hi gary as a plumber i see plenty of benefits to the system using preheated air
1, air rises its giving a push upwards
2, more heat within firebox
3, cleans glass
4, pushes the gas towards the riser
5, slow down the air input to firebox
i see 2 benefits to this system one is at primary air the other is at secondary air
many are striving towards heated primary air and using quick heat for the secondary air im also considering pushing higher heat at the secondary air with the new build
there is draw backs to this effort the heat within the riser can be very hot indeed and any metal that is used is going to take the brunt of very high heat
my idea is to use a firebrick at top of riser mainly to displace some of the vortex tube heat by deflecting some of the heat away from the top