Boiler Pressure Too High? Why It Rises and How to Fix It

What pressure should the boiler be at?

For most sealed systems, around 1 to 1.5 bar cold is right, and the manual for your boiler will give the exact figure. When the system heats up the water expands and the pressure rises, typically by half a bar or so. That rise is normal and is what the expansion vessel is there to absorb.

The upper limit is 3 bar, which is where the pressure relief valve on the boiler is set to open. If your gauge regularly goes above about 2.5 bar when hot, or sits above 2 bar when cold, something is adding water or failing to absorb expansion, and it is worth finding out which before the relief valve starts discharging.

How to bring the pressure down safely

Turn the heating off and let the system cool, because working on it hot risks scalds and gives you a false reading. Then take a radiator bleed key, a cloth and a small container to a radiator, preferably downstairs. Open the bleed valve slowly with the cloth around it, let water run into the container, and watch the boiler gauge, or have someone else watch it. Close the valve when the needle is at about 1.5 bar.

That is a safe homeowner job and it fixes the symptom. If the pressure climbs back within days, it has not fixed the cause, and bleeding it down every week is a sign to stop and get the system looked at.

Do not try to reduce pressure by opening the pressure relief valve itself or loosening fittings on the boiler. Leave the boiler case closed; anything inside it is Gas Safe work.

Cause one: the filling loop is open or passing

The filling loop is the connection between the mains water and the heating system, usually a braided hose or a small valve under the boiler, used to top the pressure up. If it was left slightly open after the last top up, mains pressure keeps pushing water into the heating until the relief valve opens.

Check both valves on the loop are fully closed. On many installations the braided hose should be disconnected once topping up is finished, which removes any chance of it letting water through. If both valves are closed and the pressure still creeps up, one of them may be passing internally, which is a cheap valve replacement.

Cause two: the expansion vessel has lost its charge

The expansion vessel is a small tank, inside the boiler on most combis or a red vessel on the pipework on some system boilers, with an air cushion on one side of a rubber diaphragm. As the heating water expands, it pushes into the vessel and compresses the air, so the pressure only rises gently.

Over time the air leaks away. With nothing to push against, the expanding water has nowhere to go and the pressure shoots up every time the heating comes on, then falls back as it cools. That swing, low when cold and very high when hot, is the classic sign. The vessel can usually be recharged with a pump through its valve, but only with the system pressure removed first, and a vessel with a split diaphragm needs replacing.

Cause three: a failed coil or heat exchanger

This is the cause that matters for hot water, and the one people least expect. In a home with an unvented hot water cylinder, the heating water runs through a coil inside the cylinder, and the stored hot water around it is at mains pressure, typically well above the pressure in the heating. If that coil develops a pinhole, water flows from the higher pressure side into the lower one: mains water pushes into the heating system and the boiler pressure rises, without anyone touching the filling loop.

A combi boiler has its own version. The plate heat exchanger, where the heating water hands heat to the tap water, keeps the two apart with thin plates. If a plate fails internally, mains water can cross into the heating circuit in the same way, and the pressure climbs most noticeably when hot water is being run.

The tell is a pressure that rises even with the filling loop confirmed closed and disconnected, sometimes along with discoloured water at the hot taps. On a cylinder, a failed coil cannot be repaired inside the vessel, so it becomes a conversation about unvented cylinder repair versus replacement. On a combi, it is a plate heat exchanger replacement.

Cause four: simply overfilled

Sometimes the answer is dull. The system was topped up to 2 bar or more while cold, then heated up and expanded past the point the relief valve accepts. If the pressure was fine until someone topped it up, and has stayed steady since you bled it down to about 1.5 bar, that was all it was.

Is high boiler pressure dangerous?

The boiler is designed for this. The pressure relief valve opens at 3 bar and discharges through a copper pipe to the outside, and most boilers lock out before any damage is done. So it is not a danger in the sense of an explosion risk. The real risks are water damage if the discharge pipe is not properly run, a relief valve that starts to let by permanently once it has opened a few times, and the underlying cause, particularly a failed coil, getting worse.

If you see water dripping from a copper pipe outside, near the boiler, that is often the relief valve discharging. On a cylinder, a similar drip from the tundish, the open funnel in the pipework near the cylinder, is a different valve with its own causes, explained in our guides to a pressure relief valve leaking and a tundish dripping.

When to call an engineer

Call if the pressure keeps rising with the filling loop closed, if it swings from low to very high between cold and hot, if water is discharging from the relief valve pipe, or if the boiler has locked out on high pressure. With a cylinder in the house, mention it when you call, because we will want to test the coil as well as the boiler.

We have 20 engineers across London and hot water systems are our core work, so a boiler pressure fault that might involve the cylinder is one we are set up to diagnose on a single visit. Our rate is on the pricing page and agreed before we attend.