Uneven flow from cold bath tap

Albermarle

We have a side extension, with a bath and an over bath shower. The water flow to the shower ( and sink) and the hot bath tap are fine. However with the cold bath tap, sometimes nothing comes out for a few seconds, then some gurgling and then a big spurt. Usually does this two or three times before settling down to a more regular flow.
Also with both hot and cold bath taps running ( into one mixer ) often you find that it is mainly hot rather than 50:50 . 
Also there is sometimes a loud bang in the pipes when the cold bath tap is shut off quickly.
I can not remember if it started when we had the extension done, or later when we changed over to an unvented DHW and mains pressure direct to all taps. I think it was the latter.
The last plumber we asked said as it was an unvented system it could not be an airlock issue, and that it was almost certainly a problem with the taps, which were quite old. So taps were replaced and made no difference. 
Our mains pressure is low rather than high, so that is not the issue either.
Any ideas?

Albermarle

Anybody any ideas?

ThisIsWeird

That's a real mystery, Albe.

Edit: having reread your post - you have 'low' mains pressure? How low?

If you have decent mains pressure, then any trapped air should be ejected very quickly, first time, and the problem should then be permanently sorted.

The 'bang' in the pipe sounds like classic pipe 'hammer', caused by the change in pressure in the supply pipe when the tap is turned off - the pipe literally swells or straightens under the pressure shock, and that's often enough to make it bang and shudder against a close joist or other pipe, for example. 
You can usually test this by turning the offending tap off very slowly - does this prevent it from banging?
This doesn't explain the strange flow, tho'. My guess, too, would have been a faulty tap, especially if it is a 'conventional' washer type, and not quarter-turn ceramic. Which type is it? But, it still doesn't make much sense here.
The 'gurgling and spurt', tho', are really bizarre in a mains-driven supply. You say it then settles down to a 'regular' flow, but what does this mean? 'Cos you also say your mains pressure is good, so I'd expect that cold tap to be gushing. Is it? (To be edited).
Keeping things simple, it's important to also not discount it being another faulty tap!
Could we have pics of this tap, please? Any chance of a video clip of what happens?
And, how familiar are you with the pipe runs to this extension - do you know where they are, and can you access any of them?

FreeBear

ThisIsWeird said: The 'gurgling and spurt', tho', are really bizarre in a mains-driven supply. You say it then settles down to a 'regular' flow, but what does this mean? 'Cos you also say your mains pressure is good, so I'd expect that cold tap to be gushing. Is it?

Sounds like the water is draining out of the pipe somewhere and air is getting in. That would explain the gurgling & spurting, but that shouldn't be happening if the pipe is at mains water pressure...

A non-return valve close to where the pipe branches off from the main feed might help. A hammer arrestor would help to reduce the banging when turning the tap off.

Albermarle

ThisIsWeird said:

That's a real mystery, Albe.

Edit: having reread your post - you have 'low' mains pressure? How low?

If you have decent mains pressure, then any trapped air should be ejected very quickly, first time, and the problem should then be permanently sorted.

The 'bang' in the pipe sounds like classic pipe 'hammer', caused by the change in pressure in the supply pipe when the tap is turned off - the pipe literally swells or straightens under the pressure shock, and that's often enough to make it bang and shudder against a close joist or other pipe, for example. 
You can usually test this by turning the offending tap off very slowly - does this prevent it from banging?
This doesn't explain the strange flow, tho'. My guess, too, would have been a faulty tap, especially if it is a 'conventional' washer type, and not quarter-turn ceramic. Which type is it? But, it still doesn't make much sense here.
The 'gurgling and spurt', tho', are really bizarre in a mains-driven supply. You say it then settles down to a 'regular' flow, but what does this mean? 'Cos you also say your mains pressure is good, so I'd expect that cold tap to be gushing. Is it? (To be edited).
Keeping things simple, it's important to also not discount it being another faulty tap!
Could we have pics of this tap, please? Any chance of a video clip of what happens?
And, how familiar are you with the pipe runs to this extension - do you know where they are, and can you access any of them?

