Combi boiler pressure explained & how to fix it

The little pressure gauge on the front of your boiler may be small, but the information it tells you is the difference between your boiler working, or having no hot water or central heating. In this article, we’ll discuss exactly what combi boiler pressure means, and what can be done to fix it if your boiler has no pressure.

Combi boiler pressure explained

In order for a combi boiler to operate, the water in the central heating must be pressurised. While the exact amount of pressure required for a boiler may vary depending on model and manufacturer, most combi boiler systems need to be filled to a pressure of 1 to 1.5 bar when they are 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 will be 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 is supplied directly from the mains. However, water regulations do not 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 – which is normally at a pressure of 1 – 3 bar. Higher pressures may be reported.

How to fix low boiler pressure

Topping up the system with water in order to bring it to the correct pressure is a very easy task. 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 have low pressure?

The answer to this question is very simple: water is escaping from the central heating system. To put it simply, you have a leak. In some cases, it may be somewhere around the house; in others, the culprit is one of the boiler’s components.

If you find that you’re having to top up the boiler pressure often, then you can’t afford not to find the source of the leak and get it fixed. 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 check the connections to your radiators to make sure that they aren’t weeping. Make sure that the radiator bleed valves are fully closed – you will need a radiator key to do this. If the radiators have been recently bled, the system may need topping up from the filling loop.

The pressure release valve

If there are no visible leaks around the home, then 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 is discharged through the PRV and out of the building via the PRV release pipe. This pipe can be identified outside. It should not be confused 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? Check the inside with a piece of tissue to see if it is wet. 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 to see if it has been wet over a continued 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, it will need to be replaced. If the PRV has been jammed open due to corrosion debris, the system will need to be powerflushed.

Faulty expansion vessel

Another component which may cause your boiler to lose pressure is the expansion vessel. On a combi boiler, this component is usually fitted 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 broken. The water will expand with nowhere to go, be discharged out of the PRV, and the boiler won’t have enough pressure to operate.

The expansion vessel is usually fitted with a Schrader valve. This is the kind of valve that’s found on car and bike tyres. A tyre pressure gauge can be used to check the pressure in the expansion vessel. If the pressure is too low (the optimum pressure is 1 bar or 15 psi), a foot pump or handheld pump can be used to add pressure. Water coming out of the Schrader valve is a good indication that the diaphragm inside the valve has a puncture. In this case, the expansion vessel will need to be replaced.

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 likely to be concealed, e.g., if pipes run under floorboards or within walls.

 

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