A boiler loses pressure because there is a water leak or a failed internal component. This can drop the pressure to below 1 bar, and at this stage the boiler can no longer provide hot water or heating. It will automatically shut down to protect itself from further damage, and display either an error code or cold radiators to indicate a fault.
Luckily, some causes are easy to fix, and you can restore the pressure yourself. Others need a Gas Safe registered engineer. Our guide takes you through how to check the pressure, find the potential cause of a problem, safely re-pressurise the boiler and know when emergency boiler repair in London is necessary.
Boiler pressure at a glance:
The pressure gauge on your boiler is the quickest way to identify a boiler losing pressure. Most sealed systems will sit at around 1 to 1.5 bar when the system is cold. This can vary by model, so always check your user manual before assuming there is a problem.
Our table shows what the pressure gauge could be telling you:
| Pressure reading | What it means | What to do |
| Below 1 bar | Pressure has dropped too low. The boiler may cut out. | Re-pressurise via the filling loop. If it keeps falling, look for a leak or fault. |
| 1 to 1.5 bar (cold) | Normal operating range | No action needed. |
| Above 2.5 bar | Over-pressurised or over-filled | Bleed a radiator to release the excess. Check the filling loop is fully closed. |
| Drops again after topping up | Persistent leak or internal fault | Call a Gas Safe engineer. Do not keep re-pressurising. |
What is boiler pressure?
Boiler pressure is the balance of water and air inside your sealed central heating system. It’s measured in bar, and for most modern boilers, it needs to stay between 1 and 1.5 bar (this can vary between boilers) to circulate hot water around your radiators safely and efficiently.
Older systems would use a water tank in the loft, but the majority of boilers today are sealed units that rely on an expansion vessel. As the boiler heats the water, it expands; the expansion vessel absorbs the change so the pressure can stay steady. When everything is working as it should, the pressure gauge will barely move. Regardless, a reading outside of the normal limits for your boiler means something needs attention.
Why is my boiler losing pressure?
Your boiler could be losing pressure for many reasons, but the most common are:
- A leak in the heating system: Usually at radiator valves, pipe joints or connections.
- Faulty boiler components: A failing expansion vessel or pressure relief valve can cause a gradual or repeated drop.
- A leak in the boiler: Corroded or worn parts, often showing as a small drip under the boiler.
- Recently bled radiator: Releasing air and a little water lowers the pressure. This is normal, and easy to top up.
How to check if your boiler pressure is low
If you think your boiler pressure is too low, grab the user manual and head directly to the boiler. Consult the manual for the correct pressure levels and check the gauge to see if it matches. If the gauge is below the recommended pressure levels, the boiler’s pressure is too low; if it is above, the pressure is too high.
On many modern boilers, a digital display shows an error code. You can then cross-reference this with the code in your user manual to establish the issue. This will help establish the right course of action and whether you need professional help.
How do I find a leak in my heating system?
To find a leak in your heating system, look for damp patches around the radiators, near the boiler and near any pipework.
Pipes are often wet due to condensation, so don’t assume it’s a leak. Wipe them dry and place something underneath to capture water. That way you can tell for sure if the pipe is leaking.
If you spot discoloured paint or flaking paint on pipes or skirting boards, water stains on the floor, or damp patches on the ceiling, you can safely assume there is a leak somewhere.
Repairing a leak often needs professional intervention, and repairing loose or broken pipework joints is not advised. Instead, contact an emergency plumber in London. Once they repair the pipes, check if your boiler is still losing pressure. If it isn’t, you’ve solved the problem; if it is, you should investigate further or contact a local boiler engineer.
My boiler is losing pressure, but I can’t find a leak
If your boiler is losing pressure but there is no evidence of a leak from the radiators or pipework, there are two possibilities:
- Boiler leak: The boiler itself may be leaking. Parts inside the boiler may have corroded over time or worked loose. Without regular servicing, you may not notice these issues until a problem becomes more apparent. These leaks are normally noticeable underneath the boiler and may be only a small drip. If this seems to be the case, contact your local boiler engineer and ask them to inspect it. Do NOT remove the boiler cover yourself.
- Faulty parts: Often a part breaks down and causes the boiler to either perform erratically or cease working altogether. As with a boiler leak, do not attempt to repair this yourself. Only Gas Safe registered engineers should remove a boiler’s cover and work on the parts behind it.
If the boiler is at fault, check whether it is still under manufacturer warranty before paying for repairs.
How do you re-pressurise your boiler?
If you feel safe doing so, and the user manual indicates you can do it without professional help, you can re-pressurise your boiler yourself. You can do this by following a few simple steps:
- Find the filling loop and pressure gauge. The filling-loop handle should be at a right angle to the flow of the pipe.
- Check the pressure gauge, and ensure you can see it as you access the filling loop. That way, you can check that you pressurise the boiler to a safe level. Remember, most sealed units operate between 1 and 1.5 bar, but check your manual first.
- Turn off your boiler.
- Turn both filling loop handles to match the direction of the pipe. Do this slowly. You should hear water flowing.
- Keep an eye on the gauge and when it reaches the correct levels, close the filling loops.
- Turn the boiler back on and monitor the pressure.
If you have exceeded the recommended pressure levels for your boiler, don’t worry. Bleed your radiators and have someone monitor the gauge to see that it drops to the correct level. If you have to re-pressurise your boiler more than two or three times a year, that’s a sign of a leak, rather than normal settling and is worth professional investigation.
Is a boiler losing pressure a landlord’s responsibility?
Yes. In a rented home, keeping the boiler and heating in working order is the landlord’s legal responsibility, not the tenant’s. Under Section 11 of the Landlord and Tenant Act 1985, landlords must keep heating and hot water systems in repair and proper working order. Landlords must also arrange the annual gas safety check by a Gas Safe registered engineer and give the tenant a copy of the record.
If you are a tenant and your boiler keeps losing pressure, contact your landlord or letting agent immediately. Topping up the pressure yourself is fine, but if you do not feel comfortable doing so, reach out to the appropriate person for property maintenance.
What to do if your boiler keeps losing pressure
If your boiler is still losing pressure after you have re-pressurised it, the problem is likely something behind the boiler cover. Internal faults like this can only be treated by a qualified Gas Safe engineer. The sooner they are called out, the better, as any internal boiler issues can be caught before they develop into something more severe and more expensive to fix.
At London Property Services, we have a team of skilled Gas Safe engineers available to attend properties throughout London 24/7. Whether in Brixton or Kensington, Walthamstow or Windsor, we can reach you quickly to make your boiler safe, restore the pressure and get the hot water working again. Contact us today to book your London gas engineer.
