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Pressure and Flow in Water Pipes

Low water pressure and flow in household water supplies

When we are designing a water system to supply water, hot and cold, to bathrooms on the upper floors of a house it is important to increase the water pump supply pressure to pump the water to the higher level.

We also need to make sure that hot water pipes are kept as short as possible and with small diameter pipe to make sure that it does not take a long time for hot water to reach the bathroom. Here we look at the technical factors we need to consider.

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water pump with pressure tank

Let us take for an example plumbing problems in a 4 storey house, on the ground floor is a water tank with a water pump to pump water around the house. Up on the 4th floor is a bathroom with the owner's shower in it. The owner has terrible problems with water pressure, while there is good pressure on the ground floor, there is very little pressure up on the 4th floor and every morning water only dribbles out of the shower head. If anyone turns on a tap on any lower floor no water at all comes out of the shower or taps on the top floor.

While the pressure, or lack of it, was bad enough, waiting for the hot water was even worse, it took forever for hot water to reach anywhere in the house.

The first factor was that the bathroom was up on the fourth floor of a very tall house. In fact the water pump was powerful enough but, set at 30 to 40 psi didn't give much pressure at the showerhead 22 metres above.

The Measurement of Water Pressure

Let us first get to grips with the figures. psi (pounds per square inch) are those old imperial units that should have died with the ark but are of a very convenient size when talking about water and air pressure and this could well be why they are often still used on tyre pressure and water pump gauges.

There are, of course, other units of measurement for pressure, for example divers (or anyone else with water in their ears) use the 'atmosphere' which is the pressure of the air at sea level and is equal to 14.7 psi. For you who think in the more progressive metric headspace 1 psi equals 6.89 kilopascals (kpa) and a bar (almost the same as an atmosphere) is 100 kilopascals or 14.5 psi.

All very interesting but let's see how this applies in practice.

The effect of height on water pressure

Water is rather heavy stuff. If a diver goes down 10 metres the weight of water pressing down on him increases the pressure by an atmosphere (an additional 14.7 pounds of weight on each square inch of his skin) on top of the air pressure at sea level. The same applies to water pumps, you will need to pump water up to a pressure of 14.7 psi to lift it 10 metres.

In our problem house with a shower head 22 metres above the surface of the water in the water tank the pump will have to pump a pressure of 32 psi to get it up there. This pressure will only just get the water up there, it won't give any additional pressure to make a dribble into a squirt.

In addition the water pump was set between 30 and 40 psi. Pumps of this type have 2 switches, the first senses when the water pressure is low and switches the pump on (in this case 30 psi) and the second when the pressure is high which switches the pump off (40 psi). It can be seen that the pressure in this system would have varied between 30 and 40 psi and most of this pressure was needed just to lift the water, no wonder the shower just dribbled.

Large Diameter pipes and a long wait for hot water

This house had a second problem in that the pipes in this house were a mass of expensive green pipes. Because the position of the pump had not been thought about carefully an additional 17 metres of pipe had been used. To add to the problem even further the inside diameter of the pipe was 20 mms (millimetres) when 12 mms would have been adequate.

As a result of 17 plus 22 metres of 20 mm diameter pipe the pipes had 12 litres of cold water in them which would have to be replaced by hot water before the hot could reach the shower, 12 litres at a very slow flow rate. By using a more sensible 22 metres of 12 mm diameter pipe only 2.5 litres of cold water would need to be replaced by hot which would bring the hot water delivery time down from around 10 minutes to a mere 20 seconds.

As a final mistake only a 50 litre hot water tank had been installed. This is too small so by the time the hot water did get there the tank was already minus a quarter of it's hot water.

Steps to reduce your water pressure problems

So, to make sense of all this:

If you have low water pressure check the pump pressure and the height difference between the surface of the water in your water tank and the height of the showerhead or other water outlet. For every 10 metres of height difference you will need about 15 psi of water pressure and then a further 15 or 20 psi to give a good healthy pressure in the water flow. You can get someone to adjust the pump to give you this.

To get hot water quickly you need good pressure and small diameter hot water pipe (12 mm internal diameter is alright). You also need a reasonably short pipe length directly from the heater to the shower. Bear in mind that if the ambient temperature is always quite warm you won't need oodles of boiling hot water for your shower so a small diameter pipe will suffice.

Finally a rule of thumb for storage water heaters, you will need a 50 litre tank for a kitchen, an 80 litre tank for a bathroom and a 100 litre tank for 2 bathrooms. Solar water heaters need larger tanks.

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Copyright © Phil Wilson October 2011
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