Yes. The Purewell VariHeat cast iron boiler is able to operate on an open vented heating system. We would advise compliance with minimum pressure requirements at boiler, reference guidance note INDG436 (Health and Safety Executive).
Open vent and cold feed pipes must be installed correctly and in compliance with BS6644. You can refer to the installer guide for detailed information on Open Vent and Cold Feed pipes.
However, Hamworthy recommends the use of sealed heating systems, where possible.
If you want to know the difference between a sealed system and open vented system, find out more here.