Why does my boiler keep losing pressure? A step-by-step diagnostic guide for 2025

A boiler that keeps losing pressure is almost always caused by one of two issues: a water leak somewhere in the central heating system or a fault with an internal boiler component, most commonly the expansion vessel. The challenge for a homeowner is to distinguish between an obvious, visible leak and a ‘phantom’ pressure drop caused by a hidden fault, which requires a more systematic diagnostic approach.

That sinking feeling: Why boiler pressure matters

There are few household sights more frustrating than the little needle on your boiler’s pressure gauge dipping into the red. A sudden drop often means no heating and no hot water, a situation that always seems to happen on the coldest day of the year. Your central heating system is a sealed loop of water. When your boiler heats this water, it expands, and the pressure in the system rises. When it cools, the pressure falls. This is why to function safely and efficiently, boilers must operate within a specified gauge, typically between 0.7 and 2.5 bar. For most modern boilers, the sweet spot when the system is cold is between 1.0 and 1.5 bar.

If the pressure drops too low—usually below 0.5 bar—a safety sensor will prevent the boiler from firing up. This is known as a boiler lockout, often accompanied by an F1 fault code or similar error. While this is a nuisance, it’s a crucial safety feature. The real question is, where has that pressure gone? Answering this is the key to a lasting fix, not just a temporary top-up.

The first step: Your 10-minute leak hunt

Before you suspect a complex internal fault, you must rule out the most common culprit: a simple water leak. Many homeowners frustrated by a “boiler losing pressure but no leak” have simply missed a very small, slow drip. This systematic checklist will help you perform a thorough inspection in just a few minutes.

A systematic checklist for visible leaks:

Inspect every radiator: Go to each radiator in your home. Check the body of the radiator for any signs of corrosion or weeping. Pay close attention to the valves at each end—the thermostatic radiator valve (TRV) you turn to adjust the heat, and the lockshield valve on the other side. Look for active drips, water stains on the floor beneath, or tell-tale signs of rust. Finding a leak in a radiator valve is a very common cause of pressure loss.
Trace the pipework: Follow all the visible pipework running to and from your radiators. Check every joint, elbow, and connection point. A slow leak might not create a puddle; instead, it can leave behind a chalky white deposit (limescale) or a green stain on copper pipes.
Check the boiler itself: Take a look underneath your appliance. Is there any evidence of water dripping from the boiler underneath? Sometimes an internal part, like a diverter valve or a heat exchanger seal, can fail and cause a leak within the casing. Also, check the small boiler auto air vent, which can sometimes weep slightly.
Revisit bleed valves: If you’ve recently bled your radiators, double-check that every bleed valve (the small square nipple, usually at the top corner) is fully tightened. It’s easy to leave one slightly open, which will cause a slow and steady pressure drop. This is a key reason why a boiler pressure drops after bleeding radiators.

If you find a leak, the solution depends on your confidence. A small weep from a valve’s compression nut might be fixed with a gentle quarter-turn from a spanner. However, if the leak is from the valve body itself or you’re in any doubt, it’s time to call a professional.

No visible leak? Welcome to the world of ‘phantom’ pressure drops

This is where the real detective work begins. If you’ve done a thorough leak hunt and found nothing, you’re likely dealing with a ‘phantom’ leak—a situation where the system is losing water, but in a way that’s not immediately obvious. There are two primary suspects.

The secretive culprit: The pressure relief valve (PRV)

The PRV is a critical safety component. If the pressure inside your boiler gets too high, this valve opens to release water and bring the pressure back down to a safe level. It typically discharges this water through a copper pipe that runs from the boiler through an external wall to the outside.

The problem is that these valves can fail. A small piece of debris can get stuck in the valve seat, or the spring mechanism can weaken over time. This causes the valve to pass a small but constant trickle of water. Because this water is being expelled outside, you’ll never see a puddle indoors. This is a classic cause for a boiler losing pressure daily.

