Combi Boiler Losing Pressure: A UK Homeowner's Guide
When your combi boiler starts losing pressure, it’s a sure sign that something’s not quite right. Often, the culprit is a hidden leak, a faulty part, or simply air escaping over time. If the pressure drops too low, your boiler will likely shut down completely, leaving you without heating or hot water.
Understanding Why Your Combi Boiler Loses Pressure
Seeing the pressure gauge on your boiler heading south can be worrying, but it’s a very common issue. Your combi boiler is the heart of a sealed central heating system, which needs a specific amount of water to work. The gauge simply shows you how much water is in the circuit.
Pressure isn’t static; it rises slightly when the heating is on and the water expands, then drops as it cools. The real tell-tale sign is what happens when the system is completely cold. If you notice a persistent drop in pressure when the boiler is off, it’s almost always a sign that water is escaping somewhere.
What is the Correct Boiler Pressure?
For most combi boilers, the sweet spot for pressure is between 1.2 and 1.5 bar when the heating is off. You’ll usually see this marked as a green zone on the gauge. Once the heating kicks in, it’s normal to see this climb towards 2.0 bar.
The problem starts when the pressure falls below a certain point, typically around 0.9 bar. Most modern boilers have a safety feature that will automatically shut the system down to prevent damage. You can find more details about boiler pressure and safety on reputable home service sites.
This lockout protects crucial components like the pump from running dry, which could cause serious damage. So, while a combi boiler losing pressure isn't an immediate danger, it needs sorting out to get your heating and hot water back on.
Primary Causes of Pressure Loss
Before fixing the problem, you need to know what’s causing it. The pressure drop usually comes down to one of a few usual suspects:
- A Leak in the System: This is the number one cause. Even a tiny leak in a radiator valve or a hidden pipe can cause a slow but steady pressure drop.
- A Faulty Expansion Vessel: Inside your boiler, an expansion vessel absorbs pressure changes. If it fails, the pressure can become erratic, shooting up when hot and plummeting when cold.
- Bleeding Radiators: If you’ve recently bled your radiators, you’ve also released a bit of water and pressure. This is normal, and the fix is as simple as topping the pressure back up.
Common Causes of Boiler Pressure Loss at a Glance
| Cause of Pressure Loss | Common Symptoms | Initial Action |
|---|---|---|
| Leak in the System | Constant need to re-pressurise, damp patches, visible drips. | Carefully inspect all visible pipework, radiators, and valves for any signs of water. |
| Faulty Expansion Vessel | Pressure gauge spikes very high when heating is on, then drops very low when off. | This usually requires a professional. |
| Faulty Pressure Relief Valve (PRV) | Water dripping from the external copper pipe that leads outside from your boiler. | Check the external pipe for drips, especially after the heating has been on. |
| Recently Bled Radiators | The pressure dropped immediately after you bled one or more radiators. | This is expected. Re-pressurise the system using the boiler's filling loop. |
Understanding these potential issues can give you a head start. While some are simple fixes, others will need a Gas Safe registered engineer.
Diagnosing the Source of the Pressure Drop
When your combi boiler loses pressure, the first job is to play detective. A simple trick used by heating engineers is the "24-hour test" to narrow down the culprits.
Conducting the 24-Hour Test
First, top up the boiler pressure to the green zone – usually around 1.5 bar. If you're not sure how, our guide on how to check water pressure can help.
Once set, note the exact reading. Then, turn the heating system off at the programmer or thermostat and leave it for 24 hours. Do not use your heating during this time. After 24 hours, check the gauge again.
Interpreting the Results
If the pressure has dropped with the heating off, it’s almost certainly a leak somewhere in the system. If the pressure held steady but drops when you use the heating again, the issue is likely a faulty component inside the boiler.
Hunting for Physical Leaks
If the test confirms a pressure drop while the system was cold, it’s time to go on a leak hunt. Look for:
- Damp Patches: Check carpets, floorboards, and ceilings around pipes.
- Visible Drips: Inspect every radiator valve and visible pipe joint.
- Staining: Look for faint green or rusty marks on copper pipes.
Work through your home methodically. Some leaks are subtle, so be thorough.
