The thorny question of when to replace the windows in a property occurs only when something serious occurs, such as small floods from leaking seals, or if a window becomes condensated inside the two glazed panes.  In one house my family owned in the mid 1980s, the frames were alluminium – quite the big thing to have in the 1970s, but although they looked sleek and attractive, they were hopeless from the point of view of keeping the room warm and dry.    The condensation would stream down in cold weather and we had drips of water forming across the top bar and down either side.  It looked terrible having to put towels down to catch the damp, but we had to employ this method so as not to have a massive problem with damp moulding carpets.  So the need for changing those windows became urgent and we sourced a supplier who would be able to advise about the irregularly dimensioned patio window in the sitting room.  We found out after moving in that it was far bigger than would be recommended for the size of the wall in was holding up . . .  However we had the problem sorted within 6 months and a few hundred pounds sterling more than envisaged.