Cold water tank overflowing after changing ballcock

[Willowandmikey]

Hi peeps

new to this forum and the world of plumbing. I've got a issue that two plumbers have so far failed to resolve and is driving me round the bend.
I currently have a cold water tank that feeds my upstairs toilet recently it started overflowing and even though I could not find anything wrong with the ball cock it was changed. Couple of hours later the new ball Vick was submerged and it was overflowing.

The ball clock has been changed for a high pressure and low pressure one and regardless after a few hours its overflowing. The ball cock seems to shut properly but after a few hours with my back turned its overflowing again. It's mains pressure fed.

The central heating runs on a combo system so I assume nothing feeds back into it one plumber secured all the pipes in the lift and put a metal plate where the water enters to stop the tank flexing and declared it fixed. Alas 3 hours after he turned his back more overflowing.

I have set the ball cock at its lowest level and watched the tank fill up to about a foot below the overflow. Then turned my back on it and a couple of hours later it was overflowing. There's no hot water tank or heating tank in the lift just the coldwater tank with one feed and one point draining out to the toilet and upstairs sink.

Hopefully one of you experts will help me sort this frustrating issue.

 

[mutley racers]

If there is no hot water tank do you have a combi boiler and this water tank just feeds the toilet?

Or do you have a cylinder in airing cupboard? If so do you have mixer taps any where? Tie your ball valve up and see if it still over flows

 

[Willowandmikey]

If there is no hot water tank do you have a combi boiler and this water tank just feeds the toilet?

Or do you have a cylinder in airing cupboard? If so do you have mixer taps any where? Tie your ball valve up and see if it still over flows

Yes it is a combi boiler with no hot water tank. The water tank feeds one toilet and basin. There is no cylinder in any airing cupboard. I will try tying it up in case the water is coming from elsewhere

 

[kay-jay]

Yes it is a combi boiler with no hot water tank. The water tank feeds one toilet and basin. There is no cylinder in any airing cupboard. I will try tying it up in case the water is coming from elsewhere

Does the basin have a mixer tap? The combi powered hw Will have more pressure than the gravity cold and may be forcing water back up to the tank

 

[Paulus]

:iagree: Simple solution is to swap feeds to toilet and washbasin to mains and scrap the tank.

 

[Willowandmikey]

Discovered the thermostatic shower mixer is also fed from the cold water tank. Is it possible this Is faulty and forcing water back into the cold water tank.

it would be simpler to swap everything to a mains feed but unfortunately if the upstairs tap was turned on it would mean no hot water downstairs due to the old combi boiler

 

[PlumbBobBob]

could well be the shower mixer passing between hot and cold

 

[Centralheatking]

Looks like you got it - the gravity fed cold from tank is being overpowered by mains pressure hot fro combi------fit mains cold to shower and an equamatic shower control

I might try a one way valve on the feed from the cold tank but it might not work

centralheatking

Discovered the thermostatic shower mixer is also fed from the cold water tank. Is it possible this Is faulty and forcing water back into the cold water tank.

it would be simpler to swap everything to a mains feed but unfortunately if the upstairs tap was turned on it would mean no hot water downstairs due to the old combi boiler

 

[Willowandmikey]

Firstly thanks for all the help people. It seems that if I set the thermostatic shower to 0 degrees it doesn't flow back into the tank. As soon as I go past 38 degrees I can hear the water rushing through. I assume the thermostatic tap is faulty and the only option is to replace it. It's one of those cheapie non brand jobbies

 

[steadyon]

Before you change it, it's worth checking if the shower has non-return valves built in to the hot and cold, many do. These can get jammed, particularly in hard water areas, and allow reverse flow. They can usually be unjammed by cleaning in de-scaling solution (or white vinegar) at virtually zero cost.