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UK WATER SYSTEMS
There are 3 main water systems used in most housing in the UK.
!). Gravity fed systems from a cold water storage tank
normally in the loft space.
2). Mains fed systems using a combination boiler
3). Mains fed systems utilizing a pressurized hot water
storage cylinder.
Most Housing stock in the UK still use gravity fed systems
and when installing a shower in a property with such a system you need to
consider the following very carefully.
Water Pressure, a commonly used expression is the force that
water exits the end of a pipe. An incredibly simplistic view, but non the less
true. What is water pressure and where does water acquire its energy from.Lets first take a look at your cold water storage tank in
the loft feeding for example the bath cold water tap in your bathroom. The
static pressure at the tap is said to be the weight of the water acting in a
downward force from the level of water at the top of the tank to the spout exit
on the bath. Lets say its 4 meters. Water is also acted upon by gravity at 9.8
meters per second per second. Gravity acts upon mass or the weight of water in
the pipe.
PRESSURE =
LENGTH OF VERTICAL PIPE X DENSITY X
GRAVITY
This calculation gives static pressure and it is referred to
in a few terms you may recognise, head of water measured in meters or feet, Newtons or pounds per
square inch or bar.
In the quoted example you have 4 meters of static pressure.
That is the vertical distance between the outlet and the level of water in the
tank. Pretty simple and all you need to know you may think. So what happens
when you turn on the tap. Water flows out but at what pressure does it flow. We
know we have a static pressure of 4 meters but the pressure diminishes as it
overcomes friction in the pipe, pipe bends and finally the tap itself.
So the pressure when the water is exiting the tap is less
than the static pressure.
Normally this is not a worry but when installing a shower it
is.
The first problem you have is that you have to stand under
the shower head so the shower head is now 1.5 meters at least above the bath
tap outlet. You now only have 2.5 meters of static pressure and this pressure
has to drive the water through the pipe, the shower valve filter, the non
return valves, the thermostat cartridge the on off control and finally the
shower head itself. This results in a very poor flow rate and little pressure.
You can maximize the situation by using 22mm pipe, bending
or curving pipe rather than using 90 degree angle bends, installing a valve
designed to run on low pressure and choosing the correct shower head. The
results however will still not be ideal.
For a reference point it takes aprox 7 times more force to
drive similar amounts of water through 15mm pipe than 22mm pipe. Using this
theory there will be more available pressure at the entrance to the shower
valve once water starts flowing using 22mm pipe than using 15mm pipe.
It is also worth noting that the density of hot water is
less than cold and hot water has more friction to overcome as the pipe runs are
longer. This will give a pressure differential at the entrance to the valve
between the hot and cold. The cold will normally have an available pressure
greater than that of the hot.
For those interested take a look at the flow rate spread
sheet. Here we have columns indicating the pressure drop available and the flow
through 15mm and 22mm pipe. The equation used to calculate this is on sheet 2.
I have assumed a worse case friction coefficient for copper of 130 and internal
pipe diameters of 13.6 and 20mm. All you have to do is key in the available
head at the top in knm and it does the rest. Remember that this does not
include friction for the fittings bends and valves but is a straight piece of
pipe 4 meters long and vertical.
It clearly shows the merits of 22mm pipe runs.
PUMPs Many people look at boosting the available static pressure
by installing a booster pump. Installation can be complex but the basic rules
are as follows.
The pump must have an uninterrupted supply from both the
cold water header tank and the hot water storage tank. The feeds away from the
pump can go of to where you like. There will be variations on these rules so
close attention needs to be paid to the pump manufacturers instructions.
The shower head must also be below the level of the cold
water tank so that water will flow naturally and trigger the flow switches in
the pump which causes it to start.
If you are having a loft extension this may not be the case
and the pump has to be a negative head pump or you have a switch that starts it
manually. If it is hard or impractical to obtain the direct feeds
needed to supply the pump you can use a single pump and cut it into the feed
leaving the shower valve and going to the head.There are certain advantages to this method the main one
being that there is never live water sitting in the feed to the pump. So should
the pump seals fail it will only leak when you have turned on the water from
the shower valve.
Mains Fed Systems.
There are some basic rules here, you cannot pump mains water
and you cannot place a pump on a water supply from a combination boiler.
Also remember that mains water will usually come in at
ground level and has to go vertically upwards to the upper floors of the house.
So if you have 20 meters equivalent head or 2 bar coming in. You will not have
2 bar when it exits your shower head as the static pressure is immediately
reduced by the vertical distance between the incoming mains and the shower head
outlet as well as the pipe runs and fittings.
When considering a combination boiler check out the hot
water flow rate. The bigger and more powerful the boiler the more hot water
will flow from its heat exchangers.
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