Backflow on Vented Gravity System?!

[Rokit]

There's something not right going on. If I can ask a simple question first and then expand on it from there if necessary.

It's easy to understand backflow caused by mains cold going through the hw side of a gravity fed system but how can gravity fed hw backflow through the cold pipes of the system when they're fed from the same tank?

 

[Puddle]

Well one way would be for there to be more or larger cold supply outlets open compared to the hot supply

 

[Rokit]

Cheers Puddle.

Right. Good points that make perfect sense. How about if very little has been changed in a bathroom refit? The room is slightly larger and pipework only marginally altered? The feeds come down from the cwst above the vented hwc in the cupboard just outside the bathroom. Iso valves are in the floor before the threshold of the door. The first branch off under the (tiled) bathroom floor goes to an en-suite. Nothing up to this point has been altered. Next tee off both pipes feeds the mono basin waterfall mixer,which has poor flow obviously on this system,but does what it needs to and mixes the hot & cold fine. Next tee off the cold feeds the wc cistern then both pipes run to and up the wall to the bath filler/shower mixer. Having fitted w/m valves as temporary taps until a suitable low pressure mixer was found I can say everything worked fine and the flow from the w/m valves to fill the bath seemed good and balanced.

However after fitting the mixer over the bath - it's sequential with one control knob going from off through cold to hot and no flow rate adjustment, after running for a period to test it appeared to only give out hot water. Even more worryingly later on I was told the kids said hot water was coming from the mixer on the basin on the cold side. Thinking it may be heat transfer from pipe proximity I tested some more and discovered that when turning the bath mixer to off the cold feed to it started to heat up. I followed it working it's way down the cold feed,flushed the loo and sure enough heated water began entering the cistern. By the time I'd finished it was heading all the way back up the feed pipe from the cwst. I've talked to my dad-in-law who's been doing bathrooms and plumbing for 50 years and my mum's fella who's a long time qualified plumber (but too far away to come have a look) and they're baffled.

I can easily fit nrv to stop the backflow but am still puzzled as to what the cause is and would like to figure it out before fitting them. With them on I'm not sure whether there'll ever be a cold flow as the hot seems to come through constantly.

More help appreciated as it's doing my brain in!

Cheers.

 

[PlumbBobBob]

sounds like you've got a crossed pipe somewhere. i don't think heat transfer will do this to the extent you describe.

 

[diamondgas]

I'd agree with steveb .. crossed pipe. If hot and cold is gravity fed from the same tank... Do you have any TMV's fitted? Just wondered if that was passing. Just a thought, is the water in the CWS Cold?

 

[PlumbBobBob]

that's a point diamond, do you have any mains cold tank hot mixer taps?

 

[Puddle]

Think you have a faulty non return valve on shower cold inlet,requires repairing,when you turn cold outlets/taps on,you are pulling cold down cold pipework and hot through hot pipework through shower mixer ,due to fault,if you like it has the same effect as having two supplies to the one outlet

If you turn shower to its coldest setting when off you should find improvement at cold taps and prove theory

 

[Rokit]

Thanks for all the replies and suggestions. First chance to get back on with a proper computer rather than the iPhone which, whilst awesome in any ways, does not always lend itself to lengthy typing sessions!

So as an update.....had already spent ages going over everything, checking, rechecking, triple checking. Called the tap manufacturer to find out any unusual requirements (none). Got advice from my Mum's fella (plumber but 70 miles away). Got advice fro Father-in-Law who did the plumbing. Got advice from you guys. Banged my head against the wall for a few hours.

Discovered the hot and cold feed to the sink mixer fed into the wrong sides so swapped them over but this had no bearing on the bath mixer. Flipped the bath mixer 180 as suggested by my Mum's fella but same problem occurred. Plumbed a spare waterfall bath mixer (which is obviously not designed for LP but was a test) and worked fine despite the poor flow rate...no backflow. So took the mixer apart as I'd tried everything else and no-one could think of anything else it could be. This had been my first thought but wanted to test everything else before dismantling a new product.

This is where it gets....odd! It had a ceramic mixing cartridge - 1/4 turn usual affair. But with it still plumbed into the hot and cold feeds I turned on each iso valve in turn. Lo and behold both hot and cold were able to come through the body of the mixer and mix before reaching the mixing cartridge. The two small inlets in the body of the mixer that go to the cartridge allowed both the hot and cold to flow through - there was no seperation. Took it off the wall and blew through each inlet and spout blocking off the necessary holes to verify this (with my Mrs present and even she could understand what was happening). My first thought had been correct, I assumed, the tap was manufactured faulty.

Called manufacturer and supplier (the supplier was more help) and revealed my discovery. No problem. Another mixer on it's way. Turned up after a week and a half.

Went back home upon it's arrival thinking would all be sorted in a couple of hours. Fitted a couple of NRV before starting just to be sure we'd get no back flow even though I'm lead to believe this is only necessary to prevent contamination of a mains cold supply which wouldn't happen from our gravity tank system. Thought 'I'll just take the cartridge out and have a look before cracking on'. Nothing different to the first mixer. Same mixing of hot and cold in the tap body - no separation. Took said dismantled tap round to In- Laws hoping my Mrs' Dad would say " You numpty, look at this....it works this way!" but, partly to my delight and partly to my sorrow, he agreed that he couldn't figure out how we'd ever be able to alter the mix between the hot and cold.

So now I'm back to square one. Can anyone explain to me how a mixer that mixes the hot and cold in the body of the tap before it reaches the ceramic cartridge would work? I've left it fitted at the moment and all we can get is varying degrees of warm! I'm over the backflow problem....I haven't discovered why this was happening but it has been easily rectified by fitting the NRV's. If I can get the mixer to work maybe I'll go back and try to figure it out but one step at a time.

The only gravity hot and mains cold, Steveb and Diamond, I'm assuming will be the kitchen. As mentioned though fitting a second mixer (a HP waterfall that mathches the sink) didn't cause any problem other than a ludicrously poor flow.

At this point I don't even care about looking a complete idiot if it means I can fit this thing and have it working. The mixer is a Pheonix Pu bath filler/shower mixer (can I put that down here?!). If anyone has fitted one or has any experience of one please let me know. Also I'd like to loose the NRV's as god forbid I have to come back and remove tiles to replace them if they foul up as they're in the wall.

Cheers chaps and any ladies. Hope you're enjoying the Christmas festivities.