Cold water tank backfilling

[ISO2000]

The last time this question was asked was in 2004, unfortunately the answers have not been any help.

My cold water tank is being filled from the hot water tank. The system has been in place about 15 years. Both tanks are in the loft with the cold water tank raised on a platform. It is an indirect system.

I know it is filling from the hot water cistern because I turned off the feed from the cold tank and the overflow pipe from the hot tank started dripping into the cold water tank. The feed pipe is also quite warm near the cold water tank, which seems odd. In fact near the hot water cistern the feed pipe is warmer than the outlet pipe at the top.

I have read that it may be a faulty mixer tap. I only have one and turned off the cold feed to it but still got the overflow.

Any ideas? Is it a faulty hot water tank? What else can it be?

 

[JohnD]

I know it is filling from the hot water cistern because I turned off the feed from the cold tank and the overflow pipe from the hot tank started dripping into the cold water tank.

When you say "tank" do you sometimes mean "cylinder?"

If not, how big are the two tanks? Which is higher? Tie up the ballcock in both of them, and observe if the water level drops in one and rises in the other.

How old is the cylinder? What colour?

 

[ISO2000]

I know it is filling from the hot water cistern because I turned off the feed from the cold tank and the overflow pipe from the hot tank started dripping into the cold water tank.

When you say "tank" do you sometimes mean "cylinder?"

If not, how big are the two tanks? Which is higher? Tie up the ballcock in both of them, and observe if the water level drops in one and rises in the other.

How old is the cylinder? What colour?

Sorry, it is the hot water cylinder, copper with the green foam lagging. It is about 15 years old.

 

[polesapart]

What you describe indicates a cross connection between a hot water distribution pipe and cold main somewhere in your house..

Shower on unbalanced feeds (gravity hot, mains cold) usual suspect.

Got to be there if you look hard enough....

 

[longdogs]

....and solved by fitting check valves.

 

[ISO2000]

What you describe indicates a cross connection between a hot water distribution pipe and cold main somewhere in your house..

Shower on unbalanced feeds (gravity hot, mains cold) usual suspect.

Got to be there if you look hard enough....

I was just about to reply that I had eliminated that but I thought I'd try again. This time I turned off the hot supply to the downstairs shower (before I thought I'd turned off the mains supply to it) and lo and behold I think the shower is the cause. I also have a water equalising valve feeding the shower so may be it is that and not the shower mixer. Not that either of these is new but I just need to narrow it down to one of these (I hope!).

 

[ISO2000]

....and solved by fitting check valves.

Are they normally fitted to shower feeds? Not sure if I should look for a fault or just try fitting check valves.