In a bit of a rush today, so not going to post a video.
Mains pressure is around 1.3/1.4 ( I know now too low for an unvented system)
The water definitely does not gush out anywhere, although the showers are adequate.
The old taps were conventional washer type, and the new ones are quarter turn cartridge. Changing them made no difference.
Just tried a little experiment ( as I personally do not use this bath that often)
It is the hot tap that bangs when closed off quickly so could be a red herring.
If you switch the cold tap on its own it runs normally ( modest flow). If you have the hot and cold tap running together and then switch off the hot, then the cold tap is running at low flow, and /or stops altogether and this spurting starts until it settles down again.
So somehow having the hot tap running 'upsets' the cold tap.
The pipe runs are largely hidden, but I have a suspicion that is where the issue is as it does not happen anywhere else, not even up in the loft conversion.

ThisIsWeird

Increasingly bizarre.
Do you have much variation in overall mains flow, say late at night and at peak times?
This bath tap is a 'mixer'? 
Do you have room, even in your garage, or for a wee purpose-built shed, for a Challis? Yes, it'll be around £1.5k, but water supply transformed.

Albermarle

ThisIsWeird said:

Increasingly bizarre.
Do you have much variation in overall mains flow, say late at night and at peak times? No
This bath tap is a 'mixer'? Yes
Do you have room, even in your garage, or for a wee purpose-built shed, for a Challis? Yes, it'll be around £1.5k, but water supply transformed. No

Just going out now but will have another play around tomorrow and report.
It has been happening for a long time and we have just got used to it, but just wondered if anybody had any bright ideas.

ThisIsWeird

Stupid stupid Q - you are 100% certain that all elements of the old CWS system were removed/isolated when you converted to unvented? 

Absolutely no way the cold is still being supplied from a stored tank, perhaps to allow all of the low mains pressure to drive the new unvented hot demand from the mains? (Even that doesn't make sense, as the refilling CWS would also take away from the mains!)

If the spluttering is caused by trapped air, in a system that is under constant mains pressure - and even 1 bar is a fair amount - then the only source of that air can surely only be internal. And the only location of an air source that I can think of is the air bubble/vessel of the HWC. Make and model of your new cylinder? This is truly grasping at straws, of course...

Albermarle

ThisIsWeird said:

Stupid stupid Q - you are 100% certain that all elements of the old CWS system were removed/isolated when you converted to unvented? Yes, we had a loft conversion and all the old system was taken out and changed to an unvented system.

Absolutely no way the cold is still being supplied from a stored tank, perhaps to allow all of the low mains pressure to drive the new unvented hot demand from the mains? (Even that doesn't make sense, as the refilling CWS would also take away from the mains!) No way 

If the spluttering is caused by trapped air, in a system that is under constant mains pressure - and even 1 bar is a fair amount - then the only source of that air can surely only be internal. And the only location of an air source that I can think of is the air bubble/vessel of the HWC. Make and model of your new cylinder? It is not new but quite a few years old , RCM Ultrasteel with a separate expansion tank ( that is newer) This is truly grasping at straws, of course...

Some more experimentation.
If you open the hot bath tap on its own you get a smooth silent operation. If you open the cold tap on its own, the tap makes a noise and there is an initial gush of water, like a higher pressure has built up and now released. Plus for a few seconds there is a bit of unstable flow and gurgling noises, before it settles down.

Then ( as said before) if you have both on ( as you would filling a bath) you often find the bathwater is hotter than expected. If you turn the hot off you can see the cold flow is now lower and will often then stop for a second or two and then suddenly spurt out . This can happen a few times before it all settles down.

It  seems to be the cold flow to that one bath tap is the issue, as no problems at any other cold outlet in the house. 

One other point is that when either the hot or cold bath taps are running, the hot and cold supply to other outlets is much reduced/zero. More than you would expect from just having not a great mains pressure.

Thanks for your comments so far. If no obvious solution it is not the end of the world, just a bit annoying.

ThisIsWeird

I'm stumped.
It's all classic of a vented system with a partial air block, but bizarre in an unvented, unless the pressure and flow is soooo pitiful that it could cause this. But even then, for it to happen on only one tap is strange (I had this with my first flat - the mains flow would drop to a trickle at times, and seemingly even draw in air, 'cos I'd sometimes get a massive cough from the kitchen cold when opened at other times, once it blew a glass out of my hand. Smashing.)
What route does the H&C take to this bathroom? Does it loop in via the loft, for example? Any potential for adding an air vent on the high spot, even a manual one?