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I'm getting a new shower installed in my house, to replace the existing mixer shower. I am going to stick with a mixer shower, not least because I don't have a water tank. So, this means I have a high pressure system.

There are 2 pieces of terminology commonly used with showers - pressure, and flow rate. As I understand it, the flow rate determines how much water can pass out of the shower per minute, and pressure determines how much you can restrict the water's egress while maintaining the flow rate.

Now, I want to get the most powerful shower possible with my current setup. I understand that I am limited to the pressure and flow rate coming in from the mains because I have a high pressure system. In my experience, though, what makes a shower feel "powerful" is a third factor - the water velocity. The velocity of the water coming out of the shower head makes a major difference - if it's really fast, that's what I call powerful, as it can give a really nice massaging feel. That, combined with a good flow rate, is what I'd like to get. So, what should I do to get the best water velocity from my shower? Additional queries:

  • Is it the shower head that would be best at giving a good water velocity? Presumably a shower head with fewer, smaller holes will restrict the water egress and force the water velocity to be faster in order to maintain the flow rate, assuming there is enough pressure? If so, what kind of shower heads would give this effect?
  • Can anyone recommend any particular showers that are good at feeling powerful (basically, having a high water velocity to give a good massaging jet of water)? Conversely, can anyone give examples of showers that are designed to have a low water velocity?
  • Is it possible that increasing my boiler size will allow a higher flow rate of hot water to pass through my shower? How can I measure what the maximum flow rate of hot water would be, and what the maximum flow rate of hot water is for my boiler?
  • Why do different showers have different flow rates (sometimes quite significantly different)? It seems to me that the flow rate is basically dictated by what goes in to the shower - does this mean that showers with lower flow rates are artificially reducing the flow, and if so, why?
  • In the UK, every store says it's "not possible" to use a pump (like you would with a low-pressure power shower system) with a high pressure system. Is there any technical reason for this, or is it purely a legality (it's illegal to pump from the mains because it would reduce everyone else's pressure/flow rate)?

The more detailed the answer, the better. I'd really like to get a good knowledge base built up of how to maximize the power of your mixer shower.

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1 Answer

Official term is "water velocity" but for domestic plumbing this isn't really a thing you need to concern yourself with stick to pressure and flow rates. To answer some of your questions all in one: The flow rate can vary between showers due to the plumbing that supply's them also the pressure can vary for the same reason, they can also have water restrictors that slow the flow rate down. Water restrictor In Australia this is due to regs stipulating maximum flow rates for water conservation reasons, ( I will always remove these as I hate a weak shower).

As for the flow rate you can use a flow cup to check the flow rate on a unrestricted tap and a pressure gauge to check pressure available.

Flow Cup

You can attach a booster pump to your water supply but if this is legal in your location I do not know, the use of a booster pump will also depend on the supply pipe you have to connect to and what the pressure is already if it is near the output of the pump there is no point installing the pump.

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I'm pretty sure it's illegal in the UK to attach a booster pump to the water supply, as anywhere you look it always says you must not attach a pump to a high-pressure system - I suspect this is purely for legal reasons, but I wanted to check whether there was any technical reason. Nobody ever gives you a straight answer on this one. :-) –  Jez Dec 4 '12 at 12:02
    
Why do you say that water velocity isn't something I need to concern myself with? Don't you agree that this is probably the most important thing in how "powerful" and "massaging" the shower feels? You can have a high flow rate but with slow water velocity, it will feel weak. –  Jez Dec 4 '12 at 12:04
    
what pressure do you classify as 'high pressure"?, .. Because water velocity is generally within a the scope of the mains unless your attaching some serious hardware to the system(eg designing a muti story building with booster pumps and reservoir tanks) if you want to get into water velocity en.wikipedia.org/wiki/Darcy-Weisbach_equation, but you don't really need to go there it's done for you, you have the flow rate and the pressure –  UNECS Dec 4 '12 at 12:44
    
When I say high-pressure, I mean the shower is connected directly to the mains and not a water tank. As for water velocity: if it is within the scope of the mains, that would mean you could not change water velocity yourself. But if you take a hose, turn the tap on full, then cover part of the nozzle, the water will come out with a higher velocity, yes? –  Jez Dec 4 '12 at 13:40
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Yes, feeble, no lie! There's evidence that people take longer showers due to low flow, especially with softer water, so the restriction ends up being counter productive. Fortunately, it's often easy to circumvent the flow restriction. And the cheapest heads that perform least well at 2.5gpm work just fine at 5gpm with the restrictor removed. Even more stupid, we have plenty of water where I live. It seems every regulation, no matter how well intentioned, gets all fouled up once executed. –  bcworkz Dec 7 '12 at 23:33
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