Building Construction, Renovation, Maintenance & Advice

Knocking in Water Pipes

"Knock knock"

Knock, knock
"Who's there?"……………

Knock, Knock, Knock
"Who's there?"……………………

Knock, knock, knocking, knock knock.
"Who is it?……………………

Don't worry it's only the water pipes.

Why do water pipes knock?

Why do they sometimes knock?

This is a phenomenon known as water hammer or, more technically, hydraulic shock and is the result of a sudden build up or release of pressure in your water pipes.

It all goes back to an inescapable quality of water - it is pretty solid really (yes I know - a solid liquid) and not compressible (well - only a tiny weeny bit), you can't squeeze it. It might flow very easily but get it into a confined space and its volume remains virtually the same. The fact that you cannot compress water is useful, it means we can pump it and we can use it to push things but no matter how much pressure we put into it its volume may change very, very slightly but to all intents and purposes remains the same.

Let us consider what this means in practice. If we pump water into a container such as a tank or a pipe, or for that matter, your water system at home it will take a certain amount of water to fill the space available. Once full we keep pumping and the pressure in the water increases. Very, very little additional water is needed to increase the pressure. This also means that it only takes a very small release of water to release the pressure. Any very small leak will immediately release the water pressure in the tank, or for that matter, your whole water system.

In some ways this can be very useful but from other points of view it can be a total pain in the proverbial posterior.

Why? Well its like this, if you have a water pump to pressurise your water system and give you a good energetic shower rather than a limp dribble then your day starts on a noticeably better note doesn't it.

For most houses we need a pump that is large enough to supply all the demands we might have at any one time. Someone might be showering while in the laundry the washing machine is running, someone has just been to the toilet so the cistern is filling, he goes off to top up the swimming pool or wash the car when Joe Muggins decides to get up, walks into the shower and, turning on the tap, gets - zero, zilch, nothing, a distinct absence of anything liquid like issuing forth from his ever so trendy but very water demanding “rain effect” shower head. Our day is no longer starting very well.

To avoid this we need a water pump that is large enough to supply all these needs at once.

Great - but now we have a problem. Usually there is only one person showering or one tap on at any one time and our somewhat large pump is pumping far more water than can physically squeeze itself out of a single shower head. Its rather like a sumo wrestler icing a cake. He can squeeze as hard as he wants but the icing can only get out of the nozzle as fast as the nozzle will allow.

To pump only a low flow of water our large pump must switch itself on and off. To do this water pumps have two pressure switches on them. They work like this: if you open a tap the water pressure in the system drops instantly, one pressure switch senses low pressure and switches the pump on. The pressure in the system increases and when it is high enough the second pressure switch switches the pump off.

Lost me – time for a cup of tea and we'll resume when your head has cleared.

We now have another problem. It will only take the removal of about as much water as is contained in a gnat's bladder to remove all the pressure from the water system. So we turn on our shower and within a split second the gnat's bladder of water is gone, the pressure has dropped and our lumbering great pump rumbles into action attempting to pump 3 gigologallons of water down that little pipe to your shower. In the time it takes a gnat to sneeze (an imperial unit of time known as a GS) the gnat's bladder of water has been replaced, the pressure is back up and the pump switches off like a fly hitting a windscreen.


Ah, so that is what makes the water hammer. It can also occur when you turn a tap off suddenly, this is particularly the case with modern quarter turn taps which tend to cut off the water flow very quickly.

But, just a minute, the shower is still running and once again a GS later the gnat's bladder of water has gone the pump switches back on. This process continues ad nauseum, the pump switches on an off like a young lover with a fretful virgin and the house shudders like a constipated elephant.

Not good

So how do we avoid this?

A pressure tank provides a reservoir cussion

Well, we add a bit of air to the situation. You see air (and all gasses for that matter), unlike water, are very, very compressible. Have you ever seen how much air you can get into a car tyre? Rather a lot I can tell you.

Not only that, because air is compressible it makes things soft and squishy, it acts like a cushion. In former times air was used in some of the more eccentric car designs for suspension: the frog's leg eaters in their citroens spring to mind along with some of the rust buckets produced by those chaps that invented the concept of eccentricity - the cowboys of British Leyland (no wonder the Germans bought them out).

As I have said in the case of water there is a very small difference in volume between high and low pressure. With air, there is a huge difference in volume between high and low pressure.

So how do we use this? If you have a look on the top of your water pump you may see a round tank about the size of a football. Small pumps may have a small tank and large systems may have very large tanks. The football size is very common.

This is a pressure tank, it is a fairly simple device that takes a lot of the load off your water supply system by introducing a rubber bag of air which acts as a cushion in our water system.

How does a pressure tank work?

The tank has a rubber bladder in it which is pumped up with air to a pressure of around 20 pounds per square inch.

As our water pump pumps up the pressure in our water system it also pumps water into this tank which progressively compresses the bladder of air. This will take time so our pump can run for a longer period before reaching the high pressure and switching off. We turn on the shower and the bladder gradually expands pushing the water out of the tank to our shower until it reaches the lower pressure when the pump switches on again. This also takes time.

The pressure tank does 2 things: it slows down the time between the water pressure pump switching on and off and it acts as a cushion stopping sudden pressure changes in the water pipe that cause that knock. By the way water hammer can damage water pipes by shock loading them.

These pressure tanks are also known as accumulators – they collectand save pressure.

Identfying pressure problems in your water system

How do you know if there is a problem?

If your water pressure pump is rapidly clicking on and off then there is probably something wrong in your water system. A bit of investigation can help to isolate the problem.

The first thing to do is to check your pump and make sure it is the water pressure pump and not the well pump that is the problem (the well pump is the one that pumps water from your well or bore to your storage tank).

Ok so you are sure it is the pressure pump.

Next make sure anything that may be using water, taps, toilets, showers, etc. are all turned off. Is the pump still clicking on and off or running constantly? It should be off. If the pump is running constantly or clicking on and off then you most likely have a leak in the system somewhere. The slower the clicking the smaller the leak.

Alright so the pressure pump is off. Now turn on a shower or a single tap. The pump should start. If it is a small pump such as a Shimizu it will probably be running constantly. If it is a larger pump it will probably be cycling, switching on and off at a slow pace as it alternately pumps up the pressure tank then allows it to empty.

If it is cycling quickly then the problem will probably be the setting of the 'on' and 'off' pressure switches on the pump or the operation of the pressure tank. These are not really things you can do yourself and you should call a plumber.

Knock, knock.”
Who's there?”
It's me, the plumber.”

Copyright © Phil Wilson 2011
This article or any part of it cannot be copied or reproduced without permission from the copyright owner.

8 February 2017 Copyright © Mr Fixit,
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