Overflowing hot water cylinder

I thought I had a leaking stop valve so I replaced the whole unit only to find the water tank is still overflowing.

The new valve seems to be working but on closer inspection it seems that water was still flowing even though the valve had shut off at the correct level. It looks like the water is somehow feeding back into the exit hole from the tank. That hole feeds into the hot water tank (heated by the boiler). Is the hot water cylinder overflowing and pushing back up the cold water one?

A selection of responses from some of our very best tradespeople:

"What's happened here is that the coil inside the tank has split and the water has nowhere else to go but into the water tank used by the boiler. The boiler indirectly warms the coil to produce heat for the central heating system and this warm water is pushed back into the tank as there is no valve on this pipe line. The cylinder just fills up and out of the overflow - I have replaced many cylinders with the same problem. The remedy is to replace the cylinder and flush the water from the tank as it's now become contaminated."

Melvyn Cockell, M&J Maintenance
Mel and his team come highly recommended on Rated People. They work in all sorts of different areas including plumbing, construction, damp proofing and plastering. They're based in Kent but cover a wide area.

"There are two possible causes and solutions for this problem:

1. The internal coil in the hot water cylinder used to heat the hot water cylinder (in conjunction with the boiler) is leaking into the primary water (the hot water that flows from the taps). This then causes water to flow back up through the feed pipe to the hot water cylinder and backfills the feed and expansion tank causing the overflow. The overflow will carry on in this case until water is drawn off from the hot water cylinder.
Remedy: replace the hot water cylinder.

2. The water level in the tank is set too high and when the hot water tank heats, and the water expands, it causes an overflow. If this is the case, you'll notice the overflow is quite minimal and should stop when the water becomes heated.
Remedy: Set the water level lower in the feed and expansion tank by adjusting the inlet valve."

Lee Richards, Water Trade Ltd.
Water Trade Ltd., operating in the Slough and Berkshire area, offer plumbing, heating and drainage services to the domestic and commercial sector. Amongst other things, Lee and his team specialise in bathroom installations and offer services such as professional tiling, electrical and carpentry works as well. Water Trade has recently moved into the world of renewable energy services such as solar water heating, ground source heating and air to water heat sourcing. Between them, Lee and his team have a combined experience of 62 years and they come highly recommended on Rated People.