How to check: Find the small copper pipe on your outside wall near the boiler. Is the pipe’s end dripping? Is the brickwork below it damp or stained? If so, you’ve likely found your culprit. A faulty PRV is a job for a Gas Safe engineer, as replacing it requires draining the system. The typical cost to replace a pressure relief valve can vary, but it’s a routine repair for a professional.

The hidden menace: Pinhole leaks in concealed pipework

Another phantom cause is a micro leak in central heating pipework under the floor or buried in a wall. These can be incredibly difficult to find. A pinhole-sized perforation in a pipe might only release a tiny amount of water. If this water drips onto a warm pipe, it often evaporates before it has a chance to pool or cause a visible damp patch on a ceiling. This is one of the most frustrating reasons for a central heating system losing pressure, as you’re left with the symptom but no obvious cause.

Signs of a hidden leak: Listen for unusual noises, check for floorboards that feel spongy or have started to creak, look for unexplained low-level damp on walls, or notice a musty smell. Finding a hidden water leak in a central heating system often requires specialist equipment, so this is definitely a job for an experienced heating engineer.

The leak mimic: Understanding the expansion vessel’s role

So, can a boiler lose pressure without a leak? Absolutely. This is where the expansion vessel comes in. Think of it as the heating system’s lung. It’s a small tank inside the boiler (or sometimes located nearby) that contains a rubber diaphragm. On one side of the diaphragm is your system water; on the other is a pressurised pocket of air or nitrogen.

When the heating comes on, the water expands. The expansion vessel’s air cushion absorbs this expansion, keeping the system pressure stable. When the vessel fails, one of two things has usually happened:

It loses its charge: The air can slowly leak out over years, just like a car tyre. Without the air cushion, there’s nowhere for the expanding water to go.
The diaphragm ruptures: The rubber diaphragm splits, allowing water to fill the entire vessel.

In either case, the vessel can no longer do its job. The result? When your heating turns on, the unmanaged expansion causes the pressure to shoot up rapidly. If it exceeds 3 bar, the PRV will open and dump water outside. Then, when the heating turns off and the system cools, the pressure plummets, often dropping to zero. This directly explains why your boiler pressure drops when radiators get hot and then falls dramatically when the heating is off. It’s the ultimate ‘phantom’ issue because the system only loses water when it’s actively over-pressurising. This is a very common cause of a faulty expansion vessel boiler pressure problem.

Comparing the culprits: Visible leaks vs. internal faults

Understanding the different characteristics of these problems can help you communicate effectively with an engineer. Here’s a side-by-side comparison.

Feature	Visible Leaks (e.g., from a radiator valve)	Internal Faults (e.g., expansion vessel or PRV)
Visible Evidence?	Yes. You can usually find a damp patch, drips, or rust stains with a careful inspection.	No (or not easily). Water is lost outside via the PRV pipe or evaporates from a hidden micro leak.
Speed of Pressure Drop	Typically slow and steady. You might find your boiler losing pressure slowly over a week or more.	Often rapid and linked to the heating cycle. The pressure might be fine when cold but drops to zero overnight after a heating cycle.
DIY Fixable?	Sometimes. A loose compression fitting can sometimes be gently tightened by a confident homeowner.	No. These issues involve opening the boiler casing or sealed system and must be diagnosed and fixed by a Gas Safe registered engineer.
Typical Symptoms	Puddles, damp patches, the sound of dripping water.	Boiler pressure drops when heating is on, water coming from boiler overflow pipe, boiler lockout, needing to top up boiler pressure too often.

Your diagnostic flowchart: What to do next

Feeling overwhelmed? Follow this step-by-step decision tree to determine your next move.

