Water Pressure – Functional Flow – What’s the Difference?

Home Inspector with Complete Home Inspections, Inc. TN Lic#050

It is not uncommon to have real estate agents or my clients follow me through the property that I am inspecting and ask questions about my findings. While inspecting an older property, I noticed that the water volume coming out of the basin faucet reduce to a trickle when I flushed the toilet.

I was asked, "What's Happening?"

I'll explain.

Pressure Gauge

Some real estate agents and my clients seem to have some confusion differentiating between functional flow and water pressure. First of all, let's address water pressure. If you attached a pressure gauge to a spigot on your house, you would record the amount of force or pressure being supplied by the utility company or well pump. Legal water pressure being supplied by a municipality must be a minimum 40 PSI. If the plumbing system is designed right, this pressure will supply adequate functional flow in most properties.

Functional flow is the amount of volume flowing through your plumbing system. A good way to test this during your initial walk through is to turn the faucets on at the basin, flush the toilet and then turn the water on to the tub or shower. Some decrease in volume is normal. A very noticeable decrease in volume is not. This is an indication that there is a plumbing issue which may need to be further investigated by a licensed plumber.

Reduced water volume

Some of the causes for weak functional flow are:

  • Galvanized pipe systems. Generally these have about a 50 year life expectancy and like your arteries - they get sclerosis. They rust from the inside out. Although the lines may be ½ or ¾ inches in diameter, due to sclerosis, the functional diameter may be ¼ inch or less. This restriction in the lines reduces the amount of volume that they pipe system can deliver even if you have 100 PSI of water pressure. The horizontal lines are the ones that are affected most. The vertical lines can last longer, but will need eventual age replacement and will be more costly to replace as more invasive measures will be needed.
  • Calcium or lime build-up on the internal walls of the pipes in hard water areas act in a similar fashion to that of the conditions for galvanized pipes.
  • Poor plumbing techniques. Most municipalities now require a minimum of ¾ inch supply line from the main to the water heater. Often times, I see the supply reduced to ½ inch supply lines at the entry point of the property. This adds more restriction to the system and can adversely affect water volume in a larger property with more than 2 baths.
  • Another poor plumbing technique is the use of too many fittings such as tees and ells. Each time water goes through a bend, some restriction is encountered. Generally, there is enough pressure to overcome this small amount of restriction. However; too many bends can lead to too many restrictions and start to impede water volume.

If you live in a property where you are experiencing water volume issues, consider having your plumbing system further reviewed by a licensed plumbing contractor.

If you find this information helpful, feel free to pass it on to a friend or colleague.

This comment has been brought to you by Michael Thornton - Tennessee's Favorite Home Inspector. Michael is an ASHI certified inspector and president of Complete Home Inspections, Inc..  We are serving the greater Nashville Metro area one satisfied customer, one house at a time.  


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Complete Home Inspections, Brentwood TN 615.661.0297

Comments (6)

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Keller Williams Realty - Ann Arbor, MI
Ann Arbor Area Real Estate
I'm bookmarking this to save it for later.  Excellent information.  Thank you!!  I did a blog about well and septic, I would love it if you did the same info for those of us who sell with well systems and water pressure issues!  Thanks again.
Dec 19, 2007 10:17 PM
Neal Bloom
Brokered by eXp Realty LLC - Weston, FL
Realtor CRS-Weston FL Real Estate
I have a property that is currently being rented...it was built in the 1950's we have had problems with the pressure and we get that smell of sulfur...it is just the whole area most likely the water district has a problem or maybe because it is close to the beach...not sure but I always wonder why the city doesn't address it.
Dec 19, 2007 10:44 PM
Larry Brewer - Benchmark Realty llc
Benchmark Realty LLc - Nashville, TN
good post. I'm sure a lot of us have seen this happen.
Dec 19, 2007 10:46 PM
Vincent McKamy
Samson Properties - Fredericksburg, VA
Realtor Fredericksburg Virginia

This is a great post - I have been asked a couple of times about this issue.  I to am going to save your post for future reference. 


Dec 19, 2007 10:51 PM
Jeannie Kontis
Lancaster, PA
This is very informative -- thank you!
Dec 19, 2007 10:51 PM
Charlottesville, VA

The last one, to many tees and elbows I see a lot as a problem. Great information and thank you.

Merry Christmas!

Dec 19, 2007 11:16 PM