So you have have an insert or you are going to install an insert, and want to know if you need a liner? Well the short answer is yes. There are two types of liners, the most common with an insert is by far the flexible stainless steel liner. Challenges can be met with the liner installations and the diameter. Some may suggest smaller lines than the flue size of the insert. Beware and NEVER go less than 1/2 inch smaller than the flue size – and for good reason. First the code states that while you can go smaller you must guarantee sufficient draft. Well good luck with that one, the installer puts in a 5 inch liner because it fits – takes your money and runs. Then you have draft problems, well good luck getting them to put in a larger liner – good luck getting your money back. First they will think of every reason they can to explain why it is not their liner size that is the problem and it is something else and they will never give back any money nor replace for a larger liner to make it work. That is the reason why the rule of thumb in the industry is to NEVER downsize unless necessary and NEVER go below 1/2 inch reduction on the liner size – so a 5.5 inch liner on a 6 inch flue size. Some may argue this but it is because they want to make a sale and frankly are not actually concerned about your future ability to use the system. But bottom line is you MUST have a liner in any insert. Because the code states it and because without one – which is known in the industry as a slammer – one can NOT ever clean them. It is not possible and while some technicians may say otherwise – they have never been trained nor do not understand the physics of how the systems work. The code was put in place for one simple reason, as the early installations did not require a liner. But it was found out that without one the systems could NOT EVER be cleaned and WERE TERRIBLY DANGEROUS. The code states: B365 : 6.5.2 When an insert or hearth-mounted stove is installed in a fireplace, a full chimney liner meeting the requirements of ULC S635, Class III, or CAN/ULC-S640 shall be installed in the chimney. The liner shall extend from the appliance to the top of the chimney and be securely attached to both. What Happens If You Don’t Have A Liner? If you don’t have a liner for an existing insert, you should have been told it is not code compliant and to ensure a liner is installed. When you take it down and see the mess, don’t assume the chimney sweep did not do their job as they very likely did it properly. Their job is not to clean up the mess above the metal plate which somebody installed – that is NOT the chimney, smoke chamber or liner. It is an improperly installed device that can not be cleaned and why you need to replace it! What you may not full appreciate is the main reason for the requirement is because it is impossible to clean the smoke shelf area and behind the insert. Some will use a metal plate to seal the old damper area and attempt to place a flue pipe into that section, assuming wrongly that this will allow the smoke to go up the chimney and be okay. Others will simply push the insert into the fireplace and let it vent. Both are wrong, both are impossible to clean and both are dangerous. The insert pushed into the fireplace is the worst and most dangerous as creosote will fall down behind the insert and accumulate. Eventually a fire is inevitable in that area, and while contained in an old fireplace it has been known to physically push the insert into the room due to the expansion of hot gases and air. The second is in theory a little safer as the fire should be contained in the smoke shelf and flue area. Either way though you do not want a chimney fire. Reality is that a chimney sweep, any chimney sweep, can not clean those areas as tools do not exist to clean that, nor should they need to be!! Bottom line is it is an illegal installation and one does not need to carry tools to clean an install that is not code compliant. It should be noted that a sweep may have cleaned it and then told you the situation. However that area in the smoke shelf will still have huge amount of creosote in it. Pulling it down will be a mess and should NEVER be attempted by a DIY homeowner.