Why does my boiler keep losing pressure?

If we had £1 for everytime we’d heard the question, “why does my boiler keep losing pressure?”, we’d have at least £247 by now!

The fact is, this is a common problem with all  poorly maintained sealed systems.

Firstly, if we explain what a sealed system is, you’ll find it much easier to understand why your boiler keeps losing pressure.

boiler pressure gauge

The term “sealed system” refers to the primary heating circuit being fed directly from the cold main via a filling loop (rather than conventional feed and expansion tanks in the loft). In most instances the filling loop is manually operated, filling the system to a specific pressure (1.5 bar ).

Once the system has been filled to 1.5 bar, any water which escapes from the system will reduce the overall  system pressure. Once the pressure in the boiler drops below 0.75 bar, a safety switch will prevent the boiler from firing, leaving you without heating and hot water.

Why does my boiler keep losing pressure?

Now let’s look at the 3 main culprits which cause the pressure in your boiler to keep dropping.

1. Leaks
2. Burst and/or discharged expansion vessel
3. Faulty pressure relief/safety valve

Leaks

This section is rather self explanatory. A leak can occur anywhere on your central heating circuit, which can make it difficult to identify the source of the leak.

Time should be taken to inspect as much of the system as practically possible, this may involve getting down on your hands and knees and feeling around your radiator valves and pipework, other places to look, are your cylinder cupboard and underneath your boiler. Run your fingers around pipe joints and connections, regularly checking your fingers to see if they are wet. You’ll be surprised how a relatively small leak can affect the system pressure over a matter of days or weeks.

Ensure that the central heating is switched off for at least an hour before commencing to ensure the pipework is not hot.

Once a leak has been identified, and you feel confident enough to do it yourself, the leak can be repaired. If you like to stay as far away from plumbing as humanly possible, just contact our friendly team on 0114 345 0014.

Burst and/or discharged expansion vessel

As the water in the primary heating circuit is heated, the water itself will expand, increasing the overall system pressure. The expansion vessel is designed to accommodate this increase in pressure.

The vessel itself is very simple in design, it’s basically a metal tank which houses a balloon full air (which is precharged to 1 bar). This balloon will slowly lose its charge over time, this is normal and is easily rectified during an annual boiler service .

When the balloon loses its charge, it is no longer able to accommodate the expansion of the system water. The system pressure will continue to increase until it reaches 3 bar, at this point the pressure relief/safety valve will open, allowing a small amount of system water to escape. When the heating system cools the pressure will be slightly less than it was originally. This process will be repeated until the pressure drops below 0.75 bar, at which point the boiler will shut down.

If the “balloon” is allowed  to lose all of its charge, this can cause the balloon to burst, resulting in costly repairs.

Expansion vessel being recharged

Faulty pressure relief/safety valve

This is a direct consequence of the over pressurisation described above.

When the pressure reaches 3 bar, the safety valve automatically opens, however, in most cases the safety valve will be unable to close properly due to system debris becoming lodged on the valve seating. This will cause the valve to persistently weep/leak resulting in a drop in system pressure.

This problem is not rectifiable by simply servicing the boiler, the valve will need replacing.

Why does my boiler keep losing pressure?

So there you have it, the three most common reasons why your boiler keeps losing pressure.

Below is a helpful video which explains how to repressurise your Worcester boiler using the integral filling key:

We hope you found this article interesting and most of all, helpful.

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If the pressure in your boiler keeps dropping, don’t put up with it,

contact Love Your Boiler and get the problem solved.

0114 345 0014