Boiler Pressure Too High? Causes & What to Do

Boiler pressure too high? Some pressure increase when your heating warms up is normal, but the gauge should not continually climb towards the top of its range.

High boiler pressure can happen after accidentally overfilling the system, because a filling loop is not fully closed, or because a pressure-control component such as the expansion vessel needs attention.

The important thing is to check whether the pressure is only higher when the heating is hot or whether it stays unusually high even after the system has cooled.

What Should Boiler Pressure Normally Be?

Many sealed heating systems operate at around 1 to 1.5 bar when cold and rise somewhat when the heating is running.

The exact pressure range depends on your boiler, so always check the manufacturer’s instructions or markings on the pressure gauge.

Vaillant advises that many boilers operate around 1 to 2 bar and should remain below the appliance’s upper pressure limit.

For a fuller explanation, see:

Normal Boiler Pressure & Pressure Adjustment

Boiler Pressure Too High: 4 Common Reasons

1. The Boiler Was Overfilled

One of the simplest causes is adding too much water when repressurising the heating system.

If you recently used the filling loop and the pressure increased immediately afterwards, this is the first thing to consider.

A common mistake is leaving the filling valves open for too long or continuing to fill the system after the correct pressure has been reached.

If the pressure was normal before repressurising and became high straight afterwards, accidental overfilling is more likely than a sudden boiler component failure.

2. The Filling Loop Is Not Fully Closed

The filling loop allows mains water into a sealed heating system when the system needs repressurising.

Once the correct pressure has been reached, the filling controls should be returned to their normal closed position.

If a filling valve is not fully closed, or if the filling device itself is allowing water through when it should not, pressure can gradually continue to rise.

Vaillant specifically lists a faulty external filling device as one possible reason for continuously increasing boiler pressure.

If you’re unsure what a filling loop looks like, see:

Boiler Filling Loop & Valve Guide

3. Expansion Vessel Problem

The expansion vessel helps manage the increase in water volume as the heating system warms up.

Water expands when heated. In a sealed heating system, the expansion vessel provides space for that change while helping keep system pressure under control.

If the expansion vessel is not working correctly, you may notice a pattern such as:

• normal pressure when the heating is cold;
• a large pressure increase once the radiators become hot;
• pressure moving towards the upper end of the gauge;
• water eventually being discharged through the pressure-relief system;
• pressure later becoming low again once the system cools.

Worcester Bosch advises that if pressure rises by more than around 1 bar as the system heats up, the expansion vessel may require attention from a service engineer.

The expansion vessel is an internal boiler or heating-system component, so this is not a homeowner repair.

4. Another Boiler or Pressure-Control Fault

If the system has not recently been overfilled and the filling loop is correctly closed, persistent high pressure may require professional diagnosis.

An engineer may need to check the expansion vessel, filling arrangement, pressure-relief system and other components involved in controlling system pressure.

Diagnostic point: high pressure is a symptom. The pressure reading tells you something is happening, but it does not automatically tell you which component has failed.

Why Does Boiler Pressure Rise When the Heating Is On?

A pressure increase while the heating warms up is normal.

As the water in the heating system becomes hotter, it expands. This causes the pressure reading to rise.

The important question is how much it rises.

For example, a system that moves from around 1.2 bar when cold to a moderately higher reading when hot may simply be operating normally.

A system that climbs dramatically towards the maximum range every time the heating runs may need investigating.

What Happens If Boiler Pressure Gets Too High?

Boilers and sealed heating systems include safety controls designed to prevent excessive pressure from continuing unchecked.

One of these is the pressure relief valve, often shortened to PRV.

If pressure reaches an excessive level, the valve can release water from the system through a discharge pipe.

You may then notice:

• water dripping from an external discharge pipe;
• pressure becoming low after the heating cools;
• repeated pressure swings between high and low;
• water around the boiler or heating system.

If you have water leaking from the appliance, see:

Boiler Leaking Water? Causes & What to Do

Can High Boiler Pressure Cause a Leak?

Excessive pressure can cause the pressure-relief system to discharge water.

It can also expose weaknesses in seals, joints or other heating-system components.

However, a leaking boiler should not automatically be blamed on pressure. There are several possible causes of boiler leaks.

If both high pressure and a leak are present, tell the engineer about both symptoms.

Can I Reduce High Boiler Pressure Myself?

First, allow the heating system to cool and check the pressure again.

If the pressure became high immediately after you accidentally overfilled the system, manufacturer guidance may allow you to reduce pressure by bleeding a radiator.

