I have a Stanley oil fired cooker in a bungalow. It currently has a fan flue which works fine but causes severe blacking of the outside wall and I think it is too close to a window.(The boiler has just been serviced and pronounced in good nik). The question is: Can I install a vitreous enamel steel flue through the loft to outside or does it have to be twin wall. There is a considerable difference in cost.