Combi Boiler Keeps Losing Pressure

Does the pressure in your Combi boiler or heating system boiler keep dropping? Can't find a leak or there's no visable leak? Here's how to fix it! We'll show you why it happens, and how to fix it. We'll show you how the filling loop works with mains pressure, we'll show you how the expansion vessel works and can fail, how the high pressure relief valve can release pressure and how a lack of inhibitor can cause air and a drop in pressure.

Watch the video below to find out more, we've even captioned it if you're hard of hearing. Enjoy!


If you boiler keeps losing pressure sometimes you will not be able to find a leak. This can be worrying and annoying, especially if the heating is off and you have no hot water or heating. 

Here's a quick guide on what to do.

1. Check that all the radiator valves are not leaking. This might sound obvious but often the leak runs down the copper pipe, obsuring it from view. If the radiator valve is leaking, click here to watch out video on fixing it.

2. Find the high pressure relief pipe. This is usually outside and not somewhere you always look. If this pipe is dripping you have a leaking pressure relief valve. At this point you'll need to get a plumber in. Check our plumber search here to find one. 

3. Sometimes the system is creating air. On most pressurised systems this is expelled automatically from an air vent. This reduces the pressure. This is due to a lack of heating system treatment inhibitor in the water. Get around this problem by adding inhibitor to the heating system. You can learn how to do that by visiting our page here.

4. Sometimes the pump speed can be set too high. If the water isn't getting away from the pump quick enough it can cause cavitation. Resulting in the 'splitting' of the water particles. Since water is part oxygen, cavitation therefore releases this oxygen. In turn the oxygen is released from the air vents and the pressure drops. Get around this by turning down your pump speed and ensuring there are no blockages in your heating system.