Boiler keeps losing pressure: 7 possible reasons why

If your boiler keeps losing pressure and you keep having to top it up via the the filling loop,. However, if your boiler pressure keeps dropping, your system may have a leak.

Boiler keeps losing pressure – what pressure should my boiler be?

While the correct amount of pressure may vary depending on model and manufacturer, most combi boiler systems require a pressure of 1 to 1.5 bar when cold.

Water expands when it’s heated, and so the pressure rises when the boiler is on. Should the pressure get too high – 3 bar is usually the limit – the water is discharged out of the system via the pressure release valve (PRV).

In a household with a combi boiler, there is no feed and expansion tank in the loft. Instead, the water inside the radiators comes directly from the mains.

However, water regulations don’t allow central heating systems to be directly connected to the mains permanently. That’s where the filling loop comes in. The filling loop is a small section of pipe on or near the boiler which provides a temporary connection to the mains. It may be a rigid pipe or a flexible one, like a shower hose. Opening up the valves on the filling loop allows you to top up your central heating system with water from the mains.

Mains water is usually between 1 – 3 bar, but higher pressures are possible.

How to fix low boiler pressure

Topping up the system with water in order to bring it to the correct pressure is very easy. Simply open the valves on the filling loop until the gauge displays the correct pressure. Be sure to close the filling loop swiftly once you’ve reached it.

Why does my boiler keep losing pressure?

The answer to this question is very simple: water is escaping from the central heating system. Basically, you have a leak. If there is apparently no leak in the home but the boiler pressure still keeps dropping, the culprit may be one of the boiler’s internal components.

If you find that you’re often having to top up the boiler this way, you can’t afford not to find the source of the leak and fix it for good. That’s because the water in your central heating contains (or should contain) inhibitor, which protects your radiators and boiler from internal corrosion. By frequently adding oxygen-rich water to your central heating system, you’ll encourage it to rust from the inside out.

Finding the leak

Check all the visible joints on your central heating pipework and the connections to your radiators. Make sure that the radiator bleed valves are fully closed – you’ll need a radiator key to do this. If the radiators have been recently bled, you may need to top up the system via the filling loop.

The pressure release valve

If there are no visible leaks around the home, the leak may have something to do with the boiler itself. A common culprit is the pressure release valve. A faulty PRV will allow water to exit the system when it shouldn’t.

In the event of high pressure, excess water flows out of the boiler via the PRV and out of the building via the PRV release pipe. Don’t confuse this pipe with the condensate pipe, which is usually white, plastic, and connected to the drain or soil stack.

Check the PRV release pipe. Is it dripping? Dab the inside with a piece of tissue to see if there’s any moisture. If the PRV is leaking very gradually, then the water may be evaporating before you can spot it. Look at the brickwork of the building for any signs of moisture over a long period of time. Signs of water may include residual lime scale and a change in the colour of the bricks.

You can also test to see if the PRV is discharging water by hooking a plastic bag over the end of the pipe, or placing a container underneath it to catch the water. Never block the pipe, as this could be dangerous.

Common pressure release valve faults include:

  • The valve has become defective and will no longer remain closed.
  • The washer inside the valve has perished, usually due to age.
  • The valve has become jammed open due to debris in the system – most likely oxides from internal corrosion.
  • The valve has become jammed open due to a faulty expansion vessel.

If the PRV is faulty, a replacement is probably necessary. If corrosion debris has jammed the PRV open, powerflushing the system could resolve the problem.

Faulty expansion vessel

Another reason why your boiler keeps losing pressure may be the expansion vessel. On a combi boiler, this component is usually inside the boiler’s casing. Its purpose is to accommodate the expansion of the water when it’s heated.

The expansion vessel consists of two internal compartments divided by a rubber diaphragm. One compartment contains water from the central heating; the other contains pressurised air. Air is much more easily compressible than water, and so when the water expands, the air is compressed.

If the air pressure inside the expansion vessel is too low, the system will have no way of accommodating the change in the water’s volume. Consequently, the water will discharge out of the system via the pressure release valve. Eventually, enough water will discharge from the system so that the boiler will report low pressure when cold, and won’t light.

The same problem can happen if the diaphragm in the expansion vessel is faulty. The water will expand, but with nowhere to go, it’ll flow straight out of the PRV. This leaves the boiler without enough pressure to operate.

The expansion vessel usually has a Schrader valve. This is the kind of air valve on bike and car tyres. You can use a tyre pressure gauge to check the pressure in the expansion vessel. If the pressure is too low (the optimum pressure is 1 bar or 15 psi), you can add pressure with a foot pump or a handheld pump. Water coming out of the Schrader valve is a good indication that the diaphragm inside the valve has a puncture. In this case, a replacement expansion vessel is necessary.

If it’s not the boiler…

If there is no evidence that the boiler and any of its components are faulty, then the leak is probably somewhere else in the house. It could be under floorboards or within walls.