what causes boiler to lose pressure: A Quick Homeowner Guide
- Luke Yeates
- 4 days ago
- 13 min read
That sinking feeling you get when you see your boiler's pressure gauge dipping into the red is a familiar one for many homeowners. But what causes a boiler to lose pressure? It's usually a lot simpler than you might think.
A boiler losing pressure nearly always points to one of two things: water is leaking out of the system, or air has managed to get in. I often tell my clients at Harrlie Plumbing and Heating to think of it like a puncture in a bicycle tyre; once the seal is broken, it just can't hold its pressure anymore.
Your Central Heating System and Boiler Pressure
Let's quickly cover the basics. Your central heating system is essentially a big, sealed loop of pipes and radiators filled with water. The boiler's job is to heat this water and pump it around your home. For that to happen efficiently, the whole system needs to be kept under a certain amount of pressure—usually between 1 and 1.5 bar when the heating is off.
This pressure is what gives the hot water the oomph it needs to travel all the way to the furthest radiator in your house. If the pressure drops too low, a clever safety feature in most modern boilers will shut the system down completely to prevent any damage. This, of course, leaves you with no heating or hot water. Here at Harrlie Plumbing and Heating, this is one of the most common calls we get from homeowners across Eastbourne, from the seafront apartments to the family homes in Hampden Park.
The Two Main Culprits Behind Pressure Loss
So, when we're trying to figure out what causes a boiler to lose pressure, our investigation almost always leads us to one of two suspects.
A Water Leak: This is by far the most common cause. Even a tiny, slow drip from a radiator valve, a hidden pipe joint, or a component inside the boiler itself will eventually cause the system pressure to fall.
Air in the System: If air gets into your radiators, it takes up space that should be filled with water. Since air is much easier to compress than water, its presence effectively lowers the overall pressure in the system.
This simple decision tree helps visualise the two main paths for diagnosing what causes a boiler to lose pressure.
As you can see, the first job is to work out if you're dealing with a leak or just trapped air, as the solution for each is completely different.
In our experience, leaks are a massive issue in UK homes. Industry data actually shows that leaks from radiators, pipework, or boiler components are responsible for up to 60% of all cases where boiler pressure drops again and again. These can be deviously hard to spot, often starting as small damp patches or a faint hissing sound you can barely hear. You can read more about boiler pressure insights to see the stats behind the most common faults.
To help you get started, I’ve put together a quick table summarising the most common causes and what to look for.
Quick Diagnosis for Boiler Pressure Loss
This table should give you a good starting point for your own detective work, but remember that some issues, especially those involving internal boiler components, should always be left to a professional like the team at Harrlie Plumbing and Heating.
How to Find a Leak in Your Heating System
If your boiler pressure keeps dropping, the odds are high that you’ve got a leak somewhere in your central heating system. Sometimes it's obvious – a puddle under a radiator is a dead giveaway. But often, leaks are much sneakier. They can be slow, tiny drips that gradually sap your system's pressure over weeks or even months.
To get to the bottom of it, you need to put on your detective hat. This means giving your entire heating system a careful, methodical once-over. Don’t just scan for puddles; you're hunting for the more subtle signs of moisture.
Your Systematic Leak Inspection Checklist
Before you start, grab a torch and a roll of kitchen paper. The paper is brilliant for this job – just wipe it around joints and valves, and it'll show up even the tiniest bit of dampness instantly. Follow these steps to cover all your bases:
Examine Every Radiator: Start with the radiator valves. Check the main valve where water enters and the lockshield valve on the other side. Give the valve body and the pipe connection a good wipe with your kitchen roll.
Inspect the Pipework: Trace the pipes running to and from each radiator. Pay extra attention to joints, elbows, and connections, as these are common weak points. Keep an eye out for any tell-tale signs of corrosion, like green or rusty stains on the pipework itself.
Check Under the Boiler: Have a good look directly underneath your boiler for any drips. You should never remove the boiler casing yourself, but you can often spot if water is escaping from the pipe connections below.
