A broken coil refers to a fault in a hot water cylinder in which the coil inside the cylinder is perforated, allowing water from the central heating circuit inside the coil to mix with water in the hot water cylinder.
An indirect hot water cylinder contains a coil of pipework which is connected to the central heating circuit. Hot water from the boiler flows into the coil at the top, transferring its heat to the water in the cylinder, and then exits the coil at the bottom. These two aspects are referred to as the flow and return.
The hot water cylinder provides hot water to domestic services in the home, such as the bath, shower, and hot taps. It should therefore never come into contact with the murky brown water that is circulated through the radiators, and is typically loaded with anti-corrosion chemicals. A broken coil will allow this to happen.
The most common symptom of a broken coil is a cistern which is continuously filling and another cistern which is continuously overflowing, even though the ball valve is working properly. The cistern overflowing is usually the feed and expansion cistern. This is because the level of water in the feed and expansion cistern is typically much lower than the water level in the cold water storage cistern due to its size – the cold water storage cistern can obviously accommodate a much greater volume of water.
A broken coil means that the two separate water systems – the domestic water supply and the central heating – effectively become a single system, and the water level in the two cisterns tries to balance out. However, the overflow in the cistern with the lower water level prevents this from happening, resulting in a cistern which is constantly overflowing, even though the ball valve is in good working order. This is due to water entering the cistern via the outlet at the bottom.
While the feed and expansion cistern is usually the cistern which overflows, the cold water storage cistern will also overflow if the water level in the feed and expansion cistern is higher.
Another symptom of a broken coil will be yellow or rust-coloured water from the hot taps. This is due to water from the central heating circuit entering the hot water cylinder via the coil. The water may have a chemical smell as a result of chemicals added to the central heating circuit, such as inhibitor and/or biocides.
Repairing a broken coil
A broken coil is usually the result of corrosion, and unfortunately the only way to repair it is to replace the hot water cylinder.