Published: September 19, 2026 · By Absolute Plumbing & Heating This Worcester Heatslave leak case study covers a real oil boiler leak repair in Exeter: an external Worcester Bosch Heatslave 2 oil boiler serving a customer in Kingsteignton, near Newton Abbot, developed a pinhole leak in the heat exchanger manifold, which caused a complete loss of hot water. Here is how the fault was diagnosed and put right, with photos from the job. The symptoms: loss of hot water The customer called because the property had lost its hot water. The boiler is a Worcester Bosch Heatslave 2, an external oil-fired unit housed in the familiar green weatherproof casing outside the house. On this type of boiler a fault developing inside the casing is easy to miss, because everything is tucked away behind the outer panels and out of sight. What the customer noticed No hot water at the taps The boiler cutting out or locking out rather than running normally A gradual drop in system pressure that had to be topped up Signs of weeping and staining inside the external casing A slow loss of pressure combined with damp, rust-coloured staining inside the case is a classic sign that water is escaping somewhere it should not be. On a sealed heating system even a tiny weep will, over time, drop the pressure far enough for the boiler to stop firing safely, which is exactly what had happened here. Diagnosis: finding the pinhole leak With the boiler isolated and the outer casing removed, the internal components could be inspected properly. The control panel and the red combustion chamber were exposed, and the telltale trail of dried water and light corrosion pointed straight towards the heat exchanger. Working back along that trail is how the source of the leak was located. The heat exchanger area inspected with the external casing removed, tracing the leak back to its source. Close inspection of the heat exchanger manifold revealed a pinhole leak – a tiny corrosion perforation in the metalwork that was letting system water escape under pressure. Pinholes like this can be difficult to spot when the boiler is cold and static, so the area was examined carefully with the system pressurised to confirm exactly where the water was coming from. A close-up of the heat exchanger manifold where the pinhole leak was confirmed as the cause of the lost pressure and lost hot water. The repair Once the leak was confirmed at the heat exchanger manifold, the boiler was stripped down far enough to reach the affected area safely. That meant removing the outer panels, taking out the components blocking access, and cleaning up the manifold so the fault could be dealt with on sound, clean metal rather than over old corrosion and debris. The Worcester Heatslave stripped down to give clear access to the leaking manifold. With the manifold accessible and cleaned back, the pinhole was repaired and the joint made good. The heat exchanger area was then checked over for any further weak spots before anything was reassembled – there is no point closing a boiler back up only to find a second leak a fortnight later. Repair work on the heat exchanger manifold, sealing the pinhole leak that had drained the system pressure. The boiler was then rebuilt, the system refilled and re-pressurised, and the unit fired and run to confirm the leak had stopped, pressure was holding, and hot water was fully restored. The external casing was refitted so the boiler was once again properly protected from the Devon weather. The external Worcester Heatslave back together and running, with hot water restored. Why Worcester Heatslave boilers can develop manifold leaks Worcester Heatslave boilers are well-built, popular oil boilers and they give many years of service, but no heat exchanger lasts forever. There are a few reasons a pinhole leak can appear in the heat exchanger or manifold over time: Corrosion from the inside Oxygen and debris in the heating water gradually attack the metal from within. Over years of heating and cooling, thin spots can perforate and produce a pinhole – often long before there is any obvious external damage. Age and thermal cycling Every time the boiler fires and cools the metal expands and contracts. On an older boiler this repeated stress, combined with any internal corrosion, is what eventually tips a weak point into a genuine leak. External exposure As an external boiler, a Heatslave 2 lives outdoors in its weatherproof casing. Damp, condensation and the Devon climate all encourage corrosion on the outside of components as well as the inside, accelerating wear over the years. Poor water treatment A system without the correct inhibitor, or one that has been topped up repeatedly with fresh (oxygenated) water because of an existing leak, corrodes far faster. Keeping the inhibitor level right is one of the best ways to protect a heat exchanger. The good news is that a single pinhole leak, caught early, does not always mean the boiler needs replacing. Diagnosing it properly and dealing with the actual cause – rather than just endlessly topping up the pressure – can get plenty more life out of a sound boiler. About the engineer Absolute Plumbing & Heating is a local Exeter business, established since 2015. This repair was carried out by the owner, who is OFTEC-trained with over 10 years' oil boiler experience and was trained by Grant UK. Please note the OFTEC registration has lapsed, so we do not carry current OFTEC registration for oil work. For gas work the business is Gas Safe registered. If you would like an honest assessment of an oil or gas boiler fault, we are always happy to take a look. Worried about your oil boiler? Losing hot water, a boiler that keeps needing topping up, or damp and staining around an external oil boiler are all worth getting checked before a small pinhole turns into a bigger problem. If you have a Worcester Heatslave or any other oil boiler playing up in the Exeter area, get in touch. Call for an oil boiler leak repair in Exeter Speak to an experienced local engineer about your Worcester Heatslave or oil boiler fault. Call 07766 501664 Learn more about our oil boiler services in Exeter. Call 07766 501664