We have all experienced poor "functional flow." This is a loss of water pressure when turning on two plumbing fixtures at the same time. (Fixtures refer to anything that uses water, e.g. showers, sinks, toilets, etc.) Home inspectors are required to check functional flow. In fact, we're required to use the term "functional flow" in our report, or describe the method we used for testing functional flow.
Functional flow is usually most noticeable in bathrooms, especially when there is a large "garden" or whirlpool bathtub. When you turn on the tub faucet, you lose pressure at the other fixtures. Sometimes there is a common plumbing wall between the tub and a separate shower stall, so the tub and shower faucets are virtually next to each other. This can cause a substantial loss of pressure at the shower when the tub faucet is turned on.
There is always some loss of pressure when turning on two faucets at the same time. What we are concerned about is a significant loss, which can actually affect the water temperature at fixtures. The worse functional flow I ever saw was when I was stationed at Chanute Air Force Base in Illinois for a few months. And, of course, the Air Force sent me from New Mexico to Illinois in the middle of winter. I had to stay in the barracks, and the large common (only) bathroom was on the second floor over a breezeway. This made the bathroom itself cold, and the cold water was about 4 degrees above freezing. The shower stall had six or eight showerheads (modesty was not a good thing to have in the military). When someone flushed a toilet, the water temperature at the showers immediately went to just below scalding. You always yelled "flush!" just before flushing a toilet. And everyone in the shower automatically stepped back out of the water. (You only forgot to yell "flush" once, because getting towel-whipped by six angry wet naked men was a hard lesson to forget).
I only recommend trying to improve poor functional flow if it is significant. In older homes, poor functional flow can be caused by "hardening of the arteries" in galvanized plumbing pipes. Galvanized pipes will corrode on the inside, making the inside diameter of the pipe smaller and smaller. I've seen 1-inch piping with so much corrosion you couldn't fit a pencil into it. In this case, an inspector is likely to recommend further review by a licensed plumber.
Poor functional flow can be hard to check if you can't see the fixtures involved. I lived in a home once with a hose faucet right outside the bathroom. If someone turned on the hose faucet while the shower was in use, the water pressure dropped a lot in the shower. There is no way to test for this in a home inspection.
Now I have to tell you that everything you just read is wrong. Functional flow has nothing to do with pressure. Functional flow is a loss of volume, not pressure. In fact, the definition of functional flow in the Standards of Professional Practice for Arizona Home Inspectors is "a reasonable flow at the highest fixture in a dwelling when another fixture is operated simultaneously."
I have used the term "pressure" so far because that's what most people call it - poor "pressure" at the sink or shower. But the pressure will be the same at every fixture in the home in a single-level home (the pressure may be slightly lower in upper levels). If the water pressure at the hose faucet is 50 psi (pounds per square inch), the water pressure at the shower, toilet and icemaker line to the freezer is also 50 psi.
I know this confuses some people. Consider it this way. You have 50 psi water pressure in your home. It takes you 10 minutes to full your bathtub using the tub faucet. Now imagine filling the bathtub with a little 1/4-inch water line, like you see for icemakers or evaporative coolers. It would obviously take much longer to fill the tub. The pressure at the 1/4-inch line is 50 psi, just like at the tub faucet. But the volume of water coming out of a 1/4-inch line is much less. This is why a large bathtub can cause poor functional flow. Large tubs usually have high-volume faucets, so they will fill faster.
Home inspectors also report on water pressure. The recommended water pressure for a home is 40 to 60 psi. Water pressure above 80 psi can be hard on plumbing lines and fixtures. Clothes washers and dishwashers are especially susceptible to high water pressure. These appliances use rubber hoses with clamps - the water lines are not soldered or glued together. They use hot water, which can make the hoses soft, and they vibrate when in use, making it more likely for a hose to break or come loose. And they are inside a home, where a broken water line can cause water damage to expensive cabinets or floor coverings.
Our water pressure is often over 80 psi. In fact, in areas it's over 100 psi (I stopped buying the 80 psi max gauges after I broke a few at homes with 115 psi).
It's easy to control your water pressure with a pressure regulator. Pressure regulators are installed in the main water line and the maximum water pressure can be adjusted. Most newer homes in our area have a pressure regulator installed. If your home does not have one, the good news is pressure regulators only cost about $30. Of course, plumbers cost about $5,000 an hour.
Randy West owns Professional Building Consultants in Prescott. He is state-certified and has performed more than 6,000 home inspections in the Prescott area. West serves on the Home Inspector Rules and Standards Committee for the Arizona Board of Technical Registration. Contact him at firstname.lastname@example.org.