"Paul Barker" wrote
| ... I'll be fitting a regular condensing boiler open
| vent with open vent dhwc this week as a new installation,
| because it is a holliday flat so the frequent changes of
| user don't have to be educated about filling loops.

I don't see why weekly or fortnightly tenants should need education about
filling loops. Pressurised systems should not need repressurising unless
there is something wrong like a leak or pressure vessel failure.

| Open vent systems though hasstle for the installer
| (particulalry if he designs his pipe runs badly) are
| more user friendly.

One particular reason for a pressurised system in your application, where
the property is, I surmise, likely to be unoccupied for periods of time, is
that if there is a leak the amount of water in a pressurised system is
finite. If a vented system leaks there is an unlimited amount of water to
cause damage because the system is continually replenished from the header
tank.

Owain