If you look online at a chimney products site, you will see that you can choose from a variety of different materials for a chimney liner. You will also see that metal chimney liners come in convenient kits, complete with a flexible metal liner and all the necessary fittings to complete your installation. You may be tempted to order an aluminum liner kit, because it is a lot cheaper than stainless steel, or you may want to order a rigid stainless steel liner kit because it is also much cheaper than a flexible stainless steel kit. But, before you buy, you need to be sure you are getting the correct liner for your particular application.
In order to be completely sure you are getting the right product, it would be wise to consult with a chimney specialist or your local building codes to see what the industry standard for your system is. In general, you will need a stainless steel liner for any application that needs to with high high heat or corrosive flue gases. You may be able to use an inexpensive rigid liner, but this will make sense only if you have a flue that is vertical from top to bottom with no offset, or bends, along the way. The labor expense and parts cost involved in connecting special fittings to a rigid liner to make bends in the flue will likely offset the higher cost of a flexible liner.
The only type of appliance you want to attach an aluminum liner to is a low efficiency gas or oil burning boiler or furnace or water heater. The combustion gases leaving a fuel burning appliance contain carbon dioxide and water, and possibly some other compounds, which tend to condense inside a cool flue and cause corrosion damage. A low efficiency appliance wastes a lot of heat by sending very hot gases up the flue, which are typically hot enough to not condense before they are exhausted outside. That is why an aluminum liner is adequate for this type of installation.
If your application will be exhausting lower temperature gases which are likely to condense before they leave the chimney, then you need to choose an appropriate grade of stainless steel for your flexible liner. High efficiency furnaces and other appliances emit lower temperature flue gases, so corrosive condensation is a problem if you do not have the right type of flue liner. For applications such as woodstoves and fireplaces, you have the potential for very high temperatures at times, and creosote and corrosive chemicals condensation at other times (especially when you cool the fire by constricting the damper to keep hot coals all night long). So, you will need a stainless steel liner to avoid corrosion and to withstand high temperatures in the event of a chimney fire ignited by creosote in the flue.
When planning your chimney liner installation, always buy materials that are safe for the use you need them for. Flexible stainless steel liners may cost more than other materials, but they will save you money in the long run and keep your house safer if they are the type of liner you need.