So you are Flueless ? Here is a bit of info to help you understand which flue you should use and why.......
These are the types of flue
316g Liners
904g Liners
Gas Flex
These come in various sizes, the most common diameter we use are 5" and 6" flues which most appliances require. Sometimes you will be limited to a 5" liner but you still have a wide range of DEFRA approved appliances to choose from that are approved to function on the small diameter flue.