish flow if you open too many fixtures at once. One sure way to kill the flow is to open up an exterior hose bib (the spigot where you connect a garden hose) during testing. I like to keep it simple and test flow by opening up every fixture in a given bathroom.

I hope this clears up some common misconceptions about water pressure and functional flow. Remember, informed homebuyers are happy homebuyers.



Dylan Chalk

Dylan Chalk is the owner of Seattle-based Orca Inspection Services LLC – www.orcainspect.com. He is the founder of ScribeWare inspection report software offering innovative and simple report-writing solutions – www.getscribeware.com. He is also the author of The Confident House Hunter – a book to teach home buyers how to look at and understand houses: Cedar Fort Press Due out July 2016 – www.dylanchalk.com

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