Central heating system pressure dropping gradually
Jason Baird 27/02/2026 - 7.56 AM
Hello, moved into a house recently built in 2007, it’s an oil heated house, not long after moving in I realised the central heating pressure was very low, around 0.5bar. My Mrs’ dad is a general builder and he took a look and noticed a couple of the rads were hot at the top half and only warm on the bottom half. Bled all rads and We topped up at the filling loop and when the heating was on it was sitting at 2 bar and when off around 1.5 bar, fast forward 2/3 months later it is now sitting at just above 1 bar when on and jus below 1 bar when off. Had a look round all the rads and no leaks visible, none either around the boiler in the airing cupboard, looking for advice before throwing money at it, cheers.
Are you a tradesperson and able to answer this question?
This sounds like a faulty expansion vessel which could be losing its charge or diaphragm is failing, can be a simple fix by recharging the vessel or worse case scenario put a new expansion vessel
The radiators being hot at the top and cool at the bottom sounds like you've got a lot of sludge in the system. Would benefit from a clean (flush the radiators with a cleaning chemical, but make sure to flush it out again so as not to destroy your system).
Re the slow drop in pressure, you've probably got a micro leak somewhere, it could be solved with a leak sealer fluid added to the system (after you've cleaned it). If that doesn't sort it, maybe look at your expansion vessel.
There must be an expansion vessel checked regularly, and top up only when disintegrated from the system. Please mind also to prevent the corrosion inside the system by implementing the inhibitors.
Answered20 February 2026
0
Anonymous user
Re pressurised again and look for an Air vent valve see if there is a leakage on thats valve.and close just a bit and unsaved.also look for any visible leakage.
If you haven’t already, I would start by having a service performed on the boiler to check that all is right there.
As per other comments, the expansion vessel needs to be set to the correct pressure. So ensure any service you buy includes a re-charge of the expansion vessel.
As we are getting closer to Spring you may not need heating for a while, so you can close the flow and return valves on you boiler which will then hold pressure in the boiler. If the pressure falls after a while it will point to the boiler. If the pressure holds however, it is likely to be the heating system. A slow leak somewhere. Check all pipes, radiators and fittings for leaks/moisture and rectify accordingly if any are found.
