--- title: "Why Do I Keep Losing Pressure in My Boiler? | R.K Plumbing & Heating" description: "A sealed heating system should not lose water. The common causes of falling boiler pressure, what you can check yourself, and when to call an engineer." canonical: "https://www.rkplumbingandheatingservices.com/blog/why-do-i-keep-losing-pressure-in-my-boiler" last_updated: "2026-08-20T22:28:41.558Z" --- # Why Do I Keep Losing Pressure in My Boiler? PublishedMonday, 17 August 2026 ![Alpha CD50 boiler with its drop-down control flap open, showing pressure gauge, dials and timer](/_next/image?url=%2Fapi%2Fmedia%2Ffile%2Frk-boiler-installation-20210330-06.jpg%3Fprefix%3Dtenants%252F6a73367026944505f092a67a%26v%3D1787045400885&w=1536&q=75&dpl=dpl_J3UkotoCWSfG7mH3JdDuNRVcD7J7) ## What boiler pressure actually is Your central heating is a sealed loop of water. Pressure is simply how firmly that water is packed into the loop, and the gauge on the front of the boiler tells you where it stands. Most boilers want somewhere around 1 to 1.5 bar when the system is cold. Your manufacturer's instructions will give the exact figure for your model, and many gauges have a green band marked on them to show the range. The reading rises a little when the heating is running and the water expands, which is perfectly normal. If it drops below the bottom of that range, the boiler will usually shut itself down rather than run, because operating a sealed system short of water risks damaging it. ## Topping it up once is normal. Doing it repeatedly is not. This is the part worth understanding, because it is where most people go wrong. A sealed heating system should not lose water. It is a closed loop. If it is losing pressure, either the water is going somewhere, or the part that manages pressure has stopped doing its job. Refilling through the filling loop will get the heating going again, and that is a perfectly reasonable thing to do once. But if you are doing it every few weeks you are treating a symptom. Something is causing it, and topping up repeatedly introduces fresh oxygenated water into the system each time, which encourages corrosion and sludge over the long run. > If you are topping up more than once or twice a year, the pressure is not the problem. Something else is. ## The usual causes **You have bled the radiators.** The most innocent explanation. Letting air out of radiators lets pressure out with it. Bleed the radiators, top the system back up, and that is the end of it. If the pressure keeps falling afterwards, this was not the cause. **There is a leak somewhere in the system.** Sometimes it is obvious: a damp patch under a radiator, a green or white crust around a valve, a stain on a ceiling below pipework. Often it is not. Pipework buried in floors or walls can weep slowly for a long time before anything shows, and a leak under a solid floor may never show at all. **The pressure relief valve is passing.** Every sealed system has a safety valve that opens if pressure climbs too high, discharging through a small copper pipe that runs outside, usually low down on an external wall. If that valve has failed, or has a piece of debris on its seat, it will let water out steadily. The telltale sign is water dripping or staining below the end of that pipe, or a pipe that is wet when it has not rained. **The expansion vessel has lost its charge.** This is the one most people have not heard of, and it is a common culprit. The vessel is a sealed container with a rubber diaphragm and a cushion of air behind it, and it absorbs the expansion as the water heats up. When it loses that charge, the pressure swings sharply, climbing high when the heating runs and dropping back low once it cools. If the needle moves a long way between hot and cold, the vessel is the first thing to suspect. **The filling loop is letting by.** Less common, but a filling loop valve that has not been closed fully, or one that is worn, can let pressure move the wrong way. ## What you can check yourself Look at the gauge when the system is cold and note where the needle sits. If it is below the marked range, top it up using the filling loop, following the instructions for your boiler, and close both valves fully afterwards. Then write the date down. That one habit tells you more than anything else, because if you are back doing the same job a fortnight later you have a fault rather than a one-off. Have a look at the outside end of the discharge pipe too. If it is wet or stained on a dry day, that points straight at the pressure relief valve. ## When it is time to call someone Call if you are repressurising more than occasionally, if you can see damp, staining or corrosion near pipework or radiators, if the discharge pipe outside is dripping, or if the gauge swings widely between hot and cold. Those are faults to be diagnosed, not managed. It is worth saying as well that a system losing water quickly can cause damage well beyond the boiler itself, particularly where the leak is under a floor. We are Gas Safe registered for domestic gas work, registration 568345. If your pressure keeps dropping, call 01273 911551 and we will talk through what it is doing before anyone comes out, and you can read more about [boiler pressure problems](/services/boiler-pressure-problems) and [boiler repair](/services/boiler-repair). We are based in Woodingdean and cover Brighton, Hove, Worthing, Lewes, Shoreham-by-Sea, Haywards Heath, Seaford, Burgess Hill and surrounding Sussex towns. From the blog ## Recent articles [All articles](/blog) [ 4 min ### Can Air Conditioning Heat Your Home? ](/blog/can-air-conditioning-heat-your-home)[ 3 min ### How Often Should Air Conditioning Be Serviced? ](/blog/how-often-should-air-conditioning-be-serviced)[ 4 min ### What Does Air Conditioning Cost to Run? ](/blog/what-does-air-conditioning-cost-to-run)