Simple explanations for plumbing problems

Coil in hot water cylinder split

Why is there a coil in my hot water cylinder?

The coil is just a long copper pipe bent into a coil inside the hot water cylinder. The water circulating in your radiators and boiler is filled with chemicals to reduce rust and sludge. This water (known as primary water) is pumped through the coil in the hot water cylinder and heats the water inside the cylinder (known as secondary water) without mixing with it.

In your attic or airing cupboard there will normally be two cisterns.

The large cistern feeds the clean secondary water into the hot water cylinder.

The smaller cistern feeds the dirty primary water into the boiler, radiators and hot water cylinder coil.

If you have a pressurised system (there is a pressure gauge and a filling loop somewhere on your system) you will not have the small primary tank.

A cut away hot water cylinder showing the coil.
A cut away hot water cylinder showing the coil.

How do I know the coil in my hot water cylinder is split

It’s quite rare but the coil that holds the water from the boiler inside your hot water tank can split.  If this is the case then either the lowest of the feed tanks will be overflowing or the boiler pressure will rise continuously.

To prove:

Tie the ball valve up in the bigger secondary tank so that no more water can enter the tank.

Run your hot taps so the level in the secondary tank is lower the level of the smaller cistern. Don’t let the whole tank drain as that can cause airlocks in the hot water system.

As long as you don’t run any taps (sometimes the cold taps in the bathroom also come from this cistern) and with the ball valve still tied up the water level should stay the same.

If the coil is still intact and keeping the primary and secondary water separate the water levels will stay as they are.

If there is a leak in the coil the water levels of the two tanks should now equalise. The water level in the partially drained larger tank should slowly rise to the same level as the smaller primary tank.

As the split in the coil may be quite small it may take a while for the water level in the overflowing tank to go down, possibly several hours.

What should I do next?

The most effective solution to this problem is to replace the cylinder.

You can use central heating sealant chemicals to attempt to repair the pin hole in the coil. CT1 specifically say on their website that this is one of the uses of the miracle seal.

These chemicals have to first leak from the primary water into the secondary water. So for at least a short time the sealant will be mixing with the hot water you will use for bathing. Personally I would not recommend that but they know their product better than anyone.

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