Outside tap mystery

[Dg8]

Hi all, I'm completely confused by my outside tap.
If I open it quarter of a turn, I get high pressure (as expected) but also high flow (I can fill a saucepan in about 5 seconds). If I continue to turn the tap, the flow suddenly slows to the point it takes about 25 seconds to fill the same saucepan. I assumed it must be a dodgy tap, so changed it, but the new one acts in the same way. If I unscrew the tap and turn the stopcock on, water just pours out of the back plate hole without any apparent pressure.
I don't think I have a pressure reducing valve on my feed to this tap, and no other cold water taps act this way.
Does anyone have any ideas? I will have to call the plumber out if it's not a simple fix, as I think I've already burnt out my pressure washer having that tap fully open and assuming it was getting a good flow.
Thanks very much in advance.

 

[Scott_D]

Is flow and pressure good at other taps?

 

[Dg8]

Is flow and pressure good at other taps?

Yes, all other taps are fine with good flow

 

[Scott_D]

If you leave the outside tap cracked open so that you get good flow and pressure, does it last or start to drop off?

 

[Dg8]

If you leave the outside tap cracked open so that you get good flow and pressure, does it last or start to drop off?

I've not tried it over a longer period of time, only seen for a couple of minutes, the water seems to just pour out rather than have any pressure behind it. It's weird, it's like something detects when there's no resistance (like a partially opened tap) and kills the pressure and flow. I just can't think what would cause this effect. If the tap has a quarter turn, the water comes out like the clappers, any more, and there's almost a 'clunk' sensation I feel on the tap and it just pours out in a rather lackluster fashion

 

[Dg8]

If you leave the outside tap cracked open so that you get good flow and pressure, does it last or start to drop off?

Just reread your question. It seems to maintain flow. If it's been off for a few minutes and you turn it straight to fully open pressure drops immediately. So I don't think it's building pressure in the pipe.
One possibility I suppose, is that there's some obstruction in the pipe the only moves to block when the pressure is high enough (fully open), at a quarter cracked it's good pressure but not sufficient to block the pipe? No idea how I'd confirm this or rectify it as all the pipework is hidden within the internal walls (fairly new build property).

 

[Chuck]

Does anyone have any ideas?

Most likely a lump of something rolling around in the pipe somewhere.

There are things called 'excess flow valves' (and about a dozen other names) that are designed to detect burst pipes and shutoff when the flow exceeds a set maximum rate, or more than a certain volume in a certain time. Not something I'd expect to see on a new build house but worth looking for.

 

[Ric2013]

Could a stopcock somewhere in the pipeline to the tap have been installed backwards such that when the flow increases it pulls the jumper towards the valve seat and restricts the flow?