Question: Hi! We would like to replace our existing furnace with a woodburning-oil combo furnace. The problem is our flue is a 6" insulcrete lined flue about 30 feet high - and the furnace specs call for an 8" flue. Can we downsize it or will that create problems? Thanks! Answer: I'm afraid that you may very well have problems. You flue is quite high, though, and may be able to handle the BTU load of the combo furnace. Add the two fuel BTU inputs together, and ask your heating man for a chart determining the capacity of the 30 foot chimney. Besides chimney capacity, you also have to consider flow, which determines whether the unit will smoke out the loading door when you are fueling. There is no real set answer to this one.