Ballcock Valve in Water Tank Query

[Alfie25]

I recently had a plumber out to replace the ballcock valve in my cold water tank in the loft as it was overflowing to the pipe outside the house. The new valve has certainly fixed that problem, with the water level set about an inch below the overflow pipe.

The problem is that when the tank fills up after I've used some hot water, the mains water gushes through the valve as normal, gradually slowing down as expected but then the water drips through the valve for hours and hours until it closes off the valve. Actually, it's probably never closed off the valve by itself, as after 10 hours it's still slowly dripping (maybe 1 drip every 10 seconds) and after that time you've probably used some hot water so the process starts again.

I've manually pushed the float arm down to fill the tank quicker and it does then shut off the valve about an inch below the overflow tank. I've also poured water into the tank from a bottle and about 1.5 litres will raise the ballcock enough to close the valve when it gets to the point of slowing down to a drip.

So... it's not really a problem in the sense that the valve will shut off eventually, but obviously the last litre or so it needs comes in at a very slow drip rate - taking maybe several days to close the valve. Is this normal for these types of valve or should I get the plumber back round to take a look/fit a better quality valve?

 

[HERTS P&D]

Is this what he fitted?

http://www.screwfix.com/p/part-2-float-valve-brass/92516

If so then it's correct.

Andy

 

[Alfie25]

Yup, that's the part he fitted. The part does do what it is intended to do, it's just the last 1.5 litres of water required to close off the valve would probably take several days to fill via a 1 drip per 10 seconds rate.

 

[TicklyT]

That is generally the nature of a conventional ballcock, and it isn't usually a problem. If the mechanism is operating smoothly, as a new valve should, they will take ages to close completely.

A theoretically 'perfect' ballcock would never close completely.

In the real world they close eventually. A 'part worn' valve will probably close quicker than a new one, as a combination of a bit of friction in the mechanism, and a bit of play between the moving parts combine so the valve 'snaps' shut instead of exactly following the movement of the float.

 

[Alfie25]

That is generally the nature of a conventional ballcock, and it isn't usually a problem. If the mechanism is operating smoothly, as a new valve should, they will take ages to close completely.

A theoretically 'perfect' ballcock would never close completely.

In the real world they close eventually. A 'part worn' valve will probably close quicker than a new one, as a combination of a bit of friction in the mechanism, and a bit of play between the moving parts combine so the valve 'snaps' shut instead of exactly following the movement of the float.

Thanks. I do like logical explanations and that makes sense, although I can't see why a perfect ballcock would never close completely. Surely once it is raised sufficiently (after several days maybe), it would close the valve?

 

[TheVictorian]

It's because the higher the float the slower the rate of water going in to the tank.

Therefore it will theoretically close, but this would also theoretically take an infinate amount of time to do so as the flow rate woudl exponentially slow down as the water level increases.

Google 'exponential decay' and look at the graphs in the images, this should give you an idea.

 

[TheVictorian]

Double post

 

[TicklyT]

That is generally the nature of a conventional ballcock, and it isn't usually a problem. If the mechanism is operating smoothly, as a new valve should, they will take ages to close completely.

A theoretically 'perfect' ballcock would never close completely.

In the real world they close eventually. A 'part worn' valve will probably close quicker than a new one, as a combination of a bit of friction in the mechanism, and a bit of play between the moving parts combine so the valve 'snaps' shut instead of exactly following the movement of the float.

Thanks. I do like logical explanations and that makes sense, although I can't see why a perfect ballcock would never close completely. Surely once it is raised sufficiently (after several days maybe), it would close the valve?

The nearer to closed the valve is, the less the flow, so the longer it takes to raise the water level to close the valve further.....
It's the same as the old snail climbing a wall problem - A snail climbs halfway up a wall in a day, but can only climb half that distance the second day, and only half that distance again the next day, etc. etc. How long does it take the snail to get to the top?

 

[Alfie25]

Nice analogy! I've never come across the problem in a toilet before, but presumably as the tank is much smaller and the tank isn't made of flexible plastic then it takes less time to fill.

Hopefully in time when it's more worn in then it'll stop dripping. Cheers for the responses.

 

[Agile]

Alfie, I think they are being overly theoretical there because the "last" drip would in theory make the difference to close off the valve!

However, I would strongly suggest that you do check the situation after say overnight when no water has been used in case you have an unseen leak in your house!

Then there is another problem. If the tank is plastic and there is no reinforcing metal plate on the outside where the valve is fitted, the tank then flexes as upward pressure is applied to the valve. This causes the shut off position to rise by say 20-30 mm making the shut off time far longer.

A good plumber would always fit the plate on a plastic cylinder in most cases unless the pipework was seen to be rigid enough not to need it.

I would also say 25mm level below the overflow is too little. As the valve ages the level will increase. I would consider 60-80 mm as a minimum for a new valve!

Tony Glazier

 

[Alfie25]

I've left it overnight (10 hours as I mention in my 1st post) and it is still dripping in the morning. If I let more water in (by pushing the float arm down) then release it shortly after, then the dripping stops as presumably the water level has increased enough to raise the float to a level that shuts the valve. Because of this, I do not suspect a leak.

The problem is that when left to its own devices, the drip is so slow that it would seemingly take days and days for the water level to finally rise to a level where the valve closes. Is this in itself a problem??

The tank is indeed made of plastic and there is no metal plate fitted. The pipework is not rigid as there is a hose coming off the top of the pipe which connects to the ballcock valve. As such there is some play in the valve. As stated, there is around 25mm between the water level and the overflow pipe when the valve has fully shut off (which takes into account the tank wall flex)

I could adjust the float arm so the water level is further under the overflow pipe however this would not solve the dripping would it.

 

[LRA]

Ever fix this?

LRA

 

[system]

I'd stop worrying Alfie, after all you were quite happy with the old one until water appeared at the overflow pipe. Life is too short mate, Merry Christmas

 

[LRA]

Who are you? LRA

 

[terryplumb]

LRA ,you should start a new thread , this one is six years old and I doubt Alfie is still worried !!!