water pressure

[momof4]

I just currently moved into a new house. We are on city water, if that has anything to do with it. The pressure coming from all of my pipes is next to nothing. I was wondering if anyone knew of an easy and inexpensive way to fix this problem without having to dig up all of the pipes and replace them.

Mary Osborn

[LazyPup]

You stated that you moved into a new house, but I suspect it may be an older house, just new to you.

There are a number of factors that can adversely effect water pressure.

In all cases begin at the main water shutoff valve and trace all the lines as well as you can, locating any shutoff or zone valves that may be on the lines. Make sure the valves are turned wide open.

If the house is an older structure that was originally built prior to 1960 it more than likely has a 3/4" galvanized iron pipe main water line from the municipal line to the structure. As galvanized iron pipes age the develope a terrible layer of mineral scale and rust on the interior walls of the pipe. The mineral scale buildup will effective reduce the working diameter of the pipe, and as diameter decreases velocity of flow must increase proportionally. The Venturi principal dictates that as the velocity of a fluid, either liquid or gas, increases the pressure will decrease proportionally. In a worst case scenario the scale can become so bad that you may only have a minute passage remaining in the lines, often down to an 1/8" inch or even less. If this is the case the only solution is to replace the line from the street to the house.

Before electing to replace the line have the pressure tested at your main water shutoff valve in the house. Under the International Residential Code the minimum acceptable water pressure is 40psi. Under the Uniform Plumbing code the minimum is 15psi.

Under both codes the maximum allowable pressure is 80psi.

Check your natural gas service contract. Many gas companies offer what they call a "Line Backer" policy on your gas bill. In exchange for a small monthly premium they guarantee the gas and water lines, even if they are not your water provider. If you have the line backer contract they will replace your main water line under the insurance contract.

Replacing the line is not nearly as difficult as it may sound. They dig a hole at the street connection and another where the line enters your house, then they use a "Ditch Witch" type trenching machine that has a blade that looks like a huge chain saw. The trenching machine cuts a trench about 4" to 6" wide and down to the required depth. In most areas they then install a 3/4" polyethelene continuos roll line from the street to the house. They then cut the old line and make the final connections on each end and backfill the trench. They can usually replace a line of 100' or less in about 4 hours with a minimal damage to the landscape.

If you already have a copper or synthetic water line it is very doubtful that you have any scale in the lines simply because scale doesn't form well in copper or synthetic pipes, however, there is the remote possibility that you have a break or crimp in the water line from rocks or tree roots in the soil. In the case of a break you can usually locate the break by looking for a wet spot in the yard, but normally it is cheaper to replace the line than it is to locate and repair a leak.

If you have copper or CPVC pipe inside the structure it is unlikely that you have any scale problems in the pipe, however, if you have galvanized iron pipe in the structure you are again confronted with the same scaling problem and the only solution is to replace the piping with copper, PEX or CPVC.

While we commonly think of the water supply as being one system, in fact we have two completely separate and parallel water supply systems, the Hot and the Cold.

In order to insure a proper pressure balance the main line from your "Main Water Shutoff Valve" to the water heater must be capable of carrying twice the capacity of the lines downstream of the water heater junction. For this reason, the line from the Main Water Shutoff Valve to the water heater junction would normally be a 3/4" line while the hot and cold lines from that point onward would be 1/2" lines. That may sound sily but a 3/4" line has more than twice the capacity of a 1/2" line.

When comparing line size we must consider the cross sectional area of the pipe rather than the diameter, therefore the formula is:

Large Diameter Squared divided by the small diameter squared.

Example: when comparing a 3/4" line and a 1/2" line first convert the fractions to decimals:

3 divied by 4 =0.75
1 divided by 2 = 0.50

(0.75 x 0.75) / (0.5 x 0.5) =
0.5625 / 0.25 = 2.25

Thus a 3/4" line can carry 2.25 times as much volume as a 1/2" line.

When determining pressure we must also consider "vertical static head"

Let us assume that your house has a basement and the water line enters near the floor of the basement and you have a two story house with a bathroom on the second floor.

The vertical rise from the basement to the shower head would then be 9' for the basement (alloying for the floor joists), another 9' for the first floor, and another 7' to the shower head. That would equal 25' vertical. We must then deduct the physical weight of the water in the pipes, which is 0.433psi/ft vertical.

25 x 0.433 = 10.825psi. static loss.

This means that the available water pressure at the shower head would be 10.82psi less than the supply pressure at the Main Water shutoff valve.

Now, the next thing we must consider is, have their been any additions made to the house since the original water line was installed? Did those additions put additional loads on the water supply, if so, the main line may be grossly undersized to meet the demand.

Now, the codes do demand some minimal water supply pressures throughout the structure, by example, a shower must have a minimum of 4gallon/min at 8psi. A tank type toilet requires 1.6gal/min at 15psi but a one piece toilet requires 6gpm at 20psi.

If you could post the layout of your house including how many plumbing fixtures on each floor and an approximation of the maximum distance from the water main to the furthest fixture I would be glad to work up the pipe sizing to give a working example of how we compute the main and distribution piping.