4 Simple Reasons For Boiler Pressure Anomalies – & How To Repressurise

Have you ever come home to find your boiler pressure is at zero, the system is off, and you can’t turn it on? Cue panic. Depending on the time of year (British weather being the fickle fo that it is) this could prove a teeth-chattering nightmare.

You see, boiler pressure is a delicate balance; It’s a magical equilibrium which must be maintained for your system to cosy your home and keep your baths hot continuously. Too much or too little water pressure inside your boiler, and it won’t work. Simple.

Even if a boiler is pretty much new, there are still things that can go awry. Fortunately, most boiler issues are easily fixed and don’t involve blowing your household budget on a new heating system.

However, from the anxiety of leaking deadly carbon monoxide and killing your entire family, to the horrible thought of exploding boiling water all over your face; when it comes to boiler maintenance, there are seemingly myriad hazards looming.

And don’t get us wrong, unless you’ve done your homework, and know what you’re doing; those threats could become horrifyingly real. If EVER in doubt, contact an expert.

In this week’s blog, we’ll demystify boiler pressure issues and help you to identify any potential problems.

How To Determine Healthy Boiler Pressure

To make it easy for you to check on your pressure levels; modern combi and system boilers have a built-in pressure gauge. This gauge may be a hydraulic pressure dial, or if you have a super fancy sexy new one, it’ll be digital. Whichever version you have – it should be a plain as day for you to read if your pressure is abnormal; i.e., red = unusual, green = healthy.

Small fluctuations in pressure are entirely normal. With a Combi boiler, pressure naturally increases when the heating is on as your boiler combusts fuel to heat the water. However, it’s important to note that after the initial fluctuation, a healthy boiler’s pressure will stabilise once the system is idled and cooled down.

What To Look For

Your pressure gauge should read 1 to 1.5 bars when your central heating is off. When your boiler is active and heating up, it should read around 2 bars. Here’s what to look for;

If you’ve noticed your central heating stopping unexpectedly, your pressure levels are probably off. Your boiler’s pressure dial will be in the red. However, panic not; fixing your pressure levels is way less scary than you think, but the first step is identifying the problem.

Reasons For High Boiler Pressure

Your pressure could be too high for a couple of simple reasons;

1. Your filling loops may be open

Check that the pressure valves (filling loops), usually located underneath your boiler, are secured tightly. Faucets can quickly come loose when knocked accidentally. If they’re open at all, your boiler will continuously be filling up, and in turn, the pressure will steadily be building.

2. An expansion vessel is flat

This one’s a job for a qualified engineer. If your boiler’s pressure rises significantly when your heating is on, then it’s an indicator that the expansion vessel is flat. We do this on every service without fail. Most boilers need a vessel pressure of 1 bar (but we follow manufacturers instructions). It’s rare we attend a service and find the boiler to have adequate pressure in the vessel.

If your boiler has a flat expansion vessel and when heated pressure rises above 3bar, it activates the pressure relief safety device, this signal means water will leave the system and exit the property through the safety device. Then once cooled down the system pressure will drop significantly. But, now it may seem like the weight is low again, and that the system needs topping up again, phew! And round and round and round we go! It’s a vicious circle. However, by doing the job correctly and not taking shortcuts, your engineer will save you time, money and inconvenience.

Reasons For Low Boiler Pressure

Has your boiler pressure recently noticeably dropped? Or is your pressure generally too low? There are several reasons for this.

1. You bled your radiators recently

Bleeding your radiators once in a while is crucial to maintaining a healthy system; however, doing so will create a natural pressure drop in your combi boiler. Although worry not, this is normal, and unless the pressure fails to regain its balance, you’re all right.

*It’s also useful to note; if your radiators are heating sufficiently, they’re hot to touch at the top, and there are no cold patches, you don’t need to bleed them. So there! One less job for the things-to-do-list.

2. You’ve sprung a leak

If you have a hole, you’ll lose pressure, so scour your home for any soggy patches. Leaks in radiators are generally simple to fix. However, occasionally, a leak may spring inside the boiler, which will require the skills of a professional engineer.

If you’ve exhausted this tick list, and your boiler is still acting crazy, it may be time to for a good ol’ re-pressurising session. Surprisingly, this is a job you can safely do yourself.

How To Repressurise Your Boiler

WOOP! The moment you’ve all been waiting for! But before diving in, guns blazing, be sure to check your boiler manual for instructions, and whether it’s ok for you to tackle yourself. Each model is unique, so it’s always good practice to double-check the facts.

Once you’re read-up and ready for action, use the following steps to complete the process:

1. Turn off your boiler and leave it to cool entirely
2. Locate the filling loop and check that both ends are tightly attached. A leaky valve may be the reason for losing pressure so look for water around the valve
3. Open both valves to let cold water enter the system, keeping a close eye on the pressure gauge
4. When the pressure gauge reads 1.5 close both valves
5. If it isn’t built-in, remove the filling loop
6. Switch the boiler on again and check the pressure

WTF Is A Filling Loop?

If you’re reading this part and thinking, WOAH! REWIND! What the heck is a filling loop?! Here you go; filling loops come in two forms: built-in and external. External ones aren’t part of the system, so you’ll need to attach them during Step 2. You’ll then have to remove it before you turn your boiler on again.

Bringing Back The Equilibrium

In re-pressurising your boiler, you’re allowing more water into the system. In the event of over-pressurising the system (easily done), panic not. A quick bleed of your radiators should bring things back to a healthy level.

What Next?

Following your sterling DIY work, be sure to keep a beady eye on the pressure. If the pressure drops again in the weeks to come, you may be leaking somewhere.


So, there you have it! If your boiler pressure is too low, no party. If your boiler pressure is too high, no party. Optimum pressure, 24/7 hot water party! Check for leaks and loose filling valves, and remember to bleed your radiators once or twice a year if necessary.

YET, now and again, your boilers problems could be more severe and way out of the DIY league. If the pressure continues to drop regardless of your fawnings, give us a shout before any permanent damage occurs, you could have a significant leak from within your walls.

On the flip side, if you’ve bled your radiators and checked your valves, and your boiler pressure continues to rise – call for professional help immediately. With nowhere for the excess pressure to escape, the internal health of your boiler is in serious jeopardy.

If your boiler is acting like an unpredictable maniac, or you’re fearful of bodging things up, or you hate DIY – we’re here! Contact us for advice, services or support on 01522 300 744.

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