Solar hot water cylinders (often called ‘twin coil’ or ‘dual fuel’ cylinder) store the heat that is produced by solar heating panels. Your solar thermal system heats the water that you use in your house. However, some cylinders may start to leak after a number of years. Unfortunately, this may mean that the replacement of the solar hot water cylinder is necessary.

The three main problems that may occur with solar cylinders

  1. When the solar coil is perforated, antifreeze under pressure travels through the coil and into the domestic hot wholesome water. This is often evident by the hot tap water smelling ‘oily and burnt’. Sometimes you may find an oily brown residue in the water itself. The antifreeze that is in the system should be non-toxic. This is, however, a serious cause for concern. In the case of a leaking coil it is nearly always recommended to replace the cylinder. Occasionally, though, other alternatives are available which we would need to assess.
  2. The cylinder itself may leak water. This is often from where the pipes are brazed to the cylinder’s copper walls. Repairs are occasionally possible but replacement is recommended.
  3. Probe pockets leak. These can normally be capped or replaced and there is often no need for a cylinder replacement.

Cylinder failures we see most often

In our experience, the cylinders most susceptible to leaking and coil perforation are made by Gledhill (these are often covered in a blue or green PVC cover).

There have been occasional cases where Newark or MCDONALD and Oso cylinders have also failed.

In some cases it is possible to replace the cylinder with a stainless steel version that is significantly more corrosion resistant.

Gledhill made cylinder failures we have encountered

Below are two videos of cylinders we have worked on. After diagnosing the failure, we cut open the cylinders for further inspection and recorded the videos below for demonstration purposes.

We had diagnosed the cylinder in the solar thermal system as faulty and decided to give it a closer inspection.

We cut open the cylinder and found it to contain an amount of lime scale which is to be expected. We pressurised the lower coil with water and found that the cylinder had failed on the lower joint to the coil.

The diagnose on this cylinder came as a bit of a surprise. We cut open open the cylinder and found it to be in good condition with very little limescale build up.

However, we pressurised the lower coil with water and found that it had failed on the upper joint to the coil, just as we had expected.

Solar Cylinder replacements – the  process

If your twin coil solar cylinder needs to be replaced, your solar system needs to be drained first. Once temporarily decommissioned, your cylinder needs to be drained, ready to be replaced. When the new cylinder is in place and reconnected, the solar thermal can be recommissioned.

This is often also the right time to assess the condition of your system and give your system a service.

We can arrange both the cylinder replacement as well as the recommissioning of your solar system.

Plumbing Insurance

We often deal with cases where the cylinder and plumbing in a customer’s property is insured by British Gas or other providers. In these cases, we work with the assigned heating engineer to make the process as seamless as possible.

How to reduce the chances of cylinder failure

Some components fail due to poor quality manufacturing and age. However, the coil in a solar cylinder is far more likely to fail if the solar antifreeze is in poor condition.

Solar antifreeze, when new, is slightly alkaline with a pH value of about 9. If the system overheats, over time, the pH will drop, eventually becoming acidic (below pH7). The resulting liquid is far more corrosive and more likely to attack the solar coil in the cylinder.

Avoiding overheating and regular servicing are the best ways to reduce the chance of cylinder failure.