Home » Resource Library » Water

Print E-Mail Share  Text Size

Water Pressure vs. Volume
(Not the same as water Flow or Pressure)

Water volume:
the amount of water that a pipe can deliver/supply per minute, regardless of water pressure.

The size of the pipe determines the amount of water available for delivery, or to put it another way, a pipe can hold a certain amount of water no matter what the pressure.

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 how many faucets are open in the house (demand), or the pressure being exerted on the water.

These principle applies not only to the service to your house, but to branch lines within your house as well.

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.


An excellent example of this is the dual-hose connector, which allows a second hose to be connected to a single outside faucet. What happens when two hoses are connected to a single faucet?

Turning on both hoses will not double the amount of water coming out of the supply pipe. In fact, the amount of water going to each hose is reduced to one half of the amount coming out of the faucet.

It will take each hose twice as long to fill a bucket, as it would if the faucet was used instead.

This principle explains why a person taking a shower will notice a sudden reduction in water 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 increase the amount of water coming out of the faucet.

It's All About Pipe Size

The causes for low water volume include:

1. Internal Plumbing Pipes Too Small
   A pipe can only hold a certain amount of water. The smaller the pipe, the less water it can hold.
   
   Water lines coming into your house are generally 3/4" or 1" pipe which can provide adequate volume and pressure. But, in many homes, internal piping is only 1/2" or less. This not only reduces the volume of water up to 75%, and can also reduce the pressure by 75%.
   
   
2. Adding Bathrooms and/or Fixtures
   If you are planning to remodel, be sure the water pipe diameter can handle the additional demand for the new bathroom and/or fixtures.
   
   
3. Plumbing Appliances
   Devices such as water softeners can increase pipe volume by dissolving deposits in the pipe. On the other hand water filters, flow restricting fixtures and decorative faucets can restrict the available volume of water.
   
   
4. Obstructions
   In older homes, when the flow drops, the cause is more likely the volume in the supply lines. 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.
   
   
5. Galvanized Pipe
   Rust and minerals can build up in older, galvanized plumbing decreasing the capacity of the pipe.
   
   Before 1980, galvanized steel plumbing pipe was widely used in many parts of the country. It is silver colored when new, dulling to gray as it ages.
   
   Over time, galvanized pipe corrodes on the inside. As water passes through, corrosion deposits build up inside and partially block water flow.
   
   Older pipes may be so corroded that even a pencil would not fit through the center of the pipe. The smaller size means less water volume . In most cases you cannot clean the inside of the pipe without causing leaks.
   
   You can test for corrosion by running 3 faucets in your house at the same time. If the water flow in the first faucet decreases significantly when you turn on the others, your pipes are probably corroded and you may need to replace pipe. Replacing galvanized pipe usually requires a plumbing permit.
   
   If the water is discolored when you first run the faucet, you may also have a corrosion problem. Let your faucet run for 30 seconds or more – until the water feels noticeably colder. This flushes standing water out of the pipes and replaces it with fresh water.
   
   

A reputable plumbing contractor can recommend solutions to increase your water volume.

 
Learn More About ...

Get to Know Your House One House ... Many Systems Water Supply		About Water Water Quality Hard vs. Soft Water Common Water Problems Water Problem Tables Water Conservation		Related Articles Cisterns Water Wells Water Pressure Water Pressure vs. Flow Water Pressure vs. Volume Water Pressure vs. Head Backflow
Components: Pipe  -  Fittings  -  Valves  -  Flanges  -  Pumps  -  Water Heaters  -  more »