I'm currently in the process of planning and material collection for the relocation of my furnace from flue vented to direct vented.
Certainly there are some minuses for switching to a direct vent (more mechanical components equals less reliability, greater electrical cost, etc.). However, after being fortunate enough to obtain a direct vent appliance, a large wood stove, and a generous supply of free wood, there are some good incentives for me to free up my basement chimney flue.
The vent that I have is a Tjernlund SS2, which is listed for zero clearance to combustibles. It will be installed between my floor joists, and has a 4 inch inlet to the blower.
The nature of my question is in regards to the piping that I should choose from the 4 inch inlet on the SS2 to the furnace (Biasi B3 with Riello burner, 58k btu, 6 inch exhaust outlet). I have been unable to find a double walled or otherwise low clearance 4" to 6" increaser (to maintain clearance to the floor joists above), and am flirting around with the idea of using a D.I. 4" pipe. I found one that Duravent manufactures (Pellet Vent Pro). Though marketed for pellet stove applications, the description says that it can be used for an oil burning stove (doesn't specify "furnace" or "boiler"), with a maximum flue gas temp of 570 degrees F.
In waiting to hear back from tech support, I was curious to see if anybody had any insight on whether there is a way to accurately estimate my flue gas temperature (to determine if 570F is acceptable) , or whether there are any other suitable low clearance 4" pipes or adapters out there on the market that I haven't yet come across...
Any feedback is appreciated, and I appreciate your taking the time to read through my post.