How to Safely Re-pressurise Your Combi Boiler
If it's just a simple pressure drop, re-pressurising your boiler is a job most homeowners can do. First, switch your boiler off and let it cool down.
Locate the Filling Loop
Find the filling loop. It’s typically a silver, braided metal hose connecting your boiler to the cold water mains, with a valve at each end. On many boilers, it's external and easy to spot underneath. On modern designs, it's often built-in. If you can’t find it, your boiler’s manual will show you where it is. For general tips, see our guide on how to work your boiler.
Gently Open the Valves
Slowly open the valves to let water back into the system. You should hear water flowing. Keep your eyes on the pressure gauge.
Safety First: Always open the valves slowly. A sudden blast of high-pressure water can cause a pressure spike and damage components.
The needle on the gauge will start to climb. Your target is the green zone, around 1.5 bar when the system is cold.
Close the Valves and Check for Leaks
When the gauge hits 1.5 bar, shut the valves immediately. Make sure they're fully closed to avoid over-pressurising the system. Check the filling loop connections for drips. You can now switch your boiler back on.
If you overfill it, don't panic. Bleed the nearest radiator until the gauge drops back into the green. If your boiler is still losing pressure, it’s a sign of a deeper issue, and it's time to call the professionals.
Is a Gradual Pressure Drop Normal?
A very slow, gradual decrease over a long period isn't always a cause for panic. It's common for a sealed system to lose a tiny amount of pressure over time. A drop from 1.5 bar to 1.0 bar over six months can be considered normal and just needs a quick top-up. This can come from undetectable micro-leaks or air settling out of the water, as you can see in discussions among other homeowners.
Gauging the Severity of the Drop
The key is how fast the pressure is dropping. If you only have to top up your boiler once or twice a year, your system is likely healthy. But if re-pressurising becomes a monthly or weekly chore, you're masking a symptom of a larger problem.
A Rule of Thumb: A drop of 0.5 bar over six months can be normal. A drop of 0.2 bar in two weeks signals something needs investigating.
When to Call a Gas Safe Registered Engineer
While topping up pressure is often a simple DIY job, there are times when you need a professional. It's a legal requirement in the UK that any work on a gas appliance must be done by a Gas Safe registered engineer. This is a critical safety measure. You can find more details in guides about LPG gas safety certificates.
Clear Signs You Need a Professional
If you find yourself in any of these situations, it’s time to call an expert:
- The Pressure Drops Immediately: If the gauge is back in the red within an hour of re-pressurising, you have a significant leak.
- You Suspect a Leak but Can't Find It: The leak could be hidden under floorboards or in a wall, where only a professional has the equipment to find it.
- Visible Water Damage Around the Boiler: Drips or rust stains from the boiler casing point to an internal fault. This is strictly off-limits for DIY.
- You're Not Confident: If you feel unsure, it's always better to be safe than sorry.
Don't Take the Risk: A boiler that keeps losing pressure is a symptom of an underlying problem. Ignoring it could lead to a far more expensive and dangerous situation.
If you're facing a stubborn pressure problem, you can book one of our Gas Safe boiler engineers for a fast, reliable, and safe resolution.
Frequently Asked Questions
How often should I top up my boiler's pressure?
In a healthy system, you should only need to top up the pressure once or twice a year, usually after bleeding radiators. If you're doing it monthly or weekly, there’s likely an underlying problem.
Can I use my boiler if the pressure is low?
No. Modern boilers have a safety lockout that kicks in when pressure is too low (around 0.9 bar). Trying to run it can cause serious damage to internal parts like the circulation pump.
What if I put too much pressure in my boiler?
Don't panic. Your boiler's Pressure Relief Valve (PRV) will automatically release excess water to keep the system safe. You may see water dripping from a small copper pipe on your outside wall. To fix it, simply bleed a nearby radiator until the pressure gauge needle drops back into the green zone (around 1.5 bar).
If you're fed up with persistent pressure problems, don't let a small annoyance turn into a major breakdown. The Go Assist team has a nationwide network of fully vetted, Gas Safe registered engineers on standby to diagnose and fix the issue for good. Book a reliable, no-fuss boiler repair online today and get your heating system back to its best.