A recent call-out we had in the Meads area of Eastbourne is a perfect example. The homeowner’s boiler was losing pressure every few days, but there were no visible signs of a leak anywhere. After a thorough investigation, we at Harrlie Plumbing and Heating found a minuscule drip from an old pipe joint hidden under the floorboards. It was so slow the water was evaporating before it could ever form a puddle, but it was enough to cause the constant pressure drops.
Spotting the Hidden Signs of a Leak
Of course, sometimes the leak isn't on an exposed pipe or radiator at all. Water can escape into your walls, ceilings, or under your floors, making it much harder to pinpoint. This is especially true in older properties, which we see a lot of in areas like Old Town Eastbourne, where pipework can be decades old.
Keep your eyes peeled for these tell-tale signs:
Damp Patches: Unexplained damp spots or discolouration on walls, ceilings, or floors.
Peeling Paint or Wallpaper: Moisture seeping through from behind will cause paint to bubble or wallpaper to peel away.
A Musty Smell: A persistent damp, musty odour in a room is a classic sign of a hidden water problem.
Warped Floorboards or Skirting Boards: Over time, wood will swell and warp when it's exposed to moisture.
Finding a leak takes patience, but a methodical approach usually gets results. For a more detailed walkthrough, you can check out our guide on how to find a water leak in your home with easy DIY tips.
If you’ve checked everywhere and still can't find the source of the problem, it’s time to call in the professionals. At Harrlie Plumbing and Heating, we have specialist equipment to trace even the most elusive leaks for our Eastbourne clients without causing unnecessary damage to their homes.
Bleeding Your Radiators to Release Trapped Air
If you’ve thoroughly checked for leaks and come up empty-handed, the next likely culprit behind your boiler losing pressure is trapped air. Think of it like a fizzy drink bottle—the air at the top takes up space that should be filled with liquid. It’s much the same inside your heating system, where pockets of air displace water and cause an overall drop in pressure.
This trapped air is also the reason for those annoying cold spots you often find at the top of your radiators. Since hot water can’t circulate properly, the radiator won’t heat your room as it should. It’s a classic issue we see in Eastbourne homes, particularly in the older Victorian properties around the town centre, but the good news is, it’s often one you can sort out yourself.
How to Safely Bleed Your Radiators
Releasing this trapped air, or "bleeding" the radiators, is a straightforward job that can make a huge difference. All you’ll need is a radiator key and an old cloth or towel to catch any drips.
Here’s our simple, step-by-step process:
Turn Off Your Heating: This is absolutely crucial. If the heating pump is running, it could pull more air into the system. Let the radiators cool down completely before you start.
Find the Bleed Valve: Look for a small square or screw-head valve, usually located at the top corner of your radiator.
Open the Valve Slowly: Place your cloth under the valve to protect your floor. Fit the radiator key onto the valve and turn it anti-clockwise—a quarter to a half turn is usually plenty. You should hear a distinct hissing sound as the trapped air escapes.
Close the Valve: The moment the hissing stops and a steady trickle of water appears, quickly turn the key clockwise to close the valve. Just be careful not to overtighten it.
Go ahead and repeat this process for any other radiators that feel cool at the top. For a more detailed walkthrough, check out our complete homeowner guide on how to bleed radiators.
Important Reminder: After you've bled your radiators, you will have released pressure from the system. It is absolutely essential to go back to your boiler and top up the pressure using the filling loop, aiming for that sweet spot between 1 and 1.5 bar.
Trapped air is one of the most common causes of boiler pressure loss in the UK. In fact, industry data from 2023 showed that around 25% of boiler pressure issues reported to heating engineers were directly linked to air in the system. This often happens after DIY work or if automatic air vents in the system have failed.
If you find yourself having to bleed your radiators frequently, it’s a strong hint that air is getting into your system from somewhere else. This could point to a deeper issue that needs an expert eye from one of our Harrlie Plumbing and Heating engineers.
When the Problem Is Inside the Boiler
So, you’ve meticulously checked your radiators for leaks, bled them until you’re blue in the face, and scoured every inch of visible pipework. If your boiler pressure is still dropping, it’s time to accept the problem probably isn’t somewhere you can see. The issue is likely hiding inside the boiler’s casing itself.
