Outside boiler pipe leaking or loosing pressure and need to top up?

Why Was My Boiler Losing Pressure? Here’s What I Did.

Last week, I was called out to a job where the customer’s boiler kept losing pressure. They were topping it up daily—clearly not normal. After a quick inspection, I found two key issues: the expansion vessel inside the boiler had failed, and the pressure relief valve (PRV) was constantly dripping, leaving a telltale wet patch below the external pipe.

Instead of replacing the internal vessel—which would have meant dismantling half the boiler—I fitted a new external expansion vessel. It’s quicker, more accessible for future maintenance, and just as effective.

Next, I stripped and cleaned the PRV. These valves often get a bit of grit stuck inside, preventing them from sealing properly. Once cleaned and reassembled, the dripping stopped.

Since the fix, the pressure has held steady, and the boiler’s been running like a dream. Just goes to show—small parts like expansion vessels and PRVs can make a big difference.