Under Pressure

You’re having a shower and somebody flushes the toilet.  Suddenly, you don’t have water at the shower any more. Or maybe you run out of cold water when the toilet flushes and get scalded. Sometimes you run out of hot water in this scenario. In any case, none of these options makes you happy. Chances are, you have a water supply problem.

The water supply may be less than ideal for a number of reasons. These reasons may apply to the service piping that brings the water from the city main to the house (or from the well, lake, or river to the house) and to the distribution piping within the house.  Let’s look at some of the causes.

  1. Excess Pipe Length:  The amount of pressure drop is dependent on pipe length. The longer the pipe, the more pressure loss there will be due to friction as the water flows through the pipe. Houses set well back from the street or a considerable distance from a well, may have lower pressure because of the long supply piping runs. The distribution piping runs in the house can be long as well, further reducing flow and pressure available to the faucets. Elbows and tees are each equivalent to several feet of pipe and convoluted systems suffer as a result.
  2. Static Pressure:  More flow means less pressure. Less flow means higher pressure. Consider a garden hose. If we take a garden hose without a nozzle on the end of it, water will plop out of the hose and fall to the ground. We wouldn’t be able to spray someone 15 feet away with the water coming out of the hose.  We would, however be able to fill up a bucket fairly quickly.  If we put a thumb over the end of the hose and just let a little bit of water by, we are reducing the flow rate and raising the pressure at the nozzle. Because only a small amount of water is moving past every point along the pipe, there is considerably less pressure loss due to friction.  This pressure would allow us to easily squirt someone 15-20 feet away.  If we could get our thumb to completely stop the water flow, the static pressure at the end of the garden hose would be the same as the city water pressure at the street. From this example, it is easy to see that the larger the flow rate, the lower the pressure. If we wanted to fill up that bucket we talked about, we’re better off to leave our thumb off at the end of the hose. We’ll get more gallons per minute flowing out of the hose at a low pressure. This analogy works well in the bathroom. To prepare a bath, we would use the tub spout because it delivers more gallons per minute and the tub will fill quickly.  Using the showerhead delivers water at a higher pressure but a lower flow rate. A shower is great for rinsing shampoo out of our hair because of the velocity that results from a higher pressure. A showerhead would be a poor way to fill a bathtub though because it would take much longer than using the tub spout. The flow of water into the home from the source can affect how the pressure is distributed throughout the home.
  3. Rusted Piping:  As mentioned briefly in the section above, galvanized steel piping is notorious for rusting on the interior. It will eventually leak, usually at the threaded joints, but before this, the water pressure will drop. The rough surface creates friction loss, and the accumulation of rust on the inside of the pipe restricts the flow by reducing the diameter of the pipe. Some people say you have to replace all the pipe to make a difference. That is not true. For every foot of pipe that is replaced, you will enjoy some improvement in water pressure and flow. If someone is only going to replace some of the piping, it makes sense to do the horizontals, which tend to rust more quickly than the vertical pipes (risers).  It also makes more sense to do the hot water piping before the cold water piing since the rusting is a chemical reaction that happens more quickly at higher temperatures.
  4. Undersized pipe:   It is obvious to most people that the bigger the pipe, the more water that you can move through it. What’s not so obvious is how dramatic this is. For example, changing the pipe diameter from a ½-inch to ¾-inch makes a very large difference. Many would say that a ¾-inch pipe is 50% larger than a ½-inch pipe and you should be able to get 50% more water through it. Actually, a ¾-inch diameter pipe is 225% of a ½-inch diameter pipe in cross-sectional area!  So now that you have an idea about how important pipe diameter is to pressure/ flow, you can appreciate some of the problems associated with corrosion in galvanized steel pipes. Not only does the surface become considerably rougher, which reduces flow, but it also reduces pipe diameter and the ability of the pipe to move water.  We will touch more on that later.  In some older homes, the supply pipe diameter may be as small as 3/8- inch.   In new houses, ¾-inch is considered minimum. In large houses or houses with more than 3 bathrooms, many jurisdictions call for a 1-inch pipe.  This pipe diameter is often reduced once water splits up into a hot and cold distribution system.
  5. Obstructions:  There may be obstructions in the pipe itself. Obstructions can also be lodged in valves. Water meters, pressure regulators, water softeners, and other treatment devices all present obstructions to flow and can trap foreign materials. A sludge build-up in a water heater will cause poor pressure on the hot water side but not on the cold side. A partially closed valve presents an obstruction to water flow. Piping that is crimped or mechanically damaged effectively reduces the pipe diameter and the loss of water.
  6. Leaking:  If the water supply pipe is leaking outside the house, the flow and pressure inside the house will be reduced. In most cases, this will be noticed, but if the water leaking from the pipe is carried away quickly from free draining soil or a storm sewer, for example, this may go undetected.
  7. Elevation Losses:  Houses built well above street level or at the top of a hill may have lower pressure and flow than houses near the bottom of a hill or houses built below street level. We have talked already about how we lose pressure when pushing water uphill. The static pressure will be slightly less on the second story than it is on the first. Pressure on the third floor will be slightly lower than on the second.


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