Changing hot tap with no isolator

[cartman1]

I need to change a hot water tap without an isolator.

I have a vented hot water system and my tank cupboard has 3 pipes running through it from floor to ceiling. One carries hot water and is connected to the top of the tank, the other 2 carry cold water - one has a label "isolation valve" it also feeds my electric shower. The other one has a label "stop valve".
My main cold water stop cock is downstairs and there are 2 cold water tanks in the loft, presumably each fed by one of those 2 pipes .

The hot tank has 4 pipes connected to it
1) the one i mentioned above
2) one labeled cylinder drain
3) one entering in the middle of the tank that comes from my boiler via a 3 way valve
4) one at the base of the tank with an unlabelled red tap on it that doesn't seem to change anything albeit it does clearly open & close with about 10 turns

Given my lack of isolator on the hot tap in question, am I likely to be able to change it without draining the entire hot tank?

 

[denso13]

Should be the one to the base of the tank. If unsure just turn off the cold and run the hot until it stops running.

Any photis?

 

[CountryFan]

Yes, you can usually bung it (the cold feed on cold water storage cistern). There’s usually one as a gate valve (not always though) from the cold feed to the cylinder, usually entering the base. Photos of setup?

 

[cartman1]

One of the pipes on the left to the loft (with the isolation valve label) is obscured, you can just see it coming through the floor.

I previously tried closing both valves to the loft and what I assumed was the gate valve to the base of the tank but the hot water kept running for ages

 

[denso13]

Need to turn off the feed, i.e., the one entering the cylinder bottom left.

 

[CountryFan]

One of the pipes on the left to the loft (with the isolation valve label) is obscured, you can just see it coming through the floor.

I previously tried closing both valves to the loft and what I assumed was the gate valve to the base of the tank but the hot water kept running for ages

View attachment 406949

That one at the back with the drain off, look above that

 

[terryplumb]

The pipe in the left corner ,which has bright copper section, just below the wooden shelf , is the feed to the cylinder. See if there is an isolation valve above the shelf.

 

[fixitflav]

3 pipes running through it from floor to ceiling

4 on the photo

 

[gas112]

just drain it , as the valve others are telling you to look for and turn off have a habit of snapping in the off position

 

[CountryFan]

just drain it , as the valve others are telling you to look for and turn off have a habit of snapping in the off position

If it’s a gate valve of course

 

[gas112]

If it’s a gate valve of course

which they usually are

 

[ChrisJP]

Given my lack of isolator on the hot tap in question, am I likely to be able to change it without draining the entire hot tank?

You won't drain the hot water tank - instead, once the cold water feed tank has drained, the flow of hot water from the hot water tank will stop.
But there'll probably be a stop cock on the outlet of the cold water tank that feeds the hot tank. If there is, close that, open the hot taps and the flow will stop after just the water in the pipes drains out.

 

[cartman1]

You won't drain the hot water tank - instead, once the cold water feed tank has drained, the flow of hot water from the hot water tank will stop.
But there'll probably be a stop cock on the outlet of the cold water tank that feeds the hot tank. If there is, close that, open the hot taps and the flow will stop after just the water in the pipes drains out.

Thanks for all of the replies & you wrote what I expected to happen, I'd just not looked all the way up that pipe previously as I'd been fixated with it being labelled cylinder drain.

That 4th pipe on the left that goes from the cylinder to the ceiling has a tap on it further up, above the shelf, as @terryplumb suggested. Sadly, when turned the water from the tank above still drains and hot water keeps flowing. All of the taps on the pipes appear to function properly, they turn both clockwise and anticlockwise and feel like they engage to both extremes - would this still happen if one of them was broken and not actually closing as appears to be the case here?

Also what's the tap on the bottom right likely to be for?

 

[terryplumb]

I assume the valve is a gate valve ,( big round red handle) ?
If it is ,they are notorious for not fully closing.
The gate valve bottom right is used to throttle the flow through the internal coil in the cylinder.

 

[cartman1]

Yes, round red handle.
I assume this means that it needs replacing and I need to either drain the whole cold tank or bung it to both for that and change the valve?

I also assume that I shouldn't try to reuse the nuts / olives currently on the pipes or is this considered ok if I can find an identical valve?