Boiler Losing Pressure: Finding the Real Cause
A sealed heating system should hold its pressure for months.
A sealed heating system should hold its pressure for months. If you are re-filling the boiler weekly, water is leaving the system somewhere — and repeatedly topping up pushes fresh oxygenated water in, which accelerates internal corrosion and sludge. Finding the cause is a short job for an engineer and it saves the much larger bill that follows years of top-ups.
Three causes cover almost every case
First, a physical leak: a radiator valve gland, a towel rail joint, a compression fitting under floorboards, or the boiler's own internal seals. Second, a failed expansion vessel — the pressure swings widely between cold and hot, then falls as the pressure relief valve lifts. Third, a pressure relief valve that no longer seats properly and weeps continuously through the outside discharge pipe.
How we tell them apart
We record cold and hot pressure, check the outside discharge pipe for signs of discharge, and test the expansion vessel charge with the system depressurised. If the vessel holds and nothing is discharging, we move to leak detection: visible joints first, then radiator valves, then concealed pipework. We do this methodically rather than guessing, because replacing the wrong part fixes nothing.
Why repeated top-ups are expensive
Every litre of mains water added brings dissolved oxygen and hardness minerals with it. Over time that produces magnetite sludge, which blocks radiator bottoms, wears pump bearings and narrows heat exchanger passages. A system that has been topped up weekly for a year usually needs a chemical clean or power flush alongside the leak repair to get its efficiency back.
Prevention once it's fixed
We recommend a correctly sized magnetic filter on the return, an inhibitor dose after any repair that drains water, and a pressure check at every annual service. Those three things together prevent the majority of repeat pressure problems we get called back to.
Common questions
How often should I need to top up my boiler?
Ideally never between annual services. An occasional top-up after bleeding radiators is normal; anything more frequent than a couple of times a year means something needs investigating.
Can you find a leak under the floor?
Yes. We isolate zones to narrow the location before lifting anything, so we open the smallest possible area of floor. Where a buried pipe run is beyond economic repair we'll quote to re-route it rather than patch it repeatedly.
What is an expansion vessel and can it be recharged?
It is a small sealed vessel with an air cushion that absorbs the expansion of heated water. If the diaphragm is intact we can re-pressurise the air side; if the diaphragm has failed the vessel needs replacing, or an external vessel fitting where there is no space inside the boiler.
Will the leak damage my boiler?
Not immediately, but the constant fresh water it forces you to add will. Corrosion and sludge from repeated top-ups is one of the most common reasons a heat exchanger fails early.