We install system boilers or combi boilers, the only time we install a regular boiler is if we can convert the central heating system to being sealed. The reason for this is all about keeping the system clean. Because of the way new condensing boilers (all new boilers are condensing whether they are combi boiler, system boiler or regular boiler) work it is even more important than ever the system is clean. Why it is perfectly possible to clean an open vented central heating system (with tanks in the loft for a regular boiler) it is much more difficult to ensure the system remains clean. This is because if the positioning of the pump and vent pipe is not perfect air can be sucked into the system through the vent pipe. This is bad because air and water cause the radiators to corrode from inside. Sludge then forms which will effect the new boiler much more than an old boiler. This will not happen with a sealed system. To convert the system to sealed we need to fit an expansion vessel and a pressure relief valve. The difference between a regular boiler and a system boiler is that system boilers come with these components built in together with a pump. This means all would be covered by the up to 10 year boiler manufacturers warranty so it makes sense to fit a system boiler.
Sometimes it is not possible to do this because of limited space, under those circumstances we would fit a regular boiler but covert the system.