Dripping tap downstairs bathroom - hot - turning off water?

[Roger Mills]

Surely this cant be gravity fed?

Does the hot water 'tank' look like a conventional copper cylinder, or is it a really sturdy looking affair?

If the latter, you may have an unvented hot cylinder with a direct feed from the cold mains, and running at near mains pressure. If so, it will have lots of pipes and other gubbins connected to it - which is why someone asked for a photo in order to be able to identify it.

If it *is* an unvented cylinder, the hot flow will stop very shortly after turning off the cold mains. Turn off the mains and open a hot tap. If the water flows unabated for a long time, it ain't an unvented cylinder. I would expect to get no more than a few litres of hot before it reduces to a dribble and then stops. If this happens, you can safely dismantle a tap to fix the leak - as long as you don't turn the mains on again until the tap's back together!

[John Rumm]

Firstly there should be no need to drain the hot water from the tank just to stop the flow - you just need to stop the flow of cold water into the bottom of the hot tank, since its the pressure of that which pushes the hot water out of the top of it.

So first job is to identify if its a vented or unvented system.

A vented cylinder is your traditional dome top cylinder like:

An unvented one is a more sophisticated looking thing surrounded by far more ancillary pipework. E.g:

If its the former, then there *may* be a tap or valve in the feed that goes to the bottom inlet of the cylinder. Turning that off should interrupt the flow of water from it. (although if its a traditional gate valve like [1] then it may still let a dribble past - leaver ball valves [2] usually work better). If there is no tap, then you will need to either stop the main header tank (loft) refilling (tap feeding the ball c*ck if there is one, or tie the ball c*ck "up" to a splint of wood laid over the top of the tank), and let that drain down, or better, plug its outlet with a rubber bung (or stuff a large carrot into the outlet!).

If its a unvented system, then there should be a valve before the safety valve assembly on the inlet:

Turn that off, and run the hot tap - a few litres (i.e. less than 10 typically) will be able to flow as the expansion vessel pushes out its volume of water.

[1] [2]