Blocked cold water pipe - but why

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Location
Croydon
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United Kingdom
Hi

After many years of trouble-free use, and with no changes to the cold plumbing for many years, the cold water failed flow through our bathroom taps (basin an bath) or to fill our WCs twice in the past 2 weeks.

I am dumbfounded by why the cold failed to flow and would appreciate any ideas. I investigated and noted the following:
  • The pipes are all copper and noboby has touched the gate valves
  • The header tank was full and there is minimal sediment in the bottom of tank
  • I have 28mm pipes from the header tank. These drop to 22mm for the long distances and present as 15mm at the hand basin and 22mm to the bath (pretty standard)
  • Hot side is working perfectly
  • My power shower was working perfectly (does use different pipes)
I resolved the issue on both occasions by running the bath with both taps fully open and my thumb over mixer-tap spout for 30 seconds. After that I let the the cold flow for 1 minute. No sediment was seen from the water.

ideas?
 
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Air lock.

Is your float in good working condition?
Is your shower booster pump emptying the cold water storage tank?
Has the mains been turned off outside/road works?

Andy
 
does the cold-water tank in the loft have a close-fitting plastic lid?

If not, how do you keep out drowned wildlife, balls of fluff, wisps of loft insulation?

What colour is your hot-water cylinder?
 
Your resolution is that for an air lock!

That would usually be caused by the cold tank emptying and allowing air to enter the outlet pipe.

You should look at the ball valve. It may be sticking closed or the flow rate may not match the demand.

You should measure the flow rate from it and compare with your usage rate.

Tony
 
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Thank you for the replies.
To address as best I can
  • No works to water mains in the area - that I am aware of
  • The cold tank is a black plastic thing and it was full when I checked. It is a very large cold header tank with a 2nd tank coupled-in to increase the capacity (The tank can feed our 3 power showers, but only two people live here).
  • With our normal lifestyle (and nothing is special about recent times) it would be impossible for my wife and I to empty the cold tank unless the feed was interrupted. (if the water mains was turned off for a long period we would notice as the kitchen tap and the garden taps would stop)
  • There is a lid to the water tank(s) and there is a loose plastic sheet covering the tank. A expansion & air release pipe from the h/w tanks feeds into the top of the cold tank and this leaves a gap of a cm or two around pipe.
  • Given it is a plastic header tank there is no way that a vacuum is forming in the top of the tank - The tank would deform and break any seal well before the water empties(IMHO)
It is an interesting idea concerning the ball valve sticking closed and allowing the tank to empty. This could then let air into the cold pipes, before the ball valve then dislodges to allow the tank to refill before we know the water is not working. How to test that?

We also have got a water softener on the house. This sits on the main feed to the cold tank. I suppose the water softener could be turning the water supply off for a few mins whilst it does its regeneration? But it is a very big tank to empty in the 15 mins re-gen cycle. Any Views?

Given that ball valves are cheap and is a job I can do - do you think I should just replace it?
 
A possible fault is ball valve not fully opening.

Since less than £10 if it looks anything less than perfect might be good to replace it.

Tony
 
Open both bath tap hot and cold and leave it running, check ballvalve is running full flow, there may be blockage or a small stone stuck in nozzle inside ballvalve.

Daniel.
 

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