Cold water tank overflowing

[choffer74]

Hi all,
looking for some advice about an overflowing cold water tank. Each morning the tank has continued filling up, so by the morning we have water coming out the overflow pipe outside.

My initial thought was either (a) the ball valve was not set to the right height to turn off the water feed; or (b) the valve itself had failed and wasn't turning off the water. But these are both fine. The water turns off well below the overflow pipe.

So the only option is that water is feeding back into the tank as there's only 3 things going in/out of the tank. (1) the incoming cold water feed (2) the over flow pipe (3) an outlet at the bottom of the tank that then goes into a water pump which feeds toilets/showers/sinks.

I thought i'd try and confirm this by last thing at night, turning off the valve in the pipe between the tank outlet and the water pump. This turns off the water to the sinks/showers if I'm changing a tap etc. I thought this would stop water feeding back up. But it didn't work - water was still overflowing by the morning. I'm going to try again tonight and ensure its fully turned off. So my first question is, should closing this valve stop any water feeding back into the tank?

House was built mid 90s. We moved in a few years ago and haven't changed any of the plumbing. This is our setup:
The mains a combi boiler downstairs, the kitchen, and the washing machine/dishwasher.
Cold water tank in a boiler cupboard on the 1st floor. i.e. not in the loft.
Cold water tank feeds the sinks/toilets/showers via a water pump
2 showers have thermostatic mixers (I've recently replaced the valve in one of the showers, but timing doesn't fit in with when we first noticed the tank overflowing) - both showers are fed from the cold water tank.

Any ideas what could be causing the problem?
thanks in advance
G

 

[Deleted member 120897]

You may have a split coil in the cylinder, or a backflow issue with something, eg your shower that was replaced. Are you able to isolate the shower feeds at all?

 

[choffer74]

You may have a split coil in the cylinder, or a backflow issue with something, eg your shower that was replaced. Are you able to isolate the shower feeds at all?

thanks for the reply.
and thats a split coil in the Combi Boiler? my googling of this issue seemed to indicate that is when you have a hot water cylinder. But may have misread it!

I've had a quick look under the bath and couldn't see any isolating valves. And not sure I can get to the pipes under the shower without taking apart some of the enclosure - but I'll take a closer look later on. The alternative would be to remove the shower unit and cap the pipes - then check if it still overflows? would that work?

 

[Deleted member 120897]

No Combi boilers don’t have coils, however some people don’t realise what boiler they have, thinking they have a Combi when they haven’t. You mentioned tanks in the loft, so naturally assumed you didn’t actually have a Combi boiler.

 

[SJB060685]

No Combi boilers don’t have coils, however some people don’t realise what boiler they have, thinking they have a Combi when they haven’t. You mentioned tanks in the loft, so naturally assumed you didn’t actually have a Combi boiler.

We used to have a couple customers with open vented oil fired combis.

 

[choffer74]

No Combi boilers don’t have coils, however some people don’t realise what boiler they have, thinking they have a Combi when they haven’t. You mentioned tanks in the loft, so naturally assumed you didn’t actually have a Combi boiler.

guess thats good news - fixing a mixer valve sounds cheaper than a new coil!

I've managed to find isolating valves for the downstairs shower - I've shut them and will monitor if it continues to fill up. If its still an issue, I'll try the turning off water to the upstairs shower
thanks

 

[choffer74]

the overflowing tank is definitely caused by the downstairs bathroom. I isolated the hot and cold water in the bathroom - this turned off water to the sink and the shower - and the overflowing stopped (turning the hot/cold water back on last night restarted the overflowing)

I removed the shower unit last night. There are plastic NRV (similar to this) internally within the shower unit on both the hot and cold water feeds. I did a quick check under the kitchen tap and they both seemed to work - just not sure how good/valid a test this is!

I could try replacing one or both of the NRV.

But could it be an issue with the bathroom sink? its just a basic mixer tap.

 

[Timmy D]

A thermostatic cartridge is passing. Hot from the boiler is mains pressure and must be passing through a dodgy cartridge in to the cold gravity/tank.

Is the basin tap thermostatic? If not, highly likely the shower.

If you turn the temp of the shower to either fully cold or fully hot, does that stop the overflowing? If so, it’s the shower.

Any reason the cold water tank/pump were not removing/by passed when the combi was installed? It shouldn’t really have been left in, removing it would put the cold up to mains pressure equal to the hot.

If it was left to try and help flow rate, I’d remove it and add an accumulator.