--- title: Common Causes of Why Does My Combi Boiler Keep Losing Pressure? description: Find out why your combi boiler keeps losing pressure, from hidden leaks and bled radiators to faulty valves and internal boiler problems. image: https://www.argcontracts.co.uk/hubfs/why-does-my-combi-boiler-keep-losing-pressure-common-causes-explained.jpg --- [![Contracts-Logo-Website](https://www.argcontracts.co.uk/hs-fs/hubfs/Act3%20Child/images/Contracts-Logo-Website.webp?width=149&height=70&name=Contracts-Logo-Website.webp "Contracts-Logo-Website")](https://www.argcontracts.co.uk/) [![Contracts-Logo-Website-1](https://www.argcontracts.co.uk/hs-fs/hubfs/Contracts-Logo-Website-1.png?width=149&height=70&name=Contracts-Logo-Website-1.png "Contracts-Logo-Website-1")](https://www.argcontracts.co.uk/) - [About](https://www.argcontracts.co.uk/about) - Services - [Boiler Installation](https://www.argcontracts.co.uk/boiler-installation) - [General Plumbing Work](https://www.argcontracts.co.uk/general-plumbers) - [Bathroom Installation](https://www.argcontracts.co.uk/bathroom-fitter) - [Boiler Repair](https://www.argcontracts.co.uk/boiler-repair) - [Boiler Servicing](https://www.argcontracts.co.uk/boiler-servicing) - Boiler Options - [Combi Boiler Installation](https://www.argcontracts.co.uk/combi-boiler-installation) - [System Boiler Installation](https://www.argcontracts.co.uk/system-boiler-installation) - [Electric Boiler Installation](https://www.argcontracts.co.uk/new-electric-boiler) - [Heat only/Regular Installation](https://www.argcontracts.co.uk/regular-boiler) - [Pricing](https://www.argcontracts.co.uk/pricing) [![Get A Quote](https://hubspot-no-cache-eu1-prod.s3.amazonaws.com/cta/default/148104347/interactive-387189251311.png)](https://www.argcontracts.co.uk/hs/cta/wi/redirect?encryptedPayload=AVxigLK%2F%2FZSlG0KdhDl9xGQusQrnmxFFjQcj5AkiAXODf0xxSzm06CeoSk2%2BmTNxHASYC0Mxmlq%2BbB2oqAk00w0H94irfg9DZPiJBLhHnveTC7bvp4ZIaCxVJDVXTQ2C2bfAd8FbdVQFJI0%2Bj3BU4Pzv1C1ovjYI5dJov%2FY0P4XJjInm%2F%2FJ5urvtGCnyZayZ6twXo75UESn4HeHrPl9d8%2FTvAYk9sBcd36I%3D&webInteractiveContentId=387189251311&portalId=148104347) [Blog](https://www.argcontracts.co.uk/blog) [![Get A Quote](https://hubspot-no-cache-eu1-prod.s3.amazonaws.com/cta/default/148104347/interactive-387189251311.png)](https://www.argcontracts.co.uk/hs/cta/wi/redirect?encryptedPayload=AVxigLK%2F%2FZSlG0KdhDl9xGQusQrnmxFFjQcj5AkiAXODf0xxSzm06CeoSk2%2BmTNxHASYC0Mxmlq%2BbB2oqAk00w0H94irfg9DZPiJBLhHnveTC7bvp4ZIaCxVJDVXTQ2C2bfAd8FbdVQFJI0%2Bj3BU4Pzv1C1ovjYI5dJov%2FY0P4XJjInm%2F%2FJ5urvtGCnyZayZ6twXo75UESn4HeHrPl9d8%2FTvAYk9sBcd36I%3D&webInteractiveContentId=387189251311&portalId=148104347) [Blog](https://www.argcontracts.co.uk/blog) - [About](https://www.argcontracts.co.uk/about) - Services - [Boiler Installation](https://www.argcontracts.co.uk/boiler-installation) - [General Plumbing Work](https://www.argcontracts.co.uk/general-plumbers) - [Bathroom Installation](https://www.argcontracts.co.uk/bathroom-fitter) - [Boiler Repair](https://www.argcontracts.co.uk/boiler-repair) - [Boiler Servicing](https://www.argcontracts.co.uk/boiler-servicing) - Boiler Options - [Combi Boiler Installation](https://www.argcontracts.co.uk/combi-boiler-installation) - [System Boiler Installation](https://www.argcontracts.co.uk/system-boiler-installation) - [Electric Boiler Installation](https://www.argcontracts.co.uk/new-electric-boiler) - [Heat only/Regular Installation](https://www.argcontracts.co.uk/regular-boiler) - [Pricing](https://www.argcontracts.co.uk/pricing) [![Contracts-Logo-Website](https://www.argcontracts.co.uk/hs-fs/hubfs/Act3%20Child/images/Contracts-Logo-Website.webp?width=174&height=82&name=Contracts-Logo-Website.webp)](https://www.argcontracts.co.uk) [020 