thescruff wrote on Jan 19th, 2016, 10:20am:If your not loosing pressure you can't be loosing water
The system pressure seems to stay mostly the same as water loss is compensated by air intake. However when I bleed the radiators the pressure drops; the last time I did this it went down from 1 to 0.5bar.
Today I took the covers off the boiler and had a good look round but there is absolutely no sign of any leak. The boiler has three manual air vents but there was no air when I opened them. It probably has an auto vent ( it's an odd design but similar shape to an auto vent) but that is on the DHW storage tank. I checked the expansion tank as best I could in that it didn't feel like it had any water in it.
So then I'm left thinking I must have a leak in the house but I have a Magnclean fitted to the return next to the boiler and when I opened the bleed valve it also had no air in it.
Completely flummoxed, so I've increased the system pressure from 1 to 1.5bar, my logic is that if there is more pressure in the system it will be less able to draw air in and this seems to have worked because this evening the boiler came on with no sounds of swirling water or bubbles rushing through the pipes. Also the higher pressure will force more water out wherever the leak is so hopefully I can find it without it bringing the ceiling down!
Thanks for all the advice, this is the most bizarre central heating fault I have ever come across and I will post again if and when I finally find out if I have a leak and where it is.