LPG Conventional boilers are also known as LPG open vent boilers or LPG regular boilers. Most of the very old LPG conventional boilers would have a copper hot water cylinder, cold water storage tank, plus a feed and expansion tank (which is usually stored in the loft above the boiler to provide a good head of water), also known as gravity fed.
These LPG conventional boilers also have a circulating pump which sits outside the boiler. Also take into consideration that if you are replacing an old LPG conventional boiler that other parts of the system may be very old also which you need to take into consideration. In many cases the heating controls will also need to be renewed when you are doing an upgrade for your new LPG boiler.