Filling loop left open

If the filling loop is left open the pressure on the boiler will go too high. This is a very common problem and happens regularly when the pressure has dropped and the boiler pressure needs to be topped up.

Why do I need a filling loop?

Your boiler has a filling loop on a boiler is used to fill the boiler, radiators and all the heating pipework (known as “the heating system”) with water.

If the water gets out of the heating system the pressure will go down. Your boiler may continue working with no pressure . Others will turn off and display a fault code.

A boiler running with lower pressure than recommended is not good. At best it causes poor performance for the radiators and hot water. At worst it contributes to burnt out components in the boiler.

The pressure in the heating system is topped up using the valves in the filling loop.

Why would the pressure go too high?

After using the valves on the filling loop they should be fully closed, the metal braided filling loop removed and the valves capped.  The majority of people don’t do this so there is always a chance that the valves can be opened accidentally or be faulty.  This can allow too much pressure to develop in the system which may cause the metal pipe near the boiler to overflow.

If you aren’t sure if your filling loop is open or closed click here to see our easy guide.

What to do if the pressure is too high.

The first step is to turn the taps so they are in the off position. A round topped tap should be turned clockwise.  Sometimes this type is very difficult to get fully turned off. Especially if it’s old. You may need to use pump pliers to turn it. If it has a lever, turn it so that is not in line and at 90 degrees with the body of the valve.  When the water is flowing you may hear it in the pipe. Closing the valve will stop the flow so you should hear it stop when the valve is in the correct position.

The filling loop below is open

Open filling loop
Open filling loop

The filling loop below is closed

closed filling loop
Closed filling loop

It doesn’t need to be as pronounced as this to cause the pressure to go up, even slightly open is enough.

Many people assume the boiler is going to explode if the pressure gets too high. However there is a safety valve in the boiler so if the boiler is installed properly this can’t happen so it’s nothing to worry about.

To fix this you need to turn both valves (or one if you only have one) off. This should make the metal pipe outside stop overflowing.

The metal overflow pipe should only overflow if the pressure is above 3. So when the pressure goes back down below 3 the water should stop.

If it doesn’t then you might need a new safety valve as they are notorious for not resealing after they have opened. This is a job for a gas safe engineer not a DIY job.

Reducing the pressure

The next step is to reduce the pressure in the system. This can be done by bleeding a radiator or opening a drain valve.

The safest (and slowest) is to bleed a radiator. Place a tray under the radiator and open the bleed point. A slow trickle of water will come out.

Leave this trickle of water gently running into the tray until the pressure drops. This will take a long time but is the least complicated and risky method.

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