Space Required
Needs space for cold and hot tanks (cylinder). This means using loft space and an airing cupboard. Well suited to larger homes.

Efficiency
So long as you have a modern condensing boiler it will almost match the combi for efficiency.

Cost of Installation
If it's a new installation, two water tanks needed and cylinder. More plumbing involved. Easiest to swap if it's just a boiler replacement.

Demand
Can supply a bath and a shower simultaneously - until the stored water is all used.

Time to Heat Water
Around 30 minutes.

Flow of Water
Hot water is taken from the hot water cylinder. Water is not at mains pressure, and you may feel you need to install a pump for added pressure when showering.

Improving Shower Flow
Can fit a Power Shower or water pump.

In The Event of Breakdown
Can use the immersion heater in the cylinder for hot water, but not central heating.

Ease/Cost of Repairs
The system as a whole is a bit larger than the others, but usually not much to choose between the three. Possibility of pipework to the tanks in the loft getting frozen if not well lagged.

Consideration Before Purchase
Do I have the space available.? How many radiators do I need and hence what size of boiler is required? If replacing a traditional boiler with a System boiler will my existing pipework have problems with the higher pressure the other system exert, or should I stick to the same boiler type? In other words should I just swap like for like?

Space Required
No tanks or cylinder needed, which saves space. Better for smaller properties.

Efficiency
Very efficient, as water is only heated when it is needed.

Cost of Installation
Cheapest to install from scratch.

Demand
Only one major water draw at a time.

Time to Heat Water
Instant hot water at any time.

Flow of Water
Water is at mains pressure, meaning that hot water comes out of the taps as fast as the cold. Unlike the gravity fed conventional boiler system.

Improving Shower Flow
Hot water is delivered from the mains - and delivered at mains pressure. Power Showers cannot be fitted.

In The Event of Breakdown
SOS, we're cold. We have no hot water, and no heating.

Ease/Cost of Repairs
Boiler itself is a bit more cramped and fiddly to work with, but otherwise much the same as the others.

Consideration Before Purchase
Is the mains water pressure in my area good? How many bathrooms are there? How many people are likely to want to bath/shower simultaneously?

Space Required
Cylinder is needed, similar to that in a conventional system, but no tanks in the loft.

Efficiency
Efficient, though it holds hot water in the cylinder like the conventional system. So, some heat loss occurs whilst it sits there waiting to be used.

Cost of Installation
No loft tanks needed, so plumbing work is cut down, though it still needs a cylinder - in the airing cupboard. System boilers will work with traditional cylinder and tank.

Demand
Similar to the conventional system, it suits multiple occupants and more than one bathroom.

Time to Heat Water
Same as a conventional system. The hot water supply is not instant - around 30 minutes.

Flow of Water
Same as the combi. Water is not gravity fed like in a conventional system, it arrives at mains pressure.

Improving Shower Flow
Same as the combi. You can't fit a Power Shower, but it shouldn't be needed.

In The Event of Breakdown
Can use the immersion heater in the cylinder for hot water, but not central heating.

Ease/Cost of Repairs
Most major components are housed in the boiler, making this as easy to maintain as the others.

Consideration Before Purchase
No worries about needing loft space here. Though good water mains water pressure is important, as with the combis. Will happily deliver to more than one bathroom, so consider the size of boiler needed to heat the number of radiators you have.If the System is replacing a conventional boiler is the old pipework up to the increased water pressure?