How to top up boiler pressure

Topping up boiler pressures can be simple . Learn how, to save on call outs and waiting times


Simple instructions to tackle the most common heating issue in winters, without the need to pay call outs or wait for engineers to attend during the busiest periods  

Explaining boiler pressure 

Boiler pressure is an essential aspect of keeping your home warm and comfortable. It refers to the pressure level inside the system that is used to circulate hot water and heat throughout your house. Adequate boiler pressure is necessary for the efficient functioning of your boiler, and it is essential to maintain it regularly.  

However, many homeowners or landlords are unsure of how to check and maintain their boiler pressure. In this post, we will provide detailed step by step instructions on how to top-up your boiler’s pressure, as well as explain the importance of maintaining proper pressure, the potential issues that can arise from low pressure, and what you can do to prevent them. 

Causes of Low Boiler Pressure  

A leak in the system, a problem with the pressure relief valve, or a problem with the expansion vessel can all cause low boiler pressure. If you have low boiler pressure, you should address it as soon as possible because it can lead to a variety of issues, including decreased efficiency, higher energy bills, and even boiler breakdowns. Similarly high pressure or over pressuring a heating system or boiler can also stop the boiler functioning and cause damage to boiler parts such as the pressure relief valve and the expansion vessel. 

Boiler Cover and help with low boiler pressure 

Purchasing boiler cover is one way to protect yourself from the costs of boiler repairs and breakdowns. Boiler cover typically includes an annual service as well as the cost of repairs and replacements in the event that something goes wrong. However, if your boiler pressure is low, it may not be covered by your boiler cover policy, and you may have to pay additional fees to repressurise your boiler. It is also critical to review the terms and conditions of your boiler cover policy to ensure that low boiler pressure is covered. 

Water Pressure supply to your property  

Water pressure in your property can have an impact on having the correct boiler pressure. If your home’s water pressure is too low, your boiler pressure not be adequately topped up to the required level. The water pressure required to fill a system needs to be at least equivalent to the optimal operating pressure of that system (typically 1.5 bar).  

A pressure gauge, which can be purchased at most home improvement and plumbing stores, can be used to check the water pressure in your home. If your home’s water pressure is too low, you should contact your local water supplier to have it adjusted. If you suspect that a leak in your home’s water system is causing the low water pressure then you should have the leak investigated by a plumber 

Boiler Service Contract  

Maintaining your boiler on a regular basis is crucial for keeping it in good working order. Purchasing a boiler service contract is one way to ensure that your boiler is properly maintained. A boiler service contract typically includes an annual service as well as the cost of any repairs or replacements that are required.  

The engineer will check the pressure level, inspect for leaks, and ensure that all of the components are working properly during the annual service. They will also provide you with a report on the condition of your boiler and recommend any repairs or replacements that are required. This can assist you in identifying and addressing any problems with your boiler before they become major issues. Furthermore, regular maintenance can help your boiler last longer, potentially saving you money in repair and replacement costs. 

How do I top up my boiler pressure? 

Before attempting to tackle low pressure issues and topping up boiler pressure it is first important to understand the system and at all times manufacturers instructions should be followed. The  most common misconception is that only combi boilers are pressurised; there are a number of heating systems within a property that utilises a pressurised system.  

Type of boiler  

  • Combi Boilers 
  • System Boilers  
  • Heat Interface Units (HIUs) 
  • Electric Central Heating systems and others 

The point from which these systems can be topped i.e the filling point can vary, These range from internal built-in filling loop or externally connected running loops.  the purpose of the filling loop is 2 to add water to the central heating system and  the location is not restricted to just within the boiler. In fact a filling loop can be placed almost anywhere within a sealed system. as long as  there is a cold water incoming pipe located somewhere near the heating pipes (the return pipe).  

Built in filling loops   

Built in filling are usually Incorporated within a combi boiler.  These are a small pipework set up typically with two valves that open to let water in from the cold water inlet into the return pipe.  are usually located conveniently below two boiler pressure gauge, for the person filling up the pressure in monitor the gauge and ensure it is not pressurised.  

In addition to built in filling loops that can be operated by hand some boilers are fitted with filling loops that can only be operated by a key. These can typically be seen  

External filling loops. 

These are usually located near the boiler or near the unvented cylinder if one has been installed. In these systems typically an external expansion vessel can also be seen. They normally consist of two feeling rolls and the flexible silver hose that connects these two vowels together.  similar to the built-in filling loops water can be filled into the system they opening both valves to allow the cold water to enter the central heating system.  in some instances The filling loop can be located near the cylinder and the pressure gauge is in a different location within the boiler.  such scenarios  utilising two people if available is a good way off keeping an eye on the pressure to avoided over pressurising.  the other alternative is to do it very slowly and carefully while constantly monitoring the pressure.  

In some instances for example certain HIU setups, There is neither a built-in filling loop nor an external one. Referring to the manufacturer’s instructions will reveal a cold water outlet points central heating return internet point.  Both  these valves usually contain an isolation valve and once the two points are connected together, Isolation valves can be opened to allow water to pass through and pressurise the system.  where most external filling Loops tend to be ½ inch connections, HIU valves tend to be the lodger ¾ inch fittings. If the Original filling hose is not available,  washing machine connection hose to be an ideal fit and long enough to do the job. 

