Why Does My Boiler Keep Losing Pressure?

A boiler that repeatedly loses pressure can leave your home without reliable heating or hot water. You may repressurise the system and find that the pressure drops again after a few hours or days.

Occasional pressure adjustment may be needed after radiators have been bled, but regular pressure loss usually indicates a problem that should be investigated.

What pressure should a boiler have?

Most sealed heating systems normally show approximately 1 to 1.5 bar when the system is cold, although the correct pressure depends on the boiler manufacturer and system design.

The pressure will usually rise slightly when the heating is running. However, if it rises towards 3 bar or falls close to zero, there may be a fault.

Always follow the instructions provided by your boiler manufacturer.

Common reasons a boiler loses pressure

1. A leak in the heating system

Even a small water leak can gradually reduce the system pressure. Check around:

• Radiator valves
• Pipe connections
• Towel rails
• The boiler
• Heating pumps and motorised valves
• Visible pipework
• Underfloor heating manifolds

Look for water, staining, corrosion or green deposits around copper connections. Some leaks only appear when the heating is hot, making them more difficult to find.

Never remove the boiler casing yourself. Internal boiler inspections must be carried out by a Gas Safe registered engineer.

2. A problem with the expansion vessel

The expansion vessel allows heated water to expand safely. If it loses its air charge or its internal diaphragm fails, the boiler pressure may rise sharply when the heating is on.

The pressure relief valve may then discharge water outside. When the system cools, the pressure can fall very low.

A common warning sign is pressure that changes considerably between a cold and hot heating system.

3. The pressure relief valve is passing water

The pressure relief valve protects the heating system from excessive pressure. After operating, it can sometimes continue to let water escape.

Check the copper discharge pipe outside the property, normally located near the boiler. If it is dripping or the wall beneath it is wet, the pressure relief valve may require attention.

Do not block, cap or modify this pipe.

4. Radiators have recently been bled

Bleeding radiators releases trapped air, but it can also reduce the system pressure. A small pressure drop after bleeding is normal, and the system may need to be repressurised.

If the pressure continues falling afterwards, there may be a leak or another underlying fault.

5. An automatic air vent is leaking

Automatic air vents help remove trapped air from the heating system. If an air vent becomes faulty, it may allow water to escape.

The leak may be small and can evaporate when the boiler is hot, leaving only staining or corrosion behind.

6. An internal boiler leak

Components inside the boiler can develop leaks, including pumps, seals, heat exchangers and hydraulic connections.

An internal leak may not always be visible beneath the boiler because water can evaporate on hot components or drain through another route. A Gas Safe engineer should inspect the appliance safely.

Can I repressurise the boiler myself?

Many boilers have a filling loop or filling mechanism that allows the pressure to be restored. Only do this if you understand the manufacturer’s instructions for your particular boiler.

Do not repeatedly add water without investigating why the pressure is being lost. Constantly introducing fresh water adds oxygen to the system and can contribute to internal corrosion and sludge.

Make sure the filling valves are completely closed after repressurising. If you are uncertain, contact a heating engineer.

For Worcester Bosch appliances, find the official user manual for your boiler model and follow its instructions. Filling mechanisms and recommended pressures vary between models.

When should I call an engineer?

Arrange an inspection if:

• The pressure drops repeatedly
• You need to repressurise the boiler every few days
• The pressure rises close to 3 bar when the heating is operating
• Water is coming from the external discharge pipe
• You can see water underneath the boiler
• Radiators or pipe connections are leaking
• The boiler is displaying a low-pressure fault code
• You have no heating or hot water

Identifying the actual cause is important. Replacing a pressure relief valve, for example, may not solve the problem if a faulty expansion vessel caused it to operate.

What should I do if water is leaking?

If water is escaping quickly and it is safe to do so, turn off the heating and electrical supply to the affected equipment. Protect the surrounding area and contact a qualified engineer.

If you smell gas, call the National Gas Emergency Service immediately on 0800 111 999. Do not operate electrical switches or attempt to repair the appliance.

Boiler pressure problems in Hastings and East Sussex

ZSU Ltd provides professional boiler fault diagnosis and boiler repairs in Hastings, St Leonards-on-Sea and surrounding parts of East Sussex.

We can inspect the boiler and heating system, locate the likely source of pressure loss and explain the available repair options clearly. We also provide central heating services, including radiator repairs, valve replacement, system balancing and circulation fault diagnosis.

For help with a boiler that keeps losing pressure, call 07963 590 223 or contact ZSU Ltd.