I didn't take any notice of the recent topics of water pressure because it
didn't affect me at the time but now I wish I had because I find myself in a
very similar situation to one of those earlier posters and now I can't find
the info posted in their threads.
Our water company are saying that to get compensation the pressure must fall
below 1.0bar but I pointed out that that makes no sense unless the person in
question has a pressure guage, and I haven't (although I'll get one now for
future use). I said that it was taking 1min 5sec to draw off 1 litre from
the kitchen tap but she just kept referring to the 1bar thing. It appears
I'm talking in apples and she's telling me in oranges and never the twain
Can anyone tell me what the pressure was likely to have been at a time when
it took 1min 5sec to get a litre of water out of the tap? We have a Triton
T80xr shower and the "Low Pressure" light was lit. According to the manual,
it needs a min of 1bar and a flow rate of 9litres/min, if that helps.
I imagine the pressure is the static pressure provided by the water
company to your property through a set diameter pipe. It cannot be
measured by flow rate because the flow will depend on the various
valves, bends etc in your pipework which will reduce the flow the and
the dynamic pressure.
You will need to get a pressure at an opening when there is no water
flow. Ie all taps turned off and tanks not filling. If this is not below
the limit try turning a tap on somewhere else in the building and seeing
if the pressure plummets. If it does then there may be something
somewhere partially restricting a pipe. That could take a while to find.
How can flow rate be a different thing to pressure? If opening the kitchen
tap usually gives a high-speed, gushing torrent of water (which it does) and
then one day I turn it on and only get a trickle out of it, I would describe
that as low water pressure and I imagine 90% or more of the population would
describe it that way too. Is that not correct?
What happened was that the water main runs along the street and the houses
are tee-d off it. From the main a two-foot long pipe goes to a stop cock in
the pavement outside our drive, and from the stop cock a piece of pipe,
probably 10 metres long runs under and along our drive, and pops up in our
kitchen, behind the washing machine, where our internal stop cock is. The
kitchen sink tap is about 4ft along from it.
It appears that something had partially blocked the 2ft section of pipe
between the external stop cock and the tee into the main, resulting in (as I
describe above) low water pressure in the house, with just a trickle coming
out of the kitchen tap. Are you saying that that situation is _not_ low
The problem has now been fixed but surely it's unrealistic of the water
company to expect the general public to have access to pressure guages - I
can't see my 84-year old granny getting a tool box out and fitting a
pressure guage anywhere. The general public perception surely must be that
if you normally get a good flow and now you haven't, then there's low
pressure. Or, another way would be to say that under normal circumstances I
can't press my thumb up against the tap to stop the flow of water, whereas
during the time with the problem, I could easily stop the flow. They are the
only diagnostics available to most of the public I would imagine.
This comes up nearly every time water pressure is discussed.
Think of a 50mm diameter water pipe with 10 bar pressure and a 50mm tap on
Think of a T connection to a reducer which leads this 50mm pipe into a 1mm
diameter pipe with a 1mm tap on the end.
Assume that there is a massive water main behind this which can maintain the
pressure when you open the tap.
Use each tap in turn to fill a bucket.
Which pipe and tap do you think would fill the bucket faster, the 50mm or
Remember that they are both operating at the same water pressure.
P.S. the drop in flow rate suggests a drop in pressure, but you can't use
flow rate to measure pressure directly.
No plan survives contact with the enemy.
[Not even bunny]
OK, thanks for that Dave, a good illustration and I now understand - I think
So, when the water company say they'll give a few quid compensation for low
water pressure, are they using semantics to avoid paying out? Like I said
earlier, you can't expect an 84-year old granny to get a toolbox out and fit
a pressure guage. The general public understanding is of the amount of water
they get out of the tap - high speed gush = good pressure, low speed trickle
= low pressure.
Did our pressure fall below 1bar - who knows? My granny only knows that she
couldn't use the shower for more than two weeks and had to go to a
neighbour's for a shower, to name just one of the inconveniences
experienced. It would seem that the water company are dangling an
unreachable carrot by saying they'll pay compensation for low water pressure
when the vast majority of people don't even know what it is, let alone how
While this may be the interpretation they put on it, it is a flawed
Look at it from the suppliers point of view. They supply water to the
property at say 3 bar. With adequate capacity in the mains to maintain
that pressure even under dynamic conditions (i.e. while water is being
drawn). However the owner of the house has 50m of badly scaled 1/2"
copper pipe between the main and his tap. The result is the static
reading will still be 3 bar at the tap, but the flow rate will likely be
inadequate. The property next door however with a 10m run of 25mm MDPE
pipe will have no such problems. It seems rather unfair to make this the
This may be the case, but realistically what else could they do?
Flow and pressure are completely different things. You can have a very
high pressure and a very low flow rate quite easily. And vice versa.
Would I be correct in saying a river has a high flow rate but no pressure?
Not quite - you will need some pressure difference to achieve a flow in
the first place. However its fair to say they probably have low pressure
and very low flow resistance - hence you get a massive flow rate for a
relatively small pressure.
(analogies with any other flow of energy will give similar results - the
current flow in a circuit will depends on the "pressure" i.e. voltage
pushing it round, and the resistance it meets along the way. The rate of
flow of heat will depend on the temperature difference, and the
resistance (i.e. insulation) to that flow etc)
On Wed, 27 Jul 2011 21:24:52 +0100, John Rumm wrote:
Is this what I've seen referred to as 'over-pressure' for air flow? The wind
is at 1 bar but exerts a force required to accelerate it around
The jet from a 'pressure washer' must be at atmospheric pressure and it's
the velocity that gives it the energy to remove crud - pressure without
velocity wouldn't do much!
Hired, as in paying money? Every time I want to check?
The water pressure here is so high that all my taps and valves were
having problems. So I've fitted a pressure reducing valve.
The water authority has no idea what the pressure is. They have no
idea about anything much.
My neighbour's water meter stopped going around about 5 years ago so
they pay nothing for water.
Imagine that you have two taps mounted on the wall. One is connected
through a very thin tube to a tank at the top of the mountain. The
other tap is connected to a water-butt the other side of the wall.
If you open the two taps a tiny crack, water will spray out of the tap
connected to the tank at the top of a mountain in fine spray. The tap
connected to the water butt will just drip slowly. This is because
the water from the mountain-top tap is at high pressure, but the water
from the butt tap is at low pressure.
Now open the taps fully: It is difficult to get water to flow quickly
through a narrow pipe, so not much will flow out of the mountain-top
tap; on the other hand, there's no particular resistance to water flow
from the butt, so it will flow quickly.
It is right, but you can't equate low flow to a certain pressure,
since it depends on the pipes and fittings in your installation.
The flow rate is whatever causes a frictional pressure loss that
equals the available pressure difference.
They know that, but if you complain about low pressure, it's not
unreasonable for them to ask what the pressure was.
The other one is the head of water. 1 bar static is equivalent to 10.2
metres head of water, so if the water isn't getting to the storage
tank in most 2 storey houses, there's probably les than 1 bar and
complaints will follow.
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