A boiler pressure that keeps falling can be frustrating, particularly when the heating suddenly stops working or the boiler displays a low-pressure warning.Read original
For many UK households with sealed central-heating systems, the pressure gauge is an important indicator of whether the system has enough water to operate properly. Manufacturers such as Worcester Bosch and Vaillant say low pressure is commonly associated with water loss, leaks or other heating-system faults.
But why does boiler pressure drop, and when is it something a homeowner can fix themselves?
What should my boiler pressure be?
The correct pressure varies between boiler models, so homeowners should check their manufacturer's instructions.
As a general guide, Worcester Bosch says a cold sealed heating system should normally show around 1 to 1.5 bar. When the heating is running, the pressure can rise as the water heats up.
Vaillant similarly explains that sealed heating systems operate within a defined pressure range and that a reading below about 1 bar can indicate that the system has lost pressure.
If the pressure is slightly lower than normal, the boiler may still operate. However, a pressure that repeatedly falls after being topped up should not simply be ignored.
1. There could be a leak somewhere in the heating system
One of the most common explanations for falling boiler pressure is that water is escaping from the sealed central-heating system.
The leak does not necessarily have to be underneath the boiler.
Water can escape from:
Radiator valves
Radiator connections
Pipe joints
Boiler components
Heating-system pipework
Underfloor pipework
Other connections around the central-heating system
Some leaks are obvious, while others are extremely small.
Worcester Bosch says so-called micro-leaks can be difficult to identify because the small quantity of escaping water may evaporate before leaving an obvious puddle. Slight discolouration around radiator valves or air vents can sometimes provide a clue.
What should you look for?
Check visible radiator valves and pipe connections for:
Dampness
Rust or corrosion
White or green deposits
Water marks
Discolouration
Small drops of water
If you find evidence of a leak, repeatedly topping up the system is not a permanent solution. The cause needs to be investigated and repaired.
2. You recently bled your radiators
Bleeding a radiator removes trapped air from the heating system.
However, bleeding can also result in some water being lost from the system. If enough water escapes, the boiler pressure can fall.
Vaillant identifies releasing air or water while bleeding radiators as one of the reasons a heating system can lose pressure.
This is why it is a good idea to check the boiler pressure after bleeding radiators.
Worcester Bosch also advises homeowners to check their boiler pressure after bleeding radiators.
3. The pressure relief valve could be releasing water
Another possible cause is the pressure relief valve, sometimes referred to in relation to the boiler's discharge or overflow pipe.
This safety component is designed to release water if the system pressure becomes excessively high.
If it is leaking or has been opening repeatedly, water can leave the heating system. Once enough water has escaped, the pressure shown on the boiler can fall.
Worcester Bosch recommends checking the discharge pipe leading through the external wall for signs of dripping. A wet or dripping pipe can indicate a problem with the pressure relief valve.
If you suspect this is happening, a qualified heating engineer should investigate it rather than simply continuing to refill the system.
4. The expansion vessel may have a problem
Modern sealed heating systems use an expansion vessel to accommodate changes in water volume as the heating system warms up and cools down.
Vaillant explains that water expands as it heats, and the expansion vessel is designed to absorb those changes in pressure.
If the expansion vessel loses its charge or develops a fault, the pressure can behave abnormally.
One warning sign is a large increase in pressure when the heating comes on, followed by a significant drop when the system cools.
Worcester Bosch says that if the pressure rises by more than 1 bar when the heating reaches operating temperature, the expansion vessel may need repressurising and a service engineer may be required.
5. The boiler itself could be leaking
Sometimes the source of the pressure loss is inside or immediately around the boiler.
A boiler leak may be caused by a worn seal, connection or component, while older equipment can also develop corrosion.
Signs can include:
Water underneath the boiler
Dampness around pipe connections
Repeated pressure loss
Rust or staining
Small drips
Boiler fault messages
Worcester Bosch notes that even a small boiler leak can affect efficiency and potentially cause water damage if it is not addressed.
Do not remove the boiler casing yourself to investigate an internal leak. Gas-boiler repairs and internal work should be left to an appropriately qualified heating professional.
