Well I have removed an old insert stove, which hardly put any heat in the room and opened up the fireplace, to fit a freestanding stove. I'm going to being doing a DIY install, signed off by building control
The chimney is regularly swept, in good condition and has a decent draw. It also has 2 x 45 deg bends, so I don't fancy trying to install a liner, so I will just be connecting with a short length of flue through a register plate.
Thing is, I can't find any guidance on what length of pipe should be inside the chimney, my gut feeling is that the longer the length of pipe, inside the chimney, the better the draw will be, however, it will be heating the chimney and not the room, so I wonder if there is an appropriate length which ensures a good draw without wasting heat?
It does seem that 300mm is appropriate when connecting to a liner but nothing about an unlined chimney?
I have been through Document J and can't find any info in there, there is limit on the maximum total length but nothing on what should be inside the chimney.
FWIW, the insert stove didn't have anything, just a bracket around the flue opening.
I'm not sure whether a [30cm] longer piece of single skin pipe above the register plate is going to make much/any difference TBH.
For what it's worth my standalone stove has a 60cm vertical pipe out of the top of the stove that then goes through the wall into the external unlined chimney - it doesn't go up the chimney at all and works fine. I have a external soot door at the bottom of the chimney for cleaning purposes.
I installed the stove and has not been signed off.
My inset stove goes into a lined [internal] flue and draws really well. The thing with inset stoves is that they put a lot of heat into the surrounding structure which makes it feel as though it's not putting out much heat, but I can feel the heat coming out of the fireplace a couple of days after the stove was last used.... so it's not as bad as you may think.
I've got a chimney funnel that's sealed into the chimney -then flue. With the register plate sat on the angles of the funnel holding it up
That ensures no build up on the register plate as I've no soot door.
I can sweep without removing anything and everything falls into the sump of the flue -in unscrew that and hoover up the mess.
No issues with draw -if anything I've too much on a windy day. An extra length of flue inside a chimney is unlikely to create extra draw as I cannot see where any extra pressure drop would be created with a pipe in a pipe. But will create voids for build up.
Be cautious about having a long flue into the chimney. You need to avoid any areas where soot can build up and be difficult to remove, deposits of soot can fuel chimney fires. I would also get your local sweep to check the condition of the flue with their camera, mine looked good from top and bottom but many years later when I did some roof work I found a gap in the mortar where fumes had been escaping into the soffits/fascia. A lucky escape!
Flue liners (get the 25/30 year guarantee one) do fit around bends and are easy to sweep and inspect.
Mine is about a foot. As above, ideally you want to be able up pull the flue out to clear debris on top of the register plate.
Last time I pulled the stove out but I suspect it would be possible to remove the fire bricks in the stove and drop the liner down into the stove if it’s not too long