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Water pressure, or lack of it

Posted by walrus (My Page) on
Tue, Feb 8, 11 at 18:40

Been living in an 1870's house for about 10 years and never really had good water pressure if two things were running at the same time, any two things. Any one thing gets fine pressure.

There's a length of 1/2" copper pipe that runs about 35 feet in the basement before any fixtures. I don't know if that's limiting the volume of water, or if it's an ancient pipe from the street which may be constricted.

Would a 35' length of 1/2" copper restrict the flow of water to multiple fixtures?

I've been thinking of adding a 3/4" or 1/2" line to the run. Would that help?


Follow-Up Postings:

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RE: Water pressure, or lack of it

If you can sweat solder why nut cut the pipe at the end of the 35 run and check the flow.


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RE: Water pressure, or lack of it

I can open a faucet to do that.

If one thing is open the pressure is fine. Two things and it's noticibly less. Three things and water almost goes to a dribble.

Could it be the long, 1/2" run in the basement?


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RE: Water pressure, or lack of it

Volume of flow is determined by the size of the orific at the load fixture.

If a line feeding the load is undersized it will continue to supply the proper volume, but in doing so it will dramatically increase both the velocity of flow and the friction loss in the line.

In order to properly determine the correct size of a line we must have a list of all fictures that are feed by that line and the maximum length of the line.

There is no doubt that the line in question is probably undersized, but even that would not cause the type of problem you are having.

Let us examine a 30foot length of 1/2" pipe:

@ 1gpm velocity equals 1.05ft/sec and pressure loss equals 0.129psi.

@ 4gpm velocity equals 4.22fps and pressure loss equals 1.68psi

@ 8gpm velocity equals 8.44fps and pressure loss equals 6.06psi

@11gpm velocity equals 11.6fps and pressure loss equals 9.162psi

If the 1/2" was increased to 3/4" at 11gpm the velocity of flow would only be 5.61fps and pressure loss would be 1.86psi.

Code requires that when sizing lines we must have a list of all fixtures served by the line and the length of the line. We must then use a fricton loss chart to determine the correct size of line insuring that the velocity of flow in a copper line does not exceed 8fps or 12fps in a plastic line. If the velocity of flow exceeds those limits we see excessing pipe wall erosion, which is the leading cause of pineholes in copper pipes.

In your case you are having problems with both volume and pressure whereas volume would remain nearly constant in an undersized pipe.

You stated that the house was built in 1870, and it is doubtful if it even had municipal water when it was first built, but I would guess that the main water line feeding the structure is a 3/4" galvanized iron pipe which has probably been in service for 50 years or more.

Over the years galvanized iron pipe developes a severe corrosion from minerals and salts in the water. In many cases that corrosion is so severe the the pipe literally full of a sand like material and the water is forced to flow through a minor pipehole down the center of the pipe or in some cases it has to flow through the corrosion like leaching through sand.

In most cases when we are dealing with an older home such as yours the only solution is to replace the main supply line to the structure.


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RE: Water pressure, or lack of it

Thanks for the scientific approach.

If I understand it correctly, the run of 1/2" copper should be capable of handling 9GPM, and in use since it's more than that distance to fixtures can assume half that. Since we're using perhaps 5GPM simultaneously at different fixtures, and suffering, it would appear the problem is mostly from the street.

We'll check into that come spring.


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