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[jonny2112]

Fitted replacement taps on the kitchen sink on Sunday. Had to turn off the water obviously and that's where it all goes wrong ish 

Turned the feed to the hot cylinder off at the cold water tank, having initially turned off the wrong valve - at the cylinder itself, though this was for the boiler feed 

Anyway, took a long time for the hot tap to stop running / drain and I found out that the cold water tank was being emptied at this point. I thought that was strange to start with but hey ho, at least the water had stopped and I could crack on.
So, taps changed, I turned the main stopcock on followed by the valve at the cold tank. All seemed well .........

Have now noticed that there is no residual hot water. Previously after the heating had been switched on, there was plenty of hot water even the next day, whereas it doesn't now stay warm very long. Also, when the hot tap is turned on the cold tank still seemingly supplies the water.
Does all this seem normal? Seems something is wrong and it's a terrible waste if the water isn't being heated when the heating is on 
I'm wondering is there an airlock maybe in the cylinder? Or does the cylinder even empty under these circumstances?
I wish I was a plumber  but any advice or guidance gratefully received 

[martin42]

Water tank fills the cylinder as hot water is drawn from tap,
Check cylinder thermostat if fitted to cylinder is in right place.
Does heating work ok?

[jonny2112]

Yes heating and immersion both work ok, and the water is hot when either is turned on. It just doesn't seem to stay warm when they're turned off? It wasn't always like this 
I thought that's how the water flow worked but it would almost give you the impression that the cold water is cooling the cylinder down. As far as the thermostat goes, I don't know. I think it's all sealed and I certainly didn't touch anything in that respect.

[martin42]

So what your saying is that the hot water doesnt stay constantly hot when running the hot tap,might have to pop up some pictures of the airing cupboard so we can see what valves etc are there.

[jonny2112]

Exactly. We would put the heating on at night for a couple of hours. During that time the water would be really hot, and if it wasn't used for a bath or shower etc then it would still be really warm the next day. Now that's not happening. Despite the heating being on for a while, the water cools down really quickly once it's turned off.
In the airing cupboard the only valve is on the boiler feed to the cylinder. The cylinder itself is encased in foam or whatever it is. There are no fittings that can be adjusted, etc. afaik.

[martin42]

In theory you should have a 22mm gate valve( red round handle) this should be the hw feed to the cylinder which is a bottom connection.
Another 22mm if cold taps are also tank fed except kitchen tap(must always be mains supplied.
Then there maybe,depending on installer a 15mm gate valve,this supplies the central heating feed.
Then where the pump is a valve either side with out a handle on,these isolate the pump for changing.
The only 1 you should have touched was the 22mm for the cylinder feed.
I really cant see why it goes cold very quickly tho,its hard to say over the internet.
What tap did you fit?

[jonny2112]

Just standard contract type taps - like for like.
The only valve is the gate one you describe, which is the bottom ch feed as stated.
I don't understand it either tbh. It was a straightforward job I thought.
Very little was disturbed or touched, and the cooling down is weird. It was a great system, albeit probably 35 years old now.

[Andy H]

2 possibilities come to mind

1. If I read it correctly the first valve you closed stopped the flow of heat from the boiler to the cylinder. - It is quite common for the screw threads in 35 year old valves to disintegrate when disturbed. You might find that the gate in the valve has only opened a fraction and is only letting a tiny amount of heat through.

2. The cylinder should have an open vent pipe that has no valves in it. If it hasn't, AND if you were able to shut all the pipes into the tank except the outlet AND you drained the contents of the tank from taps on the ground floor you may have generated a partial vacuum which can make the cylinder collapse and finish up looking like a prune.
I know this because...... my aunt & uncle actually did this to theirs about 30 years ago.

[Rods2]

Or you have a water leak on the hot side somewhere which is using up all of the hot water. I had this with a leak under my utility room floor, which took me sometime to track down. 

[Kevin Wood]

Another possibility is that you have something cross-feeding from the cold to the hot side and vice-versa. Something like a faulty shower valve or mixer tap could do it. This would explain why turning off the hot water feed didn't stop the flow, and maybe it has got worse now.

[Marks DTM Calib]

A few basic questions

1) Does the hot water get upto the temperature you expect? (e.g. is it getting as hot as before or is it now only luke warm)

2) The boiler flow gate valve you closed, did you open it the same amount or was it fully open?

3) Is the system a controlled one with motorised valves, if so is it a Y plan or S plan?

[jonny2112]

Thanks for all the info so far - it's all being thought through, slowly 

When the heating or immersion is on the water is very hot, as normal.
Although the hot tap does seem to draw hot water, after a while you would think it is actually drawing from the cold tank, that's the difference in temperature, to the point where there is condensation on the taps themselves ( as if cold feed ).
All valves were fully open afaik, and I don't believe it is motorised system.

If there is a gate valve at the cold tank to stop the feed to the cylinder, why did the hot tap draw water from the tank, emptying it, when the valve was closed? Surely it should only draw from the cylinder, and turning off the valve prevents the cylinder refilling? I would suspect the gate valve except that when I went back into the loft I could 'see' the water leaving by the other opening which I thought was the cold tap feed.
None of the pipework was touched, etc and although frustrating I'm wondering is it all coincidental?

Swmbo is going mad!

[martin42]

I think your best option given location would be to get a plumber in and sort it out,if i was local i would pop round but sadly im not 

[Kevin Wood]

OK, so if the hot water tank IS getting hot (or at least hotter than the water coming out of the tap), where is the water from the tap coming from?

I would switch on the immersion to ensure the hot water tank is up to temperature (we can discount a problem with the primary side if using the immersion).

Now run a hot tap and see if the problem occurs.

If you're getting cold/lukewarm water, compare the temperature of this water to that of the pipe emerging from the top of the hot tank. If it's hotter, follow the pipe along and you'll find it splits into a vent pipe up to the header tank and a feed off to the taps in the house. Does the temperature reduce at this point? Does the vent pipe go up to the cold tank and end well above the water level in the tank? Does it have a decent sized "loop" well above the water level in the cold tank and is there any sign that it has "flown over" into the tank recently?

Is the pipe from the header tank into the bottom of the hot water tank now stone cold?

As said, if you've got any mixer taps (especially if they have a mains cold water feed, i.e. at the kitchen sink) or shower valves, can you isolate them from the hot water supply to eliminate them?

Does the hearer tank ever overflow?

I had a fault once where a casting flaw in the kitchen mixer tap had opened up a pinhole between the hot and cold sides.

[Searcher]

Exactly. We would put the heating on at night for a couple of hours. During that time the water would be really hot, and if it wasn't used for a bath or shower etc then it would still be really warm the next day. Now that's not happening. Despite the heating being on for a while, the water cools down really quickly once it's turned off.

You are now trying to  heat a full cylinder of cold water from scratch.

It should sort itself out within a few days.