What counts as high boiler pressure
The pressure gauge on a combi boiler measures the water pressure in the sealed heating circuit. With the heating off and the radiators cold, the reading should normally be between 1 and 1.5 bar. When the heating is running, the water expands and the pressure rises, typically to around 2 bar. That is expected and nothing to worry about.
Pressure becomes a problem when it climbs towards 3 bar. At that point the pressure relief valve opens to protect the boiler and pipework, letting water out of the system. Some boilers will also show a fault code or lock out if the pressure sensor reads too high.
| Reading | Heating state | Verdict |
|---|---|---|
| 1 to 1.5 bar | Cold | Normal |
| Up to around 2 bar | Hot and running | Normal |
| 1.5 to 2.5 bar | Cold | Too high, bleed a radiator to lower it |
| Above 2.5 bar | Hot | Likely expansion vessel problem |
| Around 3 bar | Any | Relief valve may open, call an engineer if it keeps happening |
Common causes of high boiler pressure
The filling loop was left open or is letting by
The filling loop connects your cold mains supply to the heating circuit. If one of its valves was not fully closed after the last top-up, mains water keeps flowing in and the pressure slowly rises. Valves can also wear and let a little water through even when they look shut. Check the handles or screwdriver slots under the boiler are fully across the pipe. If the pressure still creeps up with both valves closed, a valve is passing and needs replacing.
Too much water was added when topping up
It is easy to overshoot when repressurising, especially if you open the valves quickly. If you topped up recently and the reading is now high, this is the most likely reason. Our guide to repressurising a combi boiler explains how to fill slowly and stop at the right point.
The expansion vessel has lost its charge
Inside the boiler is an expansion vessel, a small tank with a rubber diaphragm and an air cushion on one side. As the water heats up and expands, it pushes into the vessel and the air compresses to absorb it. If the air charge leaks away, the extra volume has nowhere to go, so the pressure shoots up whenever the heating runs. A classic sign is a reading of 1 bar cold that climbs close to 3 bar within minutes of the heating starting.
You are reading the gauge while the system is hot
A reading of 2 bar with the radiators hot is usually fine. Before you do anything, turn the heating off, let the system cool and check the gauge again.
A faulty pressure sensor or gauge
On boilers with a digital readout, the number comes from an electronic pressure sensor. If that sensor fails, the display can show a figure that does not match reality. On Vaillant ecoTEC boilers, pressure sensor and pump problems often show as fault code F75. Sensors and gauges are replaced by an engineer.
How to lower boiler pressure by bleeding a radiator
Bleeding a radiator releases water from the sealed system, which brings the pressure down. You need a radiator key (available from any DIY shop), an old towel and a small container.
- Turn the heating off and wait until the radiators are cool. Water from a hot radiator can scald.
- Make sure the filling loop valves under the boiler are fully closed.
- Choose a radiator, ideally one downstairs, and hold the towel and container under the bleed valve at the top corner.
- Fit the key and turn it slowly anticlockwise, about a quarter turn. You will hear air hiss out first, then water will start to drip.
- Keep an eye on the boiler gauge. If you cannot see it from the radiator, ask someone to watch it for you.
- When the gauge drops to between 1 and 1.5 bar, close the bleed valve by turning the key clockwise. Do not overtighten it.
- Wipe away any water and check the valve is not dripping.
If you let out too much and the pressure falls below 1 bar, simply top it up again through the filling loop.
Only open the bleed valve a small amount. It is much easier to control a slow drip than a steady flow of dirty radiator water.
The pressure relief valve and the pipe outside
Every combi has a pressure relief valve set to open at about 3 bar. It connects to a copper discharge pipe that normally runs through the wall to the outside, often ending near the ground and close to the white plastic condensate pipe. Its job is to dump water safely before the pressure can damage anything.
If the pressure has been high, check the end of that pipe. Drips, a wet patch below it or a rust stain on the wall all mean the valve has opened. That is the valve doing its job, but it tells you the pressure is reaching unsafe levels. Once a relief valve has opened, it can also fail to reseal properly and carry on dripping, which then causes low pressure. If you notice pressure drops after a spell of high readings, see our guide to a boiler leaking water.
When to call a Gas Safe registered engineer
Bleeding a radiator is a quick fix, but if the pressure keeps climbing there is an underlying fault. Call a Gas Safe registered engineer if:
- the pressure rises close to 3 bar every time the heating runs
- the pressure creeps up with the filling loop fully closed
- the relief valve pipe outside is dripping or keeps discharging
- the gauge reading does not change at all, or looks wrong
- you see water coming from inside the boiler casing
Do not remove the boiler casing or try to recharge the expansion vessel yourself. The vessel is inside the boiler, and work there must be done by a Gas Safe registered engineer.
Typical repair costs for high pressure faults
| Repair | Typical UK cost |
|---|---|
| Recharging the expansion vessel | Typically £80 to £150 |
| Replacing the expansion vessel | Typically £200 to £400 |
| Replacing a passing filling loop valve | Typically £80 to £150 |
| Replacing the pressure relief valve | Typically £150 to £250 |
Costs vary by region and boiler model. An engineer will usually check the expansion vessel charge as part of an annual service, which is a good way to catch the problem early.