Water Pressure Explained
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What is water pressure and how does it work?I have noticed that many of my customers are a bit confused about the issue of domestic water pressure and how it works. This page is going to try and explain the workings of domestic water pressure and touch on some of the problems and issues that you may have. I will also discuss how we deal with these issues. Most suburban homes in SA are connected to a main feeder pipe supplied by Rand Water or their municipality. The most common connection is via a 3/4 inch (22mm) pipe that Tee's in to the main supply. Just after the T you will find a consumption meter. After that the pipe runs towards and into your house and outbuildings. Most houses only have one of these main supply pipes and one meter unit. The pressure in the main feeder pipe (running outside on the pavement) is at a certain pressure as determined by your water supplier. This pressure is variable - but fairly constant for houses near each other on the same main line. It depends on where you are and who is suppling the water. It also depends on how many others are using the pipe at any given time and to a large degree the time of day! Some areas experience good average water pressure while others have pretty pathetic pressure - especially at 07:00 in the mornings as everyone has a shower! Here is an example. I live in Northmead, Benoni a couple of hundred metres from a water tower. I have never seen, nor will I ever see more than 1.5 Bars of static pressure. Down in Rynfield, that is significantly lower in elevation the pressure is very good. Okay, so the pressure coming into your house is variable, but you can determine a daily average pressure. Remember that between the "dead" period at night (12 to 4) the pressure is at it's highest. This is when leaks start or leak most! Some suppliers actually reduce the supply pressure at night - this reduces their leakage rate. The most important factor to consider is that pressure is not constant in any active (flowing) system. Pressure is inversely proportional to flow. The more water that flows the greater the drop in pressure. You see this if someone opens the garden tap while you are in the shower! There is a fixed quantity of water that will flow through a given diameter pipe at a certain dynamic pressure. It's actually FLOW that is the problem rather than just pressure. You measure flow in litres/minute and it's this measurement that fills your bath in so many minutes! People complain of poor water pressure - they should, strictly speaking, be complaining about poor water FLOW RATE! Dynamic pressure is the pressure (head) that drives the water from the shower rose. If you measure the actual pressure just before the (working) shower rose you will find that it is negligible. On the other hand Static pressure is that pressure that exist when there is no flow. This may be very high - say 5 Bars. The moment you open the tap the pressure drops to 1 Bar. Why? Because the restriction to flow is not allowing the pressure to be maintained in a now "open" system. WHAT I AM SAYING IS THAT YOU CAN, IN SOME CIRCUMSTANCES, HAVE A VERY GOOD STATIC PRESSURE BUT A PATHETIC FLOW RATE. What is a "good" pressure? In a domestic environment, I would say that anything over 3 Bars is great. From 2 to 3 Bars is fine and below 2 Bars is starting to get irritating. Under 1 Bar is a problem. Here I am talking static pressure. What does this mean to the home owner? You may find that the pressure at the main supply is okay, but is really poor in the house. The first thing to check is the diameter of the feed pipe to the house. Some plumbers used 15mm (1/2 inch) pipes to feed the house a couple of years ago. This is a problem - you can only have one tap open at a time. The moment you open a second tap both taps trickle. Open a third tap and they all but stop flowing. There may be other places where flow is restricted - you may need a plumber to look at it for you. What happens if my supply pressure is too low? This is a problem. There are essentially only two things you can do - increase the diameter of the feeder pipe (or add another) or install a pressure booster pump. Pressure boosters of adequate flow rates do a great job. They "suck" the water out of the main feeder and really do a good job at maintaining really good pressure (4 Bar and more) in the house. BUT they use a lot of electricity. Typically they are 750-1000 Watt units and they may switch on every time a tap is opened. You will notice an increase in your electricity bill. There is another disadvantage - the guy next door is going to get less water :-). We supply and fit booster pumps. We set them up so that they only switch on when the pressure is really low. This significantly reduces the electricity cost. However, we often find that a booster is not required because there is some other problem with the pipe work. Remember that this is just the cold water we are talking about. You have to have a good cold water pressure to enable a good hot water pressure! Cold water pressure is fine but the hot water trickles. There can be a number of things wrong. Some easy to fix and some not. Here are some - 1. Pressure control valve on the geyser faulty or filter blocked. 2. Geyser inlet and/or outlet blocked up. 3. Pipe is damaged in some way. Copper goes nice and flat when stood on hard! 4. Hot water tap/s faulty or filters blocked. 5. Wrong diameter pipes used 6. Cold water into the geyser - tap is not fully open. 7. Old geyser with header tank or low pressure (1 Bar) Latco type system. OK - most are easy to fix except the blocked pipes in the walls. We can clear them a bit but not totally. The geyser can be cleaned out but we are reluctant to clean out an old geyser. We weaken it an there is a better chance that it will fail when we are done. The answer is a new geyser if it's really badly scaled up. The most common causes are a faulty/blocked PCV, old geyser and old pipes in the wall. We do replace the pipes in the roofspace with PE plastic pipe - 4 main reasons: they flow better, they are safer, they don't freeze and they retain heat. This is a simple exercise. More complex distribution systems require a more complex approach. In a townhouse complex, building or factory there are many more variables to consider and it may be nessesary to do a flow simulation to see exactly how the water should be moving in the pipes. This is done by means of simulation software that models the flow characteristics of a specific system and will show up shortcomings in the design and construction. BBDC is able to do these simulations for you. Have a look at water flow simulation. Converting from Low Pressure (1 Bar) to high pressure 4 or 6 Bar Geysers. This does not generally pose any problems BUT there is a caveat! The new system should be pressure tested so that problems with the existing hot water pipes can be exposed. The old pipes are "used to" only 1 Bar. You may end up putting 6 Bar in them and this may cause them to fail. The pressure testing should at least go up to the working pressure of the system. High pressure "spikes" in a system ( & water hammer). This is an interesting thing to watch. It can cause problems for home owners and not many people are aware of this. In a working system where taps and valves are opening and closing the system experiences extremely high instananious pressures. This is a high pressure that is only there momentarily. In fact it's more of a "shock wave" than a pressure. Valves that close suddenly - like a washing machine cause this. I have seen spikes of pressure that go way over 10 Bars. We have high speed water pressure sensors that are able to measure spikes up to 1 Bar at a rate of about 500 samples/second. High pressure spikes often lead to a thing called "water hammer". Water hammer is caused by a pressure spike that resonates in the pipes. Water hammer can be harmfull over time because it can amplify and sustain high pressure spikes. Note that we have high speed (500 samples/sec), water pressure data recording equipment that can log water pressure over time. Contact Lee on 083 458 4836 for details.
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