Bet it would have to be pretty thick not to warp. I can see it glowing now
If the air cant move upwards because of a narrowed flue then its going to move faster; cue jet engine effect x 10.
Your exchanger would need to get rid of heat pretty quick
Normally you would control the burning rate by reducing the avilable air but thats not an option unless you seal the stove.
You could of course control it by adding small amounts of fuel often but thats a pain.
Being as you can do things metally; how about a new baffle plate inside the stove where the boiler used to live and seal the stove properly?
You can control them lovely when they are sealed.
I have a story to tell here. Many years ago i bought a Much Wenlock stove with a back boiler on it. It sat filling the inglenook for years as I could never find the time to connect the bloody thing. When I got round to it I found there was so much work involved sealing the thing, removing the back boiler (was full of dry sand from the previous owner but there were too many tales of exploding boilers). making a back plate to replace the boiler, cutting and fittign a baffle plate etc. that I just went out and bought a brand new 5kw little wenlock stove.
http://www.firesonline.co.uk/acatalo...ve.html#aagalw
The heat is insane from it. At full tilt we have to open every door in the house as its so hot. So I run it at half speed when its cold and it tops up the c/h.
I have killed *all* the draughts and dry lined the external stone walls though; this made a huge difference.
I love stoves