Shower Head Principles

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Retired Defense Industry Engineer xxx
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New England
People often complain about shower head pressure, especially if they plan multiple heads along with body sprays. In reality, what they mean is the force of the spray outlet can be weak...the pressure will be the same, regardless. Think of a soaker hose. The pressure of the water feeding the soaker hose is exactly the same as if you had a regular hose with a lawn sprinkler on the end. It's just that the soaker hose has a huge amount of small openings so that water gets dispersed out all of them.

What is really the issue is the force of the water coming out of the nozzles. Basic physics says that the force = one half of the mass times the velocity squared. From a rain shower head, the velocity will be essentially the result of gravity because there are lots of openings in the head. To improve the force, you can increase the velocity if the pressure of the system is increased, but practically, unless you have your own well with a pump, that normally isn't something people can do. There are add-on pressure pumps you can install in your home if your public water supply pressure is low, but that's sort of an uncommon response.

Pressure in a system has two viewpoints:
- static pressure (no flow)
- dynamic pressure (water flowing)
The static pressure only changes in a home with elevation caused by gravity...just like in a water tower, the height of the water varies and changes the pressure of the outlet. That variation is about 0.43 pounds per foot of elevation change. So, say your water comes into your house at 50psi in the basement, and you have a shower on the second floor that is 20' higher, if you were to measure that water pressure out of the shower (capped, so no flow), it would be 20*0.43=8.6psi lower, or 41.4 pounds just from the elevation change. Dynamic pressure varies based on frictional losses along the way in trying to move the water to its destination. Friction comes from the diameter of the pipe and any fittings that change direction or imperfections in the piping. The friction increases as the speed of the water moving increases, and in how many changes of direction you try to make it traverse. The friction also increases as the diameter of the piping decreases (trying to shove water through that smaller pipe), so a larger pipe will have less frictional losses and thus, be able to maintain the inlet pressure better. FWIW, a fire hose and a soda straw, fed from the same supply will have the same static pressure, but the dynamic pressure under operation, will have radically less pressure on the outlet and less volume out the soda straw.

This has all been leading up to the bigger factor in the force equation noted above...if you increase the mass by a factor of two, the force out of a shower head will increase by two. To get more mass, you'd need more volume. If, on the other hand, you could increase the velocity of the water by a factor of two, because it is squared, say you made it twice as fast, the force exerted by the water coming out of that shower head would be 4x more, rather than 2x (2*2=4, i.e, two squared). So, you get much more bang for the buck if you can increase the velocity than you will if you can increase the volume (and therefore mass). Some people will drill out the restrictor in a shower head or remove it, if it's removeable, to increase the force of their shower head. Up to a point, that will increase the force. It will also deplete your hot water supply quicker.

Why does a typical showerhead increase the force of the water coming out? The nozzle, because it causes a restriction, and increases the velocity of the water. This is referred to the Bernoulli effect. In a short restriction (nozzle), the water volume passing over time tries to stay the same. The only way for that to happen is for the fluid (works the same with air as liquids) to increase in velocity. Over a long distance, like say in a piping system where you go from a large pipe to a smaller branch, friction becomes the major component, and the fluid cannot maintain that velocity increase, so you just end up with a pressure reduction. But, in a shower head nozzle, because the distance is short, the velocity increases and comes out the other end faster, increasing the force as it hits you - the friction moving through the air on the outlet of the nozzle is minor, so it doesn't slow appreciably.

What does this all mean about how a showerhead 'feels'? Well, you can increase the velocity by having very small nozzles, or you can increase the supply pressure. Increasing the supply pressure is often not practical (although, changing the supply pipe size may be, to keep the dynamic pressure loss down).

For any US legal shower head, it must not flow more than 2.5gpm, so the volume (mass) of the water available in a shower head is limited. So, why do different heads have such a variation in 'feel'? It is the size and shape of the nozzles, and their quantity. The Bernoulli principle says you will only get a velocity increase when there is a restriction. That means that the area of the outlets of the nozzles must be smaller than the area of the inlet, or you're just dividing the water up into smaller streams versus running it through a restriction that will cause the velocity to increase - the difference between a rain shower head and a shower head.

This leads back to how much mass (volume) of water a typical pipe can supply. The Copper Institute, through research, has said there is a maximum flow rate through copper pipe that is safe for long-term use. That flow rate varies based on whether the water is hot or cold. The bigger factor in a shower is the hot flow rate, which is 5-fps. To keep that flow rate, the maximum volume you get from a pipe will depend on it's nominal size. For 1/2" copper pipe, that equates to about 4-gpm and for 3/4" copper, 8-gpm. As you exceed that maximum recommendation, the frictional pressure loss increases, you risk water flow noises, and you can as a result, literally erode the interior of the pipe. That last one will take awhile to notice, but when it eventually does exhibit itself, you may find pinhole leaks developing in your pipes.

So, a 1/2" pipe feeding a single US allowed 2.5-gpm shower head will have a restriction in it, regardless of the design, that could increase the velocity of the water coming out. That won't happen (much, if at all) on a rain shower head, because it has so many outlets, but will on a typical shower head. But, if you try to feed multiple heads, you risk either exceeding the flow velocity of the supply pipes, or not producing the restriction required to accelerate the water through the nozzles. Running the water quicker can also lead to water hammer, depending on how things are designed, and is one reason why some valves require them for their installation...they try to flow more than ideal based on the Copper Institutes research and guidance suggests.

How much acceleration (and resulting velocity) of the water coming out of a head is a big design tradeoff. Try to make it faster, and you increase the friction, which will decrease the volume (mass) of the water available. Keep the velocity up, will require fewer outlets, so maybe not a very good spread on the outlet. Some people love a wide spread (maybe best on a rain shower head), but others want a pressure washer to more easily get the shampoo out of their hair. There's a huge range in between, and only you can decide which is best for you. Some love the prickly sensation of a small, high-speed jet, some love the sensation of being flooded in water. There's a huge range in between, and finding one you like, given the limitations of the supply and water restrictions, can be a challenge to both the designers and the end users. Another factor is, there is no universal standard of water pressure in a home. You may have the code minimum, or maximum, and that will affect how the same head might work under those differing conditions. IOW, there is probably no definitive 'best' shower head that will satisfy everyone, because their end application may be different along with the individual preferences.

If the math involved interests you, do a search on Bernoulli Principle and read the Copper Institute's handbook where it talks about supply, volume, pressure losses, with recommendations.

I'm not going to go into head recommendations, because what's good for one person, may not be for another. The quality of the engineering plays a part in it, but the execution during manufacturing and the designer's preferences will play a big part as will how they are ultimately installed in the system.
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