Hot water being back flowing into Cold water tank

[Mark_James]

Any advice would be extremely appreciated. I have an indirect heating system. A few days ago the large cold water tank overflow started to leak. Given that I had the ball-cock replaced 6 months ago I kinda knew it was not the problem - it was not.


When the water is being heated on its own it is raising the water level in the large cold water tank up to a point where it overflows down the pipe at the roof eves.


This happens over a period of a 90 mins (water is only heated for 20 mins). Having looked in the tank I can see that water is being backfilling the large cold water tank not via the open vent pipe but the pipe at the bottom of the tank where the cold water should flow to the Cylinder.


When the hot water is off (not heating water) the large cold water tank level remains as it should.


Any ideas why this is happening???? I have had no new taps or mixers installed and this has only happened (as far as I know) in the past few days

 

[Best]

New indirect cylinder shouldn't be too expensive. Fairly straightforward job for a plumber, but will usually take several hours.
Just tie the ballvalve on the small heating tank (f&e tank) and see if the water drops in that. If it does lower, then has to be coil in cylinder perished.
You might want to consider if you could have stainless steel vented cylinder, or perhaps even if an unvented cylinder could be suitable for your needs, home and mains supply

 

[moonlight]

If you go down the invented root, there will be more costs. But no cold water tank in the roof, and if you decide at some point to change your boiler (I don't know what boiler you have) the cylinder would be t energy efficient .and suitabke for purpose. Just make sure the person who fits it is G3 certified and does all the appropriate checks. Good luck

 

[jtsplumbing]

From what I am reading you have a faulty hot water cylinder, replace it with a stainless steel one, (Nice long warranty) are you in a hard water area ?

 

[nickbrookes76]

i had a similar problem at an old house, and it turned out to be a mixer tap wasn't able to cope with the incoming hot water pressure and was leaking cold water into the hot supply. I know this doesn't exactly fit your symptoms, but just thought it may help.

 

[macplumb]

Can't be the coil due to overflow stopping. The overflowing would be constant due to a common return. so the coil would be subject to the same pressures when 3 port is open or closed.

Check the new float valve has not slipped (should be a part 2 float valve so can be ajusted. Put the ball to the lowest point) and bend the arm down a bit.
When water is heated it expands and hot water is less dense so will rise back to the tank by stratification giving you the effect you can see from the feed.

 

[king of pipes]

It most likely to be the mains cold water over taking the hot supply through a mixer tap or shower mixer you can bungee the new float valve up so no water enters the header tank and monitor the height of water in it when a mixer is used if it raises then theres your problem if not its due to fitting and height of the ballvalve give it a try regards kop.

 

[cr0ft]

Only two possibilities for this are: -

1.) Mixer tap/mixer shower connected to mains pressure cold and hot water from the cylinder sending mains pressure cold water back up into the cold water storage cistern (large tank in your loft).

2.) Pinhole leak in the heat exchanger coil in your hot water cyinder. For this to be back-filling your cold water storage cistern the water level in the feed/expansion cistern in the loft (smaller plastic tank attached to the central heating system) must normally be HIGHER (not LOWER) than the cold water storage cistern. With a pinholed heat exchanger coil it's always the cistern with the lower water level that overflows, not the higher one.

This is very likely to only happen when hot water is running initially. Over time it will become constant. Initially it's a pinhole leak that only opens up as the coil expands due to warming up. This is exactly how heat exchangers in boilers fail too. A pinhole leak that opens up when the heat exchanger warms up.

From the symptoms you've mentioned I think that it's very unlikely to be a mixer tap and incredibly likely to be the coil in the cylinder as it's only happening when hot water is running from what you say.

Is the feed/expansion water level higher than the cold water storage cistern water level?

 

[student plumber]

I have had to replace a couple of these due to this problem, though must have been larger than a pinhole as it was constant.

If the F&E (Feed & Expansion, 4 gallon small cistern) is higher it will exert more pressure on the CWSC (Cold Water Storage Cistern, 25 gallon large cistern).
The Feed & Expansion will constantly be trying to fill up as it will constantly empty into heating system, and will leak out of the heating pipes via the coil into the DHW, (Domestic Hot Water), inside your Hot Water Cylinder.

That will push against the water that should be coming down the feed pipe from the CWSC, but instead it goes up fills the tank and out the overflow.

It is possible the feed is connected near the cylinder, (but this can vary). So after a while it will be fairly clear looking water coming out as this will have more pressure behind it as it has come down from the loft; as opposed to radiators with inhibitor in on the same floor as the cylinder as they will be near equal pressure.

If hot water is going up as earlier described you will see it, as it will be shimmering as it comes out of pipe into CWSC.
So the water level in F&E will be to the feed pipe (about 2" in the bottom of cistern); and in the CWSC it will be to the overflow.

I have seen set ups like this.

But is more common to see both cisterns on the same level, e.g. a platform.

In which case the CWSC will be trying to re-fill, going into the coil in the cylinder, filling up the heating water, going up into the F&E, and out the overflow.
The water level in both cisterns will be to the overflow in the F&E.

A hot water cylinder is basically a big kettle.
Instead of electricity going into the coil in a kettle and heating the water (which must be kept seperate); in a cylinder you have heating system water going into the coil and heats the DHW (which must also be kept seperate).

I could give an idea of costs here, but as there are a number of factors to consider, IF IT IS THIS PROBLEM, I wouldn't want to mislead you.

Incidentally, I spoke to a cylinder manufacturer last year and they said they only guarantee their copper cylinders for 2 years, this one had lasted for ten.