For most of us, our water comes from a municipal water system, where it's usually extracted from various groundwater sources and treated to remove impurities. It is then pumped to large water towers that rise above than the highest delivery point in the system. Gravity provides the pressure that forces the water through the water mains and eventually into our homes.

Remember that water in a closed system will possess the same characteristics as it does in a lake or a reservoir. That is to say that the water pressure at a given elevation in any pipes carrying it away from its source will be the same as that found at a comparable elevation directly below the surface level at the source.

This is another way of saying that water seeks its own level. It can be demonstrated by siphoning fluid from one container to another with a rubber hose; it only works when the source is at a higher elevation.

A cubic foot of water weighs 62.4 lb. and exerts .4333 lbs of pressure per square inch on the bottom of its container at a depth of one foot.

Therefore, the discharge pressure of a water storage tower 100 ft. tall will be 43.33 psi. If your house is in a valley 50 ft. below the bottom of the tower, the theoretical water pressure at your house will be approximately 65 psi, reduced by the friction in the pipes.

The pressure of the water delivered to your home will vary depending on your elevation relative to the source, as well as the complexity and condition of the piping system that delivers it. But once established, the pressure will be constant, and will not change based on demand.

Typical residential systems are designed to function under a pressure of 40-60 psi. If the water pressure entering your home exceeds this level, you may need to install a pressure regulator in the line to reduce the pressure to an acceptable range. This protects the equipment in your home from damage caused by excess force exerted by the water.

On the other hand, if the pressure coming from the main is below 40 psi, you may need a pressure-booster to bring it up to acceptable levels.

Do not confuse water pressure with water flow.
Water flow is the result of pressure on volume. Volume is the amount of water available for delivery, and pressure is the force exerted on it. There is a big difference between the two.

If the low pressure in your system is caused by an obstruction in a supply line, or if the line to your house is undersized, pressure may not be able to be increased.

A very old pipe may contain mineral build-up or corrosion that reduces its internal diameter by half or more. This would be like a fireman trying to douse a five-alarm fire with a garden hose. It can't be done.

Adding a second hose to the same spigot will only reduce the flow in both by half. Pressure is a one-way street. It can always be reduced, but it cannot be increased unless there is sufficient volume to sustain it, and the volume is strictly a factor of pipe capacity.

Think of your plumbing system as an hourglass, with the city water main on the top, and the fixtures in your house being at the bottom.

Under a constant pressure, such as in a city water main, only so much water can pass through the supply line in the middle in a given amount of time. It is not affected by demand, so adding a booster pump on the bottom will not affect it.

The standard residential supply line is ¾ inches. This can feed two ½ inch lines to full capacity simultaneously.

But most mechanical designs call for a ½ inch line to each bathroom. They can supply two 3/8-inch lines to capacity, but not another ½ inch line at the same time. That is why a person taking a shower will notice a decreased water flow when someone flushes a toilet in the same room.

'Carwash' showers and oversized whirlpools may need increased water flow to operate the way you expect them to, but if the available volume is insufficient, increasing the pressure will not help.

That's why it's important to identify your needs before construction begins so that an adequate water supply is provided where you need it.

In new construction, it's easy to tell whether your home's water flow is adequate because the pipes are new, so all you have to do is measure it.

But in older homes, when the flow drops, it's more difficult to identify the cause. If your neighbors all have adequate water flow and you don't, then the chances are there is something wrong either with your supply line or the branch lines inside your house, and the only solution is to replace them.

See Also:

• Cisterns
  Texas Guide to Rainwater Harvesting  (PDF) - although this document was written for Texas, almost everything in it applies everywhere
  
  
• Water Wells
  Wellowner.org
  Nat'l Groundwater Association
  
  
• State of Kentucky
  Building a Cistern for Home Water Supply
  Cisterns for Kentucky
  Management of Wells for Drinking Water
  Assesing and Reducing the Risk of Groundwater Contamination from Drinking Well Water Condition
  
  
• State of Ohio
  State of Ohio Rules on Private Water Systems - These rules apply to both wells and cisterns.
  
  
• Graywater (for non-drinking use)
  Home Use of Graywater, Rainwater - from the State of Arizona
  
  

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