Is it difficult? How much does it cost? Can i do it myself?
This article is intended to cover the main points you need to know, but as always you should consult a qualifed installer to get a definitive answer.
Lining your chimney with a flexible stainless steel flue or chimney is common practice for most HETAS qualifed installers, but we still get people coming to our showroom and asking why they can't just fit a register plate, put a vitreous flue pipe through it into the chimney and just stop there? This is even more common from people who are replacing an open fire with a log burner, as they reason that if the chimney was OK for that, why not for a log burner stove?
Basically, the answer is that an open fire was sending 80% or more of it's heat straight up the chimney and in doing so it was driving the smoke upwards - thus aiding the natural "draw" of the chimney. However, modern wood burning or multi fuel stoves are 75-80% or more heat efficient - meaning that much less of the heat is going up the chimney and is therefore not as efficient in driving the smoke and flue gases safely up and out.
A typical stove fitted with just a flue pipe poking up through a register plate is sending flue gases into a cold chimney, where they can quickly cool and, instead of going up the chimney, they settle back on top of the register plate just above the stove. These gases are still flammable, so any tiny spark getting into the chimney can immediately ignite the fumes and start a deadly chimney fire.
Also, a natural by-product of burning solid fuels is Carbon Monoxide - an odourless and colourless gas which is deadly and kills dozens of people every year. If that is sitting above the register plate and your seals are not intact, it can easily seep back into the room.
Most reputable HETAS installers will not even consider doing an installation if the owner isn't lining the chimney, as they will not be willing to put the signature on the required safety certificate with the risks above already known.
So yes - it's a very good idea to line your chimney. It doesn't even cost that much - you can buy a complete liner and a matching install kit for a medium sized chimney for under £350 and it's money that can quite possibly save your life.
Fitting a liner is quite dangerous, since the liner has to be fed down the chimney from the top, and therefore someone has to go up on the roof. For this reason we would always recommend getting a professional in to do the job.