Topping up boiler pressure Step by Step guide: 

When you’re ready to begin topping the pressure, take these steps:  

  • Turn off your boiler and let it finish cooling.  
  • Look for any external leaks present around the boiler or on the heating system 
  • Locate the filling loop and make sure both ends are firmly joined.  
  • With both valves open, let cold water into the system while monitoring the pressure gauge.  
  • When the pressure gauge reads 1.5 bar, close both valves.  

If radiators need to be bled then repeat the process after each radiator has been bled. 

It is important to allow the system to cool down as a running, hot system will display a higher false reading and the recommended 1.5 bar of pressure should be added at room temperature.  

What happens if I Over-pressurise the system? 

If possible this is something that should be avoided but can easily happen by accident.  over pressurising the system is not ideal as it may have an effect on other components of a border for example can trigger the pressure release valve on the boiler. the pressure release valve is a component either built into the boiler or externally Added to the system as a safety measure. It opens to relieve this pressure To avoid damage to the heating system or danger to the occupants of the property.  in hard water areas limescale is deposited around the pressure relief valve,  and once opened it may not see you again fully. This would mean that the pressure relief valve will now constantly leak or drip and it’s something that needs to be replaced by a heating engineer. Therefore over pressurising the system could become a costly mistake, however  this can be avoided and you today it should be rectified by realising the mistake and not running their heating system.  then please points on the radiators can be used to drain excess water from the system and return it to the recommended 1.5 bar pressure.  

The main reason the pressure drops on a central heating system? 

Although they can be many factors for the pressure drop the most common are leaks or trapped air within the central heating system.  


All the components of the pressurised system are connected together and should be considered as one,  there for a leak on any part of the system affects the whole setup and results in a pressure drop.  candy from a number of locations such as boiler parts, connections, radiators and pipework. Whilst most leaks can be found on close examination, some may not be visible such as concealed pipework within walls or under floorboards or also be the pressure release valve as mentioned above which are usually piped to the outside of the property and the continuous dripping maybe missed. It is important to find and repair leaks before topping up the pressure on any system. 

Trapped Air 

These consist  in systems for a number of reasons but can easily be resolved  by removing it from various points that should be installed on any good central heating system.  Typically the bleed valves on the radiators are the usual go to points,but large towel radiators are somewhere where you can get a lot of trapped air as  these tend to be higher than the typical radiators and trapped air rises to the highest point. Problematic system could be 1 with drop down radiator pipes that run perhaps through the loft and drop down to each individual radiator.  these systems can be notoriously difficult to bleed and should have multiple bleed valves fitted to the highest points.  it had these points that the trapped air can be taken out. 

What if I cant find my filling loop? 

Filling loops for pressurised systems are generally located under the boiler or near the unvented cylinder. This so the filling loop can be used whilst the user can see the pressure gauge to avoid under or over pressurising. Unfortunately this is not always the case and sometimes filling loops can be in hidden or concealed locations. A filling loop can effectively be fitted in any location where there is a cold water supply pipe in close vicinity to the central heating return pipe, and in some instances installers can fit them in awkward locations. What is important to remember is that if you have pressurised system that has at some point been functional then you definitely have a filling loop and it may require some searching to locate it. 

Internal and External Built in Filling Loops  for combi boilers  

Another reason you may not be able to find your filling loop easily could be that it is an internal filling loop. 

Internal filling loops are fitted within the boiler and usually operated by a special key. These are most commonly seen on Vaillant and Worcester-Bosch boilers. They are easy to identified and you may need to refer to your manufacturers instructions for guidance. Generally one side of the filling loops is a valve operated by hand and the key can be used to open the second valve, and these are accessible on the underside of the boiler.  

External built in filling loops are valves provided by the manufacturer that can be fitted during or after the installation of the boiler. Like internal filling loops above they are compact and located on the underside of the boiler looping the cold water supply to heating return pipe, separated by two valves. These can be difficult to fit due to lack of manoeuvring space under the boiler but when fitted can be space saving and easy to operate. In addition they are also located in a convenient place directly below the boiler pressure gauge. They can also be convenient if the boiler pipes are not easily accessible e.g. behind a tiled wall in the kitchen. This means a filling loop can be fitted without removing tiles to access the pipes in order to fit a large filling loop that can stick out, or having to find an inconvenient location away from the boiler.  

Final Thoughts  

Maintaining proper boiler pressure is critical for your boiler’s efficiency and the comfort of your home. Topping up the pressure in your boiler is a simple task that can be completed in a few simple steps, but it is important that you do it correctly to avoid any damage to the boiler. Regular maintenance, including annual service, is also essential for keeping your boiler in good working order and avoiding costly breakdowns.  

We have provided detailed instructions on how to top-up your boiler pressure, explained the importance of maintaining proper pressure, discussed potential problems that can arise from low pressure, and suggested ways to avoid them in this post. It’s important to monitor your boiler pressure, have regular boiler services, and have the proper boiler cover. Remember that a properly maintained boiler will not only save you money in the long run, but it will also keep you warm and comfortable during the cold winter months. 


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