cold tank back filling.

[paulga]

Hello all,
heres one to get all you so called plumbers thinking, i have been called to a job where the cold water storage tank in the loft is being back filled via the cold feed pipe into the hot water cylinder and thus over flowing the cold tank to the outside, and it really gushs out.

• Turning on/off the central heating or hot water seems to have no effect.
• The hot feed from the top of the cylinder is very cold as if it is mains water flowing in.
• The only thing that stops the problem is turning off the main stopcock.
• The kitchen mono mixer tap has been isolated (thinking that the mains is crossing over in the tap) but still the water flows up the cold feed pipe from the hot cylinder.
• There are no obvious cross overs in the system (eg. mixer taps, unbalanced showers etc where the water could back flow the system.)
• The problem has been on off for about a year, (the hot water cylinder has been changed a year ago) but in the last few weeks it has got worse so much so that the customer has to leave a bath tap running to stop the cold tank over flowing and switch the mains off at night.
• The central heating system is gravity fed so there is no pressure from any faulty mains fed filling loops.
• aprevious plumber seems to think the coil may have gone in the cylinder, but there is no dirty water entering the cold tank plus the problem still happens when the heating is off.

My initial thoughts are that there is mains water crossing over into the hot water pipework and forcing itself up into the cold tank just as you would when you force out an air block hence the hot water outlet pipe on top of the hot water cylinder feeling very cold.
Does any one have any ideas about this as i may have to admit defeat on this one, and no plumber likes to do that, unless i can come up with an answer.

Very ......

 

[danielp]

still is the level of the header tank higher if the coil had gone it will still push through best way to test is tie up ball valve and if it emptys thats it

 

[paulga]

Thanks for the reply Daniel,
So your saying that i should tie up the ball valve in the header tank and then switch the heating on or leave the heating off.
Then if the header tank drains out (and fills the cold tank) it means the coil has gone.
Paul.

Daniel,
P.S The header tank is on the same level as the cold tank but water level of header tank is lower than the cold.

 

[helpsy]

Surely with the water level lower in the F&E tank normally than the CWSC,if the cylinder coil was gone the F&E would be filling up and the CWSC filling and transfering to the F&E?

 

[bobpape]

Hi Paulga
Explain how you know the water is coming back up the cold feed.

 

[REDSAW]

dont overlook the obvious.
start a logical sequence as though its all knacked.

first isolate both tank valves, isolate boiler.
does it change?
turn on boiler, does it change?
turn of gate valves etc
go through it properly, and dont discount faulty cylinder.

 

[paulga]

Hi Paulga
Explain how you know the water is coming back up the cold feed.

Hello Bobpape,
I could feel warm water slowly rising up the cold feed pipe and also see warm water (a wavy stream like the air you see from the back of a jet engine) coming into the cold tank. I assume that because the hot outlet from the top of the cylinder was cold then the warm water was being forced down the cylinder and up the cold feed.

Surely with the water level lower in the F&E tank normally than the CWSC,if the cylinder coil was gone the F&E would be filling up and the CWSC filling and transfering to the F&E?

Yes, i think your right.

Hello Redsaw,

There are no gate valves on the system except for one on the coil return just off from the cylinder.
Paulga.

 

[traineedrip]

are there any appliances with mixer valves in the kitchen connected to the cold main?

 

[bobpape]

Hi again Paulga
Have you looked at the possibility that feed expansion pipe is blocked, it could even be airlocked,

 

[REDSAW]

how you are discribing this is that,
if the hw / expansion draw of pipe was connected to the mains water supply from below the cylinder somewhere and the cold feed inlet at the bottom of the cylinder was connected to the bottom feed of the cws tank, this whould cause the same effect if the water pressure from the mains was ballanced, right?.
try turning the mains on at 1/2 or 1 turn only and see what happens/.
who put in the cylinder a year ago?.

 

[paulga]

are there any appliances with mixer valves in the kitchen connected to the cold main?

Only a mixer tap in kitchen, no mixer valves.

 

[GreenArmy]

When physically seeing/feeling the hot entering the tank from the cold feed and feeling that the hot draw off from top of cylinder is cold. What happens when you draw off hot water from tap? Does the hot draw off pipe get hot (obviously it will) quickly and then cool quickly i.e hot water starting to re-enter tank? If so it would sound like cold main is somehow entering hot supply. Or as bobpage suggests have you looked at expansion pipe?

 

[paulga]

When physically seeing/feeling the hot entering the tank from the cold feed and feeling that the hot draw off from top of cylinder is cold. What happens when you draw off hot water from tap? Does the hot draw off pipe get hot (obviously it will) quickly and then cool quickly i.e hot water starting to re-enter tank? If so it would sound like cold main is somehow entering hot supply. Or as bobpage suggests have you looked at expansion pipe?

Mmmmm, good point on the blocked expansion pipe but there is just too much water coming out of the overflow for it to be just expanded hot water, but i will check it out.

 

[traineedrip]

Watch it doing it with the heating and hot water off. If the water isnt coming in via the float valves it has to be directly from the cold main.

so theres a mixing valve somewhere, shower? old w/m or dishwasher?

I wouldnt take anything the customer has told you until you see it yourself. turn everything off and watch.

Puzzelling

 

[stevetheplumber]

if it stops when you turn the main of its definatley crossing over some where
think i would turn of the main ,drain everything including the cylinder tie up the ball valves then turn on the main see if you can hear where its crossing over
cant be from the heating as the level of that tanks lower and wouldnt flow into higher tank
washing macine is a possibilty as well as thats preminantly connected to both hot and main although they have an internal air gap to stop this