Are you going to put a double glazed unit in, I have done a number of these on Victorian houses and with the glass being so thick back then I have managed to get a unit in with thinner glass and a small thickness spacer bar and the unit weight has been roughly the same so no problem with using the original cast iron weights.
That's always a choice that has to be made when refurbishing. In this instance I was working with 3mm Glass and the client decided to stay with the same arrangement. Yes thin insulated glass is now available so the difference in weight is probably negligible. I take each case on its own merits, if Double glazing then I would generally advise new Box Sash frames with a wider aspect to accept thicker Sashes. Weights can be added to or alternatively the use of counterbalance systems. Hope this is of interest. Will