Only do this:

• when the heating system has cooled;
• if you are comfortable bleeding a radiator;
• in accordance with your boiler manufacturer’s instructions;
• while carefully monitoring the pressure gauge.

Do not remove the boiler casing or attempt to adjust the expansion vessel, pressure-relief valve or other internal components.

If the pressure rises again after you reduce it, the underlying cause needs investigating.

Why Does My Boiler Pressure Keep Going From High to Low?

This pattern is particularly useful when diagnosing a heating system.

For example:

1. the boiler starts at normal pressure when cold;
2. the pressure rises substantially as the heating gets hot;
3. water is released through the pressure-relief system;
4. the heating cools;
5. the boiler is then left with low pressure.

An expansion or pressure-control problem may need investigating if you see this repeated pattern.

If your main problem is pressure continually falling, see:

Why Does My Boiler Pressure Keep Dropping?

Is High Boiler Pressure Dangerous?

A sealed boiler system is designed with pressure controls and safety devices, so a high reading does not automatically mean the boiler is about to become dangerous.

However, persistently excessive pressure should not be ignored.

If the pressure remains high after the boiler has cooled, continually rises by itself or repeatedly causes water to discharge, arrange professional diagnosis.

Should I Switch the Boiler Off?

If the pressure is only slightly above its normal cold range after accidentally overfilling the system, check your manufacturer’s instructions.

If the gauge is unusually high, continues rising, the boiler is leaking or you are unsure whether the system is safe to operate, switch the appliance off using its normal controls if you can do so safely and obtain professional advice.

Why Is My Boiler Pressure High Even When It Is Cold?

If the system has completely cooled but the pressure remains significantly above the recommended cold range, normal thermal expansion is unlikely to explain it.

Check whether the system was recently topped up and whether the filling controls are completely closed.

If the pressure continues increasing without you adding water, the filling device or another system fault may need investigating.

When Should I Call a Boiler Engineer?

Arrange professional diagnosis if:

• pressure keeps increasing without you adding water;
• pressure rises dramatically whenever the heating runs;
• the pressure stays unusually high after the system cools;
• water is coming from the pressure-relief discharge pipe;
• the pressure repeatedly goes from high to low;
• the boiler is leaking;
• the boiler locks out because of pressure;
• reducing the pressure does not solve the problem.

For professional fault diagnosis, visit:

Boiler Repairs

High Boiler Pressure in Cardiff?

If you’re in Cardiff and your boiler pressure keeps rising, water is being discharged or the system repeatedly goes from high to low pressure, visit:

Boiler Repair in Cardiff

When booking, tell us:

• your boiler make and model;
• the pressure when the system is cold;
• the highest pressure reached when the heating is running;
• whether the system was recently repressurised;
• whether water is dripping from an outside discharge pipe;
• whether the pressure later falls again.

Quick Questions

Is 2 bar too high for a boiler?

Not necessarily. Boiler pressure normally rises as the heating warms up, and some systems may operate around this level when hot. Check the recommended range for your specific boiler.

Is 3 bar too high for a boiler?

A reading approaching 3 bar is generally towards the upper pressure limit of many domestic sealed heating systems. If the pressure is repeatedly reaching this level, allow the system to cool and arrange diagnosis if the problem continues.

Why does my boiler pressure rise to 3 bar when the heating is on?

A large rise when the system heats can indicate an expansion or pressure-control problem. The expansion vessel is one component an engineer may need to check.

Can an open filling loop cause high pressure?

Yes. If a filling control has not been fully closed, water can continue entering the sealed heating system and cause pressure to increase.

Can bleeding a radiator reduce boiler pressure?

Yes. Releasing air and a small amount of water from a radiator can reduce system pressure. Only do this when the system is cool and follow the instructions for your heating system.

Why is my boiler pressure high one day and low the next?

A large pressure rise while hot followed by low pressure after cooling can be associated with an expansion or pressure-relief problem and should be investigated if it keeps happening.

Need Help With High Boiler Pressure?

Let’s Heat can diagnose high boiler pressure, repeated pressure changes, leaks and pressure-related boiler lockouts.

You do not need to know which component is causing the problem before booking.

Book Boiler Repair →

Sources Checked

This guide has been checked against current homeowner heating guidance from Vaillant and Worcester Bosch.

Vaillant – What Pressure Should My Boiler Be?

Worcester Bosch – Boiler Pressure Guidance