Bye law 30 cold water storage tank

[danmickread]

Hi All,

I recently changed the tank in my loft as I am fitting a new digital power shower and the old one wasn't big enough and was made of asbestos.
So I bought a Polytank 50 gallon tank and fitted it according to the instructions.
My question is - is the dip tube on the overflow supposed to sit in the water at normal water level when the system is cold, I assume if it is that when the system heats up and the water expands the dip tube length and height of the overflow outlet is set up so that when the water level rises it's not enough to go over the overflow?
Secondly during testing of overflow it didn't seem like the overflow couldn't take the water away as quick as it was coming in until the level got high enough that it completely submerged the overflow outlet and then it starts siphoning and sucking the water out quicker than it comes in. Then the water level starts to drop and I guess when it gets low enough air will get into the dip tube and stop it from siphoning and then the process would repeat. Does this sound right? Are the overflows meant to siphon? Is this to get rid of water as quickly as it comes in?

I have tried googling this but can't find much information on it could someone please help me as I would really like to know that the system is working as it should.

Thanks in advance

Dan

 

[chris watkins]

Yes the dip tube on the overflow is supposed to sit in the water at normal water level it is used to block cold air being driven up the warning / overflow pipe & out of the vent thus cooling the inside of an insulated cistern.

The water level will not rise noticeable due to expansion in the hot water system as 4% increase in volume is not a great amount compared with the volume of the CWSC.

The ability of the 19mm combined warning / overflow pipe to take away the water from a catastrophic failure of the float valve has been question for a long time, thankfully full flow failures are rare but the Water Regs, I believe, states that the overflow should be capable of achieving this so it is I am afraid down to you to ensure it can achieved, with or without siphonage (which it has not been designed to do).

You could increase the its size or install an additional pipe so you have separate warning & overflow pipes all as per the Water Regulations which is why you really should have the ticket before completing this kind of installation.

 

[Deleted member 25704]

The reason why cisterns should be installed by professionals:

Water tank fault that scalded baby girl to death 'was known to be unreliable' | Daily Mail Online

Make sure the cistern is supported across its base, with 150mm of board showing all the way around.

https://www.wras.co.uk/downloads/pu...ote__cold_water_service_storage_cisterns.pdf/

 

[AWheating]

the warning pipe is there to warn you hence why it needs to terminate in a location to make you notice it. if you want an overflow pipe with enough flow to take away the water from a fully open supply you will need a larger pipe to be sure it will cope.

 

[danmickread]

Thanks for all the replies however I am now more concerned than before!

Chris,
So are you saying that they make these tanks as kits with 19mm combined overflow/warning pipes that are not designed to cope with taking away the water at a rate to cope with the inlet flow if there was a catastrophic failure of the float valve and water was coming into the tank a full flow? This seems very odd to me and surely as you say doesn't comply with regs?
If I had a separate warning pipe which terminated through the fascia and say for instance the failure happened at night or when no one was home and this was 19mm then surely all its going to do is rise up past this until it reaches the overflow and then the total warning and overflow pipe outlet flow may cope with the inlet flow as there is two. So I fail to see why you would have a separate warning pipe. Can someone please educate me on this?

Clanger,
Thanks for the link I have read about this before but this was not directly related to my post this was a problem with the hot water cylinder immersion heater I believe which I will check also as the tank is now plastic.
The tank is fully supported in the attic to take the weight also.

A Wheating,
Where would you suggest normally terminating a warning pipe, straight out of the fascia like the overflow. As I said earlier you may not be around or asleep and would even know he water had reached the warning pipe so what's the benefit.
Whats your view on the overflows and warning pipes siphoning, if they do and that is taking the water away quick enough is this ok?
I have a central heating tank as well which needs upgrading so all together if having separate overflow and warning pipes I would have 4 pipes coming from loft out of fascia this seems excessive to me.

 

[PlumbBobBob]

what we call the warning pipe is the overflow pipe one only not two

 

[tamz]

When i 1st went down to England to work (i'm Scottish btw) i was shocked to find 3/4 overflows were the standard (among many other things) as that is what we termed a warning pipe. Up here overflow pipes were sized as minimum at 2 times the size of the incoming supply eg 1/2" - 1 1/14" overflow 3/4" - 1 1/2" overflow. This "overflow" pipe was classed as a combined warning and overflow.
Larger cisterns (tanks) over 100 gallon had seperate warning and overflow pipes, the warning pipe slightly lower than the overflow.
Most "overflows" up here are also standing wastes which can completely empty the cistern very quickly.
There is nothing to stop you from fitting an upgraded pipe using a McAlpine R40 or R45

 

[chris watkins]

Chris,
So are you saying that they make these tanks as kits with 19mm combined overflow/warning pipes that are not designed to cope with taking away the water at a rate to cope with the inlet flow if there was a catastrophic failure of the float valve and water was coming into the tank a full flow? This seems very odd to me and surely as you say doesn't comply with regs?

Well I don't know Dan to be able to tell you that I will need some more information.
Lets start with the following-
What size orifice is fitted to your float valve (taking it that this doesn't come out under the fault condition ?
What is the dynamic flow rate at the failed float valve, remembering that this will be dependant on the current mains water pressure at the time.
The size & frictional resistance of the the supply right up to the valve.
The height the float is above the mains.
Thats before we even start with the overflow pipe work!!

This Plumbing lark is not so easy as it first seems is it.
Products can only be produced to meet most eventualities it would be impractical and certainly not cost effective to make them so but this is where the skill and judgment of an experienced Plumber comes into its own, they would know whether your new cistern would require an upgraded overflow pipe or not.

 

[danmickread]

Ok Chris I think you've made your point that this obviously can be very technical but I would just like a steer in the right direction that's all.

Tamz,
Like you say I have always seen tanks fitted with one standard 22mm overflow and never though much of it. It's only until I fitted this new tank of mine and actually tested it by holding the ball valve down that I realised the existing 22mm overflow pipe can't cope. So these would be ok with a gradual increase in water level and are there really as a warning pipe but could never cope with a major failure of the ball valve.


My mains feed is 15mm to start with.
Now on the new tank if the standard 19mm combined overflow/warning pipe empties the tank fast enough once it starts siphoning is this acceptable?
If it's not acceptable then what's best to do - make the existing combined overflow/warning pipe one large one (1 1/4"- 1 1/2") and just run this outside as existing (which as some one else said Earlier leaves a nice whole that may look inviting to birds etc and doesn't look every nice) or run a large seperate overflow and leave existing as a warning pipe.
If I went with the option of having a separate large overflow then would this be better to run to a drain as there is a SVP running up through the loft from the bathroom and leave the warning pipe running outside where it's visible?

Thanks for everyone's valued help so far.

 

[chris watkins]

OMG, you have tested it and the tank doesn't overflow out of the lid, however the water is removed that is the main point.

The chances for a catastrophic failure of the float valve are very, very, very small which is why they are supplied with a 19mm connection.

So please Dan stop worrying about it and get on with the rest of your life. I am out of here :waving: