Hot water tank leaking cold water into cold water tank

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Hello

This is a problem causing scratched heads not only to me (a total non-expert when it comes to plumbing) but also to British Gas (I have HomeCare 200 cover).

Basic facts:

1. I have an indirect system.
2. On Friday morning (last day of 2010!), my cold water storage tank started to overflow through the warning pipe.
3. I turned off the ball valve to shut off the inflow to the tank.
4. The overflow continued.
5. I then noticed that steam was rising off the tank, stuck my hand in the "cold" water - and found that it was almost warm enough to take a bath in.
6. Obvious assumption: the hot water expansion pipe was venting from the hot water cylinder into the cold water tank. But it wasn't.
7. So I reached into the tank ... and discovered that hot water was flowing back up into the tank through the pipe that supplies cold water to the hot water cylinder.
8. I contacted British Gas, who sent out a Dyno-Rod engineer. (I'll cut the next stage short, as it involved Dyno-Road and BG batting the problem back-and-forth - i.e. was it a plumbing problem or a central heating problem?)
9. In the end (on Saturday) a BG engineer decided that it's all due to the thermostat on the hot water cylinder being knackered, causing the water to overheat and vent up into the cold water tank. (Though he didn't/couldn't explain why it was flowing up the cold water supply pipe rather than expansion pipe designed for the job.) Unfortunately, he didn't have a replacement thermostat, and we agreed that he would come back Wednesday morning.
10. In the meantime, the storage tank continued to overflow, even with the cold water inflow still turned off - and even with the entire heating system turned off.
11. So last thing last night, as an experiment, I drained the cold water storage tank by running the hot water taps only. Then I switched off the entire system - no heating, no hot water. This morning, the level of the tank had risen by 8 inches - of cold water!

My guess is that the heat exchanger coil in the hot water cylinder has ruptured, and that water from the heating system - whether hot or cold - is filling the cylinder and backing up into the cold water tank through the easiest available route, the cold water supply pipe. The only strike against this theory is that the water rising into the cold water tank is totally clear and sweet as a daisy - not a sign of contamination.

Any advice would be greatly appreciated!

Paul
 
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You might have a mixer tap (or shower)letting water through from the cold to the hot especially if the cold is mains fed.
 
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Firstly, many thanks for responding so quickly, bobthebender and twgas.

twgas, I really like your theory - a mixer tap is much easier (and cheaper!) to replace than a hot water cylinder.

As the only exit pipe from the cold water storage tank leads down directly into the hot water cylinder, I'm assuming that all of the cold water taps etc. in my house are mains fed. I'm guessing this is unusual as all the system diagrams I've found on the net over the past couple of days show a second pipe from the storage tank feeding taps etc.

I have a mixer tap in the kitchen that's definitely mains fed - it's my source for drinking water - and a downstairs shower with a mixer control that's probably ditto. And I think one of these might be the problem. During one of my many recent Google searches, I found a discussion about this on another forum, but didn't really understand it. I think the problem was solved by hooking the mixer tap(s) up to the cold water storage tank rather than being supplied by the mains.

Once again, thanks for your help.
 
I'd go with the leaking mixer. Ceramic mixers, and showers, are most often the culprit. To test, run a mixer on hot for a while, then close it tight and put your hand on the hot pipe under the tap. If you find one that goes very cold in a minute or two, that is probably the culprit.
 
JohnD - I followed your suggestion this morning. The hot water pipe feeding the mixer tap in the kitchen stayed hot for several minutes. When I tried the same with my shower, the hot water pipe went cold in about 2 seconds.

Sorted!

Many thanks to everyone who helped with this.

Hmmmmm. I pay through the nose for an annual service contract with British Gas, but I get far more constructive help and advice on a free DIY forum. Makes you think ....
 
Please send me £80






:p

BTW it is incorrect to have a mixer shower fed with hot water from a gravity tank, and cold water at mains pressure. It is difficult, expensive and unsatisfactory trying to get a shower to mix like that. You need to get a cold feed from the cold tank.
 

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