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How to Repressurise a Boiler?

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The last thing you would want happening to you in winter is finding your boiler incapable of heating the house or supplying hot water due to low pressure.

It would be best if you did not panic, though. As with most boiler problems, you can easily solve low boiler pressure without involving a certified gas engineer. All you have to do is repressurise it, and it will return to its peak operating conditions.

There are conventionally two approaches to repressurise a boiler on your own without having to call a tradesman. The first method uses the filing key to add water to the boiler, while the other entails opening the water-filling valves.

In this guide, we are going to show you the right steps to follow in each approach, and with little luck, you will be able to restore your boiler’s pressure and get it working again.

How Can I Repressurise My Boiler Using a Filling Key?

Turn off the boiler

Before attempting to repressurise your boiler, ensure you turn off the power to the heating system. After turning it off, allow it to cool for about 4 to 6 hours to avoid damaging the system or scalding yourself with hot water when you are repressurizing.

Pull out the tray beneath the boiler and remove the filling key

Both old and new boiler models have a tray mounted on their lower side. This tray is primarily metallic in old boiler models but can be plastic on newer models.

Upon pulling out the tray, you will notice a plastic or metallic key attached to the tray by some clip. Gently remove the key from this clip, ensuring it sustains no damage.

Insert the filling key into the manifold keyhole and turn it to the unlocked position

Trace the manifold keyhole, which is often located next to the manifold nut. Ideally, if your boiler is an old model having a metallic tray and a metallic filling key, then the manifold keyhole will also be metallic.

On the other hand, if your boiler is a fairly modern one, the tray, filling key and manifold keyhole will be plastic.

Insert the key into the manifold, ensuring that the arrows on the key are pointing towards the manifold. The manifold usually has a padlock, lock or another symbol that indicates the closed position. Twist the key at a 45 degrees angle away from this symbol to unlock the manifold.

Rotate the manifold nut in an anticlockwise direction.

Once the manifold is unlocked, pick up a spanner and turn the nut next to the manifold keyhole gently and slowly in an anticlockwise direction. Upon completing a half-turn, the nut should be sufficiently loose, and you will hear water entering the boiler system. The boiler pressure should then begin to rise.

Wait for the boiler pressure to reach 1.5 bars and tighten the manifold nut.

As the water continues to enter the boiler, your boiler’s pressure should continually rise. This rise in pressure will be indicated by the arm on the boiler gauge slowly moving up. Since the recommended pressure for most boilers is around 1.5 bars, you should not exceed it to avoid over pressurising your boiler.

Understandably, different boiler models have different recommended ideal pressure. Therefore, refer to your owner’s manual to get the exact boiler pressure that you should stop repressurising.

Once your boiler attains the recommended ideal pressure, turn the manifold nut in a clockwise direction until the water stops entering the boiler. Once done, check to ensure the pressure has stabilized at this value.

Bleed the nearest radiator in case of too much pressure.

It is no guarantee that you will manage to close the manifold nut when the pressure is at exactly the recommended boiler pressure. If you over pressurise the boiler and the arrow on the pressure gauge indicates a value in the red zone or somewhere close, there is no need to panic.

You can release the excess pressure in the system by bleeding a nearby radiator. To achieve this, turn the release knob on the radiator slowly while ensuring you are standing clear of the release valve. The logic behind this is simple, the radiator may release hot pressurised air, and you wouldn’t want to be caught by its trajectory.

Bleeding the radiator will lower the pressure in your boiler within seconds. When the boiler pressure nears 1.5 bars or any other recommended value, slowly turn off the release valve to avoid releasing too much pressure.

Lock the manifold.

Gently turn the manifold key back to the locked position. Locking the manifold will prevent someone else from accidentally turning the manifold nut and increasing the amount of water in the boiler.

Pull out the filling key and return it to the tray

Remove the key from the manifold keyhole by gently pulling it out.  Once done, place it back into the tray and slide the tray back to its original position beneath the boiler.

Normally, when you remove the filling key, a few drops of water may drip down the manifold keyhole. However, if the manifold nut is not closed or tightened properly, the dripping will not stop. If this is the case, you may want to tighten the manifold nut to prevent more water from entering the boiler.

As a rule of thumb, if the manifold key gets stuck when you are trying to remove it, do not forcefully remove it as you may damage the boiler.  When this happens, you should call a certified boiler tradesman right away.

Turn the boiler back on.

Once the repressurising is complete, hit the main power switch and power the boiler back on. You will notice that the pressure fluctuates for a little bit before stabilising, but that is normal. However, if your boiler starts to depressurise again, you may want to call a certified boiler tradesman. 

How Can I Repressurise My Boiler by Adding Water Through the Water Filling Valve?

Switch off the boiler

Power down your boiler and wait for 4 to 6 hours, or if you have the time, even longer for the system to cool down completely.

Locate the water-filling valves and open them

The water filling valves are normally located at the connection between the hoses and the boiler. Using a screwdriver, turn the screw heads on the valves in a counterclockwise direction to open them.  Opening the water-filling valves will allow water to enter the boiler, and you should begin to hear the water flowing in.

If the valves are difficult to turn, do not force them as this may damage them or strip the screw heads. Instead, spray them with WD 40 and allow them to soak for a few minutes before attempting to open them again.

Close the filling valves when the pressure reaches 1.5 bars

As the cold water continues to enter the boiler, the boiler pressure will steadily increase. This increase in pressure will result in the needle on the pressure gauge steadily rising from the zero mark.

When the pressure reaches the recommended optimum value for your boiler, which is ideally 1.5 bars, turn the valves in an anticlockwise direction to close them.

Closing the water filling valves will cut off the water flow into the boiler, and your boiler’s pressure will stabilise after around 30 seconds.

Bleed excess pressure and fire on the boiler

If your repressurising efforts resulted in too much pressure in your boiler, you might consider bleeding some radiators to bring down the pressure to recommended levels. Once this is done, and the pressure has stabilised around 1.5 bars, power the boiler back on.

Your boiler should now work properly, but it is advisable to turn it off and contact a certified boiler tradesman immediately if it starts to lose pressure again.

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