Unvented cylinder loses heat super quickly

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Hi guys

Great forum. I have an unvented cylinder which is a Telford Tempest 250 litres with external expansion vessel, which we have the hot water on from 6am - 10pm (gas boiler). But within 2 hours of the hot water going off, the water from the cylinder is stone cold.

If I turn off the hot water, 5 minutes later the pump stops (just the normal overrun I assume) but there is a quiet rattling noise from the cylinder which persists afterwards, and the cold water pipe going into the combination valve is making a faint whooshing sound, as if water is flowing through it. Does that sound right? Here's the suspicious thing - if I turn off the cold water going in (there's a blue tap on the pipe), the rattle stops and the whooshing sound stops.

Any ideas what is going wrong, why cold water might be flowing in (if it is, as I might be wrong) and why the cylinder is stone cold after 2 hours? I should add that the tundish is dry. I have no plumbing experience and am happy to call someone in, but the first heating engineer didnt know and just told me to leave it on overnight.

The hot water flows fine during the day when the hot water is set to on.

The picture shows the pipe which is making the faint whooshing sound, which is connected to the blue tap behind, which I assume is the cold water coming in
 

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Only way it could be going cold is if the insulation was missing or if the hot was being replaced by cold.

As suggested it sounds like the latter and the only way that could be without a hot tap being open, is a leak on the hot somewhere. trouble will be finding it but losing all heat in 2 hours and being able to hear it through the valve suggests it's a pretty significant leak.
 
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250 litres is a significant amount of water if it's loosing it and should show somewhere if leaking, anything running when the inspection cover outside is lifted . Is this a new event?
 
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Lostinthelight - great point. I've just been out and when I lifted the inspection cover, the meter was turning. Slowly, but turning, and nothing in the house was on. So this suggests a leak then. But there is no evidence of a leak. And if we were losing 250 litres in 2 hours our water bill would be super high?

We've noticed this problem for about 6 months and been going back and forth with attempted solutions.

We had the bathrooms and new heating cylinder put in a year ago. I've got the original builder coming in next Wednesday so will put all of this to him. Maybe the bathroom wall needs to come down to get at the pipes inside?
 
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We had the bathrooms and new heating cylinder put in a year ago. I've got the original builder coming in next Wednesday so will put all of this to him. Maybe the bathroom wall needs to come down to get at the pipes inside?

Are there any pipes under the ground floor which might be leaking unseen?
 
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There is only 1 part of the house where a pipe goes underground, and now I think of it, the wall next to that pipe had always had a slight damp problem. It's in the utility room / garage which is next to the house, and is much colder than the house, so I just assumed this damp was an old problem which hadn't dried out and wasn't ever going to dry out. That's definitely worth investigating.

My neighbour suggested a mixer valve could be locked open somewhere, so the hot water is running around the pipes losing its heat. That's another good idea but it wouldn't explain the cold water coming in constantly.

Great suggestions everyone
 
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Thermal camera is a great idea. Thanks Herts P&D.

I timed the water meter outside and it's consuming 2 litres a minute, with everything turned off inside. Yikes
 
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Think @lostinthelight meant open the inspection cover on your drains- good idea checking the meter, it confirms you've got a lot of water going somewhere- now to find out where it's going.
Given your rapidly cooling cylinder sketch it seems very likely that hot water is going somewhere. If you don't run a hot tap the pipework leading to it will cool off quickly-2l/min will keep the pipe warm so a touch test may be a starting point. Don't assume your builder/installer has done a perfect job- check feed pipes to toilets and outside taps, mistakes have been made in the past :)
 
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Thanks everyone. Great suggestions on the leak and looking for warm spots oldbutnotdead. We have noticed 2 patches in the floor in our kitchen and utility room stay warm through the day even when the hot water hasn't been used. My guess is that this is where the leak is happening. This matches what our neighbours said about their houses - over the years everyone has had their underfloor pipes cut off and rerouted due to leaks. I thought ours had all been done (they mostly come down the walls now and don't go underground), but the kitchen and utility room taps have their hot water pipe...underground.

Huge thanks for the input everyone. This was a big help in getting us to think about leaks, and I'm 99% sure this is the cause now. A plumber has been called. Thanks again.
 
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I was called to a property where they thought they had a leak, no water meter fitted but just low pressure. I found out they had a leaking water main, leaking into next doors sewage drain line. This confused everyone as no one knew where the water was going as there was no damp patch.

Andy
 
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Hi everyone
Just to let you know the result. It was indeed a water leak in the utility room, in the concrete floor next to a wall with a slight damp problem (should have guessed, really). Immediately underneath it is a drain so the water was probably going straight into it unnoticed. It took a bit of fixing (pipes behind kitchen cupboards etc) but all done now.
The picture shows the leak. It was behind and under the built-in freezer and you can see the damp on the wall.
And the water bill came through. £850 for 6 months! The insurance company say they will help.
Big thanks to everyone here. We never would have worked it out were it not for your suggestions.
Great forum
 

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