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

 

[bobpape]

I have re-read each post again and I feel we are all close to finding the problem,

I now think the problem will be found in the kitchen mixer,

 

[helpsy]

shouldn't kitchen mixer have NRV's fitted?

 

[traineedrip]

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

I thought they had an internal check valve, didnt realise it was an air gap.
Still, easy enough to isolate and test.

 

[Hugh]

Could it be that, the new hot water cylinder that was fitted is smaller than the previous one and the domestic hot water vent is now in the tank of the domestic cold water as the plumber might now have extended the vent ? seen an example of this before where the vent was drawing water and it was circulating it thru the cold feed bk to storage tank.

 

[paulga]

I have re-read each post again and I feel we are all close to finding the problem,

I now think the problem will be found in the kitchen mixer,

I have isolated the kitchen mixer tap but i didn't do any thing.

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

Mmmm, another good idea, it really helps to get all this info from fresh eyes.

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

Thats seems to be the only logical explanation the amount of water going up the cold feed pipe into the cold tank is continous until the mains is switched off so it stands to reason that there is a cross over but as usual it is a big house which has been extended and plumbing was done by the previous owner, not a pipe clip in site or gate valves and the tank in the tiny, tiny loft which is almost inacessible, i have to lie down and crawl over to it.
Maybe i should call in a plumber !!!

 

[Puddle]

know it sounds silly but came across this once

At diyer had connected the mains through to the hot supply, with just a stopcock for isolation ,he had done this as he used to get air locks filling system up so would just turn stopcock on near kichen sink and blast out air lock.like we do with a hose sometimes but this was perminant
Maybe this has been done here and valve left open slightly
However this would be perminant flow and not intermitant
we have checked mono block taps,showers
possible washing machine fault,isolate valves
As steve says drain down and see what happens
Also what goes one way should go the other isolate mains and hot water should start coming out kitchen cold tap,this may be a quicker way to trace,also you have advantage of tracing mains warm pipework

 

[tonyn]

Had the same problems a year back the problem turned out to be a incorrectly fitted/faulty mixer shower the cold feed to the shower had been plumbed in from the cold main and the hot was gravity fed. The valve was passing allowing the cold feed to get past the checheck vavle on the hotside of the valve thus sending mains cold back up through the cylinder and back into the storage tank in the loft causing overflow

 

[emergencyman]

As above, I found this problem once and it was the mixer valve to a shower.

 

[chrissyposs]

hi, ive just had this problem, it turned out to be the kitchen mixer tap the cold mains was leaking into the hot supply and back presuring. look for any conection on the system of mains cold supply and low presure hot supply, turn on the hot and let it run till the supply pipe is hot then turn off and hold the pipe if you feel it cool quickly thats the one.

 

[ChipDoctor]

What was the outcome to this problem? Was it the kitchen tap or the shower mixer? Mysteries bother me!

 

[Bernie2]

How about the washing machine or dish washer? The ones with hot and cold supplies could have the potential to cross flow internally.

 

[tackleburger]

Turn off everything and cap off the hot water outlet at the cylinder both sides then turn on if that stops it you have cold mains connected to a hot supply somewhere. You can test this by opening the cap to the hot supply pipework or even better connect a pressure gauge to it instead of a cap end.