Why Does My Boiler Keep Losing Pressure?
If your boiler pressure keeps dropping, topping it back up may get the heating working again, but it does not explain why the pressure was lost in the first place.
On most sealed heating systems, the pressure should usually sit at around 1 to 1.5 bar when the system is cold. A small change over time can be normal, particularly after radiators have been bled, but if you are having to repressurise the boiler regularly, there is normally an underlying cause that needs investigating. Worcester Bosch, for example, advises contacting an installer if a system needs topping up much more frequently than once or twice a year.
After more than 35 years working with heating systems, the important question is not simply “How do I put the pressure back up?” It is “Where is the pressure going?”
Here are some of the things I would look at.
A small leak somewhere in the heating system
One of the first things to rule out is water escaping from the system.
The leak does not have to be dramatic. A small amount of water lost from a radiator valve, pipe joint or another part of the heating system can gradually reduce the pressure without leaving an obvious puddle.
Have a look around:
Radiator valves and connections
Visible pipe joints
Underneath radiators
Skirting boards and flooring around heating pipes
Underneath the boiler
You may notice a small drip, damp patch, staining, corrosion or residue around a connection.
Some leaks are hidden beneath floors or inside walls, so the absence of visible water does not necessarily mean the system is leak-free. Vaillant similarly identifies leaks at radiators, system components or within the appliance as common causes of pressure loss.
Have you recently bled the radiators?
If you have recently bled one or more radiators, a drop in boiler pressure afterwards can be completely understandable.
Bleeding a radiator releases trapped air, but some water can also leave the sealed system. The boiler may therefore need repressurising afterwards.
The important distinction is what happens next.
If you restore the correct pressure and it remains stable, there may be nothing further to investigate. If it falls again shortly afterwards, something else is likely causing the loss.
The expansion vessel may not be working properly
This is one of the causes homeowners are less likely to spot themselves.
As the water in your heating system warms up, it expands. The expansion vessel inside the system is designed to accommodate that change and keep the pressure within a safe range.
If the vessel loses its air charge or develops a fault, the pressure can rise significantly when the heating is running and then fall again as the system cools.
One clue is a boiler that appears reasonably normal when cold but climbs significantly in pressure once the heating gets hot. Worcester Bosch advises that a pressure rise of more than around 1 bar as the system reaches temperature can indicate that the expansion vessel needs attention.
This requires an engineer to investigate.
The pressure relief valve may be letting water out
The pressure relief valve, often shortened to PRV, is a safety component designed to release water if pressure becomes too high.
If it has operated or is no longer sealing correctly, a small amount of water can continue escaping from the heating system. That gradually reduces the system pressure.
There may be no water visible inside the property because the discharge pipe normally runs outside.
If your boiler repeatedly loses pressure, checking whether water is being discharged externally can therefore provide an important clue. The valve itself should be assessed by a qualified engineer rather than dismantled or repaired by the homeowner.
There could be a fault inside the boiler
Sometimes the heating system and radiators appear completely dry because the problem is actually within the boiler.
Internal seals, valves or other components can deteriorate with age and allow water to escape. You might see a drip beneath the boiler, but some internal faults are not immediately visible.
Do not remove the boiler casing to investigate this yourself. If an internal boiler fault is suspected, it should be checked by a Gas Safe registered engineer.
How often should you need to top up boiler pressure?
This is the part I would pay attention to.
Having to repressurise a system occasionally does not automatically mean there is a serious problem. Manufacturers acknowledge that an occasional top-up may be necessary.
But regularly topping the boiler up every few days or weeks is not a long-term fix.
If the pressure repeatedly falls after being restored, water or pressure is being lost somewhere and the reason should be found rather than continually adding more water to the system. Worcester Bosch states that needing to repressurise substantially more often than once or twice a year warrants contacting an installer.
Can I repressurise the boiler myself?
Many modern boilers allow homeowners to restore system pressure using a filling loop or built-in filling mechanism.
However, the correct method varies by boiler, so always follow the manufacturer's instructions for your particular model rather than assuming every system works the same way.
For many sealed systems, the normal cold pressure is around 1 to 1.5 bar, but your boiler manual should be the reference for your specific appliance.
If you are unsure how to repressurise your boiler, or you find yourself doing it repeatedly, it is better to have the system checked.
When should you call an engineer?
I would recommend having the system investigated if:
The pressure falls again shortly after you top it up
You are regularly having to repressurise the boiler
You can see water leaking from the boiler, radiators or pipework
The pressure rises significantly when the heating comes on
Water appears to be coming from the boiler's external discharge pipe
The boiler displays a recurring low-pressure fault
You have no heating or hot water because the boiler will not operate
The important thing is to identify the cause, rather than repeatedly treating the pressure gauge as the problem.
Boiler pressure problems in West London
West Gas Service has more than 35 years of experience diagnosing boiler and heating faults across West London and surrounding areas.
If your boiler keeps losing pressure, we can check the heating system, identify where the pressure is being lost and explain what actually needs repairing before any work begins.
Call 07866 567112 or email us to arrange a visit.
