Boiler Losing Pressure But No Leaks: Hidden Causes Explained Possible

A boiler losing pressure without any leaks can leave homeowners confused. Normally, a drop in pressure indicates that water is leaking from somewhere within your central heating system. But when no visible leaks are found, this can feel quite frustrating.

If your boiler keeps losing pressure without any leaks, it could be due to a problem with your expansion vessel, a pressure relief valve allowing water out, trapped air, or even small amounts of water that quickly evaporate.

These issues can significantly reduce boiler pressure, making your boiler less efficient at producing hot water. Let’s discuss the common issues which can cause a boiler to lose pressure without any visible leaks.

Can a boiler lose pressure without a leak?

Yes, boilers can lose pressure without any evident leaks. This can happen due to a variety of reasons. If your Boiler pressure keeps dropping without any leaks, it’s most likely due to an internal component leak or fault. Furthermore, there can be hidden pipework issues like corrosion or sludge build-up.

Having prior knowledge of low pressure allows one to act early to prevent costly breakdowns. Even though one-off boiler repair is always an option, a boiler cover provides long-term protection for your heating system year-round. Let’s explore why your boiler is losing pressure without any visible leaks.

Reasons Why Your Boiler Pressure Is Low Without Any Leaks

The following are some of the reasons why this happens:

Fault in Your Pressure Relief Valve

A boiler can lose pressure when its pressure relief valve is worn or malfunctioning. The pressure relief valve is a safety device that releases water when the system’s pressure exceeds a preset limit.

Over time, the valve can wear out, which causes it to release small amounts of water. Because the outlet pipe is outside, these water leaks can go unnoticed, significantly lowering your boiler’s pressure. This is why you are seeing a drop in your boiler’s pressure gauge without any visible leaks.

If you notice this issue, it might be time to call a Gas Safe engineer for a boiler repair.

Solution

A Gas Safe Registered engineer will fix this issue by assessing the discharge pipe connected to the PRV and checking signs of water release. If the valve is leaking or failing to reseal properly, the engineer will simply replace it with a new unit per the boiler’s specifications.

Faulty Expansion Vessel .

Another reason your boiler is losing pressure is a problem with its expansion vessel. The expansion vessel plays a major role in balancing the water pressure of your boiler when it heats up and cools down.

An expansion vessel consists of a resilient membrane and an air chamber that absorbs these changes. If the membrane or air chamber becomes inefficient, the vessel becomes faulty, and the boiler’s pressure rises during heating.

This forces the water out of the safety valve, and the pressure drops too low upon the system cooling. This causes the system to lose pressure, even though no visible leaks are found.

Solution

A gas safe engineer can fix this problem by testing the vessel’s air charge using a pressure gauge. If the charge is low, they will re-pressurise it with air in accordance with the manufacturer’s specifications.

Air in the System or Recently Bled Radiators:

Your boiler can lose pressure due to air trapped in the system or after bleeding your radiators. When air pockets form in pipes or radiators, they disrupt the normal flow of water, reducing your boiler’s efficiency.

Bleeding your radiators releases the air, but it can also lower the overall water density in the system. This causes the pressure gauge to show dropped readings. If there is a lot of air remaining in the system or if the system is fully filled after bleeding, your boiler can lose pressure.

This might seem like a leak, but it’s simply an issue caused by trapped air from recently bled radiators.

Solution

A gas safe engineer can fix this issue by bleeding all radiators and ensuring air pockets are completely removed. After bleeding, the system is re-pressurised to restore the pressure level between 1 and 1.5 bar.

Leaking Schraeder Valve

The Schraeder Valve is a small valve fitted to the expansion vessel of your boiler. It allows an engineer to add or assess the air charge inside the vessel. This helps balance the system’s water pressure as it heats and cools.

If this valve doesn’t seal properly or is worn, it can slowly leak air from the expansion vessel. When this air escapes, the vessel cannot efficiently absorb pressure changes. Therefore, the boiler starts pushing water out of the pressure relief valve whenever the system heats up. Similarly, it drops too low when the boiler cools down.

This process keeps happening, and the consistent water discharge makes it appear the boiler is losing pressure without any obvious leaks.

Solution

The engineer will remove the faulty valve, fit a new one, and refill the vessel with the correct air pressure for proper function. If water comes out of the valve instead of air, it indicates that the internal diaphragm has ruptured, and the entire expansion vessel must be replaced.

Sludge or Corrosion Buildup

A combi boiler can lose pressure without any leaks. Over time, the pipes and radiators in your central heating system can corrode. This results in a muddy substance called sludge. Sludge can be found at the bottom of radiators and pipework as well. This blocks water flow, leading to improper circulation. This makes the boiler work harder, and pressure levels start to drop.

If there is corrosion or sludge buildup in your heating system, then you will notice:

• Radiators are cold at the bottom and warm at the top
• Discoloured water upon bleeding radiators
• Frequent need to fill the boiler pressure
• Boiler makes noises while working

Solution

An engineer can perform a system flush to remove sludge and add an inhibitor to slow further corrosion. Also, regular boiler service helps to keep your system clean and prevents any pressure-related problems.

Faulty Pressure Gauge

There can also be an issue with your boiler’s pressure gauge. It happens when the pressure gauge is clogged with debris or worn over time. It might seem like your boiler is losing pressure, but your system is working fine.

Additionally, a faulty gauge can show pressure dropping, sticking at one reading, or suddenly jumping. As the gauge is the main way to assess boiler pressure, a malfunction can be confusing and lead you to think there’s a leak.

Solution

An engineer can test the actual pressure of your boiler system against the pressure gauge’s readings. If it’s faulty, replacing the gauge is likely the solution to restoring accurate pressure readings.

DIY Fixes

There are a few DIY fixes that homeowners can try when they encounter a boiler pressure loss. The key thing here is that anything beyond basic checks is the work of a Gas Safe Registered Engineer. The following are some basic DIY fixes homeowners can try:

Pressure Gauge: Check if the gauge reading is accurate. If the gauge looks stuck, you will need a replacement. Open it and let water inside the system until the gauge reads between 1-1.5 bar.

Bleed Radiators: If radiators have trapped air, pressure can drop. Use a radiator key to bleed them until water flows steadily. After bleeding is completed, you’ll need to top up the pressure again.

Check for Visible Leaks: Closely inspect radiators, pipes and the boiler for drips or any damp patches. Small leaks can cause pressure loss.

Top Up the Pressure Using a Filling Loop: Most boilers have a filling loop (a small lever or tap), that you can use to top up the pressure of your boiler.

Boiler Pressure Ranges

Here’s a table showing boiler pressure ranges and what they mean:

Note that if the pressure drops below 1.0 bar, the boiler will not function properly, and you’ll need to repressurise it.

Final Words

In conclusion, a boiler can lose pressure even without a visible leak. Whether it’s due to a faulty expansion vessel, trapped air, or sludge buildup, acting promptly can keep your boiler free of major problems. Remember to always contact a Gas Safe engineer if the pressure keeps falling.

Lastly, understanding the causes of pressure loss is important, but knowing how to safely increase boiler pressure ensures safety and efficiency.