Physics question

[Iamcazzy]

The water supply in a house enters the property via a rising main on the ground floor. The water is distributed around the property via 15mm diameter copper pipe. On the first floor of the property, a supply was taken from the main pipe via a T piece to supply an appliance. The appliance has subsequently been removed and the pipework was cut back following the appliance removal to a 5cm stub of pipe that's capped with a stop end to prevent water escaping.
Bearing in mind that water is frequently drawn out of the system via other taps (faucets) in the system and that the pipework is continually full of water, would it be true to state that the water contained in the 5cm stub is the same water that was there after the stub was filled immediately following capping? ie does the water physically flow from the stub when a tap is turned on further along the circuit or is the water in the stub stagnant?

Thank you

[regulo]

Uncle had a boat, which had in it's pipework a capped off section of pipe from when a shower outlet was removed. Had awful trouble with stinking water, til we removed the dead section just leaving 2" or so of stub. Previous answers seem to bear this out and I agree, turbulence should shift any "dead water" from such a short stub.

[vascop]

Wouldn't it depend on whether the stub is horizontal/sloping upwards/sloping downwards?

[Iamcazzy]

It's an interesting idea vascop. I think it's assumed that in this case the pipe is horizontal. All the same, I'm intrigued by the importance of the length of the stub. Would there be anyway of establishing for example if a 7cm stub would be more prone to harbouring bacteria than a 5cm stub?

[ll_billym]

Looks like you need to make yourself some piping and get the stopwatch out. I would put colouring in the water and not bacteria though if I was you.

[Iamcazzy]

ll_billym, whilst I appreciate your suggestion, the technicalities escape me. I would have to employ 15mm clear plastic pipe to observe the movement of dye in the system and there's also the problem of where and when the dye should be introduced. In addition, the rate of flow in the pipe would have to be precisely as it is in the copper pipe for any meaningful figures to emerge.

[Iamcazzy]

Offhand, I can't see how it's possible to stopend a 5cm or 7cm stub of 15mm clear plastic pipe and somehow introduce a dye into the system.

[wildwood]

I know this is going nowhere but, to investigate you'd have to insert a stopcock valve on the 'stub' and with that open, fill the system with coloured water and close the stop cock thus keeping the coloured water in the stub. Then flush the system with clean water and keep it flowing. Slowly open the valve on the stub to see the effect of the flow in the stub.

Next thing you know we'll have all the worlds trouble fixed on AB. lol

[Iamcazzy]

Wildwood, I couldn't agree more that there is a finite quantity of contributions that can be made to this question and I'm very grateful to all contributors.

Filling of the system with coloured water is unquestionably a major undertaking with many practical difficulties. This is before we even consider the practical difficulties of replacing the copper stub with a transparent 15mm section to see if the coloured water actually moves out of the stub.

In view of the answers given so far, I'd really like to know how long the stub has to be before the risk of harbouring legionella bacteria becomes a reality. Surely there must be a maximum recommended/permitted length for such stubs within the range of normal water pressure within a domestic property?

[Vagrant]

Is it worth asking the question in home and garden? or maybe the Screwfix forums? A plumber would know the permitted length - I presume this sort of thing is covered by water authority regulations, who have probably done the research.

[ll_billym]

The recommendation is not to leave stubs.

[regulo]

If you're going to go to all that trouble to see what happens, why not remove the stub and replace it with a straight connector? Problem solved!