Cold water storage tank issue.

[Prezza]

Hello all. I have a gravity fed system and my cold water storage tank started overfilling yesterday. I replaced the ballcock valve just in case it was a simple fix, but no joy. When I looked at the water outlet I could see the ripple of warm water being pushed back up into the tank, which leads me to believe the shower mixer in the upstairs bathroom may have failed, and the higher pressure cold water is forcing its way back up through the hot water cylinder and, ultimately, all the way into the CWST.
So I have two questions:
1. Is this the most likely reason for this to occur, as it happens even when the shower controls are off?
2. What type of mixer controls would prevent this from happening again?

Any help much appreciated.

Neil

 

[Johntheo5]

Have you HW from a unvented HW cylinder mixing with gravity fed cold from the CWST or is HW from a vented HW cylinder mixing with mains cold?.

 

[Prezza]

Hmmm! Not sure. If I turn the water off at the mains I get nothing from the upstairs cold bathroom taps, which suggests to me the cold is coming directly from the mains. Does that sound right? The cold water pressure is definitely much higher than the hot water pressure.

 

[Johntheo5]

When you turn off the mains and if the HW tap(s) flow does not stop then you have a vented HW cylinder (fed from the CWST) and I would think that cold mains could be filling the CWST as you suspect.

 

[Prezza]

That's the case. Just realised there's also no expansion tank. Assuming it's faulty shower shower controls, what type should I be replacing with when there's a big difference between hot and cold pressure?

 

[Madrab]

When you turn off the mains does Cold Water Storage Cistern (CWSC) stop overflowing? Do you have access to the shower feeds and can you isolate the hot? There should be Non return valves (NRV's) fitted to the shower, type dependent, that would stop the backflow along the hot.

That being said they may have been removed from the hot side of the shower, as it's gravity fed, to maximise flow.

This is the problem when plumbers don't follow the balanced supplies 1o1 ... what shower do you have?

 

[Johntheo5]

Can't help you there.

The problem with different cold/hot pressures is that the mixing will take place at the lower pressure resulting in a poor mixed (shower) flow rate, some thermostatic showers just won't work if the differential pressure is very high, some install a PRV set to 1.0 bar on the cold feed immediately before the shower but this probably won't cure your problem.

 

[Madrab]

... I'll get me coat then

 

[Prezza]

Hi Madrab. The flow of water back into the CWST stops when the mains are off. Not sure about access to the shower feeds. I can remove the existing mixer in order to replace it, but the pipes that feed it are inaccessible. The attached pic is style of mixer fitted.

 

[Madrab]

It just allows you to prove that it's the shower that's backflowing, it may not be. That and if you can, then installing a single check valve on the hot side may solve the problem but again as it's gravity then there's no guarantee that it won't impact the flow.

You may find it difficult to obtain a shower that can handle unbalanced supplies. You need to look for a shower that's designed for gravity supplies and you may need to restrict the cold supply.

 

[Prezza]

Thanks for all the advice. Much appreciated.

 

[terryplumb]

Do you have any other mixer taps in the property ? If you do, it could be one of them and not the shower.

 

[Prezza]

Kitchen has a mixer tap. Is there an easy way to check which is most likely to be the problem?

 

[terryplumb]

Is there an isolation valve on cold supply to kitchen tap ? If so ,close it and see if loft tank problem stops.

 

[Prezza]

Yes, I'm pretty sure there is. I'll give that a go. Many thanks.