Boiler Pressure Keeps Dropping? A Step-by-Step Guide to Repressurising (And When to Call an Engineer)

Boiler Pressure Keeps Dropping?

It’s one of the most common boiler faults we see. You notice the radiators aren’t getting quite as hot as they used to, or you wake up to no heating at all. You glance at the little dial on the front of your boiler, and there it is: the pressure gauge needle is in the red, well below the recommended 1 bar.

Don’t panic. In most cases, this is something you can safely fix yourself in about five minutes. A modern combi boiler is a sealed system, and over time, it can lose a tiny amount of pressure naturally. Topping it up is a routine maintenance task.

This guide will walk you through exactly how to do it safely. But we’ll also cover the crucial follow-up question: what does it mean if it keeps happening?

First, Understand Boiler Pressure: The Goldilocks Zone

Think of the water in your heating system like the air in a car tyre. It needs to be at the right pressure to work properly.

• Too Low (below 1 bar): The boiler’s safety sensors will detect there isn’t enough water to circulate safely and will shut the system down to prevent damage. This is the most common reason for a low-pressure fault.
• Too High (above 2.5 bar): This puts unnecessary strain on the system’s components and can cause water to leak from the pressure relief valve.

The “Goldilocks Zone” is right in the middle, usually marked as a green section on the gauge, between 1 and 1.5 bar when the system is cold.

The 5-Step Guide to Safely Repressurising Your Boiler

Ready? Let’s get this sorted. You’re looking for something called the “filling loop.” It’s usually a silver, braided hose with one or two small taps or levers connecting your boiler to your mains water pipework.

Step 1: Turn Off the Boiler This is a crucial first step. Turn the power to the boiler off at the main switch or the fused spur on the wall. The system should be cool and inactive before you start.

Step 2: Locate the Filling Loop Look underneath your boiler. You should see a network of copper pipes. The filling loop is the only flexible, braided hose. It will have a small tap or lever at each end. On some very modern boilers, the filling loop is built-in, and you’ll just have one or two levers to operate.

Step 3: Open the Valves Slowly You need to open the valves to let mains water into the sealed heating system. Sometimes you need to turn both, sometimes just one.

• For levers: They are open when they are in line with the pipe. Turn them slowly a quarter-turn until they are aligned.
• For taps: Turn them anti-clockwise, just like a normal tap. As soon as you open them, you should hear the sound of water flowing into the system.

Step 4: Watch the Gauge! This is the most important part. Keep your eyes fixed on the pressure gauge on the front of the boiler. You will see the needle start to rise. Let it climb steadily until it reaches the middle of the green zone, around 1.5 bar.

Step 5: Close the Valves and Reset As soon as the needle hits 1.5 bar, immediately and firmly close the valves by turning them back to their original position. It’s vital to close them tightly to prevent the system from over-pressurising. If you have an external filling loop, it’s good practice to disconnect it at one end to ensure it cannot leak.

Now, you can turn the power back on to your boiler. It will often need to be reset. And that’s it! Your heating should fire back up.

The Crucial Question: “Why Does It Keep Happening?”

Topping up your boiler once or twice a year is normal. Topping it up every few weeks is not.

If you find the pressure is constantly dropping, it’s a clear sign that water is escaping from the “sealed” system somewhere. In other words, you have a leak.

It might not be a big, visible puddle. It could be a tiny pinhole leak from a radiator valve, a joint under the floorboards, or from the boiler itself. While small, it’s enough to cause the pressure to drop and will only get worse over time, potentially causing water damage.

This is when you must call an engineer. Finding a hidden leak in a central heating system is a job for a professional. We have the tools and experience to trace the source of the pressure loss and fix it properly.

So, feel empowered to top up your system. But be smart enough to know when it’s a symptom of a bigger problem.

If your boiler pressure keeps dropping, or if you’re not comfortable carrying out the steps above, give the experts at Boiler Repairs R US a call. We’ll diagnose the underlying issue and get your system back to being properly sealed, safe, and reliable.

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