Boiler pressure too high — over 3 bar
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ Do not operate the boiler if the pressure is above 3 bar. Overpressure can cause the pressure relief valve to discharge scalding hot water. If the pressure relief valve is continuously discharging, call a Gas Safe engineer immediately.
High boiler pressure is one of the most common issues UK homeowners encounter, and the good news is it's usually straightforward to fix. Your heating system is sealed, which means when water heats up it expands — if that expansion has nowhere to go, pressure builds and the gauge creeps into the red zone above 3 bar. This typically happens because the filling loop (the braided hose under your boiler) has been left open after a recent top-up, or occasionally because the expansion vessel inside the boiler has worn out over time. Most cases can be resolved in under an hour without calling an engineer, though if the pressure won't come down or keeps rising after heating, you'll need a Gas Safe registered engineer to investigate further.
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Most likely cause & what to check
Switch the boiler off and allow the system to cool for 30 minutes. Pressure will often drop slightly as the system cools.
Locate the pressure relief valve (PRV) — a brass valve with a pipe leading outside the property (the discharge pipe). If it is dripping, pressure has already been relieved — this is normal behaviour of a safety device, but indicates the system is overpressured.
Check that the filling loop (the braided hose connecting two valves beneath the boiler) is not left open — this is the most common cause of overpressure. Both valves on the filling loop should be closed when not in use.
If the system is overpressured and the filling loop is already closed, you can bleed several radiators simultaneously to release some system water and bring pressure down to 1–1.5 bar.
If pressure continues to rise after the system heats up, the expansion vessel has failed — this requires a Gas Safe engineer to replace it.
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Frequently asked questions
Why is my boiler pressure so high?
The most common culprit is a filling loop left in the open position — this allows mains water to keep topping up the system as it expands with heat. Less commonly, a failed expansion vessel can't absorb the expanding water properly, causing pressure to climb. If you've recently had the system serviced or topped up, check the filling loop valves first.
Is high boiler pressure dangerous?
Yes, it can be. Above 3 bar, the pressure relief valve will open to protect the system, discharging scalding hot water through an external pipe — this is a safety feature but indicates you need to act. Never ignore continuous dripping from the relief valve, as this means your system is under dangerous pressure and needs attention.
Can I just bleed radiators to fix this?
Bleeding radiators to release air is different from releasing water to reduce pressure — yes, you can bleed several radiators simultaneously to lower pressure by a small amount, but this only works if the filling loop is already closed. If the loop is open, you'll need to close both valves first, otherwise the pressure will just climb again.
How long does it take for boiler pressure to drop?
Pressure will drop slightly as the boiler cools naturally over 30 minutes, but don't expect a dramatic change. If you've closed the filling loop and bled a few radiators, you should see the gauge move towards the safe zone (1–1.5 bar) within an hour or two as the system stabilises.
What if pressure keeps rising after I fix it?
If the pressure climbs again after you've closed the filling loop and allowed cooling, the expansion vessel has likely failed and can no longer absorb the expanding water. This requires a Gas Safe engineer to replace, as it's a sealed component inside the boiler and cannot be repaired at home.
Where is the filling loop on my boiler?
The filling loop is always located beneath the boiler and consists of a braided hose connecting two isolation valves — one connected to the cold water supply and one to the system. In most homes, you'll find it within arm's reach under the unit, and both valves should be turned clockwise (fully closed) when not in use.