Why Is My Boiler Losing Pressure? Common Causes and Quick Fixes

If your boiler pressure keeps dropping, you're not alone — it's one of the most common issues UK homeowners face, especially during colder months. But what does it actually mean, and is it a cause for concern? Let's break down the main reasons your boiler might be losing pressure, and what you can do about it.

Boiler pressure refers to the balance of water and air in your heating system. It needs to stay within a specific range — usually between 1 and 1.5 bar — for your boiler to work efficiently. Too low, and your radiators may stop heating properly. Too high, and the system can become dangerous.

1. Water Leak in the System

A leak anywhere in your central heating — whether from pipework, radiators, or the boiler itself — will reduce the pressure. Look out for damp patches, corrosion, or dripping valves.

2. Bleeding Radiators Recently?

Letting air out of your radiators can lower system pressure. It’s a normal side effect, but you’ll need to top up the pressure via the boiler’s filling loop.

3. Faulty Pressure Relief Valve

If your boiler’s pressure rises too high, the safety valve releases excess water — which can cause pressure to drop too low afterwards. This valve may be faulty or sticking.

4. Expansion Vessel Failure

The expansion vessel absorbs excess pressure when the system heats up. If it fails, pressure fluctuations become extreme, leading to pressure loss over time.

5. Air in the System

Trapped air can circulate through the pipes, disrupting pressure and leading to uneven heating or banging sounds (known as “kettling”).

If it’s a one-off drop in pressure, you can usually fix it by following these steps:

1. Locate the filling loop (a small silver hose beneath your boiler).
2. Open the valves slowly until the pressure gauge hits 1–1.5 bar.
3. Close the valves tightly.
4. Reset the boiler if necessary.

⚠️ Important: If the pressure drops again shortly after, don’t keep topping it up — you likely have a leak or a faulty component. Call in a qualified engineer.

Low boiler pressure isn’t dangerous in itself, but it will stop your heating and hot water from working properly. What’s more concerning is the underlying cause, which could lead to water damage, gas leaks, or carbon monoxide exposure if left unresolved.

If your pressure keeps dropping or you suspect a hidden leak, it’s best to get a certified Gas Safe engineer to take a look. At Pleasant Plumbers, we provide fast, affordable boiler diagnostics across London — whether you're in a flat, a terraced house, or managing multiple commercial properties.

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