I am just beginning the technical design stage of a retrofit and am looking at providing an air supply for the existing wood stove.
I've noticed a couple of firms supplying chimney systems that draw the air for the stove down the chimney alongside the exahust flue (eg POUJOULAT efficience). These are all rigid systems for new build. Has anyone seen a flexible flue system for retrofit?
I'd imagine them having a specialist terminal, then dropping a second smaller pipe for the air supply down alongside the flue liner.
There are probably very good reasons why they don't, the care needed to make certain the flue is large enough, the care needed to avoid kinks when fitting etc. But it would be a lot simpler than running a air pipe through the sub floor to the outside.

I once fitted a "hole in the wall/picture frame surround", type gas fire for a client. Quite a big machine and very heavy with remote controls. I had to construct a fire resistant chamber in the false stud wall to accommodate it, I digress; however, the balanced flue was just as you describe, with one twinwall flexible pipe inside a larger one. I can't remember what the terminal was like but it all worked fine, I think the exhaust flue was the inner one. In the case of a woodstove I guess it would have to be to facilitate sweeping, and with many wood stoves requiring 5" or 6" flues that would be a start point, and work up. Then deciding if the area remaining of your outer was enough to provide sufficient combustion air.

The bottom, stove end finishing detail may be fun and games though.

I read a translated Finnish study on using external supply air for stoves. The efficiency of the stove dropped by 15% when O Degree external air was used, the conclusion was that there was no advantage to using external air.

Posted By: Viking HouseI read a translated Finnish study on using external supply air for stoves. The efficiency of the stove dropped by 15% when O Degree external air was used, the conclusion was that there was no advantage to using external air.

I guess that would be a plus for the balanced "flue in flue" approach, as the incoming air would be pre-warmed.

Posted By: gyrogeartherefore less dense, therefore less oxygen (one supposes...)

Yes, normally one would expect combustion to go better with colder, denser air so the Finnish report is unusual. One would have to look at the exact circumstances to understand why.

Posted By: gravelld

Posted By: Viking House[...] the conclusion was that there was no advantage to using external air.

(assuming you mean a passive air brick or vent)... while the stove is running? Pretty clear advantage when it isn't running (which is going to be 99% of the time in a low energy house) imo.

Yes, I'd agree. Having changed the passive seal on the outside of the chimney, it seems that there is a constant warm air draft through the chimney itself when the stove is not in use.