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How to Measure Water Pressure?
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kingkong


Posts: 61
Joined: Oct 2006
Post: #1
07-11-2007 04:58 PM

Has anyone had any problems with mains water pressure at their property?

I understand that the water company is legally required to provide water at mains pressure of a minimum of 1 bar (which is often the minimum required to use electric showers or combination boilers). Could anyone advise if this is the case or not?

If it is the case, and if my water pressure is below 1 bar, what would you recommend my course of action should be?

Thanks,
KK

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Ian


Posts: 75
Joined: Oct 2007
Post: #2
07-11-2007 08:54 PM

They are digging up the south circular and putting a new main in.
I suggest you go down to where they are working and tell them to hurry up.


One loud voice can make a difference !
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Applespider


Posts: 283
Joined: Feb 2006
Post: #3
08-11-2007 11:17 AM

Several years ago, the plumber who was fitting my new bathroom told me that the water company is obliged to provide 1 bar of pressure to the ground floor but not to any floor above that - and no, you don't get a discount if you're the first/second floor flat on the water bill.

Mine fell just short of 1 bar so that I couldn't have that electric shower but luckily he was able to do something 'clever' with the boiler and cold water pressure so that I get a pretty reasonable shower from the taps...

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sydenham74


Posts: 5
Joined: Nov 2007
Post: #4
08-11-2007 12:13 PM

My electric shower has been complaining of low water pressure since August this year, as it was under warranty I called out the engineer to take a look. The standing pressure measures 1 bar, but when it comes to running pressure this drops.

I'm in Lower Sydenham, anyone else from this area experienced the same issue?

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Triangle


Posts: 133
Joined: May 2007
Post: #5
08-11-2007 03:47 PM

As I understand it, when you increase the velocity of a fluid, you decrease its pressure. Therefore, if you were to increase the diameter of the pipe supplying water to the shower, then for the same flowrate you would reduce the velocity of the fluid and hence increase the pressure. But changing the pipe size is easier said than done!

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Ooperlooper


Posts: 104
Joined: Jun 2006
Post: #6
08-11-2007 09:11 PM

I'm not a plumber but from what I remember of the physics I learnt in school...

The velocity of a liquid ('fluid' includes gases as well as liquids) has nothing to do with pressure (at least not in the true senses of the words 'velocity' and 'pressure').

Velocity means the rate (and direction) of movement; pressure is force per unit area.

I suspect nothing you do to the cross sectional area of your pipe is likely to affect the pressure of the water (though it may affect flow rate) - pressure is determined by force being applied to the water, either because of a pump or the effect of gravity making any water above press down.

Flowrate will, I reckon, be mainly determined by a) pressure and b) the narrowest point (smallest cross-sectional area) in the tube delivering the water (though I guess it might also be affected by friction in the tube).

Okay...that's not of much practical help, I know.

I do know that the pressure reading on our combi boiler gradually, over every few months, creeps down towards 1 bar and occasionally I need to open a tap in a pipe that runs up into the boiler for a few seconds in order to get the pressure gauge to go back up to about 1.5, where it is supposed to be for optimum operation.

Someone in the boiler manufacturer's helpline call centre talked me through how to do that.

Not sure whether that affects the flow rate through the shower though. Doesn't seem to, though I never really thought about it until now.

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Sherwood


Posts: 1,355
Joined: Mar 2005
Post: #7
08-11-2007 10:11 PM

I thought showers were fed from the cold water tank above.
The higher the tank is above the shower head the greater the pressure and so the greater the flow.

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Sherwood


Posts: 1,355
Joined: Mar 2005
Post: #8
08-11-2007 10:17 PM

Customers can make changes to their internal plumbing to increase their water pressure - for example by ensuring their stop tap is fully open, or checking that any systems that depend on the pressure of water reaching the property are set to the statutory minimum level of 1 bar/10 metres head.

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Ooperlooper


Posts: 104
Joined: Jun 2006
Post: #9
08-11-2007 10:23 PM

The cold water tank in our place is no longer in use. I guess you don't need one if you've got a combi.

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Triangle


Posts: 133
Joined: May 2007
Post: #10
09-11-2007 11:20 AM

The flowrate of a fluid in a pipe is the mean velocity of the fluid multiplied by the cross sectional area of the pipe. If you are pumping fluid through a 50 mm pipe which is then connected to a 25 mm pipe then the flowrate actually remains the same... All that happens is that the velocity of the fluid increases when it flows through the smaller pipe. Logical really, you're trying to squeeze the same quantity of water through a smaller area, so it has to pass through quicker.

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Perryman


Posts: 809
Joined: Dec 2006
Post: #11
09-11-2007 03:14 PM

My 2ps worth:
Under static conditions, the pressure will be the same whether it is a 1mm2 pipe or 1m2 pipe.
ie You will get the same water pressure out of a shower nozzle connected to either sized pipe.

Pressure = force per area

However, due to friction, once the flow has started, the pressure will drop off with the narrower pipe = less flow rate.

I'm no plumber, but the trick maybe was to make the h/w system closed - ie at mains pressure. ie nothing to do with pipe size.

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Triangle


Posts: 133
Joined: May 2007
Post: #12
09-11-2007 03:50 PM

For those interested in fluid flow may I suggest you go to Wikipedia and type in "Orifice plate", which is a device I occasionally have to use at my place of work for measuring the rate of fluid flow. It explains what I've been trying to convey about the relationship between flowrate, velocity and pressure quite nicely.

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Perryman


Posts: 809
Joined: Dec 2006
Post: #13
09-11-2007 04:55 PM

Triangle,
thanks, will check that out.
Your earlier post was quite correct, but gave the impression that the pressure increased with a smaller pipe. The pressure stays the same (in a static situation).

Putting your finger over the tap allows the water to flow out closer to the static pressure, as you are reducing the flow. That is my understanding anyway.

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