While it’s tempting to want to solve the mystery, this is where any DIY investigation has to stop. Working on a boiler's internal parts is strictly a job for a Gas Safe registered engineer. It’s not just about having the right tools; it’s a critical matter of safety.
The Telltale Sign of a Faulty Pressure Relief Valve
One of the most common culprits inside the boiler is the Pressure Relief Valve, or PRV. Think of it as your heating system's bouncer. If the pressure gets too high, the PRV opens the door and lets some water out to bring things back to a safe level.
You can often spot a faulty PRV without even opening the boiler. Look for a small copper pipe coming out of your external wall, usually near the boiler's location. If you see that pipe constantly dripping or letting out a steady trickle of water, it’s a dead giveaway. The PRV is either broken or something else is wrong, forcing it to keep opening.
Understanding the Expansion Vessel
Another key component is the expansion vessel. This clever bit of kit acts like a shock absorber for your heating system. Water expands when it gets hot, and the expansion vessel has a pocket of air inside to absorb that extra volume, keeping your system pressure nice and stable.
When this vessel fails or loses its air charge, it can't do its job anymore. You’ll see the pressure gauge on your boiler shoot up when the heating kicks on, then plummet right back down as it cools. This wild, rollercoaster-like swing in pressure is a classic symptom of a faulty expansion vessel.
At Harrlie Plumbing and Heating, when we get a call about an erratic pressure gauge in an Eastbourne home, the expansion vessel is one of the first things our engineers check. A faulty vessel forces the PRV to constantly release water, creating a cycle of pressure loss that can be confusing for homeowners.
Fixing either the PRV or the expansion vessel is a job for a qualified pro. An engineer will need to safely shut down and drain the system before they can replace the faulty part. The good news is that these are very common repairs. Our Harrlie Plumbing and Heating team carries a wide stock of PRVs and expansion vessels for major boiler brands like Worcester Bosch, Vaillant, and Ideal. This means we can often diagnose and fix the problem for our Eastbourne clients in a single visit, getting your heating and hot water back on safely and without delay.
A Step-by-Step Guide to Re-Pressurising Your Boiler
So, you’ve successfully bled your radiators or sorted out that tiny leak. You're on the home straight! The last job is to get your boiler's pressure back to the right level so it can happily get back to work.
This is known as re-pressurising, and it’s a straightforward but vital skill every homeowner should have in their back pocket.
Think of it like pumping up a bicycle tyre after fixing a puncture. You’ve let some air out (by bleeding the rads) or sealed a hole, and now you need to top it up to the correct operating pressure. For your boiler, this is done using a component called the filling loop.
Finding and Using Your Filling Loop
The filling loop is usually a silver, braided metal hose lurking underneath your boiler, connecting two pipes. You’ll spot small taps or levers on one or both ends. Don’t panic if yours looks a little different—the principle is always the same.
Before you even think about touching anything, you absolutely must turn off your boiler at the main power switch. This stops the pump from running while you’re making adjustments.
Safety First: Always make sure the boiler is completely powered down before you begin. Adjusting the pressure while the system is active can cause damage or lead to over-pressurisation.
Once the boiler is off, you’re good to go. Just follow these simple steps to get the job done safely and correctly. It’s a task we often guide Eastbourne residents through over the phone, and it’s much less daunting than it sounds.
Locate the Filling Loop and Taps: Find the braided hose and its little levers or taps. Some systems have just one tap; others have two.
Open the Taps Slowly: Give the tap (or taps) a quarter turn to open them up. You should hear the sound of water flowing into the system—that’s mains water topping up your sealed heating circuit.
Watch the Pressure Gauge: Now, keep your eyes glued to the pressure gauge on the front of your boiler. You’re looking for the needle to climb back into the green zone, aiming for a sweet spot of between 1 and 1.5 bar.
Close the Taps Firmly: The second the needle hits your target, immediately and firmly close the taps in the reverse order you opened them. It's critical that they are fully shut to stop the system from taking on too much water.
