Usually this is a simple job that need to be performed once in a while, sudden loss of pressure or pressure that can't be maintained once topped up indicates a leak in the central heating system or a faulty pressure relief valve amongst several other faults that could need attention. Topping up the pressure will depend on how your boiler was installed, but the most common procedure is as follows.
First of all locate the "filling loop" - This may mean removing some pipe boxing near the boiler, or sometimes looking in the back of the kitchen cupboard below the boiler, but essentially find the major pipes that run into the boiler. The Filling loop normally looks like a 12 inch long silver metal braided flexible hose that is connected to two seperate pipes. At each end of this braided hose will be a small plastic tap or isolation valve that you turn with a flat screwdriver. The tap or valve should normally be set closed on both ends, (tap or slot in valve will run at 90 degrees to the flow of water in the pipe)
Once you have located this filling loop hose, open one of the taps. Then concentrate on the second one, slowly open it to allow water to fill the sysytem - you will hear the water flowing as you do this. Always be in a position where you can shut this tap off quickly by keeping the screwdriver in position or fingers on the tap, then observe whatever pressure guage you can see, this maybe attached to the pipes nearby or on the boiler itself. When the pressure reaches at least 1.5 BAR and no more than 2.0 BAR, turn off the second tap quickly to stop the flow of water. Then turn off the first tap, so that both taps are closed. Leaving one open may cause the system to over-pressure and trigger the pressure release valve which can be a pain in the arse to reset.
Next job will be to make sure there is no air trapped in the system by bleeding all the radiators. If you find a lot of air in the radiators when you bleed them, you will need to top up the pressure again by following same procedure above. The job is complete once all the air is bled from all the radiators and pressure topped up to 1.5 BAR.
Some filling loops may only have one tap, so don't worry if this is the case, just make sure the single tap or valve is properly closed when you're done.
If you follow this procedure, you should be back up and running.
As a foot note to other readers, some boilers have an inbuilt filling loop that require turning valves located at the very bottom of the boiler and suggest reading the boiler instructions on how to use these. Other central heating systems, may have a separate expansion vessel located away from the boiler, often a red or white metal tank the size of a watermelon with a little pressure dial nearby. These expansion vessels often have a tap nearby to top up the pressure. Once again aim for the dial to indicate 1.5 to 2.0 bar when topping up. There may also be a green zone or red indicator needle that shows what the pressure should be with a separate black needle that shows the actual pressure.
Rupert, December 2019
