Expansion vessel failure — boiler pressure keeps rising
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Post a job — we'll find you an engineer →⚠️ If the pressure relief valve is discharging hot water inside the property (into a tundish or bucket), do not use the heating system until the expansion vessel and PRV have been assessed by a Gas Safe engineer.
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Most likely cause & what to check
Every sealed (pressurised) central heating system has an expansion vessel — a steel sphere containing a pressurised air/nitrogen charge separated from the system water by a rubber diaphragm. As the heating water expands when it warms up, the vessel absorbs the extra volume. When the diaphragm fails, it cannot absorb expansion and system pressure rises rapidly.
Symptoms of expansion vessel failure: pressure rising above 2.5 bar when the heating is hot (normal cold pressure 1–1.5 bar rising to no more than 2 bar hot), the pressure relief valve discharging, or pressure gauge moving noticeably each time the heating fires.
A Gas Safe engineer can test the vessel using a tyre pressure gauge on the Schrader valve (a small valve like a car tyre valve on the expansion vessel body). Cold system pressure should be 0 bar when tested — this is the air charge side. A reading of 0 bar means the charge has been lost and the diaphragm has failed.
The engineer can recharge the vessel with a foot pump or compressor (if the diaphragm is still intact) — this is a temporary fix if the diaphragm has not ruptured. If the vessel is waterlogged (water comes out of the Schrader valve), the diaphragm has failed and the vessel must be replaced.
Replacement vessels cost £25–£80 depending on size (6, 8, 12, or 18 litre are the most common); fitting requires draining the system pressure and takes 1–2 hours. A new vessel should be pre-charged to the correct pressure before fitting — typically 0.75–1 bar depending on system height.
After replacement, the system must be refilled to the correct pressure and the engineer should monitor it through a full heating cycle to confirm stable pressure (between 1.0 and 2.0 bar from cold to hot).
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