— Chelsea advice
Boiler Pressure Too High? How to Lower It Safely
To lower boiler pressure, turn the heating off, let the system cool, then bleed water from a radiator with a radiator key until the gauge drops back to around 1 to 1.5 bar. That fixes pressure that was simply overfilled. If it creeps back up again, the usual causes are a filling loop valve that is letting water through or an expansion vessel that has lost its air charge, and both need a heating engineer. This guide explains what normal pressure looks like, how to bring it down, and what a rising gauge is telling you. Emergency Plumber Chelsea covers SW3, SW10 and nearby postcodes on 020 3002 6667.
Updated July 2026 · Emergency Plumber Chelsea
Most combi and system boilers run a sealed heating circuit, and the gauge on the front shows the pressure of the water inside it. With the heating cold, a normal reading is usually around 1 to 1.5 bar. When the system heats up the water expands, so the needle rises, and a reading around 2 bar while the radiators are hot is normally fine. Once it goes above about 2.5 bar, it is getting close to the point where the pressure relief valve opens, which on most domestic boilers is 3 bar. Check your boiler manual for the figures for your model, as they vary slightly.
High pressure is not usually dangerous in itself, because the relief valve is there to stop the system going any higher. The problem is what happens when that valve opens. It discharges water through a small copper pipe, often out through the wall and down the side of the building, and once a relief valve has opened a few times it frequently fails to reseat properly and starts dripping. The system then loses water, the pressure drops, and you end up with a low pressure fault instead. Some boilers will also lock out and show a fault code when the pressure goes too high.
The simple fix for overfilled pressure works on almost any system. Turn the heating off and let the radiators cool, because the reading on a cold system is the one that matters. Put a towel and a container under a radiator, ideally one downstairs. Use a radiator key on the bleed valve at the top corner, and open it slowly. You will hear air first if there is any, and then water will start to come out. Keep an eye on the boiler gauge, or ask someone to watch it, and close the bleed valve once the pressure is back to around 1 to 1.5 bar. Only let out a little at a time, because it is easy to overshoot and end up too low. Our radiator bleeding guide has the step-by-step detail.
Some boilers also have a drain-off valve at the lowest point of the system or a small drain point on the boiler itself, and an engineer may use those instead of a radiator. If you are not comfortable opening valves, or you cannot find a radiator bleed valve that will open, it is better to leave it and call. Never try to lower the pressure by opening the pressure relief valve by hand. It may not close again properly afterwards, and then you have a dripping discharge pipe to deal with as well as the original fault.
If the pressure rises again after you have lowered it, something is adding water to the system or stopping it from coping with expansion. The first thing to check is the filling loop, the braided hose or built-in valve under the boiler that you use to top up. Both valves should be fully closed after every top-up. If one has been left slightly open, or the valve has worn and lets water past, mains water slowly feeds into the heating circuit and the pressure climbs over hours or days. Many engineers disconnect external filling loop hoses after use for exactly this reason. A worn filling loop valve needs replacing.
The second common cause is the expansion vessel. This is a small tank, usually inside the boiler casing on a combi, with a rubber diaphragm and a cushion of air or nitrogen on one side. When the water heats and expands, it pushes into the vessel instead of raising the pressure. Over time the air charge leaks away, so the vessel stops absorbing the expansion, and you see the pressure jump sharply every time the heating comes on, then drop back when it cools. That big swing on the gauge is the giveaway. An engineer can check and recharge the vessel with a pump, which involves draining down the pressure on the heating side first. If the diaphragm has failed, the vessel needs replacing.
On combi boilers there is a third, less common cause. The plate heat exchanger that heats your hot water sits between the mains cold supply and the heating circuit. If it develops an internal leak, mains pressure water can push through into the heating side, and the pressure keeps rising whenever you run hot water. Diagnosing this needs an engineer, and on an older combi it is one of the repairs that can tip the decision towards replacing the boiler.
Watch for signs that the relief valve has already been working. Look outside where the small copper discharge pipe comes through the wall. If it is dripping, or there is staining on the wall below it, the valve has been opening. That usually means the pressure has been going too high, and it also means the valve itself may now need replacing once the cause of the high pressure is fixed. If the pressure is now falling instead, our guide to why a boiler keeps losing pressure explains what to check. Our guide to a leaking boiler pressure relief valve covers why the valve keeps dripping and what the repair involves.
In Chelsea's mansion blocks and converted period houses, boilers often sit in cupboards and kitchens where the discharge pipe runs a long way to an outside wall, or into a tundish, and a dripping relief valve can go unnoticed for weeks while water quietly damages a cupboard floor or the flat below. If you see water around the base of the boiler, treat it as a leak and see our guide on what to do if your boiler leaks.
If your boiler pressure keeps rising, the relief valve is dripping, or you cannot get the pressure down safely, call Emergency Plumber Chelsea on 020 3002 6667. Our Gas Safe registered engineers check the filling loop, the expansion vessel and the relief valve and put right whichever has failed, as part of our boiler repair service across SW3, SW10, Knightsbridge, South Kensington and Fulham. There is no call-out fee on jobs over 3 hours.
Quick tips
- Read the gauge with the heating cold: around 1 to 1.5 bar is typical, and about 2 bar when hot is usually fine.
- Lower overfilled pressure by bleeding a little water from a radiator with the system cold, checking the gauge as you go.
- Close both filling loop valves fully after every top-up; a valve left slightly open is a common cause of creeping pressure.
- A big jump on the gauge every time the heating comes on points to a flat expansion vessel.
- Never open the pressure relief valve by hand; it may not reseal and will start dripping.
FAQs
What should my boiler pressure be?
On most combi and system boilers, around 1 to 1.5 bar with the heating cold, rising to around 2 bar when hot. Above about 2.5 bar is too high, and the relief valve on most boilers opens at 3 bar. Check your manual, as figures vary slightly by model.
Is high boiler pressure dangerous?
Not usually, because the pressure relief valve stops it going higher by releasing water. The risk is that the valve then starts dripping and the boiler loses pressure or locks out. High pressure that keeps coming back should be looked at by an engineer.
Why does my boiler pressure go up when the heating is on?
Water expands as it heats, so a small rise is normal. A big jump, for example from 1 bar to nearly 3 bar, usually means the expansion vessel has lost its air charge and can no longer absorb the expansion. An engineer can recharge or replace it.
Why does my boiler pressure keep rising when the heating is off?
If pressure climbs while the system is cold, water is getting in. The most likely causes are a filling loop valve that is letting water through or, on a combi, an internal leak in the plate heat exchanger. Both need an engineer. Call 020 3002 6667.
Need an engineer in Chelsea?
Call 020 3002 6667 for 24/7 emergency dispatch across SW3, SW10 and nearby postcodes.