How to Repressurise a Boiler [Without Paying an Engineer or Boiler Expert]

Welcome to our 5-minute guide on how to repressurise a boiler. Here, we’ll explain how to top up your boiler pressure, and more importantly, why your boiler is having pressure problems in the first place.

How to repressurise a boiler

Key Steps on How to Repressurise a Boiler

Whether you’re wondering how to repressurise a Worcester boiler, or one from any other brand, the steps below will walk you through the process. The steps below should work for an external, or keyless internal filling loop.

Keep in mind, though — following these steps may restore normal boiler pressure for a short while, but your boiler will keep losing pressure unless you fix the underlying issue (typically a leak).

Step 1 – Close All Vents to Seal in Water Pressure

Before you begin adjusting the boiler pressure, double check that all bleed valves on radiators and towel rails are fully closed.

If they aren’t closed, they’ll leak water and the boiler will lose even more pressure.

Step 2 – Find and Check Your Filling Loop

Most new combi boilers will have an external filling loop for top up the boiler with water, which increases the pressure.

It’s a braided hose with a small valve on it, and is usually located below the boiler casing.

Step 3 – Open the Boiler Filling Loop

As you open the filling loop, you’ll see the boiler pressure rise. Refer to your boiler owner’s manual to find the exact pressure your boiler needs; however, most boilers operate at around 1.5 bar of pressure. Check the pressure on the water pressure gauge and wait till it reaches 1.5 bar.

Step 4 – Close the Filling Loop

Make sure you close the boiler’s filling loop completely after repressurising. If you don’t, it will allow additional water into the boiler, thus increasing the pressure and causing the boiler to lock out

If the Filling Loop is on the Hot Water Storage Tank

If you don’t have a combi boiler, you may have a setup with a cylinder that serves as a hot water storage tank.

Although the filling loop shouldn’t be fitted there (they’re typically located in view of pressure gauges), there are instances where they are installed on the cylinder. Again, you’re looking for a braided hose with a small fill valve on it; you can follow the same procedure as above to fill up the tank and pressurise your boiler.

How to Repressurise a Boiler When There’s No Filling Loop

You may have a boiler with an internal filling loop, you’ll need to use a filling key to top up the pressure. You can follow the steps below:

Step 1 — Turn the Boiler Off

Make sure that the boiler is turned off and has had some time to cool down.

Step 2 — Find the Filling Key

Most boiler makes and models (like Worcester Bosch, for example) will have this key secured inside the tray at the bottom of the boiler’s casing. You can open the tray and remove the key.

Step 3 — Find the Manifold

Next, you’ll have to find the manifold into which you’ll insert the key. Typically, it’s at the bottom of the boiler, right next to a white, plastic nut.

Once you’ve located the manifold, go ahead and insert the key with its arrow aligned with the open padlock symbol on the manifold.

Step 4 — Turn the Filling Key

After inserting the key, turn it to align the arrow with the closed padlock symbol.

Step 5 — Repressurise the Boiler

Now, you can turn the nut adjacent to the manifold anticlockwise until you feel water rushing into the central heating system. Keep an eye on the pressure gauge, and close the nut once the reading shows 1.5 bar; keep turning until the water stops flowing.

Step 6 — Turn the Nut to Repressurise the Boiler

Return the filling key to the open padlock position and remove it from the manifold (some water may drip out at this point). Then, place the key back in the tray so you don’t lose it. That’s it, you’re done pressurising your boiler!

What to Do If You Repressurise a Boiler Too Much

If you repressurise a boiler too much and the pressure gauge is in the red zone, you’ll want to drain the boiler system. If you don’t you’ll put undue stress on the central heating components and the boiler’s pressure relief valve will be forced to leak some of the water out. However, if the pressure is only high by 0.5 bar or less, you can rectify the problem by bleeding radiators.

Using the bleed key, a bucket to catch the water, and towels to protect carpets in your home, open the bleed valve slowly. This will allow water to escape, and boiler pressure should climb down to its normal working level.

Here’s Why You Shouldn’t Repressurise a Boiler on Your Own

Whilst we explained how to repressurise a boiler if you need to, we’ve also mentioned that doing so doesn’t solve the root cause behind your boiler losing pressure. Unless you fix the underlying pressure issue, your boiler will continue to lose pressure no matter how ofter you repressurise it.

Here’s why.

Central Heating System Leaks Cause Low Pressure

If you are constantly having to repressurise your boiler, you’re dealing with a leak. This leak isn’t necessarily in the boiler; it could be in any part of the central heating system (for instance, a radiator with a pinhole in it).

However, if it’s in the boiler, there’s a chance the water could damage expensive electrical parts, such as the PCB (the boiler’s Printed Circuit Board).

We’ve created guides relating to boiler’s losing pressure, and boiler leaks, in our boiler fault finding guide.

If you’re still struggling with boiler pressure, get a Gas Safe heating engineer to help fix the problem.

Diluting Heating Inhibitor

Inhibitor is a liquid that protects your heating system from sludge.

Over time, the inside of radiators, towel rails, and pipework rust. This rust breaks off, gets broken down, and forms central heating sludge. This sludge can lodge itself in radiators (causing them not to work), the pump, and even the heat exchanger. Those are all things that are expensive to fix.

By constantly topping up a boiler, you’ll be diluting this inhibitor, and leaving your heating system exposed to sludge build up.

To summarise — Fix the leak first, then repressurise your boiler.

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What’s Next?

If you’re having to repressurise your boiler often, seek help from a Gas Safe heating engineer. They should be able to find the leak and fix it for you.

Do you have any questions? Have you ever experienced consistent pressure issues with your boiler? Leave us a comment below and we’ll get back to you as soon as we can.

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