I am fully familiar with gravity feed hot water systems and vented primary heating circuits run from my trivial-to-maintain ancient gas boiler. I keep reading here about expansion vessels, leaking pressure relief valves etc and can't fathom why modern systems seem to have pressurised primaries (ignoring what appear to be devil-spawn Combi boilers).
I know that cars have pressurised circuits to elevate the boiling point of the coolant but central heating flow temperatures are much lower than this especially with condensing temperatures.
Surely all the apparent problems with pressurised systems cant solely be justified by the convenience of not having a little header tank somewhere.
So are there other good reasons for pressurising the primary circuit?
One of these days I expect to have to change my boiler so I'd like to make the leap in understanding that I appear to need to choose, fit and maintain a modern device.
Are there any good text books on domestic heating out there?
TIA
Bob