cold water tank in attic overflowing but ballvalve OK

[topazstar]

A few weeks ago we had a new kitchen sink fitted. Immediately afterwards the cold water tank in attic started overflowing. Fitted new ballvalve but did not help.

We have had 2 plumbers look at the problem and they do not know what the problem is.
We do not have central heating so the cold water tank in the attic just feeds the immersion heater.
Overflow pours out even when immersion heater is off and water is cold.

When the new sink was fitted they did not go into attic at all, just turned off cold water supply under sink and in the airing cupboard above the immersion heater.

The cold water tank in attic is filling despite the ballvalve being new and working fine.

Does anyone have any ideas as to how we can fix this?

 

[JCplumb]

It's backfilling, cold water is passing through your new mixer tap and going up the hot water pipe, through your hot water cylinder and up into the cwt in the loft.
The tap installed was not fit to go on your system, the 2 'plumbers' should have known that, guessing it's a monobloc with one handle that controls temp and flow.
A non return valve on the hot feed to the new tap will probably sort it although that is a mechanical device that may fail in the future, also, you may not have enough hot water pressure to operate the tap properly afterwards.
Best bet is to get the right tap - the type you would ideally want is called a dual flow.
Who chose the tap?

 

[chris watkins]

Perfect answer JC, especially at that time in the morning. :santa5:

I would re-enforce your assumption that the two who attended were not plumbers despite what they may claim, so topazstar please don't pay them to come back.

 

[joni os]

Initially the water from overflow would have been warm, that should have been the clue.

 

[topazstar]

Thank you so much for your help - I purchased the tap but the plumber never queried it or said it would not be suitable. I am now purchasing a DUAL FLOW

 

[joni os]

Check pressure rating on tap. Must be suitable to work at 0.1 bar anything greater will struggle on open vent.

 

[topazstar]

I am still stuck with this - purchased new dual flow taps but the new plumber said they are not needed and best to fit a non-return valve - these are the taps currently on the sink - I am now totally confused as they have two handles, not one:

 

[Subby]

Have the plumber come back and fit non return valves on the hot and cold pipes to the new tap, this will stop the tank in the attic backfilling.

 

[Dotty]

Have the plumber come back and fit non return valves on the hot and cold pipes to the new tap, this will stop the tank in the attic backfilling.

And any other mixer style outlets....