Hot water being back flowing into Cold water tank

[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.