Backflow problems with mixer taps/showers

[CTE1]

Hi,
I have a gravity fed traditional system (header tank in loft, hot water cylinder below, gas boiler on ground floor),
The problem is an intermittent header tank overflow, that has been on and off for the last 2 years (since new bathroom installed on ground floor).
Sometimes there can be no problems for 2 or 3 months then out of the blue the overflow pipe begins to trickle out water. In the past we have turned the cold mains pressure down to 1 bar to stop it, changed the ballvalve in the header tank etc).

The culprit I am almost certain is a backflow problem from the new bathroom, although our plumber insists that a non return valve was part of the mixer shower he fitted, and that he has installed a second non return valve on the middle floor which should stop any cold water running back up the hot water pipes.

We suspected the heating system at first but we have a new hot water cylinder, plus the overflow can happen when the Central Heating and Hot Water are switched off , so we have discounted this asnot the likely cause.

Big question: Should the mixer shower and mixer tap in the groundfloor bathroom be connected to the header tank cold water supply or the rising mains cold water supply? Ours is connected to the cold water rising mains supply.

Another factor is the cold water header tank water is very slightly lukewarm (only slightly) which suggests it has passed through the hot water cylinder while back flowing.

Can anyone help, and maybe explain why it is an intermittent fault rather than a constant one please?

Baffled! Thanks

 

[justlead1]

Hi. Its not to clear as to the cistern that is overflowing. The small cistern feeds heating and large the hot water system and possibly the cold. I would suggest you refer to small and large to get a solution to the problem.

 

[CTE1]

Hi sorry, yes its the big cistern - the main cold water storage tank (not the small expansion tank that sits next to it) that is overflowing.

 

[Plumbstar Tom]

it should not be hot and can only get hot from expansion from the expansion pipe or if it is back feeding from a mixer.

A non return valve could be at fault here

A lot of shower mixers have them built in but they can fail, if your plumber fitted one in line anyway this would eliminate this so ask where it is so it can be checked as they surely would have put it in an accessible place?

 

[CTE1]

Hi Tom,
Thats great advice - I'll ask the plumber to check the mixer shower in-built non return valve, and the extra one he put in, to see if they are faulty.
Have you heard of these types of faults being intermittent though? This is the one piece of the jigsaw that is baffling - if a valve is faulty, but works some times and doesn't others?

 

[Plumbstar Tom]

it could just be a bit of debris blocking the NRV

What makes you so sure it is the shower is at fault?

Is the cylinder over heating perhaps?

 

[CTE1]

The reason I dont think its the cylinder overheating is that this has happened when the water and heating have been off all day, or overnight when neither are switched on.

Is this the correct assumption do you think?

 

[CTE1]

Further to this leaking cold water storage tank overflow pipe problem:
Last night I turned the mains water pressure down to 0.9 bar, so that it was omnly a very light flow from the taps. The idea was to see if we still had an overflow pipe leak even when the mains water pressure was very low.

This morning I awoke to exactly the same leak as normal from the cold water storage tank in the loft.
As our bathroom mixer taps and shower are fed by the mains cold supply instead of the cold storage tank, our plumber has suggested switching this round so that they are fed by the cold storage tank - he says this will eliminate the chance of them backflowing. This is all very well except that last night the mains pressure was so lw (0.9 bars) that surely water pressure at this level cant force its way back up the hot water pipes and into the cold water tank in the loft - can it?

A couple of questions that probably have very simple answers:
1) Can backflow occur when taps or showers arent being used (ie arent mixing)? I ask this because everything is fine when we go to bed, but when we wake up after 8 hours of inactivity, the overflow pipe is leaking.

2) Can check valves fail? Our plumber has installed one and says it should be working - could it have failed, and would it be a good idea to put a 2nd one in just in case?

Baffled and frustrated!

Only other possibility is faulty coil in water cylinder - but if this is the problem why does it happen when the water and central heating are switched off?

 

[WaterTight]

This kind of problem often is intermittent. It's because when your up and about and using the water, the tank is emptying and re-filling. The backflow problem will be slow and gradual and need a full tank to be left full for a good while before it can fill it to reach the overflow.

Also, bit beyond my level of experience, but I don't think the heating would need to be on for a leaking coil to produce this problem. It would just need to leak into the cylinder until the cold water tank overflowed and then the small f+e tank would re-fill itself. I think.

I'm not sure what the quickest and most logical route to discovering the answer is but things I would try if left to my own devices would be :

-check visually to make sure the ball-valve in the cold water tank is definetly shutting off properly.

-turn off any isolator valve on the cold feed to the mixer if there is one. this would prevent any backflow and if you did it while it's trickling out the overflow (and it was to blame) it would stop almost immediately.

-if no joy with other ideas...tie up ball valve arm in f+e tank to stop central heating circuit filling by itself and see if problem goes away - if it does - prob leaking coil

 

[CTE1]

Hi that is a great help thanks.

So regarding the backflow problem i assume it can happen even when the taps and showers arent being used? I thought I had read somewhere that the cold water only escapes and backs up the hot pipe when you are using the tap/shower?
If it can backfflow all the time then that would explain why it starts overflowing early morning (about 5am) when all has been quiet for several hours....

 

[mikeygas]

I would look at balacing your mixer shower properly while your at it, as the cold water into your shower should come from your large cold water tank and not fed directly from your mains. What colour is the water in your large tank i.e is it brown/reddish, as this could indicate you have a leak in your cylinder coil.

 

[CTE1]

Fortunately the colour is clear, so am hoping its not the coil, and will be resolved by the balancing.

The plumber is switching the cold supply from mains to tank tomorrow, so fingers crossed...

 

[mikeygas]

All the best, post a reply to let us know how you get on.

 

[macplumb]

If you want to check for a leak the coil have a look at the hot water is it clear or slightly yellowish? If yellowish could be a coil leak. Lowering the pressure to .9 bar is still 9m of head so water can still make it's way up to the tank. I would say it's a mixer passing on tap or shower have you seen the secondry nrv it might not realy be there. Wouldn't think it was the coil remember water always finds it's own level unless it is a sealed heating system and vented hot water. When you say water in the cistern is warm this will be due to expansion and lite warm water riseing above the heavier cold water. Hope this helps.

 

[CTE1]

Thanks for everyone's help, we have now finally nailed this problem!

Our plumber balanced to system by connecting all mixer taps/showers to the cold water tank feed as opposed to the mains water feed. Apparently the balanced feed pipe had been capped by a previous plumber, and this was hidden under floorboards near the bathroom. So he reconnected this, and just left the mains water supplying the kitchen taps and washing machine.

 

[Deleted member 95995]

Just a quick 'thank you' to participants in this thread. I had the same problem with my elderly mother's "Aquability" shower. They finally came out and fitted a new non-return valve and a pressure reducer on the mains feed. But why the over-engineering? Why not use the balanced cold feed from the header tank in the first place? Do they just not like going into lofts because there might be spiders up there?