My unvented system is losing pressure. I have been checking to see exactly when this happens and it seems to be losing quickest when washing machine or dishwasher are in use. Pressure drops from just above 1 bar to almost zero over a period of a few hours. I have to bleed the towel rail (highest point of the system) every so often so must be a leak somewhere. The pressure stays constant when heating is on and when boiler is running and also stays constant overnight when there is nothing happening except or the ocassional toilet flush. I am just wondering what I should do to start checking. Thanks.