Decreasing water pressure!

[Megaflo-Experts]

Tested the static pressure at an garden tap, and it read 5 bar, but gradually decreased down to 2.5 bar within about 5 seconds. Tested this several times with different pressure gauges, and the same thing happened every time. No other taps were being used.

Can anyone explain/suggest what may cause this?

 

[AWheating]

is there a check valve on the tap or supply?

tried another point on the system?

 

[Megaflo-Experts]

No check valve, and unfortunately that was the only test point available at the time.

is there a check valve on the tap or supply?

tried another point on the system?

 

[townfanjon]

pain I know, but could you test at washing machine appliance valve.

 

[Megaflo-Experts]

Yes, will try and test at another point, but trying to think of a scenario that may cause this to happen.

pain I know, but could you test at washing machine appliance valve.

 

[Ermintrude]

Next door opened tap. Nothing between u and them!

 

[Megaflo-Experts]

They are on their own supply (with meter), and I tested the pressure several times, and every time the pressure dropped the same amount, and at the same rate!

Next door opened tap. Nothing between u and them!

 

[gasmanrob]

There is a check valve in the suppliers valve outside that and a couple of flexes or plastic pipe could account for it

 

[Megaflo-Experts]

The outdoor tap was supplied via a plastic pipe, but surely once the pipe has expanded it should stabilise at the static pressure, which I assume to be the initial higher figure of 5 bar...or is that just wishful thinking!

There is a check valve in the suppliers valve outside that and a couple of flexes or plastic pipe could account for it

 

[Ermintrude]

Small leak under floor? Boost up to five and turn off stop tap see what happens

 

[zzzjim]

Does sound like a non return is remebering higest pressure , is there a slow weep on tap stem ?
once a small flow has happened you get the true current pressure .

( Modern water meters work like nrv don't they ?) ( ..or am I over thinking ..)

 

[Megaflo-Experts]

I didn't notice any leakage, but don't understand how a nrv (faulty or not) can cause the pressure to drop.
Each time I did the test, the initial reading was 5 bar, and then steadily dropped down to 2.5 bar!

Does sound like a non return is remebering higest pressure , is there a slow weep on tap stem ?
once a small flow has happened you get the true current pressure .

( Modern water meters work like nrv don't they ?) ( ..or am I over thinking ..)

 

[Ch4 plumbing]

Is their a small expansion vessel on the cold main somewhere acting like an accumulator?
or theirs a pressure reducing valve set to 3 bar on the main letting by?

 

[zzzjim]

...... don't understand how a nrv .......

My un metered supply happily bounces along as neighbors use water , much like water in a manometer bouncing about ..settling slowly .
If a valve is present it lets water in ..but not back. As the system is likely to have some springiness in it a high pressure surge gets remebered behind the valve stretching the springyness in the system .
The high pressure surge could just be seen by letting a large flow thru , then suddenly stopping it . ( Giving a FAKE -Peak pressure ) as the water suddenly stopped - stretches the "Springiness" in the system .

Or the high pressure is just remebered from another time that day , and no water has been used ?

 

[Megaflo-Experts]

Don't think there is a PRV, expansion vessel or accumulator. There is a combi boiler on the system which has a expansion vessel on the primary circuit.

Is their a small expansion vessel on the cold main somewhere acting like an accumulator?
or theirs a pressure reducing valve set to 3 bar on the main letting by?

 

[Ch4 plumbing]

Don't think it's anything to do with the boiler, otherwise the PRV would blow.
Theirs not a pump on the incoming main is their? Salamander or something?

 

[Megaflo-Experts]

Thanks for your explanation, and I understand what you're saying. If the neighbours or someone else was drawing off some water, I can understand the pressure dropping, but I did the test several times.
I also know what you mean about the springiness, but is more likely if there was air (or a gas) in the system. If the pressure did surge to 5 bar, that pressure must have come from somewhere.

Thinking along those lines, I wonder if the heating was on, and some residual heat in the secondary heat exchanger warmed up the cold water main, with a faulty nrv, and therefore increased the pressure. Could that be the explaination?

My un metered supply happily bounces along as neighbors use water , much like water in a manometer bouncing about ..settling slowly .
If a valve is present it lets water in ..but not back. As the system is likely to have some springiness in it a high pressure surge gets remebered behind the valve stretching the springyness in the system .
The high pressure surge could just be seen by letting a large flow thru , then suddenly stopping it . ( Giving a FAKE -Peak pressure ) as the water suddenly stopped - stretches the "Springiness" in the system .

Or the high pressure is just remebered from another time that day , and no water has been used ?

 

[PlumbBobBob]

is it possible your pressure measuring device is faulty?

 

[Megaflo-Experts]

No booster pump, I don't think. They did have poor flow though - only 10l/m, but probably not relevant.

Don't think it's anything to do with the boiler, otherwise the PRV would blow.
Theirs not a pump on the incoming main is their? Salamander or something?

 

[Megaflo-Experts]

Good thinking, but I considered that and tried with another gauge, with exactly the same readings.

is it possible your pressure measuring device is faulty?

 

[zzzjim]

Not expecting heating to be pressurising things with all protection fitted ( but a poor flow thru a nearly clogged plate HEX may be worth considering ) ..or would that just fade when cooled ?

Volume of water in high pressure spurt should be a clue to volume of springy reservoir
(suspects include --accumulator vessel / flexis / trapped air ..)

Surge in pressure from arrested fast motion in a long run of pipe is likely , its how water hammer gets going !

( Removing this fake surge may need an accumulator if volume change is only small )
-another thought how complicated/long would feed within house need to be to have enough expansion happening due to a well heated house,effect would repeat again if ran off warmed water , till cold and wait till warm -too quickly & may be a routing, insulation issue-

--Poor flow sounds like biggest-constriction issue - if pressure dives once any flow happens --