Does your boiler or central heating keep losing pressure?

Topping up your boiler / central heating system is perfectly normal from time to time and could also be required if:

  • You have recently bled a radiator
  • The system has been drained to replace a radiator

However, if you find that you need to top up your boiler pressure weekly, daily or even hourly then there is a problem that needs to be fixed.

Read on as we explore some of the reasons why your boiler could be losing pressure and what to do to resolve them.


What Is Boiler Pressure & How Do I Top It Up?

All Combi boilers, system boilers and boilers on a sealed system are filled through a manually operated filling point somewhere in your central heating system. This may be a filling loop which incorporates 2 valves to allow you to turn on and off connected with a braided hose or some boilers have filling points attached and instructions on how to work these can be found in the manufacturer’s instructions.

You should always bleed the heating system when the water has cooled and once you have bled the radiators then top up the pressure between 1-1.5 bar of pressure on the display gauge on your boiler or system, the actual pressure you set it at will depend on the number of radiators you have and the amount of pipework on the heating system, a rule of thumb is if you have a smaller system the pressure can be set at 1.5 bar and for a larger system 1.0 bar would be better.

The reason we top up the system when it has cooled is that when the central heating heats up the water expands and causes the pressure to increase, this is normal and no need to worry, but if you have the initial pressure too high it means when the water heats up it will over pressurise and will then discharge through the pressure relief valve to outside and this will drop the system pressure to zero or near zero which you will then have to top up the pressure and if you over pressurise it again without being aware of where it should be set  you will be caught in a cycle of topping up the water pressure.

Some heating systems that are bigger may require an extra expansion vessel to allow for the extra expansion of water, always seek advice if you think this may be a case.

Frequently topping up the pressure will eventually lose all the inhibitor from the system which can lead to corrosion within the heating system.

Causes of Frequently Dropping Pressure

Sometimes other things can cause the pressure to drop frequently but these causes will need to be dealt with by a qualified heating engineer but here is what to look out for:

Problem: Leaks

You could have a leak inside your boiler or somewhere on your central heating system and this will cause the system pressure to drop.

What You Can Do

Check for any visible wet marks in the house, check radiators and radiator valves for leaks, check ceilings for damp patches and check there is no leaks under the boiler if you can, unfortunately if the leak is under the floor you will not see it.

Problem: Faulty Pressure Relief Valve 

This valve is generally within your boiler and when it is faulty it will let water out of the system and cause the pressure to drop.

What You Can Do

The pipework from this valve will go to outside so just trace the pipe that goes outside and see if there is any water dripping or running from it which would indicate a faulty valve.

Problem: Flat Or Faulty Expansion Vessel

Generally an expansion vessel is fitted within the boiler but can be fitted external to the boiler as well, an expansion vessel is pressurised with air and allows the expansion of the heating water, when the vessel is flat or faulty it won’t allow the water to expand which will make the system over pressurise and will discharge outside and drop the pressure to zero or near zero.

What You Can Do

You can monitor the pressure gauge on your boiler or system when it is cold, turn on the central heating and if your gauge rises quicker and higher than normal it suggests your vessel needs to be re-pressurised or replaced.

Need more help?

For any pressure problems or other issues with your heating always use a qualified heating engineer.

The above information is so you are more aware of how and why you would need to top up your heating frequently. For more expert advice please call 0141 952 0707.