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Plumbing in the Home/Whole House Water FIlter Piping


QUESTION: I have a GE GXWH20S Whole House Water Filter housing with a sediment filter.  The in/out pipes are 1/2" PVC.  I would like to replace this unit with a Big Blue filter housing and a carbon filter.  The in/out pipes must be 3/4 inch to fit the new unit.  My plan was to install a 3/4" piece of PVC pipe into each side of the new unit, add a coupler to downsize the 3/4" pipe to 1/2" and then connect to the existing 1/2" piping.  Will this result in any change to water pressure in the house?  Also, the water source pipe from the street is 1/2" copper (at least the part that comes up from the ground near the house) and this connects to the 1/2" PVC.  Would changing all of the PVC pipe to 3/4" or 1" change the water pressure in the house?  Thank you.

You, like many, have a misunderstanding about the difference between flow rate and pressure. You can develop 200 PSI of pressure through a 1/4 inch pipe but you wouldn't be happy with the flow rate that it provided.  

Effective flow rate is measured in gallons per minute of flow. This rate is a function of pipe size and available pressure. Unless you are on a private well system where you can adjust the pressure, you are pretty much stuck with whatever the city delivers to you.

Every fixture in a house requires a certain amount of flow rate to function correctly and acceptably. This is usually calculated in "fixture units". A fixture unit is nominally 7.5 gallons per minute of flow.
As an example: a toilet requires three fixture units, a shower 2-3 fixture units, a lavatory sink is one fixture unit. To determine the proper pipe sizing, you must calculate the entire fixture unit demand of the plumbing system and size the pipe accordingly.

Having said that, 1/2 inch pipe is not enough to run a typical house plumbing system. 3/4" is the typical minimum size for a nominal water pressure of 40 to 60 psi at the source. If your water pressure is very low, increasing the pipe sizing throughout the system will help deliver acceptable flow rate to the fixtures.

Assuming typical working pressures of 40 to 60 psi, a half-inch pipe will only supply 4 fixture units. A typical one bathroom house, including a kitchen and a laundry requires a minimum of 12 fixture units. Do the can see the problem.

If your main house supply line is only 1/2 inch, you are already at a disadvantage as far as maximum flow rate and will probably experience flow rate fluctuations if more than one fixture is used at the same time.
This phenomenon is illustrated very well by the experience of getting scalded or frozen in the shower if somebody flushes the toilet or turns on a dishwasher. Water will take the path of least resistance and most of the flow will go to the first demand online and your shower will suffer.

There is also the consideration that the actual filter has a flow rating as well and will have an impact. Every restriction such as a filter, a valve, an elbow fitting or any rise in elevation above above the source will reduce the flow rate.

Good Luck,

---------- FOLLOW-UP ----------

QUESTION: Thanks Dana for the education and good information.  You are right about 1/2' pipe and the flow rate fluctuating when more than one fixture is used in the house at the same time.  I experience that now, and, short of replacing all the piping, must live with this.  

I'd like to go back to my original question. The Big Blue filter that I would like to purchase/install only accepts 3/4" pipe.  If I insert two short lengths (e.g., 3-4" each) of pipe into the inlet and outlet holes on the filter, and then connect up to the 1/2 pipe with connectors, will this reduce the flow rate to the house?  From your answer, it appears that it will not -- but please confirm.  

Also, I said the copper source pipe was 1/2'.  I assume that it is based on the external appearance of the pipe. The copper pipe looks like the same diameter as the PVC pipe to which it is connected (which is 1/2 inch).  However, when I think more about this, the copper pipe has thinner walls than the PVC pipe so maybe the interior diameter of the copper pipe is actually 3/4".  Do you think this may be correct? If this were so, changing out all the PVC which carries water from the copper pipe, through the filter and water-softener, and then into the house, may improve flow rate in the house. Correct?

Thank you in advance for your follow-up answers!

1/2 inch copper pipe as an outside diameter of 5/8 inches. Three-quarter inch copper pipe has an outside that matter of 7/8 inches. This would be similar to the outside diameter of half-inch PVC due to difference in wall thickness. You may have three-quarter copper pipe supply already. You just need the correct adapters.

Working with copper pipe can be tricky unless you know how to sweat copper. There are new systems available called "sharkbite" that you can use without needing to sweat copper piping. Just check with your local supplier or Google them to see what fittings and adapters you will need to rig this system.

It is also noted that PVC piping should not be exposed to sunlight. If it is, it must be paid to protect it from the ultraviolet or it will quickly degrade and become brittle. I'm not a big fan of PVC piping myself but I know it is used extensively in other areas of the country.
Good Luck,

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Dana Bostick


Pretty much any residential plumbing questions. For "item specific" details such as a specific model of fixture, I will need to research and there may not be any useful information available. Note: I live and work in Southern California. We do not, as a rule, use hot water or steam heating systems, oil fired boilers or private water wells so my knowledge in those areas is pretty limited. There are others here on AllExerts that can probably answer those questions better.


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