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Why is my boiler losing pressure?

Someone pointing at a Pressure Gauge
Pressure Gauge

We thought we would put together this blog as we are asked a lot about boilers losing pressure. Whether it’s a leak in the system or a faulty pressure relief valve, there are a number of potential reasons why your boiler pressure keeps dropping. We’ll look at the most common reasons your boiler keeps losing pressure, how to troubleshoot the issue, and when it’s best to seek professional help. And of course, if you’re unsure of anything we’ve outlined below, don’t hesitate to get in touch with us today. Our experienced Gas Safe engineers are available and can sort your boiler issues out.

Reasons Why Your Boiler Is Losing Pressure

1) Leaks

The majority of the time, pressure loss is due to a leak in the boiler itself or somewhere in the heating system. Unless the dial sits at 0 even when topped up (which could indicate a faulty pressure gauge), it’s likely a leak is the culprit — even if you can’t pinpoint it! Take a look around your home for signs of leakage, such as stains or damp patches on your ceiling, around pipes, radiators and the boiler. Joints in the pipework are a common place to find leaks, along with the points where the pipes connect to the radiator. Just be careful not to mistake condensation for a leak!

Also, bear in mind: even if you can’t see any evidence of water leaking from your boiler, it may still be happening in a spot that you can’t access, like under the floor.

2) Bleeding A Radiator

Aside from a leak, another common reason you’re experiencing reduced system pressure could be the result of a bleeding radiator. This is especially true if you’ve recently had some work done to your central heating system, or if you’ve been dealing with radiators that are not heating properly. Bleeding a radiator allows the air that has collected in the system to escape — and it may also cause a drop in your boiler pressure. Now, if you haven’t bled any radiators recently, and you can’t locate any signs of a water leak, then the problem may be within the boiler itself. If that’s the case, we’d strongly recommend calling in the experts!

How to Fix Low Boiler Pressure

The most effective way to top up the pressure on your boiler is by re-pressurising the system. Depending on the age and type of boiler, you may be able to re-pressurise it yourself, but you should always check the user manual first to ensure your model allows this.

If the manual gives you the go-ahead, follow these steps to re-pressurise your boiler:

  1. Switch off your boiler. Hit the main power switch and allow your boiler to cool.

  2. Find your filling loop. Most new combi boilers will have an external filling loop that’s used to top up the boiler with water and increase the pressure. It looks like a braided hose or flexible metal pipe with a valve at either end, and it’s typically located below the boiler casing.

  3. Check to see whether the filling loop hoses are securely attached. Tighten the hoses if they’re loose (you may need a wrench to tighten them fully).

  4. Use a screwdriver to open the filling valves. The filling valves are located next to where the hoses connect to the boiler. As soon as you open the valves, you should begin to hear the water flowing.

  5. Close the filling valves when the pressure gauge reads 1.5 bar. The arm on the pressure gauge should start to move as cold water enters the boiler. When it hits 1.5 bar, turn the valves into the off position and make sure the filling loop is completely closed. (Most boilers will operate around 1.5 bar, but you should refer to your owner’s manual for the exact pressure required.)

  6. Switch the boiler back on. If your boiler maintains the correct pressure, you can rest assured knowing everything is back in order.

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