Chimney-Flue question.. pic

I just purchased a bosca spirit 500 and my efforts to install on the main floor have met defeat with
my wife. Time for a new.....................................................approach. Wife must stay.

My existing clay flue and metal triple wall chimney supported a LOPI wood stove for many years.
( see attached pic). The Metal part out the roof is 6" iirc dual or triple wall and sits atop a clay flue surrounded by cinder-block. This setup goes to the basement where the stove attached about 50" off the floor so there is approx. 40" unseen in clay flue surrounded by concrete. A short horizontal run in thru the concrete wall to a brick wall ( no combustible materials on that wall)
My question is about the size of flue to use coming off the Bosca Pellet 3" into a 4" system with two 90*
elbows and in through the concrete wall into the clay flue..there would be about 2-3 ft of 4" flue at the rear of the stove and the 2 elbows to get it into the existing flue.
IS this the way to approach or would you run a new 4"flue within a flue all the way out? would be difficult at best and expensive.
Anyway looking for opinions on how to approach.
It would be so much easier to put on the main floor and exhaust out through the outside wall... but that's not gonna happen.

Yes putting the stove on the main floor would be the cheapest. I do not recommend putting a pellet stove in the basement unless you live in the basement. The stove is designed to be a spot heat system not a whole house unit.

Eric

I agree with Eric. Keep it upstairs and you can use 3" Pellet vent and keep it cheap and easy. It will heat your home better and the vent will be much easier to clean.

Look up Leafblower Trick here or on YouTube. This is an invaluable tool... Priceless to me. Makes for a very happy and very clean stove. Using your existing chimney will male the LBT impossible and also make heating your house very hard (basement).