Check the Pressure Gauge: Is the reading below the 1.0 bar mark? If so, you need to take action. As we’ve established, a boiler pressure reading below 1 bar requires attention.
Perform the 10-Minute Leak Hunt: Systematically follow the checklist above. Did you find a visible leak?
- YES: If it’s a simple loose joint you’re confident in tightening, do so. If not, or if the leak persists, call a Gas Safe engineer.
- NO: Proceed to the next step.
Repressurise the System: You will need to top up the boiler pressure. Use the boiler’s filling loop (usually a silver braided hose with one or two small taps) to slowly introduce mains water until the gauge reads 1.5 bar. Be careful not to over-pressurise. Once done, ensure the filling loop taps are fully closed and the hose is disconnected if required.
Monitor the Pressure: Now, observe. The speed of the pressure drop is your biggest clue.
- If it drops within a day: You have a significant issue. Check the PRV pipe outside for drips. If it’s wet, suspect the PRV or expansion vessel. If it’s dry, you may have a substantial hidden leak. In either case, it’s time to call an engineer for an emergency boiler repair due to low pressure.
- If it drops over a week: This points to a smaller, slower issue like a micro leak or a weeping valve. While not an emergency, it won’t fix itself and needs professional attention. Continuously topping up adds fresh, oxygenated water to the system, which can accelerate corrosion.
- If it fluctuates wildly with heating: High pressure when hot, very low when cold. This is the classic symptom of a faulty expansion vessel. Call an engineer and explain the symptoms clearly.

Remember that leakages and boiler faults are a common cause of low boiler pressure, so following a logical process is the best way to find the root cause.

Making the right choice for your needs

The right course of action depends on your level of confidence and the symptoms you’re seeing. Here’s tailored advice for different types of homeowners.

For the DIY troubleshooter:

You’re comfortable with basic checks and want to avoid an unnecessary call-out. Your first port of call is the 10-Minute Leak Hunt. Be methodical. Check every single radiator, valve, and visible pipe. If you find a tiny weep from a compression fitting that you can fix with a spanner, do so, then repressurise and monitor. If the pressure now holds steady for a week, you’ve successfully solved it. If it continues to drop, or if you find no leak at all, you’ve done your due diligence. Now you can call an engineer and confidently tell them, “I have a boiler losing pressure with no visible leak; I’ve checked all radiators and pipework.” This saves them time and points them towards an internal fault.

For the cautious homeowner:

Your priority is safety, not diagnostics. Your job is simple: spot the warning signs. Look at the pressure gauge on your boiler. Is it in the red zone (below 1 bar)? Look directly underneath the boiler casing. Do you see any water, even a small drip? If the answer to either of these questions is yes, your next and only step should be to call a qualified Gas Safe engineer. Don’t worry about the ‘why’. You have identified a problem that requires a professional. Describing the symptom (“the pressure is low” or “there’s water under the boiler”) is all the information they need to get started.

For the ‘phantom leak’ sufferer:

You’re beyond the basics. You’ve already checked for obvious leaks multiple times and are frustrated that you have to keep topping up your boiler. Your focus must now shift to the behaviour of the pressure. Repressurise the system to 1.5 bar. Now, watch it like a hawk. Does the pressure shoot up to 3 bar when the heating runs, followed by a drip from the external PRV pipe? This screams ‘expansion vessel fault’. Does it just drop slowly and steadily, day after day, with no other symptoms? This points towards a hidden leak or a weeping PRV. Providing this detailed, behavioural information to your engineer is invaluable. It transforms your call from “my boiler is losing pressure” to “my boiler pressure is fluctuating significantly with the heating cycle, I suspect an expansion vessel issue,” helping them arrive with the right parts and diagnose the problem far more efficiently.

Understanding the difference between a simple, visible leak and a more complex internal fault is the key to resolving boiler pressure problems effectively. By following a structured approach, you can either solve the problem yourself or provide an engineer with the high-quality information they need to fix it fast. At BestHeating, we’re dedicated to empowering you with the knowledge to understand your heating system. With decades of experience helping customers nationwide, we know that a persistent pressure drop is a problem that needs a reliable solution.

For a more detailed assessment of your system or to find a trusted Gas Safe engineer in your area, contact our expert team today.