Boiler Keeps Losing Pressure? Find Out Why and What to Do Next March 10, 2026 If your boiler pressure keeps dropping, it is more than just an annoying fault. Falling pressure is your boiler telling you that something in the heating system is not quite right, and ignoring it can lead to bigger breakdowns and higher bills. What normal boiler pressure looks like Most modern combi and system boilers are designed to run at relatively low pressure. As a rough guide, many sit around 1 to 1.5 bar when the system is cold, then rise slightly when hot. Your boiler manual should give the correct range for your model, so always check that first. The pressure gauge is usually on the front of the boiler fascia and may have a green zone to show the normal operating range. When people say the pressure is "dropping", they usually mean the gauge falls below 1 bar, or that it gradually falls over days or weeks and needs topping up repeatedly. Occasional top ups after maintenance can be normal, but frequent loss of pressure is almost always a fault. Main reasons boiler pressure keeps dropping Repeated pressure loss is usually caused by water escaping somewhere in the heating system or by a problem with how the system is controlling expansion. Sometimes the cause is visible, other times it is hidden in pipework or inside the boiler casing. Small leaks on radiators, valves or pipe joints Pressure relief valve (PRV) discharging water outside Expansion vessel losing its charge or failing Recently bled radiators without re-pressurising the system Filling loop not fully closed or used incorrectly Internal boiler faults, including limescale related wear Small leaks on radiators, valves or pipe joints One of the most common causes is a slow drip somewhere in the system. This might be a radiator valve that only weeps when hot, a corroded pipe under a floor, or a fitting in an airing cupboard that has started to loosen. Because the leak is often tiny, the water can evaporate quickly or soak into floors or plaster, so you might not see obvious puddles. Over time though, even a small leak will pull the pressure down and force you to keep topping up. Pressure relief valve and outside overflow pipe Your boiler has a pressure relief valve designed as a safety feature. If the pressure gets too high, it opens and dumps water through a copper discharge pipe to outside the building. If this valve is faulty or has been forced to open repeatedly, it can start to leak constantly. You may notice a dripping copper pipe outdoors, often close to where the boiler is installed. Continuous discharge here will steadily drop the boiler pressure. Expansion vessel and repeated pressure swings As water heats up it expands, and the expansion vessel absorbs that movement. It contains a rubber membrane and an air or nitrogen charge on one side of the diaphragm. If the vessel loses its charge or the membrane fails, pressure can spike too high when the heating is on, then fall sharply when it cools. This often triggers the pressure relief valve and leads to very noticeable pressure fluctuations on the gauge. Bleeding radiators and topping up incorrectly Bleeding radiators to remove trapped air is good maintenance, but it also releases water from the system. If you bleed several radiators and do not top the system back up correctly, the gauge can drop below its normal range. It is also possible to overfill the system when refilling, which then causes high pressure when hot. That can in turn open the PRV and start a cycle of pressure loss. Filling loop left open or used the wrong way The filling loop is the flexible or fixed silver hose that lets you add water to the boiler from the mains. When you are not actively topping up, both valves on the filling loop should be fully closed. If the valves are left partially open or fitted incorrectly, the boiler can keep taking in fresh water. This can upset the balance of the system, flush out protective chemicals and sometimes lead to pressure issues that an engineer will need to correct. Internal faults, hard water and system debris In hard water areas, limescale can build up inside heat exchangers and pipework. This puts extra stress on internal components and can lead to overheating, noisy operation and pressure problems over time. Sludge and debris from corrosion inside radiators can also block narrow passages in the boiler. Regular servicing, checking inhibitor levels and cleaning filters helps protect the boiler and can prevent pressure related faults from developing. Safe checks you can do yourself There are some simple, non intrusive checks you can carry out without removing the boiler casing or touching gas components. If at any point you feel unsure, stop and speak to a professional. Look for obvious leaks: check around radiator valves, towel rails, visible pipe joints and ceilings below pipe runs. Check outside pipes: locate the copper PRV discharge pipe and see if it is wet or dripping when the heating is on. Monitor the gauge: note the pressure when cold, then again when hot, and record how much it drops over 24 hours. If you have recently bled radiators, make sure the system has been re-pressurised back into the correct range as shown in your boiler manual. Never exceed the maximum pressure marked on the gauge. When to stop and call a Gas Safe engineer Some symptoms are clear warning signs that you should not keep resetting or topping up the boiler. Carrying on in these cases can damage the boiler or put your home at risk. Stop attempting DIY fixes and contact a Gas Safe registered engineer if you notice any of the following: Pressure drops back down soon after topping up, especially within hours. Visible water around the base of the boiler or from pipework beneath it. Regular boiler lockouts, fault codes or the boiler cutting out on heating or hot water. Burning or scorching marks near the boiler or flue. Internal problems with expansion vessels, PRVs, heat exchangers and gas components must only be worked on by qualified engineers. They have the tools to test and recharge vessels, replace valves and safely check for hidden leaks. Local water conditions and ongoing care If you live in a hard water area, your boiler and pipework are under extra strain. Limescale and sludge can increase operating temperatures and pressures, which then accelerates wear on seals and safety valves. A regular boiler service, along with checking inhibitor levels and system filters, helps keep everything running smoothly. It can also pick up early signs of pressure problems before they become a breakdown. FAQ: boiler pressure dropping Is it safe to keep repressurising my boiler? Topping up occasionally after maintenance is normally fine, but if you are doing it more than a few times a year there is likely a fault. Repeatedly adding fresh water can increase corrosion and should be checked by an engineer. Can low boiler pressure damage the boiler? Running at very low pressure can cause overheating and may trigger safety cut outs. Short periods are unlikely to cause serious harm, but it should be corrected quickly to avoid long term damage. Will a boiler service fix pressure loss? A proper service includes checks on key safety components and can often identify the cause of pressure loss. Some issues, such as failed expansion vessels or leaks, may require additional repair work. Need help with a boiler losing pressure? If your boiler pressure keeps dropping and you are unsure what to do next, it is usually best to get a professional opinion. An experienced engineer can test the system, trace hidden leaks and repair or replace faulty parts before they fail completely. For fast, friendly help, contact 7 Day Plumbing Services Ltd on 07896487603. You can book a repair visit or arrange a boiler check via our Boiler Repair and Boiler Servicing pages and get your heating running reliably again.