Turn the Boiler Back On: With the taps securely closed, you can switch the power back on. The boiler should now fire up without flashing any low-pressure warnings at you.
And that’s it! You’ve successfully re-pressurised your boiler. If you feel unsure at any point, or if the pressure drops again soon after, don’t hesitate to call in a professional. The Harrlie Plumbing and Heating team is always on hand to help our neighbours in Eastbourne and the surrounding areas.
Proactive Care to Prevent Future Pressure Drops
Let’s be honest, the best way to fix a boiler that keeps losing pressure is to stop it from happening in the first place. Rather than reacting to a freezing house and a flashing error code, a bit of proactive care will keep your heating system running reliably all year round.
Think of it like the MOT for your car. You wouldn't dream of waiting for the engine to seize up before getting it looked at, and the same logic applies perfectly to your boiler. An annual service is, without a doubt, the single best thing you can do to prevent pressure drops and other nasty surprises.
The Value of an Annual Boiler Service
During a yearly service, a Gas Safe registered engineer does a lot more than just tick a few safety boxes. It's a proper health check for your entire central heating system. When a Harrlie Plumbing and Heating engineer visits a home here in Eastbourne, they're on a mission to spot trouble long before it starts.
This preventative mindset involves several key steps:
Leak Detection: We meticulously inspect all the key components, joints, and seals for those tiny, slow leaks that are so often the real reason a boiler loses pressure over time.
Expansion Vessel Test: We check the charge of the expansion vessel. This little component is crucial for absorbing pressure changes as the water heats and cools, and if it's not right, it puts unnecessary strain on the whole system.
Component Check: Critical parts are cleaned and tested to make sure they’re working as they should, catching wear and tear before it leads to a full-blown failure.
“An annual boiler service isn’t an expense; it’s an investment in reliability. We find that the vast majority of emergency call-outs in the winter to homes across Eastbourne could have been avoided with regular maintenance carried out during the warmer months.”
This small, regular investment saves you from the stress—and the higher cost—of an emergency repair on a bitter winter's day. It also keeps your boiler running at peak efficiency, which can help trim down your energy bills. To see exactly what’s involved, you can learn more about how to service a boiler in our Eastbourne homeowner guide. By catching small issues early, a professional service ensures your heating system is there for you when you need it most.
Your Boiler Pressure Questions, Answered
Got questions about your boiler's pressure? You're not alone. Here are some quick, straightforward answers to the queries our team in Eastbourne hears most often.
How Often Should I Re-Pressurise My Boiler?
In an ideal world, you'd almost never have to. You'll definitely need to re-pressurise the system after you've bled your radiators to get rid of trapped air, but that should be about it.
If you find yourself topping up the pressure every few weeks or even months, that’s a big red flag. This pattern nearly always points to a slow leak somewhere in your heating system or a faulty part inside the boiler itself. It's a clear sign you need a Harrlie Plumbing and Heating engineer to take a look.
Is a Boiler Losing Pressure Dangerous?
Thankfully, low pressure on its own isn't dangerous. Modern boilers are built with a safety cut-out feature, which means the system will simply shut down if the pressure drops too low. This is a clever design that protects it from damaging itself.
The real danger can come from whatever is causing the pressure drop. A hidden water leak, for example, could lead to serious damage to your property over time, which we've seen happen in homes across Eastbourne. That’s why it’s so important to investigate what causes a boiler to lose pressure instead of just topping it up and hoping for the best.
Why Does My Boiler Pressure Drop When The Heating Is On?
This is an interesting one. It's perfectly normal for the pressure to rise a little when your heating is on, simply because heating water makes it expand.
However, if you notice the pressure gauge dropping while the heating is active, it usually signals a more significant leak. The increased pressure and temperature from the hot water can turn a tiny, otherwise unnoticeable leak into a more obvious one, pushing water out of the system much faster. It's a definite cue to call in a professional to track down the source.
If you're still wrestling with a boiler that just won't hold its pressure, don't leave it to guesswork. The Harrlie Plumbing and Heating team is ready to help across Eastbourne and the surrounding areas to find and fix the problem, fast. Contact us today for a free, no-obligation quote.
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