8804 8008](tel:02088048008) [![Get A Quote](https://hubspot-no-cache-eu1-prod.s3.amazonaws.com/cta/default/148104347/interactive-387189251311.png)](https://www.argcontracts.co.uk/hs/cta/wi/redirect?encryptedPayload=AVxigLK%2F%2FZSlG0KdhDl9xGQusQrnmxFFjQcj5AkiAXODf0xxSzm06CeoSk2%2BmTNxHASYC0Mxmlq%2BbB2oqAk00w0H94irfg9DZPiJBLhHnveTC7bvp4ZIaCxVJDVXTQ2C2bfAd8FbdVQFJI0%2Bj3BU4Pzv1C1ovjYI5dJov%2FY0P4XJjInm%2F%2FJ5urvtGCnyZayZ6twXo75UESn4HeHrPl9d8%2FTvAYk9sBcd36I%3D&webInteractiveContentId=387189251311&portalId=148104347) [020 8804 8008](tel:020%208804%208008)[contracts@arggroup](mailto:contracts@arggroup) ARG Group, Unit 2, New Ford Road Waltham Cross, Herts EN8 7PH # Why Does My Combi Boiler Keep Losing Pressure? Common Causes Explained By [**ARG Contracts**](https://www.argcontracts.co.uk/blog/author/arg-contracts) on Oct 1, 2026, 11:00:01 AM ![Homeowner checking a combi boiler after experiencing problems with the boiler losing pressure.](https://www.argcontracts.co.uk/hubfs/why-does-my-combi-boiler-keep-losing-pressure-common-causes-explained.jpg) If you've ever woken up to a chilly home and trudged downstairs to find a flashing error code on your boiler display, you're not alone. [![Get in touch ](https://hubspot-no-cache-eu1-prod.s3.amazonaws.com/cta/default/148104347/interactive-387189316825.png)](https://www.argcontracts.co.uk/hs/cta/wi/redirect?encryptedPayload=AVxigLK34MjPfBURqjYNqgj3wDu4V6L7H%2FziZzpAzcPZio4TC8r%2FOiULBcZyBftm5XcXUzH80zXEIQGnsDBtLzmlRhCFCCSaDUDUBqQXBvBUJRTLgUWWKTu3JMTDuIVZVBSntmd3xqlJuh%2FGpqDwMWeO2%2Fzk73Vz8tCmPyf%2F%2Fspn1kc6B5n58OApJU9WbpnV9jFx0682nZktvxCty%2F0wUD2gfvw6m5A%3D&webInteractiveContentId=387189316825&portalId=148104347) It's incredibly frustrating when your central heating stops working. When you check the display panel, you often find the pressure gauge has dropped into the red zone. If this happens to you regularly, you'll naturally find yourself wondering, "why does my [combi boiler keep losing pressure](https://www.argcontracts.co.uk/boiler-repair)?" Topping up the pressure manually is a fairly easy fix in the short-term. But it may only be a matter of time until that pressure falls again. At ARG Contracts, we spend our days [fixing heating systems](https://www.argcontracts.co.uk/blog/most-common-boiler-problems-guide) across Essex, Hertfordshire, and London. We know exactly how disruptive a failing boiler can be to your daily routine. Understanding the root causes of pressure loss can help you decide when a simple homeowner fix may be enough and when it's time to call a professional heating engineer. ## **Understanding Normal Boiler Pressure** Your combi boiler operates using a sealed system of water that runs through your pipes and radiators. For the water to circulate properly, the system needs to maintain a consistent level of pressure. On most models, the pressure gauge should sit between 1.0 and 1.5 bar when the heating is turned off. If the needle drops below 1.0 bar, your boiler will automatically shut itself down as a built-in safety precaution. ## **The Main Reasons Your Combi Boiler Loses Pressure** Common causes of pressure loss include: - Hidden leaks in the heating system - Recently bled radiators - A faulty pressure relief valve - A failing expansion vessel or other internal boiler fault Water pressure doesn't usually disappear without a reason. If you keep noticing the pressure dropping on your combi boiler, it usually points to a leak, a recently bled radiator, or a fault within the sealed system. Here are some of the most common culprits. ### **Hidden Leaks In The Heating System** Leaks are one of the most common reasons a [combi boiler keep losing pressure](https://www.argcontracts.co.uk/boiler-repair) over time. Because