Remove the two screws holding the bottom section of the frame. There is one screw either side of the bottom section. Once these have been removed the bottom section can be prized away from the rest of the frame. Be careful not to lever against the glass or you could shatter it. When the bottom section is clear from the rest of the frame you will be able to remove the wheels. Some wheels have a screw holding them in and some are pressed into a holding bracket. If the retaining screws have rusted you can carefully drill them out, taking care not to make the hole in the bottom section any bigger. Contact your local Hardware Store, Glazing Shop or search on Google for replacement wheels.
A visible "kitemark" on the glass is the consumers assurance that the double-glazed unit has been manufactured to British standards. The BSI has numerous standards, including: U-value verification, Window Energy Rating and Window Installation. The lower the U-value, the better the thermal performance and most local authorities will require this to be 1.8 or better to meet building regulations.
Patio doors comprise an outer frame plus individual door frames. These can be made from wood (soft- and/or hard-wood), metal or alloys (usually aluminium), pvc (polyvinyl chloride, a thermo-plastic polymer - the u stands for unplasticised) or a composite material, which may comprise any of the foregoing materials plus grp (glass reinforced polymer). There is also a style known as frameless, where the vertical sides of each door have no frames.
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