There is a large difference in the different types of bifold doors available. While this will sound strange coming from a double glazing company but-
Living Rooms do not supply and fit upvc bifold doors
We believe that upvc just expands and contracts in heat and cold too much to manufacture a trouble free bifold door. Our carefully selected aluminium bifold doors are stable, fully thermally broken with hidden pop out handles and guarantee trouble free operation for years to come.
Aluminium is still surprisingly affordable compared to upvc. We wouldn't risk it in our houses, therefore we wouldn't risk it in yours, either.