the system is sealed, even a tiny pinhole leak will gradually reduce the water volume and cause the pressure gauge to drop. You might spot water dripping from a radiator valve or a small puddle forming underneath the boiler unit itself. However, many leaks are completely hidden under floorboards or behind skirting boards. Over weeks or months, a slow, invisible drip will eventually cause the system to lock out. These should be repaired promptly to help prevent damage to your home. ### **Recently Bled Radiators** Sometimes, pressure drops are perfectly normal. For instance, if you have recently bled your radiators to remove trapped air, you have physically released volume from the sealed system. Releasing this air naturally causes the pressure to drop temporarily. Repressurising the system using the filling loop underneath the boiler is usually required afterwards. Once you top it back up to 1.5 bar, the pressure should hold steady. ### **Faulty Pressure Relief Valves** The pressure relief valve is a crucial safety component designed to release water if the system becomes over-pressurised. If this valve becomes faulty or gets blocked by debris, it might start discharging water constantly, even when the pressure is completely normal. These components require professional replacement by a Gas Safe registered engineer, as they are a vital safety feature of your central heating system. ### **Internal Boiler Faults** If there are no visible leaks around the house, internal boiler faults can also cause pressure problems. A failing expansion vessel is a very common example. The expansion vessel contains a rubber diaphragm that absorbs the extra pressure created when water heats up and expands. If the rubber perishes or the vessel loses its air charge, the pressure will spike rapidly when the heating turns on. This forces the safety valve to open and release water outside the system, resulting in the pressure dropping on [combi boiler systems](https://www.argcontracts.co.uk/blog/regular-boiler-vs-combi-boiler-guide) as soon as they cool down. ## **Why You Must Stop Constantly Repressurising** When the heating goes off, the temptation is simply to open the filling loop, top up the pressure, and ignore the problem. While doing this once or twice is fine, repeated pressure loss usually indicates an underlying fault. Constant repressurisation should not be relied upon as a long-term solution. Every time you introduce fresh mains water into your central heating system, you also introduce fresh oxygen. This oxygen reacts with the metal radiators, causing them to rust from the inside out. This creates a thick black sludge that clogs up your pipework and forces your boiler to work much harder. Eventually, this sludge will destroy the internal heat exchanger, leading to an expensive repair bill. ## **When To Call Your Local Heating Specialist** While you can safely check your radiators for visible leaks, professional diagnosis is often needed when pressure continues to drop. Removing the boiler casing to inspect or repair internal components should only be carried out by a Gas Safe registered engineer. Identifying the root cause helps prevent future issues and protects your expensive heating equipment from permanent damage. As a local, owner-led heating specialist, ARG Contracts provides honest pricing and expert fitting you can rely on. Our Worcester Bosch Accredited engineers have the skills to track down hidden faults and repair them safely. We stand by our work with a solid two-year guarantee. Boiler pressure keeps dropping? [Contact ARG Contracts](https://www.argcontracts.co.uk/get-in-touch) for expert diagnosis and reliable repairs. 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