Thanks To ChrisR
(See also HERE)
Most Combi Boilers and other boilers are Pressurised. The pressure is usually shown on a gauge marked 0 -4 bar. There is often an adjustable plastic marker on the gauge which does nothing!
If the pressure gets to 3 bar the “Pressure Relief Valve” should open and allow water to escape down a pipe to outside. Pressurized boilers cannot explode if they are installed properly!
When water is heated it expands and squashes a volume of air contained in a “Pressure Vessel” usually inside the boiler. The pressure will rise on heating, eg from 1 bar to 1.5 bar, and drop again on cooling. The volume of air will deplete over the course of several years and its pressure should be checked at every service.
1) Pressure TOO LOW
Over time, water will escape from the system which will drop the pressure, as will bleeding air from a radiator. This normally happens at a rate below 0.5 bar in 6 months. When it gets to 0.5 bar or lower the boiler may stop.
If the pressure drops quicker than this you have a leak.
This may be at rad valve spindles or through the Pressure Relief Valve or through the Automatic Air Vent (these MUST be left open, replace if leaking, and do not leave screwed shut). Old PRV’s and AAV’s often need changing because they won’t stop leaking.
Once a PRV has passed water, whether from opening by hand OR overpressure it will tend to keep leaking, usually because of dirt on the valve seat.
Sometimes you can get them to shut better by twisting the knob so they open wide then “snap” shut but usually they will need replacing.
2) Pressure TOO HIGH
This can occur if the Filling loop has been left connected and not quite shut, or the Heat Exchanger in the boiler has perforated and is letting tap water fill the system.
3) Pressure going right UP then right DOWN
If all the air in the PV has gone, the water has nothing to squash. When it expands on heating, the boiler pressure rises more than the usual half bar, eventually to 3 bar. This activates the PRV and water exits through the prv to outside.
When the system cools, the pressure drops right down and may go so low that the boiler turns off. This is often noticed when the Heating is turned on in Autumn because more water is being heated so there’s more expansion of water to accommodate. A behaviour to note is that the pressure rises very quickly on opening the filling loop tap.
Solution is to Repressurise the Pressure Vessel. See faq 7.
Another fault which gives the same symptoms occurs where the PV is fine but the pipe or hose connecting to the PV in the boiler is blocked. (Esp French boilers and Pumas!)
