The increased pressure in a sealed system, increases the likelihood of a leak and the amount of water which will be lost from a leak in the system.
I'm firmly on Bernard's side with this...
An open vented system is more complex to install, more expensive to install, but has several advantages over a sealed system.
1. Reduced chances of leakages and hence the system shutting down as a result of tiny amounts of water loss.
2. Generally DIY repairs are simpler, because more of the systems components are easier to access/not in the boiler.
3. Depending on the type of water use, an open vented system can be more economic in providing hot water, because the hot can come through to the tap quicker. Then too, both hot and cold taps can be run at the same time.
4. Often a larger gas supply pipe will be required when an opened vented is replaced with a combi.
My open vented system was installed in around 1984-ish and has had two replacement boilers. Each time the installer has wanted to convert my system to a sealed system. On one occasion a potential installer would not take on the job, because he only fitted combi boilers.
Open vented do have disadvantages..
1. They are much more expensive to install from scratch.
2. Need much more space.
3. A combi can be a cheaper source of hot water, when the owner's life is less predictable and HW use is unpredicatable.
In an earlier life, I was first responder to hundreds of commercial premises, which had heatings systems, ranging between house sized to large offices. The combi boilers gave far more issues than the open vented, the combi systems also took longer to fix and get back in service.
