Maintenance to your sash windows every 5 years should be enough to make them last a lifetime
. If on inspection they need sanding and filling then arrange to do this promptly and also paint the windows after preparation with a good quality paint.
If your sash windows have been painted shut so that you are unable to slide the sashes they can be easily released so they can be moved freely. You have 2 choices depending on how much paint has been applied to your sash windows.
Firstly you can tap a chisel around the window where it has been painted and also use a sharp Stanley knife to gently but firmly cut around where the window has been painted shut, you can now start to try sliding the sashes up and down, if they start to move then keep using the knife to cut around the paint and keep moving the sashes up and down. Once the sashes are moving it is always a good idea to sand down the internal edges of the frame where the paint had built up. Finally spraying WD40 down the inside edges is always a good idea as it will reduces the friction between the sashes and the frame.
If with the above work you still cannot get the sashes to slide you will be left with no option but to remove the sashes, sand down and refit. This is a more time consuming way of freeing the sashes but if you are going to remove the sashes then you may as well thoroughly sand down the sashes, fill and re-sand, Prime the sashes with a quality wood primer and apply a further undercoat. Also fill and sand the box frame and prime and undercoat. Re-fit your sashes and give a final top coat of paint.
Larger maintenance work to wood sash windows can involve draught excluding the sashes and fitting new staff and parting beads to the windows which incorporate a draught brush, this process will eliminate draughts. Slim double glazed units can now be filled with heavy gasses giving a much better u-value than previous standard argon filled units. When making choices on double glazing options new complete windows should have 24mm argon filled units using a low e glass. If just sashes are being replaced you would usually need to go for a slimmer unit and if you wanted to lower the u-value then you can use a heavier (krypton) gas inserted into the unit.