A common misconception is that measuring water pressure should be simple. People think a person can just screw a gauge onto a hose bib, see the measurement, and have a fact.
Unfortunately, residential plumbing systems aren’t quite that simple.
Water pressure isn’t a permanent characteristic of a house like its square footage. It’s a measurement of a plumbing system at a particular location, at a particular moment, under a particular set of conditions.
A pressure gauge can give a perfectly accurate reading while that reading doesn’t accurately represent the pressure elsewhere in the house. The pressure can also change depending on water usage, municipal supply conditions, the operation of the water heater, and other factors.
So, when someone says, “The water pressure is at certain PSI,” that is not the whole story.
Where You Measure Matters
This is probably the biggest issue with measuring residential water pressure.
Many homes have a water pressure regulator, also called a pressure-reducing valve or PRV. Its purpose is to reduce the incoming municipal water pressure to a more appropriate pressure before supplying water to the interior plumbing.
But the regulator doesn’t necessarily control every water line on the property.
On many houses, the water service often splits into two pipes before reaching the pressure regulator. One pipe goes into the house, and one controls the hose bibs and irrigation system. So often, exterior hose bibs and irrigation systems can therefore receive the higher, unregulated municipal pressure while the sinks, toilets, showers, and appliances inside the house receive reduced pressure.
For example, we could attach a gauge to an exterior hose bib and get a reading of 110 PSI while the pressure regulator is supplying the interior plumbing at 60 PSI.
Neither measurement is wrong.
They’re simply measurements of two different parts of the plumbing system.
This is especially important because exterior hose bibs are generally the easiest places to attach a standard water pressure gauge.
Unfortunately, the easiest place to take a measurement isn’t necessarily the place that gives us the information we’re looking for.
Why Not Just Measure at the Kitchen Sink?
That sounds like the obvious solution.
Unfortunately, standard water pressure gauges are generally designed to screw onto a threaded hose connection. Kitchen and bathroom faucets typically don’t provide that type of connection.
Obtaining a pressure reading directly from the interior plumbing may require disconnecting supply lines, removing plumbing components, or installing adapters.
Home inspections are visual and non-invasive. We don’t disassemble plumbing systems simply to create a location for a pressure gauge. Besides exceeding the scope of a home inspection, taking apart plumbing introduces the possibility of creating a leak or damaging a component that was functioning normally before we touched it.
So we end up with an interesting problem:
The easiest places to measure water pressure may not accurately represent the water pressure being supplied to the interior fixtures.
Pressure and Flow Aren’t the Same Thing
There’s another common misconception surrounding water pressure.
Turn on a shower and most of us can immediately tell you whether it has “good pressure.”
But what we’re actually experiencing at the showerhead is largely water flow.
Pressure and flow are related, but they aren’t the same thing.
We’ve written about this before in our article about why a shower can have poor water flow even when the water pressure to the home is perfectly normal.
A restrictive showerhead, mineral buildup, partially closed valve, aging or restricted piping, or other conditions can dramatically affect the amount of water coming from an individual fixture.
That means a home can have perfectly acceptable static water pressure and still have lousy flow at the upstairs shower.
The opposite can also happen.
A fixture can appear to have completely normal flow even though static pressure elsewhere in the plumbing system is elevated.
Think about what that means.
If a pressure measurement doesn’t necessarily predict how an individual fixture will perform, the performance of an individual fixture doesn’t necessarily tell us what the static pressure is elsewhere in the plumbing system.
They’re related, but they’re not interchangeable.
Static Pressure Is Only a Snapshot
There’s also a difference between static pressure and what happens when the plumbing system is being used.
A pressure gauge connected to a plumbing system while no water is being used measures static pressure at that particular location.
Start operating fixtures and conditions change.
Someone takes a shower. A toilet is flushed. The washing machine begins filling. The dishwasher calls for water. An irrigation system turns on.
Now the plumbing system is operating under demand.
That’s one reason the conditions in an occupied house can be different from those in a vacant house. During an inspection of an occupied property, people or appliances may be using water elsewhere in the home. At a vacant property, virtually nothing may be using water except the fixtures being operated by the inspector.
A single pressure measurement doesn’t describe every one of those conditions.
Even the City’s Water Pressure Can Change
The water arriving at the property isn’t necessarily being supplied at exactly the same pressure every hour of every day either.
Municipal water systems are dynamic.
Pressure can be influenced by neighborhood demand, elevation, pressure zones, reservoir levels, pumping operations, utility operations, and other conditions within the distribution system.
Consider something as simple as neighborhood water use.
At 4:00 in the morning, relatively few people may be using significant amounts of water. A few hours later, thousands of people may be showering, doing laundry, running dishwashers, filling pools, and operating irrigation systems.
Municipal systems are designed to accommodate changing demand, but that doesn’t mean the pressure at an individual property remains perfectly constant throughout the day.
This becomes even more apparent when communities impose watering schedules. When large numbers of irrigation systems are programmed to operate during the same permitted periods, neighborhood water demand can change substantially.
Again, a pressure reading is only what is happening when the measurement was taken.
The Water Heater Can Affect Pressure Too
There’s another variable that many homeowners don’t realize exists: thermal expansion.
Water expands as it is heated.
In some plumbing systems, that expanded water can push backward toward the municipal water supply without significantly increasing the pressure inside the house.
But many modern plumbing systems effectively become “closed” systems because of pressure regulators, check valves, backflow prevention devices, or other components.
Now the expanding water may have nowhere to go.
As a tank-style water heater heats a large volume of water, thermal expansion can temporarily increase pressure within the home’s plumbing system.
This is one reason expansion tanks are sometimes installed with water heaters. The expansion tank provides a place for that additional volume to go rather than allowing pressure within the plumbing system to increase excessively.
It also gives us another example of why one pressure measurement isn’t necessarily definitive.
A measurement taken under one set of conditions may be different after the water heater has heated a tank full of water in a closed plumbing system.
Same house. Same plumbing.
Different conditions.
Pressure Regulators Don’t Last Forever
The pressure regulator itself introduces another variable.
Like most mechanical plumbing components, pressure regulators can deteriorate with age. They can fail, become less effective, or operate inconsistently.
A regulator that is maintaining appropriate pressure today isn’t guaranteed to maintain exactly the same pressure indefinitely. Or even until tomorrow.
Likewise, discovering elevated pressure dweeks or months later doesn’t necessarily establish what the regulator was doing at an earlier point in time.
That’s an important distinction whenever we’re comparing pressure measurements taken on different dates.
What Does This Mean During a Home Inspection?
During a home inspection, we operate the accessible plumbing fixtures throughout the property.
We run sinks, showers, and tubs, flush toilets, and observe drainage. Multiple fixtures may be operated simultaneously to look for significant changes in performance while the plumbing system is under demand. We also look for leaks and abnormal operation and test the water heater when conditions allow.
However, water pressure is a great example of why home inspections aren’t simply about collecting numbers.
The location of the measurement matters. So does whether it was taken before or after the pressure regulator. Static pressure can differ from conditions when the plumbing system is under demand, and municipal supply conditions can change throughout the day. Thermal expansion may also affect the system.
Because water pressure isn’t a permanent characteristic of a house like its square footage.
It’s a measurement of a plumbing system at a particular location, at a particular moment, under a particular set of conditions.