6. The pressure is being topped up incorrectly
If the pressure has fallen, many sealed heating systems can be repressurised using a filling loop or another manufacturer's specified filling mechanism.
The exact procedure depends on the boiler.
For example, Worcester Bosch's guidance describes different systems, including external filling loops and internal/keyless filling arrangements. Its general instructions aim to bring the pressure to approximately 1 to 1.5 bar for a cold system.
However, homeowners should always follow the instructions for their particular boiler rather than assuming every filling system works in exactly the same way.
How often should boiler pressure need topping up?
An occasional pressure adjustment can happen.
But frequent pressure loss is not something to regard as normal.
Worcester Bosch says a heating system will usually need topping up only once or twice a year and advises contacting an installer if repressurisation is required much more frequently.
Therefore, if you are adding water every few days or weeks, there is likely to be an underlying issue that needs investigating.
What should I do if my boiler pressure is low?
If the pressure is low but there are no obvious signs of a serious problem, check your boiler's manual to find out how its system should be repressurised.
For systems that use a filling loop, the pressure can generally be increased by allowing water into the sealed heating system while monitoring the pressure gauge.
Worcester Bosch advises bringing the pressure to around 1 to 1.5 bar when the system is cold.
Do not keep adding water indefinitely if the pressure continues to fall.
That could simply mask a leak or another fault.
What if the boiler pressure keeps dropping after I top it up?
This is the point at which homeowners should investigate the underlying cause rather than repeatedly repressurising the boiler.
Check visible:
Radiator valves
Pipe connections
Boiler connections
Areas around the heating system
The external pressure-relief discharge pipe
Worcester Bosch specifically recommends checking visible joints and looking for water discharged from the pressure relief valve when a system is constantly losing pressure.
If nothing obvious can be found, a heating engineer may need to carry out further checks because the leak could be hidden or a component such as the expansion vessel or pressure relief valve could be involved.
What if the pressure is too high instead?
A boiler pressure problem is not always caused by pressure being too low.
Pressure can also become too high, particularly if the system has been overfilled or if there is an issue with expansion.
Worcester Bosch says that pressure around 2.75 bar or higher can indicate excessive pressure, while a rise of more than 1 bar as the heating reaches temperature can point towards an expansion-vessel problem.
Homeowners should therefore avoid assuming that adding more water is always the answer.
Can I continue using a boiler with low pressure?
It depends on how low the pressure is and what the boiler manufacturer's instructions say.
A slightly low reading may result in the boiler continuing to operate, while a sufficiently low pressure can cause the boiler to stop working or display a fault.
If the pressure repeatedly falls, however, the important issue is why it is losing pressure.
Continuing to refill the system without fixing the cause can allow a leak or component fault to remain unresolved.
When should you call a heating engineer?
You should consider getting professional help if:
The pressure repeatedly drops after topping up.
You find water leaking from the boiler.
A radiator valve is leaking.
The external discharge pipe is continuously dripping.
Pressure rises dramatically when the heating comes on.
The boiler repeatedly locks out because of low pressure.
You cannot identify the cause of the pressure loss.
You suspect an internal boiler fault.
For gas boilers, use an appropriately qualified professional for gas-related work.
The key warning sign UK homeowners should not ignore
A boiler that loses pressure once is not necessarily a major problem. The pressure may have fallen following radiator bleeding or another routine event.
But a boiler that repeatedly falls from normal pressure to low pressure is different.
It usually indicates that water is leaving the sealed heating system or that a component responsible for maintaining pressure needs attention.
As Worcester Bosch explains, a permanent or significant decrease in pressure should prompt contact with the installer or maintenance engineer.
Bottom line
If your boiler pressure keeps dropping, the most likely explanations include a leak, water loss after bleeding radiators, a faulty pressure relief valve or an expansion-vessel problem.
Check the pressure against your boiler manufacturer's recommended range, inspect accessible areas for signs of water loss and repressurise the system only according to the manufacturer's instructions.
If you repeatedly need to top it up, don't treat this as normal maintenance. Find and fix the reason for the pressure loss.
This article is for general information. Boiler systems vary, so homeowners should follow the manufacturer's instructions and use a qualified heating professional where necessary.
