"Ron" wrote in message
...
Would it cost more to run central heating with an unvented hot water
cylinder and system boiler or a combi boiler with no HWC?

R.

The issue is not vented v unvented, that is neither here nor there.
Any system with a stored water vessel will have standing losses (ie constant
heat loss from the heat store)
This is negated if you generate water as required with a combi boiler.
However they cost more and are less reliable.

A new installation is cheaper to install with a combi boiler, which is the
main reason there are so many about.
However, it's debatable whether it's a good idea to rip out an existing
vented sytem and put in an unvented combi.

However if you install a some sort of solar collector, (either thermal or
PV ) you get free hot water for part of the year. But you need the heat
store.

There are further savings if you have local hot water generators (eg
undersink heaters either small storage or instantaneous.).

The big debate with hot water storage vessels in Summer is "Is it cheaper to
turn on the CH or use an immersion heater?"