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My Boiler Pressure's Too Low. Follow Our Easy Step By Step Guide To Get Your Day back On Track


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Hot water and heating are the two things we always need and generally take for granted; until the day we hear those famous four words “There’s no hot water!!!!” Instantly you start to worry that a call out from your local plumber will cost a small fortune. The good news is, low pressure on a boiler is more common than you’d think and isn’t always a result of a bigger issue. So before you reach for the phone, try avoiding that call out fee by trying to re-pressurise the boiler yourself, with a little help from our step by step guide.

***Please note there are many different, make, models and types of heating systems, this is just a simplified approach that will usually help with most mainstream domestic boilers. If in doubt always refer to the product manual. It’s important that you never tamper, use any tools or open up a boiler, you should only ever need to interact with the control panel. For any other issues, a registered Gas Safe Engineer should be called. This guide is for issues with your boiler pressure only.


How will I know if I have an issue with my boiler pressure?


There are a few signs to look out for, the main ones being no hot water or heating. You can also check your boiler and take a pressure reading. The most common type of household boilers will either have a digital pressure gauge or circular analogue pressure gauge, see how to identify the pressure on both below:


Analogue Gauge

  1. The circular pressure gauge more often than not will be marked with red and green zones.

  2. The needle should be settled between 1 and 1.5 bar (bar is the metric used to measure pressure) when the boiler is off.

  3. If the needle is in the red zone or below 1 bar it may be because your boiler has lost water which then causes low pressure.

  4. If you have the opposite issue and the pressure gauge reads above 2.75 bar (when the boiler is off) you’ll need to reduce this to 1.5 bar. Bleeding the radiators will reduce the pressure, if this isn’t something you feel comfortable with call an engineer to assist as the water will be extremely hot.

Digital Gauge

  1. If you have a digital gauge its even easier to assess the water pressure. The numbers on screen will flash if the pressure is too low or high.


How do I fix a boiler with low pressure?


The easiest and most cost-effective way to solve the issue is to re-pressurise the boiler yourself***

To do this you just need to add more water. There are two different approaches depending on the type of boiler you have:


Approach 1

Internal filling loop (keyed) boilers – This is predominantly found on Worcester boilers. You’ll have a white plastic key that lives next to a small plastic cube underneath your boiler.


Approach 2

External filling loop boilers – Most domestic boilers will have this. This looks like a silver braided metal hose with connectors which attaches the mains water supply to the central heating system.



Here’s your step-by-step guide to re-pressuring a boiler with an internal filling loop (keyed)


  1. First things first, check the pressure gauge on the control panel. If the needle is below 1 bar and in the red you’ll need to add pressure.

  2. Before starting the re-pressurising process make sure to turn off the electrics connecting the boiler, ensure the boiler has cooled down first if it’s been on and is still hot.

  3. With an internal filling loop a special key is required to re-pressurise. This is usually located underneath the boiler. It could be living under a tray so the tray will have to be removed to see the key.

  4. Once you have the key you’ll see it has two lugs on the top and the sides, these will fit into the slots in the filling link. The filling link is found next to a small white cube, which is located under the boiler. Make sure you give the key a good push (they are made to be a tight fit) until the end of the arrow is almost in line with the black plastic.

  5. The next step is to turn the key so it’s facing you.

  6. Then slowly and gently turn the white cube, this is when you’ll start to hear the water flowing into the boiler (this sound is normal and should happen).

  7. Whilst the water is filling keep a close eye on the pressure dial. When the needle is just under the 1.5 bar mark turn the little square cube off.

  8. Yeaaahhhhh, well done, take a sigh of relief, no need for a call out just yet, your boiler is now at the correct pressure. Now turn the key back and take it out.

  9. Last but not least, return your key to it’s home so you know where to find it next time!!!



Here’s your step-by-step guide to re-pressuring a boiler with an external filling loop


  1. First things first, check the pressure gauge on your control panel. If the needle is below the 1 bar and in the red zone you’ll need to add pressure.

  2. Always turn off the electrics to the boiler before re-pressurising so the boiler can cool down first should it still be hot.

  3. You’ll see the filling loop pipe (the silver braided hose) below or next to your boiler.

  4. Before proceeding make sure you check that both ends of the filling loop are tight and secure. One end will be connected to your central heating and the other to your mains water supply.

  5. There should be two handles on your filing loop, one at the top and one at the bottom, both should be at a 90 degree angle to the hose. Turn both these handles slowly, this is when you’ll start to hear the water entering the system. ( or sometimes it may have an isolation type valve , in this case it may be necessary to have to use a flat head type screwdriver driver to open and close the valve).

  6. Make sure to keep an eye on the pressure gauge while the boiler is filling with water. When the needle is just under the 1.5 bar turn both handles off (back to their original 90 degree angle position).

  7. Switch the boiler back on. You may need to press the reset button (check your boiler manual as the boiler may automatically do this when powered back on)

  8. Yeaaahhhhh, well done, take a sigh of relief no need for a call out just yet, your boiler is now at the correct pressure.


Why does my boiler keep losing pressure?


As we’ve talked about above, when you have an issue with low pressure it means that we need to add water to your central heating system (your boiler) so if you keep having to consistently (over short periods of time) re-pressurise your boiler there’s most probably a leak somewhere within your system or in the pressure relief valve.

This could be caused by a faulty expansion vessel, corroded pipes or an airlock in the system. Unfortunately for any of these issues you will need to call out a qualified Gas Safe Registered Engineer.


If you live in the West Yorkshire area feel free to give us a call for advice on 01924 404333

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The Home Design Studio is a trading name of West Yorkshire Plumbing & Heating Services Ltd based in the United Kingdom